Power line protection devices

Abstract

To correctly determine the faulty phase even when CT saturation occurs in a two-phase short circuit fault.SOLUTION: In a power transmission line protection device, a first projection value calculation unit 72 shifts the phase of each of three line-to-line current vectors by a phase angle corresponding to the line angle, orthogonally projects the three phase-shifted line-to-line current vectors onto the corresponding line-to-line voltage vectors, and calculates the lengths of the orthogonal projection vectors as first projection values. A first projection value determination unit 73 determines whether the ratio of the maximum first projection value among the three calculated first projection values to another first projection value is equal to or greater than a first threshold. If the ratio of the maximum first projection value is equal to or greater than the first threshold, the first projection value determination unit 73 determines that two phases related to the line-to-line current vector used in calculating the maximum first projection value are short-circuit fault phases.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present disclosure relates to power line protection devices. [Background technology]

[0002] Distance relays used to protect three-phase AC power transmission lines consist of a total of six distance relay elements: three short-circuit distance relay elements for phases BC, CA, and AB, and three earth fault distance relay elements for phases A, B, and C. For example, for a short-circuit BC phase fault, the short-circuit BC phase distance relay element can correctly measure the fault location on the transmission line, and for an earth fault A phase fault, the earth fault A phase distance relay element can correctly measure the fault location on the transmission line.

[0003] In this way, distance relay elements of the same phase as the faulty phase can make a correct judgment, but distance relay elements of a phase different from the faulty phase cannot make a correct judgment, resulting in a malfunction such as overreach operation (i.e., operation in response to a fault that exceeds the protection area). As a countermeasure, for example, a faulty phase is determined when a fault occurs using a faulty phase determination circuit, and the malfunction is prevented by taking the logical product of the determination result of the faulty phase determination circuit and the operation output of the distance relay element. For example, a circuit disclosed in the following document is known as a faulty phase determination circuit.

[0004] Japanese Patent Laid-Open Publication No. 2004-364376 (Patent Document 1) discloses a ground fault distance relay that determines the faulty phase in the event of a ground fault. Specifically, the ground fault distance relay in this document includes a minimum voltage phase detection means that receives phase voltages as input and detects the minimum voltage phase, a phase determination means that receives three-phase negative-sequence currents and zero-sequence currents as input and determines the negative-sequence current phase that has the smallest vector phase difference from the zero-sequence current, and a phase agreement determination means that determines the faulty phase based on the output of the minimum voltage phase detection means and the output of the phase determination means.

[0005] The fault phase selection device disclosed in Japanese Patent Laid-Open Publication No. 2010-268658 (Patent Document 2) utilizes the fact that in the case of a single-phase-to-ground fault, the line current between two healthy phases does not change before and after the fault. Specifically, the fault phase selection device in this document calculates the difference between the phase currents detected at different times for each phase and calculates the scalar value of the differential line current between each phase based on the calculated differential phase current. The fault phase selection device then calculates a differential scalar value by subtracting the previous scalar value from the current scalar value. If only one of the differential scalar values ​​is below a threshold, it determines the fault phase as a single-phase-to-ground fault. In the case of a two-phase short-circuit fault, the fault phase selection device determines the phase with the largest amplitude among the three phase voltages as the healthy phase and the other two phases as the fault phases. Alternatively, since the impedance of the two fault phases is smaller than that of the other healthy phases in the case of a two-wire short-circuit fault, the fault phase selection device calculates the impedance between each phase and determines the two phases with the smallest impedance as the fault phase. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2004-364376 [Patent Document 2] Japanese Patent Application Laid-Open No. 2010-268658 Summary of the Invention [Problem to be solved by the invention]

[0007] However, the phase determination means in the above-mentioned Japanese Patent Application Laid-Open Publication No. 2004-364376 (Patent Document 1), which uses three-phase negative-sequence currents and zero-sequence current, may make an erroneous determination when CT (Current Transformer) saturation occurs due to a large fault current, such as a short-circuit fault. When CT saturation occurs, the current phase advances and the current magnitude decreases due to the influence of the filter inside the relay, resulting in unnecessary zero-sequence current. As a result, a short-circuit fault may be erroneously determined as a one-phase ground fault.

[0008] Furthermore, the fault phase selection device of JP 2010-268658 A (Patent Document 2) determines the faulty phase in the case of a two-phase short-circuit fault or a two-phase ground fault by comparing three-phase voltages or impedances. Therefore, in the case of a distant fault or a high-resistance fault, the voltage drop and fault current are small, making it difficult to determine the fault.

[0009] This disclosure has been made in consideration of the above problems, and one of its objectives is to provide a power transmission line protection device that has the function of correctly determining the faulted phase even when CT saturation occurs in a two-phase short-circuit fault, and even when a distant fault or a high-resistance fault causes a small voltage change. A typical application example of this disclosure is a distance relay with a faulted phase determination function, but the technology of this disclosure can also be widely applied to other devices for protecting power transmission lines, such as fault location assessment devices. [Means for solving the problem]

[0010] In one embodiment, the power transmission line protection device includes a first projection value calculation unit and a first projection value determination unit. The first projection value calculation unit phase-shifts each of three line-to-line current vectors by a phase angle corresponding to the line angle based on time-series data of currents and voltages detected from a three-phase transmission line, orthogonally projects the three phase-shifted line-to-line current vectors in the directions of the corresponding line voltage vectors, and calculates the lengths of the orthogonal projection vectors as first projection values. The first projection value determination unit determines whether a ratio of a maximum first projection value among the three calculated first projection values ​​to the other first projection values ​​is equal to or greater than a first threshold. If the ratio of the maximum first projection value is equal to or greater than the first threshold, the first projection value determination unit determines that two phases related to the line-to-line current vector used to calculate the maximum first projection value are short-circuit fault phases. [Effects of the Invention]

[0011] According to the above embodiment, the short-circuit fault phase is determined based on the first projection value, so that the fault phase can be correctly determined even when CT saturation occurs in a two-phase short-circuit fault, and even in the case of a distant fault or a high-resistance fault in which the voltage change is small. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a diagram for explaining an overview of a power transmission line protection system for a power system using distance relays. [Figure 2] 2 is a block diagram showing an example of a hardware configuration of the distance relay 10 of FIG. 1. [Figure 3] 3 is a block diagram showing the functional configuration of a calculation processing unit 40 in FIG. 2. FIG. [Figure 4] This is a vector diagram when there is a two-phase short-circuit fault in the BC phase and there is no load current. [Figure 5] FIG. 5 is a diagram showing the relationship between the projection values ​​of the line current vectors after three phase shifts in the vector diagram of FIG. 4. [Figure 6] This is a vector diagram when there is a two-phase short-circuit fault in the BC phase and the load current is forward flowing. [Figure 7] FIG. 7 is a diagram showing the relationship between the projection values ​​of the line current vectors after three phase shifts in the vector diagram of FIG. 6. [Figure 8] This is a vector diagram showing the load current flowing backward due to a two-phase short circuit fault in the BC phase. [Figure 9] FIG. 9 is a diagram showing the relationship between the projection values ​​of three line current vectors in the vector diagram of FIG. 8. [Figure 10] 10 is a diagram for explaining an AC current waveform measured on the secondary side of a CT when the CT is saturated. FIG. [Figure 11] This is a vector diagram of the case where CT saturation occurs in phase B when there is a two-phase short-circuit fault in phase B and there is a load current with forward power flow. [Figure 12] FIG. 12 is a diagram showing the relationship between the projection values ​​of the line current vectors after three phase shifts in the vector diagram of FIG. 11. [Figure 13] This is a vector diagram of the case where CT saturation occurs in the C phase when there is a two-phase short-circuit fault in the BC phase and there is a load current with forward power flow. [Figure 14] FIG. 14 is a diagram showing the relationship between the projection values ​​of the line current vectors after three phase shifts in the case of the vector diagram of FIG. 13. [Figure 15]This is a vector diagram when there is a two-phase ground fault on the BC phase and there is no load current. [Figure 16] FIG. 16 is a diagram showing the relationship between the projection values ​​of the line current vectors after three phase shifts in the vector diagram of FIG. 15. [Figure 17] FIG. 10 is a diagram for explaining a method for estimating the phase of a line voltage vector of the BC phase by using a voltage vector of the A phase or a positive sequence voltage vector in the case of a two-phase short circuit fault in the BC phase. [Figure 18] 4 is a flowchart showing a procedure for determining a faulty phase in the distance relay 10 of the first embodiment. [Figure 19] 10 is a block diagram showing the functional configuration of a processing unit 40 of FIG. 2 in a distance relay according to a second embodiment. FIG. [Figure 20] This is an equivalent circuit in symmetric coordinates for a one-phase ground fault. [Figure 21] This is a vector diagram when there is a one-phase ground fault on phase A and the load current is forward flowing. [Figure 22] FIG. 22 is a diagram showing the relationship between the projection values ​​of three phase-shifted phase current vectors in the vector diagram of FIG. 21. [Figure 23] This is an example of a vector diagram when there is a single-phase ground fault on phase A and there is a load current with backward flow. [Figure 24] FIG. 24 is a diagram showing the relationship between the projection values ​​of three phase-shifted phase current vectors in the vector diagram of FIG. 23. [Figure 25] This is a vector diagram for the case where there is a single-phase ground fault on phase A, there is a backward flow of load current, and the ground fault resistance is high. [Figure 26] FIG. 26 is a diagram showing the relationship between the projection values ​​of three phase current vectors after phase shift in the vector diagram of FIG. 25. [Figure 27] 10 is a flowchart showing a procedure for determining a faulty phase in the distance relay 10 of the second embodiment. [Figure 28] 10 is a flowchart showing a procedure for determining a faulty phase in the distance relay 10 of the second embodiment. [Figure 29] 10 is a block diagram showing the functional configuration of a processing unit 40 of FIG. 2 in a distance relay according to a fourth embodiment. FIG. [Figure 30] 10 is a flowchart showing a procedure for determining a faulty phase in the distance relay 10 of the fourth embodiment. [Figure 31] 3 is a block diagram showing the functional configuration of a processing unit 40 of FIG. 2 in a distance relay according to a modification of the first embodiment. FIG. [Figure 32] 10 is a flowchart showing a procedure for determining a fault in the distance relay according to a modification of the first embodiment. [Figure 33] FIG. 10 is a block diagram showing the functional configuration of the arithmetic processing unit 40 of FIG. 2 in the distance relay according to a modification of the second embodiment. [Figure 34] 10 is a flowchart showing a procedure for determining a fault in a distance relay according to a modification of the second embodiment. [Figure 35] 10 is a flowchart showing a procedure for determining a fault in a distance relay according to a modification of the second embodiment. [Figure 36] FIG. 10 is a block diagram showing the functional configuration of the arithmetic processing unit 40 of FIG. 2 in the distance relay according to a modification of the fourth embodiment. [Figure 37] 10 is a flowchart showing a procedure for determining a fault in a distance relay according to a modification of the fourth embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0013] Each embodiment will be described in detail below with reference to the drawings. In the following, a distance relay will be used as an example of a power line protection device, but the application of the technology disclosed herein is not limited to distance relays. Note that the same or corresponding parts will be given the same reference numerals, and their description will not be repeated.

[0014] Embodiment 1 [Outline of a transmission line protection system using distance relays] Figure 1 is a diagram for explaining the outline of a power transmission line protection system for a power grid using distance relays. In Figure 1, for simplicity, a power transmission line 2 is shown as a single line, but in reality it is a three-phase line.

[0015] Referring to Fig. 1, a transmission line 2 transmits power from a generator 1. A distance relay 10 is a relay for protecting the transmission line of the transmission line 2 from a local substation 3 where the distance relay 10 is installed to a remote substation 4, as well as the transmission line after the remote substation 4. As will be described later with reference to Fig. 3, the distance relay 10 includes a fault phase determination unit 70, a short-circuit distance relay element 81 between each line, and a ground fault distance relay element 82 for each phase.

[0016] The distance relay 10 in Figure 1 acquires information on instantaneous current values ​​from a current transformer (CT) 5 installed on the transmission line 2, and acquires information on instantaneous voltage values ​​from a voltage transformer (CT) 6 installed on the transmission line 2. The distance relay 10 detects a fault in the protection area of ​​the transmission line 2 based on the acquired information on the current and voltage of the transmission line. When the distance relay 10 detects a fault in the protection area of ​​the transmission line 2 (i.e., the operating area of ​​the distance relay 10), it outputs a trip signal to the circuit breaker 7.

[0017] A plurality of zones are set as the operating zone of the distance relay 10. Typically, the operating zone is determined based on the location of the distance relay 10 (more specifically, the location of the current transformer 5 in the local substation 3) and the range of points to be protected. Specifically, zone 1 refers to the zone up to about 80% of the impedance of the transmission line 2 from the local substation 3 to the remote substation 4. Zone 2 refers to the zone up to about 120 to 150% of the impedance of the transmission line 2 from the local substation 3 to the remote substation 4. Zone 2 encompasses zone 1.

[0018] If a fault is detected in zone 1, the distance relay 10 operates immediately. If a fault is detected in zone 2 other than zone 1, the distance relay 10 performs a timed operation (also called a timer operation) in which it operates after a time set by a timer has elapsed.

[0019] A distance relay element of the same phase as the faulty phase can make a correct judgment, but a distance relay element of a phase different from the faulty phase cannot make a correct judgment, resulting in an overreach operation, for example, by erroneously judging zone 2 as zone 1. Therefore, the faulty phase is judged by the faulty phase judgment unit when a fault occurs, and erroneous operation is prevented by taking the logical product of the judgment result of the faulty phase judgment unit and the operation output of the distance relay element.

[0020] [Example of distance relay hardware configuration] Fig. 2 is a block diagram showing an example of the hardware configuration of the distance relay 10 in Fig. 1. The distance relay 10 in Fig. 2 has a configuration similar to that of a so-called digital relay device. Specifically, referring to Fig. 2, the distance relay 10 includes an input conversion unit 20, an A / D (Analog to Digital) conversion unit 30, an arithmetic processing unit 40, and an I / O (Input and Output) unit 50.

[0021] Input conversion unit 20 includes auxiliary transformers 21_1, 21_2, ... for each input channel. Signals representing A-phase current Ia, B-phase current Ib, and C-phase current Ic output from A-phase, B-phase, and C-phase current transformers 5 of FIG. 1, respectively, are input to input conversion unit 20. Signals representing A-phase voltage Va, B-phase voltage Vb, and C-phase voltage Vc output from A-phase, B-phase, and C-phase voltage transformers 6 of FIG. 1, respectively, are also input to input conversion unit 20. Each auxiliary transformer 21 converts the signal input from the corresponding phase of the corresponding one of current transformer 5 and voltage transformer 6 into a signal with a voltage level suitable for signal processing in A / D conversion unit 30 and arithmetic processing unit 40.

[0022] The A / D conversion unit 30 includes analog filters (AF) 31_1, 31_2, ... and sample hold circuits (S / H) 32_1, 32_2, .... The A / D conversion unit 30 further includes a multiplexer (MPX) 33 and an A / D converter (Analog to Digital Converter) 34. The analog filter 31 and the sample hold circuit 32 are provided for each channel of the input signal.

[0023] Each analog filter 31 is a low-pass filter provided to remove aliasing errors during A / D conversion. Each sample-and-hold circuit 32 samples and holds the signal that has passed through the corresponding analog filter 31 at a specified sampling frequency. The sampling frequency is, for example, 4800 Hz. A multiplexer 33 sequentially selects the voltage signals held in the sample-and-hold circuits 32_1, 32_2, .... An A / D converter 34 converts the signal selected by the multiplexer 33 into a digital value.

[0024] The arithmetic processing unit 40 includes a CPU (Central Processing Unit) 41, a RAM (Random Access Memory) 42, a ROM (Read Only Memory) 43, a non-volatile memory 44, and a bus 45 connecting these. The CPU 41 controls the overall operation of the distance relay 10. The RAM 42 and ROM 43 are used as the main memory of the CPU 41. The non-volatile memory 44 is rewritable, such as a flash memory. The rewritable non-volatile memory 44 can store programs, setting values ​​for signal processing, and the like.

[0025] At least a part of the arithmetic processing unit 40 may be configured by a PLD (Programmable Logic Device) such as an FPGA (Field Programmable Gate Array), or may be configured by a dedicated circuit such as an ASIC (Application Specific Integrated Circuit), or may be configured by a combination of a PLD and a dedicated circuit.

[0026] The I / O unit 50 includes a digital input (D / I) circuit 51 and a digital output (D / O) circuit 52. The digital input circuit 51 and the digital output circuit 52 are interface circuits used when exchanging signals between the CPU 41 and an external device. For example, a trip signal is output from the digital output circuit 52 to the circuit breaker 7 in FIG. 1 .

[0027] [Functional configuration of the calculation processing unit] Fig. 3 is a block diagram showing the functional configuration of the arithmetic processing unit 40 of Fig. 2. As shown in Fig. 3, the arithmetic processing unit 40 functionally includes an input data holding unit 60, a faulty phase determination unit 70, a distance relay element 80, and a logic operation unit 90.

[0028] The input data holding unit 60 stores time series data of instantaneous voltage values ​​of each phase and time series data of instantaneous current values ​​of each phase calculated for each sampling period by the A / D conversion unit 30 in Fig. 2 from the present time up to a certain time in the past. The time series data stored in the input data holding unit 60 is updated over time. The input data holding unit 60 corresponds to the RAM 42 in Fig. 2.

[0029] The faulty phase determination unit 70 identifies the phase of a short-circuit fault and the phase of a one-phase ground fault. In the present disclosure, short-circuit faults include two-phase short-circuit faults, two-phase ground faults, and three-phase short-circuit faults. The faulty phase determination unit 70 is realized, for example, by the CPU 41 in FIG. 2 operating in accordance with a program.

[0030] More specifically, as shown in FIG. 3, the fault phase determination unit 70 includes a current change amount determination unit 71, a projection value calculation unit 72, and a projection value determination unit 73.

[0031] The current change amount determination unit 71 determines whether or not a fault exists based on the change in the effective value (or amplitude value) of each line current, and also determines whether or not a single-phase ground fault has occurred. Specifically, the current change amount determination unit 71 calculates the absolute value of the change in the effective value of the line current between before and after the fault occurs. Then, when the absolute values ​​of the change in the effective value of two line currents are equal to or greater than a threshold and the absolute value of the change in the effective value of another line current is less than the threshold, the current change amount determination unit 71 determines that the two phases related to the line current less than the threshold are healthy phases, and the other phase is a ground fault phase. In the present disclosure, the effective values ​​or amplitude values ​​are collectively referred to as "magnitude."

[0032] The projection value calculation unit 72 calculates vectors by shifting the phase of each of the three line-to-line current vectors by the line angle of the transmission line. Here, the line angle is the angle θ of the arctangent function using the resistance component R and the reactance component jX, where R+jX is the positive-sequence impedance of the transmission line. In general, the line angle is between approximately 70° and 89°, and varies depending on the transmission line.

[0033] Next, the projection value calculation unit 72 orthogonally projects the phase-shifted line current vector in the direction of the in-phase line voltage vector and calculates the length of the orthogonally projected vector as a projection value. A projection value is calculated for each of the three line current vectors.

[0034] The projection value determination unit 73 compares the maximum projection value of the three projection values ​​calculated by the projection value calculation unit 72 with each of the other two projection values. If the ratio of the maximum projection value to each of the other projection values ​​is equal to or greater than a threshold, the projection value determination unit 73 determines that the two phases related to the line-to-line current vector used to calculate the maximum projection value are short-circuit fault phases.

[0035] The distance relay element 80 measures the impedance value up to the fault point and determines whether the value is within the protection range. As shown in Fig. 3, the distance relay element 80 includes a short-circuit distance relay element 81 between each line (i.e., AB phase, BC phase, CA phase) and a ground fault distance relay element 82 for each phase (i.e., A phase, B phase, C phase).

[0036] The logic operation unit 90 detects the occurrence of a fault based on the logical AND of the determination result of the fault phase determination unit 70 and the determination result of the distance relay element 80, and outputs a trip signal to open the circuit breaker when the occurrence of a fault is detected. For example, if the fault phase determination unit 70 determines that there is a short-circuit fault in the BC phase, and the BC phase short-circuit distance relay element 81 of the distance relay element 80 determines that the fault is within the protection range, the result of the logical AND operation is true, and the logic operation unit 90 outputs a trip signal.

[0037] [Operation principle of the faulty phase determination unit in the case of a short-circuit fault] Next, the operating principles of the projection value calculation unit 72 and the projection value determination unit 73 of the fault phase determination unit 70 in Fig. 3 will be described in the case of a short-circuit fault. First, a short-circuit fault when there is no load current, i.e., the simplest case, will be described. Next, a short-circuit fault when there is load current (forward power flow, backward power flow) will be described. After that, a case where the current transformer is saturated due to the short-circuit fault current will be described.

[0038] Note that forward current refers to current flowing forward from the relay installation point (i.e., toward the protected area of ​​Zone 1 of the distance relay), and backward current refers to current flowing backward from the relay installation point.

[0039] In the following description, a two-phase short-circuit fault in the BC phase is referred to as 2φSBC, a two-phase ground fault in the BC phase is referred to as 2φGBC, and a one-phase ground fault in the A phase is referred to as 1φGA.

[0040] <<1. When there is a two-phase short circuit fault on the BC phase and no load current>> Figure 4 is a vector diagram for the case where there is a two-phase short-circuit fault in phase BC and no load current. In Figure 4, the voltage vectors of phases A, B, and C before the fault are Va', Vb', and Vc', respectively, and the voltage vectors of phases A, B, and C during the fault are Va, Vb, and Vc, respectively. As shown in Figure 4, Va = Va'. Furthermore, since Vb + Vc = Vb' + Vc' = -Va', the end points of the voltage vector Vb during the fault and the voltage vector Vc during the fault are located on the line segment connecting the end points of the voltage vector Vb' before the fault and the voltage vector Vc' before the fault.

[0041] Figure 4 also shows the line voltage vector Vab = Va-Vb of the AB phase during the fault, the line voltage vector Vbc = Vb-Vc of the BC phase during the fault, and the line voltage vector Vca = Vc-Va of the CA phase during the fault.

[0042] On the other hand, the load current vectors of phases A, B, and C before the fault are 0. If the fault current vector of phase B is Ibf and the fault current vector of phase C is Icf, the current vector Ia during the fault in phase A is 0, the current vector Ib during the fault in phase B is Ibf, and the current vector Ic during the fault in phase C is Icf. The magnitude of the fault current vector Ibf in phase B is equal to the magnitude of the fault current vector Icf in phase C.

[0043] In this case, the line-to-line current vector Iab = Ia - Ib of the AB phase during the fault is -Ibf, the line-to-line current vector Ibc = Ib - Ic of the BC phase during the fault is Ibf - Icf = 2Ibf, and the line-to-line current vector Ica = Ic - Ia of the CA phase during the fault is Icf = -Ibf. Therefore, the magnitude of the line-to-line current vector Ibc of the short-circuit fault phase is twice the magnitude of the other line-to-line current vectors Iab and Ica.

[0044] Here, since the fault current flows from the B-phase transmission line to the C-phase transmission line through the short-circuit fault point, the angle between the line-to-line current Ibc of the BC phase during the fault and the line-to-line voltage of the BC phase during the fault is equal to the line angle θ. In other words, when there is no load current, the phase of the vector obtained by shifting the line-to-line current vector of the short-circuit fault phase by the line angle coincides with the phase of the line-to-line voltage vector of the short-circuit fault phase.

[0045] FIG. 5 is a diagram showing the relationship between the projection values ​​of the line current vectors after three phase shifts in the vector diagram of FIG.

[0046] In Figure 5, the directions of the line-to-line current vectors Iab, Ibc, and Ica are aligned to the right in the figure (i.e., 0° from the origin O). Using this direction as a reference, the following are shown: vector Iab∠θ, which is obtained by phase-shifting the AB-phase line-to-line current vector Iab by the line angle θ; vector Ibc∠θ, which is obtained by phase-shifting the BC-phase line-to-line current vector Ibc by the line angle θ; and vector Ica∠θ, which is obtained by phase-shifting the CA-phase line-to-line current vector Ica by the line angle θ. As shown in Figure 5, the magnitude of the phase-shifted line-to-line current vector Ibc∠θ of the faulted BC phase is twice the magnitude of each of the phase-shifted line-to-line current vector Iab∠θ of the AB phase and the phase-shifted line-to-line current vector Ica∠θ of the CA phase.

[0047] Furthermore, in Figure 5, the AB phase line voltage vector Vab is shown based on the direction of the AB phase line current vector Iab (i.e., the 0° direction), the BC phase line voltage vector Vbc is shown based on the direction of the BC phase line current vector Ibc (i.e., the 0° direction), and the CA phase line voltage vector Vca is shown based on the direction of the CA phase line current vector Ica (i.e., the 0° direction).

[0048] 5 further shows the length Mab of the orthogonal projection of the phase-shifted line current vector Iab∠θ in the direction of the line voltage vector Vab for the AB phase, the length Mbc of the orthogonal projection of the phase-shifted line current vector Ibc∠θ in the direction of the line voltage vector Vbc for the BC phase, and the length Mca of the orthogonal projection of the phase-shifted line current vector Ica∠θ in the direction of the line voltage vector Vca for the CA phase. In this disclosure, the orthogonal projection lengths Mab, Mbc, and Mca are also referred to as projection values. Note that, for ease of illustration, FIG. 5 indicates symbols for the projection values ​​at the foot of a perpendicular line drawn from the end point of the phase-shifted line current vector to the corresponding line voltage vector and its extension.

[0049] As shown in Figure 5, the length Mbc of the orthogonal projection of the line-to-line current vector Ibc∠θ after the BC phase shift in the direction of the line voltage vector Vbc is equal to the magnitude of the vector Ibc∠θ. On the other hand, the length Mab of the orthogonal projection of the line-to-line current vector Iab∠θ after the AB phase shift in the direction of the line voltage vector Vab is smaller than the magnitude of the vector Iab∠θ. Similarly, the length Mca of the orthogonal projection of the line-to-line current vector Ica∠θ after the CA phase shift in the direction of the line voltage vector Vca is smaller than the magnitude of the vector Ica∠θ.

[0050] The reason for this is that, first, the magnitude of the post-phase-shift line-to-line current vector Ibc∠θ of the faulted phase BC is twice the magnitude of the post-phase-shift line-to-line current vector Iab∠θ of the faulted phase AB and the post-phase-shift line-to-line current vector Ica∠θ of the faulted phase CA. Second, for the faulted phase BC, the post-phase-shift line-to-line current vector Ibc∠θ and the line-to-line voltage vector Vbc are in phase, whereas for the non-faulted phases AB and CA, the phase difference between the post-phase-shift line-to-line current vectors Iab∠θ, Ica∠θ and the line-to-line voltage vectors Vab, Vca is larger. Therefore, when comparing the projection values ​​Mab, Mbc, and Mca, the projection value Mbc of the faulted phase is more than twice the projection values ​​Mab and Mca of the other phases.

[0051] From the above considerations, in the case of a short circuit fault in the BC phase, the projection values ​​Mab, Mbc, and Mc are as follows: Mbc ≥ α · Mca …(1A) Mbc ≥ α · Mab …(1B) The following relational expression holds. In the above expressions (1A) and (1B), α>1, and for example, α is selected in the range of 1.2 to 1.5 with a margin. When there is no load current as described above, α=2 provides sufficient margin, but considering the presence of a load current and CT saturation, α is set to 1.2 to 1.5 with an additional margin. Therefore, when both the above expressions (1A) and (1B) hold, i.e., when the ratio of the maximum projection value Mbc to the other projection values ​​Mab and Mc is equal to or greater than the threshold value α, the projection value determination unit 73 in FIG. 3 determines that a two-phase short-circuit fault has occurred in the BC phase corresponding to the maximum projection value Mbc.

[0052] <<2. When there is a load current (forward power flow) due to a two-phase short-circuit fault on the BC phase>> FIG. 6 is a vector diagram showing the case where a two-phase short-circuit fault occurs in the BC phase and the load current flows forward.

[0053] Referring to Figure 6, the voltage vectors are the same as in Figure 4. That is, the voltage vectors before the fault for phases A, B, and C are Va', Vb', and Vc', respectively, and the voltage vectors during the fault for phases A, B, and C are Va, Vb, and Vc, respectively. In this case, Va = Va', and Vb + Vc = Vb' + Vc'. Figure 6 also shows the line-to-line voltage vectors Vab, Vbc, and Vca during the fault, as in Figure 4.

[0054] Regarding the current vectors, the current vectors before the fault (i.e., the load current vectors of the forward power flow) for phases A, B, and C are Ia', Ib', and Ic', respectively, and the current vectors during the fault for phases A, B, and C are Ia, Ib, and Ic, respectively. Furthermore, the fault current vector for phase B is Ibf, and the fault current vector for phase C is Icf. As in the case of Figure 4, Ibf = -Icf holds.

[0055] In this case, for phase A, which is the healthy phase, Ia = Ia'. The current vector Ib during a fault in phase B is equal to the vector sum of the load current vector Ib' of phase B and the fault current vector Ibf of phase B. That is, Ib = Ib' + Ibf. The current vector Ib during a fault in phase B when there is forward power flow is a leading phase of the current vector Ibf during a fault in phase B when there is no load current. Similarly, the current vector Ic during a fault in phase C is equal to the vector sum of the load current vector Ic' of phase C and the fault current vector Icf of phase C. That is, Ic = Ic' + Icf. The current vector Ic during a fault in phase C when there is forward power flow is a leading phase of the current vector Icf during a fault in phase C when there is no load current.

[0056] Figure 6 also shows the line-to-line current vector Iab = Ia - Ib of the AB phase during the fault, the line-to-line current vector Ibc = Ib - Ic of the BC phase during the fault, and the line-to-line current vector Ica = Ic - Ia of the CA phase during the fault. As shown in Figure 6, the magnitude of the line-to-line current vector Ibc of the BC phase, which is the faulty phase, is larger than the magnitudes of the other line-to-line current vectors Ica and Iab.

[0057] FIG. 7 is a diagram showing the relationship between the projection values ​​of the line current vectors after three phase shifts in the vector diagram of FIG.

[0058] In Fig. 7, as in Fig. 5, the directions of the line-to-line current vectors Iab, Ibc, and Ica are aligned to the right of the figure (i.e., the direction 0° from the origin O). Then, using this direction as a reference, vectors Iab∠θ, Ibc∠θ, and Ica∠θ are shown, which are obtained by shifting the line-to-line current vectors Iab, Ibc, and Ica by the line angle θ. As shown in Fig. 7, the magnitude of the phase-shifted line-to-line current vector Ibc∠θ of the faulted phase, BC phase, is larger than the magnitudes of the other phase-shifted line-to-line current vectors Ica∠θ and Iab∠θ.

[0059] Furthermore, as in the case of Fig. 5, the line voltage vectors Vab, Vbc, and Vca are shown relative to the directions of their corresponding line current vectors Iab, Ibc, and Ica (the 0° direction in Fig. 7). Unlike the case of Fig. 5, due to the presence of a forward power flow load current, the direction of the phase-shifted line current vector Ibc∠θ of the faulted phase, BC phase, is not the same as the direction of the corresponding line voltage vector Vbc, but is shifted in the leading phase direction relative to the line voltage vector Vbc. However, the phase difference between the phase-shifted line current vector Ibc∠θ and the line voltage vector Vbc is smaller than the phase differences between the other phase-shifted line current vectors Iab∠θ, Ica∠θ and the corresponding line voltage vectors Vab, Vca.

[0060] 7 further illustrates, as in FIG. 5, the length Mab of the orthogonal projection of the phase-shifted line current vector Iab∠θ in the direction of the line voltage vector Vab for the AB phase, the length Mbc of the orthogonal projection of the phase-shifted line current vector Ibc∠θ in the direction of the line voltage vector Vbc for the BC phase, and the length Mca of the orthogonal projection of the phase-shifted line current vector Ica∠θ in the direction of the line voltage vector Vca for the CA phase. In this disclosure, the orthogonal projection lengths Mab, Mbc, and Mca are also referred to as projection values. For ease of illustration, symbols for the projection values ​​are written at the foot of a perpendicular line drawn from the end point of the phase-shifted line current vector to the corresponding line voltage vector and its extension.

[0061] As shown in Fig. 7, the above-mentioned formulas (1A) and (1B) hold true for the projection values ​​Mab, Mbc, and Mca. Even when a load current is present, the formulas hold true when α is set to 1.2 to 1.5. Therefore, when both formulas (1A) and (1B) hold true, i.e., when the ratio of the maximum projection value Mbc to the other projection values ​​Mab and Mca is equal to or greater than the threshold value α, the projection value determination unit 73 in Fig. 3 can determine that a two-phase short-circuit fault has occurred in the BC phase corresponding to the maximum projection value Mbc.

[0062] The reason why the above judgment formula holds is that, first, even when there is a load current with forward power flow, the magnitude of the line-to-line current vector of the BC phase, which is the short-circuit fault phase, is larger than the magnitude of the other line-to-line current vectors. Second, with regard to the phase difference between the line-to-line current vector after the phase shift and the corresponding line voltage vector, even when there is a load current with forward power flow, the phase difference in the case of the BC phase, which is the short-circuit fault phase, is closer to 0° than the phase differences in other cases.

[0063] <<3. When there is a load current (backward power flow) due to a two-phase short-circuit fault on the BC phase>> Fig. 8 is a vector diagram for the case where a two-phase short-circuit fault in the BC phase occurs and a load current flows backward. The voltage vectors in Fig. 8 are the same as those in Fig. 6 for the case where a load current flows forward, and therefore will not be described again.

[0064] Regarding the current vectors, the current vectors before the fault in phases A, B, and C (i.e., the load current vectors of the backward flow) are denoted as Ia", Ib", and Ic", respectively, and the current vectors during the fault in phases A, B, and C are denoted as Ia, Ib, and Ic, respectively. As an example of backward flow, we will explain the case where the load current vectors Ia", Ib", and Ic" are equal in magnitude but opposite in direction to the load current vectors Ia', Ib', and Ic' of the forward flow described above. In this case, Ia" = -Ia', Ib" = -Ib', and Ic" = -Ic. Furthermore, in Figure 8, the fault current vector of phase B is denoted as Ibf, and the fault current vector of phase C is denoted as Icf. As in Figures 4 and 6, Ibf = -Icf holds.

[0065] In this case, for phase A, which is the healthy phase, Ia = Ia" . The current vector Ib during a fault in phase B is equal to the vector sum of the load current vector Ib" of phase B and the fault current vector Ibf of phase B. That is, Ib = Ib" + Ibf . The current vector Ib during a fault in phase B when there is backward power flow is lagging in phase compared to the current vector Ibf during a fault in phase B when there is no load current. Similarly, the current vector Ic during a fault in phase C is equal to the vector sum of the load current vector Ic" of phase C and the fault current vector Icf of phase C. That is, Ic = Ic" + Icf . The current vector Ic during a fault in phase C when there is backward power flow is lagging in phase compared to the current vector Icf during a fault in phase C when there is no load current.

[0066] Figure 8 also shows the line-to-line current vector Iab = Ia - Ib of the AB phase during the fault, the line-to-line current vector Ibc = Ib - Ic of the BC phase during the fault, and the line-to-line current vector Ica = Ic - Ia of the CA phase during the fault. As shown in Figure 8, the magnitude of the line-to-line current vector Ibc of the BC phase, which is the faulty phase, is larger than the magnitudes of the other line-to-line current vectors Ica and Iab.

[0067] FIG. 9 is a diagram showing the relationship between the projection values ​​of three line current vectors in the vector diagram of FIG.

[0068] 5 and 7, in Fig. 9, the directions of the line current vectors Iab, Ibc, and Ica are aligned to the right in the figure (i.e., the direction of 0° from the origin O). Then, using this direction as a reference, vectors Iab∠θ, Ibc∠θ, and Ica∠θ are shown, which are obtained by shifting the line current vectors Iab, Ibc, and Ica by the line angle θ, respectively.

[0069] 5 and 7, in Fig. 9, the line voltage vectors Vab, Vbc, and Vca are shown relative to the directions of the corresponding line current vectors Iab, Ibc, and Ica (the 0° direction in Fig. 9). Unlike in Fig. 7, in the case of a load current with a backward power flow, the direction of the line current vector Ibc∠θ after a phase shift of the faulted phase BC phase is shifted in the lagging phase direction relative to the corresponding line voltage vector Vbc.

[0070] 5 and 7, FIG. 9 also shows the length Mab of the orthogonal projection of the phase-shifted line current vector Iab∠θ in the direction of the line voltage vector Vab for the AB phase, the length Mbc of the orthogonal projection of the phase-shifted line current vector Ibc∠θ in the direction of the line voltage vector Vbc for the BC phase, and the length Mca of the orthogonal projection of the phase-shifted line current vector Ica∠θ in the direction of the line voltage vector Vca for the CA phase. In this disclosure, the orthogonal projection lengths Mab, Mbc, and Mca are also referred to as projection values. For ease of illustration, symbols for the projection values ​​are written at the foot of a perpendicular line drawn from the end point of the phase-shifted line current vector to the corresponding line voltage vector and its extension.

[0071] As shown in Figure 9, the above-mentioned formulas (1A) and (1B) hold true for the projection values ​​Mab, Mbc, and Mca. Furthermore, even when there is a backward load current, the formulas hold true when α = 1.2 to 1.5. Therefore, when both formulas (1A) and (1B) hold true, i.e., when the ratio of the maximum projection value Mbc to the other projection values ​​Mab and Mca is greater than the threshold value α (α > 1), the projection value determination unit 73 in Figure 3 can determine that a two-phase short-circuit fault has occurred in the BC phase corresponding to the maximum projection value Mbc.

[0072] The reason why the above judgment formula holds is that, first, even when there is a backward flow load current, the magnitude of the line-to-line current vector of the BC phase, which is the short-circuit fault phase, is larger than the magnitude of the other line-to-line current vectors. Second, with regard to the phase difference between the line-to-line current vector after phase shift and the corresponding line voltage vector, even when there is a backward flow load current, the phase difference in the case of the BC phase, which is the short-circuit fault phase, is closer to 0° than the phase differences in other cases.

[0073] <<4. When there is a two-phase short circuit fault on the B-phase and the B-phase CT is saturated with forward power flow>> Next, we will explain what happens when CT saturation occurs. CT saturation refers to the magnetic flux saturation that occurs in the iron core of the current transformer 5 used to protect the transmission line when a large current flows through the transmission line 2 due to a short-circuit fault or the like. When the iron core becomes magnetically saturated, the primary current waveform of the CT is not transmitted to the secondary side, and the secondary current drops. As a result, the voltage generated on the secondary side of the current transformer 5 (i.e., the signal representing the AC current flowing through the transmission line 2) becomes close to zero.

[0074] Figure 10 explains the AC current waveform measured on the secondary side of a CT when the CT is saturated. Referring to Figure 10, the thin solid line waveform (a) is the current waveform when there is no CT saturation during a short-circuit fault. The dashed line waveform (b) is the current waveform when CT saturation occurs in the case of current waveform (a). As shown in current waveform (b), when a fault current containing DC occurs, the magnetic flux in the CT increases from the CT's residual magnetic flux as the current increases, and when the saturation magnetic flux is reached, the CT's secondary current waveform drops to near zero. When the fault current is in the opposite phase, saturation is eliminated, and waveform (b) becomes equal to waveform (a). The magnetic flux in the CT decreases due to the opposite phase current, but then increases again when the fault current phase returns to its original state. When the saturation magnetic flux is reached, the CT is again saturated.

[0075] Whether or not CT saturation occurs depends on the polarity and magnitude of the CT residual magnetic flux and the current phase when the fault occurs. Therefore, when a B-phase short-circuit fault causes currents of the same magnitude but opposite phase to flow in the B-phase and C-phase transmission lines, it is more likely that CT saturation will occur in one of the phases than in both the B-phase and C-phase CTs.

[0076] The thick solid waveform c) is the current waveform after the current signal having waveform b) passes through the filter inside the relay. It can be seen that the filter has caused the current magnitude to decrease and the current phase to advance.

[0077] Figure 11 is a vector diagram showing the case where CT saturation occurs in phase B when there is a forward load current due to a two-phase short-circuit fault in phase B. The voltage vectors Va', Vb', Vc' before the fault in each phase, the voltage vectors Va, Vb, Vc during the fault in each phase, the load current vectors Ia', Ib', Ic' before the fault in each phase, the fault current vectors Ibf, Icf in phases B and C, and the current vector Ic = Icf + Ic' during the fault in phase C are the same as in Figure 6, so the description will not be repeated.

[0078] On the other hand, the current vector Ib during the B-phase fault decreases in magnitude compared to the current vector Ib = Ibf + Ib' during the fault when the CT is not saturated, and becomes a vector whose phase is shifted in the leading direction. As a result, the line-to-line current vector Ica of the CA phase is the same as in Figure 6 when there is no CT saturation, but the line-to-line current vectors Iab and Ibc of the AB and BC phases decrease in magnitude and advance in phase compared to Figure 6 when there is no CT saturation.

[0079] Fig. 12 is a diagram showing the relationship between the projection values ​​of three phase-shifted line current vectors in the vector diagram of Fig. 11. As in the case of Fig. 7 where there is no CT saturation, the directions of the line current vectors Iab, Ibc, and Ica are aligned to the right in the diagram (i.e., the direction 0° from the origin O). Then, with this direction as the reference, vectors Iab∠θ, Ibc∠θ, and Ica∠θ are shown, which are obtained by shifting the line current vectors Iab, Ibc, and Ica by the line angle θ, respectively. Furthermore, line voltage vectors Vab, Vbc, and Vca are shown with the directions of the corresponding line current vectors Iab, Ibc, and Ica (the 0° direction in Fig. 12) as the reference.

[0080] Because CT saturation occurs in the B phase, the lagging phase angle of the AB phase line voltage vector Vab in FIG. 12 relative to the corresponding post-phase-shift line current vector Iab∠θ increases compared to the case in FIG. 7. Also, the lagging phase angle of the BC phase line voltage vector Vbc in FIG. 12 relative to the corresponding post-phase-shift line current vector Ibc∠θ increases compared to the case in FIG. 7.

[0081] However, it can be seen that the projection value Mbc corresponding to the BC phase, which is the short-circuit fault phase, is sufficiently larger than the projection value Mab corresponding to the AB phase and the projection value Mca corresponding to the CA phase. That is, in the case of a two-phase short-circuit fault in the BC phase, even if CT saturation of the degree shown in FIG. 10 occurs in the B phase, the above-mentioned judgment formulas (1A) and (1B) hold when α = 1.2 to 1.5. Therefore, the projection value judgment unit 73 in FIG. 3 can determine that a two-phase short-circuit fault has occurred in the BC phase corresponding to the maximum projection value Mbc when both of the above formulas (1A) and (1B) hold, i.e., when the ratio of the maximum projection value Mbc to the other projection values ​​Mab and Mca is greater than the threshold α (where α > 1).

[0082] <<5. When a two-phase short circuit fault occurs on the B-C phase and the C-phase current is saturated with forward power flow>> Figure 13 is a vector diagram showing the case where CT saturation occurs in phase C when there is a forward load current due to a two-phase short-circuit fault in phase B. The voltage vectors Va', Vb', Vc' before the fault in each phase, the voltage vectors Va, Vb, Vc during the fault in each phase, the load current vectors Ia', Ib', Ic' before the fault in each phase, the fault current vectors Ibf, Icf of phases B and C, and the current vector Ib = Ibf + Ib' during the fault in phase B are the same as in Figure 6, and therefore will not be described again.

[0083] On the other hand, the magnitude of the current vector Ic during the C-phase fault, which is the current vector Ic = Icf + Ic' during the fault when the CT is not saturated, decreases and the vector is shifted in the phase leading direction. As a result, the line-to-line current vector Iab of the AB phase is the same as in Figure 6 when there is no CT saturation, but the line-to-line current vectors Ibc and Ica of the BC and CA phases decrease in magnitude and advance in phase compared to Figure 6 when there is no CT saturation.

[0084] Fig. 14 is a diagram showing the relationship between the projection values ​​of three phase-shifted line current vectors in the vector diagram of Fig. 13. As in the case of Fig. 7 where there is no CT saturation, the directions of the line current vectors Iab, Ibc, and Ica are aligned to the right in the diagram (i.e., the direction 0° from the origin O). Then, with this direction as the reference, vectors Iab∠θ, Ibc∠θ, and Ica∠θ are shown, which are obtained by shifting the line current vectors Iab, Ibc, and Ica by the line angle θ, respectively. Furthermore, line voltage vectors Vab, Vbc, and Vca are shown with the directions of the corresponding line current vectors Iab, Ibc, and Ica (the 0° direction in Fig. 14) as the reference.

[0085] Because CT saturation occurs in the C phase, for the BC phase line voltage vector Vbc in FIG. 14, the lagging phase angle of the corresponding post-phase-shift line current vector Ibc∠θ increases compared to the case in FIG. 7. Also, for the CA phase line voltage vector Vca in FIG. 14, the leading phase angle of the corresponding post-phase-shift line current vector Ica∠θ decreases compared to the case in FIG. 7.

[0086] However, it can be seen that the projection value Mbc corresponding to the BC phase, which is the short-circuit fault phase, is sufficiently larger than the projection value Mab corresponding to the AB phase and the projection value Mca corresponding to the CA phase. That is, in the case of a two-phase short-circuit fault in the BC phase, even if CT saturation of the degree shown in FIG. 10 occurs in the C phase, the above-mentioned judgment formulas (1A) and (1B) hold when α = 1.2 to 1.5. Therefore, the projection value judgment unit 73 in FIG. 3 can determine that a two-phase short-circuit fault has occurred in the BC phase corresponding to the maximum projection value Mbc when both of the above formulas (1A) and (1B) hold, i.e., when the ratio of the maximum projection value Mbc to the other projection values ​​Mab and Mca is greater than the threshold α (where α > 1).

[0087] <<6. When there is a two-phase ground fault on the BC phase and no load current>> Next, we will explain the case of a two-phase ground fault. The fault current of a two-phase ground fault can be considered as the sum of the short-circuit fault current and the ground fault current.

[0088] Figure 15 is a vector diagram for the case where there is a two-phase ground fault in phase B and no load current. In Figure 15, the voltage vectors before the fault in phases A, B, and C are Va', Vb', and Vc', respectively, and the voltage vectors during the fault in phases A, B, and C are Va, Vb, and Vc, respectively. As shown in Figure 15, Va = Va'. The voltage vectors Vb and Vc during the fault in phases B and C represent the voltages generated by the ground fault resistance, and their magnitudes are smaller than in the case of a two-phase short-circuit fault.

[0089] In Figure 15, the short-circuit fault current vectors Ib", Ic" of phases B and C are similar to the fault current vectors Ibf, Icf in the case of a two-phase short-circuit fault shown in Figure 4, for example. On the other hand, the earth fault current flowing to the ground is divided into phases B and C and flows separately. Therefore, the fault current vector Ibf of phase B is equal to the vector sum of the short-circuit fault current vector Ib" of phase B and the zero-phase current vector I0, i.e., Ib" + I0. The fault current vector Icf of phase C is equal to the vector sum of the short-circuit fault current vector Ic" of phase C and the zero-phase current vector I0, i.e., Ic" + I0.

[0090] Therefore, the line-to-line current vector Iab of the AB phase during the fault is −Ibf, the line-to-line current vector Ibc of the BC phase during the fault is Ibf−Icf, and the line-to-line current vector Ica of the CA phase during the fault is Icf.

[0091] FIG. 16 is a diagram showing the relationship between the projection values ​​of the line current vectors after three phase shifts in the vector diagram of FIG.

[0092] 16, the directions of the line-to-line current vectors Iab, Ibc, and Ica are aligned to the right in the figure (i.e., the direction of 0° from the origin O), as in the case of a two-phase short-circuit fault shown in Fig. 5. Then, with this direction as the reference, the vector Iab∠θ obtained by phase-shifting the AB-phase line-to-line current vector Iab by the line angle θ, the vector Ibc∠θ obtained by phase-shifting the BC-phase line-to-line current vector Ibc by the line angle θ, and the vector Ica∠θ obtained by phase-shifting the CA-phase line-to-line current vector Ica by the line angle θ are shown.

[0093] Furthermore, in FIG. 16 , similarly to FIG. 5 , the line voltage vectors Vab, Vbc, and Vca are shown relative to the directions of the corresponding line current vectors Iab, Ibc, and Ica (the 0° direction in FIG. 16 ). Also shown are the length Mab of the orthogonal projection of the phase-shifted line current vector Iab∠θ in the direction of the line voltage vector Vab for the AB phase, the length Mbc of the orthogonal projection of the phase-shifted line current vector Ibc∠θ in the direction of the line voltage vector Vbc for the BC phase, and the length Mca of the orthogonal projection of the phase-shifted line current vector Ica∠θ in the direction of the line voltage vector Vca for the CA phase. In the present disclosure, the orthogonal projection lengths Mab, Mbc, and Mca are also referred to as projection values. For ease of illustration, symbols for the projection values ​​are indicated at the foot of a perpendicular line drawn from the end point of the phase-shifted line current vector to the corresponding line voltage vector and its extension.

[0094] As shown in Figure 16, the projection value Mbc corresponding to the faulty phase, BC phase, is sufficiently larger than the projection values ​​Mca and Mab corresponding to the other CA and AB phases, and it can be seen that the judgment formulas (1A) and (1B) described above hold when α = 1.2 to 1.5.

[0095] In the case of a three-phase fault, the projection values ​​Mab, Mbc, and Mca will be almost the same, so the aforementioned formulas (1A) and (1B), which state that one projection value will be larger than the other two projection values, do not hold. However, in the case of a three-phase fault, all distance relay elements correctly determine the distance, so no problem will arise even if the faulty phase is incorrectly determined to be a short-circuit fault in two phases.

[0096] <<7. In the case of a two-phase short circuit at the nearest end>> If a two-phase short-circuit fault occurs at the closest end to the relay installation point, the magnitude of the line voltage of the faulted phase will be close to 0 V. This makes it difficult to orthogonally project the line current vector of the faulted phase after phase shift in the direction of the line voltage vector of the faulted phase.

[0097] As a practical measure, in calculating each line voltage vector, the line voltage N cycles ago (N is a positive integer of 2 or 3) is added to the current line voltage, and the line voltage vector is calculated using the line voltage after addition. For example, in the case of a two-phase short circuit fault in the BC phase, the voltage vector Vbc_pol(t) after the fault occurs is calculated by adding the line voltage vector Vbc(t) at the current time to the line voltage vector Vbc(tN cycles) N cycles ago (N is 2 or 3). That is, Vbc_pol(t) = Vbc(t) + Vbc(tN cycles) … (2) Calculate the line voltage vector according to

[0098] As shown in Figures 4, 6, 8, 11, and 13, the line voltage vector Vbc' = Vb' - Vc' before the fault and the line voltage vector Vbc during the fault are in phase, so even if the line voltage vector is calculated according to equation (2) above, the orthogonal projection in the direction of the line voltage vector can be correctly calculated. Even if a short-circuit fault occurs at the near end of the BC phase and the line voltage Vbc(t) falls close to zero, the line voltage of the faulted phase can be maintained for N cycles, so the faulted phase can be determined during that time. After N cycles have elapsed, the faulted phase determination result is maintained.

[0099] Instead of the above method, the phase of the line voltage vector of the short-circuit faulted phase may be calculated using the voltage vector or the positive-sequence voltage vector of a healthy phase that is not a faulted phase. Hereinafter, an example of a two-phase short-circuit fault in the BC phase will be explained.

[0100] FIG. 17 is a diagram for explaining a method for estimating the phase of the line voltage vector of the BC phase by using the voltage vector of the A phase or the positive sequence voltage vector in the case of a two-phase short circuit fault in the BC phase.

[0101] In Figure 17, the voltage vectors of phases A, B, and C before the fault are Va', Vb', and Vc', respectively, and the voltage vectors of phases A, B, and C during the fault are Va, Vb, and Vc, respectively. As shown in Figure 4, Va = Va'.

[0102] Here, the positive sequence voltage vectors V1a, V1b, and V1c with phases A, B, and C as references are as follows: 3×V1a=Va+Vb∠120°+Vc∠240° …(3A) 3×V1b=Va∠240°+Vb+Vc∠120° …(3B) 3×V1c=Va∠120°+Vb∠240°+Vc …(3C) In the above equation, Vb∠120° is a vector obtained by shifting the B-phase voltage vector Vb by 120°, and Vc∠240° is a vector obtained by shifting the C-phase voltage vector Vc by 240°.

[0103] 17, the positive-sequence voltage vector V1a with respect to the A-phase is in the same direction as the A-phase voltage vector Va. Therefore, the vector Vbc_pol(t), which is in phase with the current BC-phase interphase voltage vector Vbc, can be calculated as the vector Va(t)∠-90° obtained by shifting the current A-phase voltage vector Va(t) by -90°, or as the vector V1a(t)∠-90° obtained by shifting the current A-phase positive-sequence voltage vector V1a(t) by -90°.

[0104] [Operation principle of the faulty phase determination unit in the case of a single-phase ground fault] Next, we will explain how to determine a single-phase-to-ground fault. In a single-phase-to-ground fault, for two healthy phases, the effective values ​​of the phase currents during the fault change from the effective values ​​of the phase currents of the same phases before the fault occurred, but the effective values ​​of the line-to-line currents of the two healthy phases do not change before and after the fault occurs.

[0105] Therefore, the current change amount determination unit 71 in FIG. 3 determines whether the change in the effective value of the line current over a certain time T is equal to or greater than a threshold. The certain time T is set as a time during which the effective value of the load current does not change when there is no transmission line fault, for example, 2 to 3 cycles of an AC system. Specifically, if the change in the effective value of two line currents is equal to or greater than a threshold and the change in the effective value of another line current is less than the threshold, the current change amount determination unit 71 determines the two phases related to the line current that is less than the threshold as healthy phases and the other phase as a ground fault phase. Note that if the fault continues even after the certain time T has elapsed after the occurrence of the fault, the effective value before the certain time T and the current effective value are both the same as the effective value during the fault, and therefore cannot be used for fault determination. Therefore, the current change amount determination unit 71 stores the effective value from the certain time T before the fault occurred, and continues to use the stored effective value as the pre-fault effective value while the fault continues and compares it with the current effective value.

[0106] [Summary of operation of fault phase determination unit] Fig. 18 is a flowchart showing the procedure for determining a faulty phase in the distance relay 10 of embodiment 1. The operation of the faulty phase determination unit 70 of the arithmetic processing unit 40 of Fig. 3 described above will be summarized below with reference to Fig. 18.

[0107] In step S10, the current change amount determination unit 71 of the fault phase determination unit 70 calculates the absolute value of the difference between the effective value Iab(t)rms of the A and B phase line current at the current time t and the effective value Iab(tT)rms of the A and B phase line current a certain time T before the current time t, i.e., |ΔIab(t)rms|=|Iab(t)rms-Iab(t‐T)rms| …(4A) Similarly, the current change amount determination unit 71 calculates the absolute value of the change amount of the line current for the BC phase and the CA phase, i.e., |ΔIbc(t)rms|=|Ibc(t)rms-Ibc(t‐T)rms| …(4B) |ΔIca(t)rms|=|Ica(t)rms-Ica(t‐T)rms| …(4C) The above-mentioned fixed time T is a time interval during which the load current can be considered unchanged, for example, 2 to 3 cycles of an AC system.

[0108] In the next step S20, the current change amount determination unit 71 determines whether the absolute values ​​of the changes in the calculated effective values ​​of the three line currents, i.e., |ΔIab(t)rms|, |ΔIbc(t)rms|, and |ΔIca(t)rms|, are each equal to or greater than a threshold value A1. For example, a value approximately 10 to 20% of the CT rated current is selected as the threshold value A1.

[0109] If the determination result of step S20 above is that the absolute value of the change in the effective value of only one line current is equal to or greater than threshold value A1, or if the absolute value of the change in the effective value of all line currents is less than threshold value A1, the current change amount determination unit 71 determines that there is no fault in the transmission line (step S22). Note that the absolute value of the change in the effective value of only one line current is never equal to or greater than threshold value A1 during a fault, and this is likely due to a problem with detection sensitivity. For example, if the detection sensitivity is set too high, the absolute value of the change in the effective value of one line current may be equal to or greater than threshold value A1 due to an unbalanced fluctuation in the load rather than a transmission line fault.

[0110] If the result of the determination in step S20 above shows that the absolute values ​​of the changes in the effective values ​​of only two line currents are equal to or greater than threshold value A1, current change amount determination unit 71 determines that a one-phase ground fault has occurred (step S24). In this case, current change amount determination unit 71 determines that the two phases related to the line currents whose absolute values ​​of the changes in the effective values ​​are less than threshold value A1 are healthy phases, and determines the remaining phase as a faulty phase.

[0111] If the result of the determination in step S20 above shows that the absolute value of the change in the effective value of any of the three line currents is equal to or greater than the threshold value A1, the current change determination unit 71 determines that there is a two-phase fault (short circuit or ground fault) or a three-phase fault (short circuit or ground fault) (step S26). In this case, the faulty phase determination unit 70 proceeds to the next step S30.

[0112] If a single-phase ground fault (step S24) or a two-or-more-phase fault (step S26) is determined, the fault phase determination unit 70 stores the effective value of the line-to-line current a certain time T before the current time as the load current in the absence of a fault, while the fault continues. Thereafter, while the fault continues, the fault phase determination unit 70 uses this stored data as the effective value of the line-to-line current a certain time T before the current time in the calculations of the above-mentioned equations (4A), (4B), and (4C).

[0113] In step S30, the projection value calculation unit 72 of the fault phase determination unit 70 shifts the phase of each of the line current vectors of the AB phase, BC phase, and CA phase by the line angle θ. Then, the projection value calculation unit 72 orthogonally projects each of the phase-shifted line current vectors in the direction of the line voltage vector of the same phase, thereby calculating the length of the orthogonal projection vector as a projection value. As a result, projection values ​​are calculated for each of the AB phase, BC phase, and CA phase.

[0114] In the next step S32, the projection value determination unit 73 of the fault phase determination unit 70 determines whether the maximum projection value of the three calculated projection values ​​is α times (for example, α=1.2 to 1.5) or more of any of the other projection values. If the maximum projection value is α times or more of any of the other projection values ​​(YES in step S32), the projection value determination unit 73 determines that the two phases corresponding to the line-to-line currents used in calculating the maximum projection value are faulty phases (step S34).

[0115] On the other hand, if the maximum projection value is not α times or more the other projection values ​​(NO in step S32), that is, if the three projection values ​​are approximately the same magnitude, the projection value determination unit 73 determines that there is a three-phase fault (step S36). This completes the fault phase determination process.

[0116] [Effects of the First Embodiment] As described above, the fault phase determination unit 70 of the distance relay 10 according to the first embodiment determines the fault phase in the case of a short-circuit fault based on the projection value calculated for each line current. This makes it possible to achieve fault phase determination that does not erroneously determine the fault phase even when CT saturation occurs. Furthermore, the fault phase determination unit 70 described above can correctly determine the fault phase even in the case of a distant fault or a high-resistance fault in which the voltage change is small.

[0117] Embodiment 2 The faulty phase determination unit of the distance relay in embodiment 2 is an improvement over embodiment 1, and is capable of determining a single-phase-to-ground fault phase with higher sensitivity. In embodiment 1, the faulty phase determination unit 70 of the distance relay 10 determined that there was no fault when there was no change in the effective values ​​of any of the line currents. However, depending on the relationship between the load current and the single-phase-to-ground fault current, there may be cases where there is almost no change in the effective values ​​of any of the line currents. For this reason, it is necessary to make the threshold value A1 as small as possible, but there are limitations due to fluctuations in the load current of the power system and errors in the relay device. In embodiment 2, this problem is improved. This will be described in detail below with reference to the drawings.

[0118] [Functional configuration of the calculation processing unit] Fig. 19 is a block diagram showing the functional configuration of the arithmetic processing unit 40 of Fig. 2 in the distance relay of embodiment 2. In the arithmetic processing unit 40 of Fig. 19, the configuration of the faulty phase determination unit 70A is different from the configuration of the faulty phase determination unit 70 of Fig. 3.

[0119] 19, the fault phase determination unit 70A includes a current change amount determination unit 71A, a first projection value calculation unit 72, a first projection value determination unit 73, a second projection value calculation unit 74, and a second projection value determination unit 75. The first projection value calculation unit 72 and the first projection value determination unit 73 perform the same processing as the projection value calculation unit 72 and the projection value determination unit 73 in FIG. 3 according to the first embodiment, respectively.

[0120] The current change amount determination unit 71A first determines with high sensitivity whether or not a fault has occurred based on the change in each of the phase currents of the A phase, the B phase, and the C phase.

[0121] For example, the current change amount determination unit 71A calculates the phase current change amount ΔIx(t) by adding the instantaneous value Ix(t) (where x is a, b, or c) of the phase current at the current time point to the instantaneous value Ix (t-0.5 cycles) of the phase current half a cycle before the current time point. Then, the current change amount determination unit 71A determines whether the effective value of the calculated phase current change amount ΔIx(t) is equal to or greater than the threshold value A2. That is, for each phase, |ΔIx(t)|rms = |Ix(t) + Ix(t-0.5 cycle)|rms ≧ A2 … (5) It is determined whether the above formula holds. Since a load current without a fault has a sinusoidal waveform, the above addition result is zero. For example, a value of about 10 to 20% of the CT rated current is selected as threshold A2.

[0122] When a fault occurs in a transmission line, a change appears in at least one of phases A, B, and C, and the effective value of the amount of change in at least one of the phase currents becomes equal to or greater than threshold value A2. However, once 0.5 cycles have passed since the fault occurred, equation (5) no longer holds, so current change amount determination unit 71A continues to determine that a fault has occurred for a period of time equivalent to the duration of the fault (for example, 100 to 200 ms).

[0123] Instead of 0.5 cycles in the above formula (5), 1.5 cycles or 2.5 cycles may be used. Also, instead of the above formula (5), the difference between the instantaneous value Ix(t) of the phase current at the present time and the value n cycles before the present time (n is, for example, 1, 2, or 3) may be used as the phase current change amount ΔIx(t). In this case, the current change amount determination unit 71A calculates for each phase, |ΔIx(t)|rms = |Ix(t) - Ix(tn cycles)|rms ≧ A2 … (6) In this case as well, once n cycles have passed since the occurrence of the fault, both Ix(t) and Ix(tn cycles) in the above formula (6) become data after the fault has occurred, making the determination meaningless, and therefore the current change amount determination unit 71A continues to determine that a fault has occurred for a period of time equivalent to the duration of the fault (for example, 100 to 200 ms).

[0124] In the above, the change in the instantaneous value of the phase current is used, but the change in the instantaneous value of the line current may also be used to determine the presence or absence of a fault, and the presence or absence of a fault can be determined with high sensitivity, similar to the case of the phase current.

[0125] If the effective value of the phase current change amount for any phase is less than the threshold value A2, the current change amount determination unit 71A determines that no fault has occurred in the power transmission line. In this case, the fault phase determination unit 70 determines that no fault has occurred in the power transmission line and ends the process.

[0126] On the other hand, when the effective value of the amount of change in the phase current for at least one phase is equal to or greater than threshold value A2, current change amount determination unit 71A then determines the presence or absence of a fault based on the change in the effective value (or amplitude value) of each line current, as in embodiment 1. In this case, the processing differs depending on the number of line currents whose absolute value of the amount of change in the effective value is equal to or greater than threshold value A1.

[0127] Specifically, if the absolute value of the change in the effective value of each of the three line currents is equal to or greater than threshold value A1, a two-phase or three-phase fault is determined, as in the case of embodiment 1. In this case, as in the case of embodiment 1, first projection value calculation unit 72 calculates the projection value, and first projection value determination unit 73 determines the faulty phase based on the calculated projection value.

[0128] If the absolute values ​​of the changes in the effective values ​​of only two line currents are equal to or greater than threshold value A1, a one-phase ground fault is determined, as in embodiment 1. In this case, fault phase determination unit 70 determines that the two phases related to the line currents that are less than threshold value A1 are healthy phases, and determines the other phase as a ground fault phase.

[0129] If the absolute value of the change in the effective value of only one line current is equal to or greater than the threshold value A1, or if the absolute value of the change in the effective value of all line currents is less than the threshold value A1, the second projection value calculation unit 74 calculates new projection values ​​for the phase current vectors of each phase, and the second projection value determination unit 75 determines whether or not there is a single-phase ground fault and, in the case of a single-phase ground fault, the faulty phase based on the calculated projection values.

[0130] Specifically, the second projection value calculation unit 74 shifts the phase of each phase current vector by the line angle of the power transmission line 2. Then, the second projection value calculation unit 74 orthogonally projects each phase current vector after the phase shift in the direction of the phase voltage vector of the same phase, and calculates the length of the orthogonal projection vector as a projection value. A projection value is calculated for each of the three phase current vectors.

[0131] The second projection value determination unit 75 compares the maximum projection value of the three projection values ​​calculated by the second projection value calculation unit 74 with each of the other two projection values. If the ratio of the maximum projection value to each of the other projection values ​​is equal to or greater than threshold value β, the second projection value determination unit 75 determines that the phase of the phase current vector corresponding to the maximum projection value is a one-phase-to-ground fault phase. On the other hand, if the ratio of the maximum projection value to each of the other projection values ​​is less than threshold value β, the second projection value determination unit 75 determines that no fault has occurred in the power transmission line 2. The threshold value β is selected to be, for example, 1.2 to 1.5.

[0132] In the calculation of the orthogonal projection vector by the second projection value calculation unit 74, if the phase voltage of the faulted phase approaches zero due to a ground fault, it may seem difficult to calculate the orthogonal projection in the direction of the voltage vector of the faulted phase. However, the calculation by the second projection value calculation unit 74 is performed only when the current change amount determination unit 71A determines that the effective value of the phase current change of at least one phase is equal to or greater than the threshold value A2, and the absolute value of the change in the effective value of all line currents is less than the threshold value A1, or the absolute value of the change in the effective value of only one line current is equal to or greater than A1. This corresponds to a case where the ground fault phase cannot be identified using the change in the effective value of the line current due to the relationship between the load current and the one-phase ground fault current. As will be described later with reference to Figures 25 and 26, in this case, the fault current is not large, about three times the load current, so the phase voltage of the ground fault phase does not drop significantly. Therefore, the magnitude of the phase voltage required for the orthogonal projection calculation is ensured, and this does not pose a problem.

[0133] [Operation of the second projection value calculation unit and the second projection value determination unit in the case of a ground fault] The operation of the second projection value calculation unit 74 and the second projection value determination unit 75 in Fig. 19 will be described in more detail below with reference to vector diagrams. First, an equivalent circuit for a single-phase ground fault will be described below, and then a vector diagram for a single-phase ground fault will be described based on this equivalent circuit.

[0134] Figure 20 shows the equivalent circuit of a single-phase to ground fault in symmetric coordinates. In the equivalent circuit of Figure 20, it is assumed that the other end is connected to an infinite bus in a direct grounding system. Therefore, there is no load current and no power supply at the other end.

[0135] In Figure 20, E is the rear power supply voltage and Rf is the fault point resistance. Z1s, Z2s, and Z0s are the positive-sequence rear impedance, negative-sequence rear impedance, and zero-sequence rear impedance, respectively. Z1f, Z2f, and Z0f are the positive-sequence transmission line impedance, negative-sequence transmission line impedance, and zero-sequence transmission line impedance, respectively. V1, V2, and V0 are the positive-sequence voltage, negative-sequence voltage, and zero-sequence voltage, respectively. I1, I2, and I0 are the positive-sequence current, negative-sequence current, and zero-sequence current, respectively. Generally, Z1s = Z2s, and Z1f = Z2f.

[0136] The voltage vectors Va, Vb, and Vc of phases A, B, and C are expressed as follows by using the vector operator "a": Va=V0+V1+V2 …(7A) Vb=V0+a 2 ·V1+a·V2 …(7B) Vc=V0+a V1+a 2 V2...(7C) The vector operator "a" represents a phase rotation of 120°. 2 represents a phase rotation of 240°. 2 There is a relationship of =0.

[0137] In the above equations (7A) to (7C), if Rf=0 is set, and V0=-Zos·I0, V1=E-Z1s·I1, and V2=-Z2s·I2 are substituted, and further, the relationships I0=I1=I2 and Z1s=Z2s are used, then Va=E-(Z0s+2·Z1s)·I1 …(8A) Vb=a 2 E-(Z0s+(a 2 +a)·Z1s)·I1 =a 2 ·E-(Z0s-Z1s)·I1 …(8B) Vc=a·E-(Z0s+(a+a 2 )·Z1s)·I1 =a·E-(Z0s-Z1s)·I1 …(8C) However, I1=E / (2·Z1s+2·Z1f+Z0s+Zof+Rf) …(9) is.

[0138] If the pre-fault voltage of healthy phase B is Vb' and the pre-fault voltage of healthy phase C is Vc', then from the above equations (8B) and (8C), Vb = Vb' - (Z0s - Z1s) · I1 and Vc = Vc' - (Z0s - Z1s) · I1 hold. Therefore, the line-to-line voltage Vbc of healthy phases during a single-phase-to-ground fault does not change from the line-to-line voltage Vbc' before the fault occurred, but the voltages Vb and Vc of healthy phases during the fault change from the pre-fault voltages Vb' and Vc'.

[0139] Furthermore, according to equation (9), when the fault point resistance Rf is not zero, the angle between the positive-sequence current I1 and the A-phase voltage Va is smaller than when Rf = 0. Furthermore, since the fault current I1 is also smaller than when Rf = 0, the drop in the fault phase voltage Va is smaller than when Rf = 0, and the phase angle of the fault phase voltage Va also changes according to equation (8A). Furthermore, even if there is no load current at one end of the power supply, the ground fault current also flows to the healthy phase via the neutral point (not shown) of the transformer at the other end. Therefore, after a single-phase ground fault occurs, a small amount of current (zero-phase current) of the same magnitude flows in the same phase in phases B and C. This zero-phase current cancels out the line-to-line current Ibc, so the line-to-line current Ibc is not affected by the ground fault current.

[0140] <<1. When there is a load current (forward power flow) due to a single-phase ground fault on phase A>> FIG. 21 is a vector diagram for the case where there is a single-phase ground fault in phase A and there is a load current with a forward power flow. Referring to FIG. 21, the voltage vectors of phases A, B, and C before the fault are Va', Vb', and Vc', respectively, and the voltage vectors of phases A, B, and C during the fault are Va, Vb, and Vc, respectively. In this case, as explained with reference to FIG. 20, Vbc = Vbc' holds for the line voltages of phases B and C, which are healthy phases. Furthermore, the magnitude of the voltage vector Va of phase A, which is a ground fault, decreases depending on the voltage vector Va' before the fault, the impedance from the power source to the relay installation point (back impedance), and the magnitude of the fault current.

[0141] Regarding the current vectors, the current vectors before the fault (i.e., the load current vectors of the forward power flow) in phases A, B, and C are defined as Ia', Ib', and Ic', respectively, and the current vectors during the fault in phases A, B, and C are defined as Ia, Ib, and Ic, respectively. Furthermore, the fault current vector in phase A is defined as Iaf.

[0142] In this case, for the healthy phases B and C, Ib = Ib' and Ic = Ic' are true. The current vector Ia during the fault in phase A is equal to the vector sum of the load current vector Ia' in phase A and the fault current vector Iaf in phase A. That is, Ia = Ia' + Iaf is true.

[0143] FIG. 22 is a diagram showing the relationship between the projection values ​​of the phase current vectors after three phase shifts in the vector diagram of FIG.

[0144] In FIG. 22, the directions of the phase current vectors Ia, Ib, and Ic are aligned to the right of the figure (i.e., the direction 0° from the origin O). Then, with this direction as a reference, vectors Ia∠θ, Ib∠θ, and Ic∠θ are shown, which are obtained by shifting the phase current vectors Ia, Ib, and Ic by the line angle θ. Furthermore, phase voltage vectors Va, Vb, and Vc are shown with the directions of the corresponding phase current vectors Ia, Ib, and Ic (the direction 0° in FIG. 22) as a reference.

[0145] 22 also shows the length Ma of the orthogonal projection vector when the phase current vector Ia∠θ of the A phase after the phase shift is orthogonally projected in the direction of the phase voltage vector Va of the A phase, the length Mb of the orthogonal projection vector when the phase current vector Ib∠θ of the B phase after the phase shift is orthogonally projected in the direction of the phase voltage vector Vb of the B phase, and the length Mc of the orthogonal projection vector when the phase current vector Ic∠θ of the C phase after the phase shift is orthogonally projected in the direction of the phase voltage vector Vc of the C phase. In this disclosure, the lengths Ma, Mb, and Mc of the orthogonal projection vectors are also referred to as projection values. For ease of illustration, symbols for the projection values ​​are written at the foot of a perpendicular line drawn from the end point of the phase current vector after the phase shift to the corresponding phase voltage vector and its extension.

[0146] As shown in FIG. 22, for the projection values ​​Ma, Mb, and Mc, the threshold value β is set to 1.2 to 1.5, Ma ≥ β Mb …(10A) Ma ≥ β Mc …(10B) The second projection value determination unit 75 in Fig. 19 can determine that a single-line-to-ground fault has occurred in the A phase corresponding to the maximum projection value Ma when both of the above equations (10A) and (10B) are satisfied, that is, when the ratio of the maximum projection value Ma to the other projection values ​​Mb and Mc is equal to or greater than the threshold value β.

[0147] The reasons why the above-mentioned judgment formulas (10A) and (10B) hold are, first, that the magnitude of the phase current vector of the A phase, which is the ground fault phase, is larger than the magnitude of the other phase current vectors, and second, that the phase difference between the phase current vector after the phase shift and the corresponding phase voltage vector in the A phase, which is the ground fault phase, is closer to 0° than the phase difference in the other phases.

[0148] <<2. When there is a load current (backward current) due to a single-phase ground fault on phase A>> Fig. 23 is an example of a vector diagram when there is a load current with a backward flow due to a single-phase ground fault in phase A. The voltage vectors in Fig. 23 are the same as those in Fig. 21 when there is a load current with a forward flow, and therefore the description will not be repeated.

[0149] Regarding the current vectors, the current vectors before the fault in phases A, B, and C (i.e., the load current vectors of the backward flow) are designated Ia", Ib", and Ic", respectively, and the current vectors during the fault in phases A, B, and C are designated Ia, Ib, and Ic, respectively.As an example of backward flow, we will explain the case where the load current vectors Ia", Ib", and Ic" are equal in magnitude but opposite in direction to the load current vectors Ia', Ib', and Ic' of the forward flow shown in Figure 21.In this case, Ia" = -Ia', Ib" = -Ib', and Ic" = -Ic.In Figure 23, the fault current vector of phase A is designated Iaf.

[0150] In this case, for the healthy phases B and C, Ib = Ib" and Ic = Ic" hold. The current vector Ia during the fault in phase A is equal to the vector sum of the load current vector Ia" of phase A and the fault current vector Iaf of phase A. In other words, Ia = Ia" + Iaf holds.

[0151] FIG. 24 is a diagram showing the relationship between the projection values ​​of the phase current vectors after three phase shifts in the vector diagram of FIG.

[0152] 22, in Fig. 24, the directions of the phase current vectors Ia, Ib, and Ic are aligned to the right of the figure (i.e., the direction 0° from the origin O). Then, based on this direction, vectors Ia∠θ, Ib∠θ, and Ic∠θ, which are obtained by shifting the phase current vectors Ia, Ib, and Ic by the line angle θ, and phase voltage vectors Va, Vb, and Vc are shown.

[0153] 22, FIG. 24 also shows the length Ma of the orthogonal projection of the A-phase current vector Ia∠θ in the direction of the A-phase voltage vector Va, the length Mb of the orthogonal projection of the B-phase current vector Ib∠θ in the direction of the B-phase voltage vector Vb, and the length Mc of the orthogonal projection of the C-phase current vector Ic∠θ in the direction of the C-phase voltage vector Vc. In this disclosure, the orthogonal projection vector lengths Ma, Mb, and Mc are also referred to as projection values. For ease of illustration, symbols for the projection values ​​are written at the foot of a perpendicular line drawn from the end point of the phase-shifted phase current vector to the corresponding phase voltage vector and its extension.

[0154] As shown in Fig. 24, for the projection values ​​Ma, Mb, and Mc, the above-mentioned equations (10A) and (10B) hold when the threshold value β is set to 1.2 to 1.5. Therefore, the second projection value determination unit 75 in Fig. 19 can determine that a single-line-to-ground fault has occurred in the A phase corresponding to the maximum projection value Ma.

[0155] The reasons why the above-mentioned judgment formulas (10A) and (10B) hold are, first, that even in the case of backward power flow, the magnitude of the phase current vector of phase A, which is the earth fault phase, is larger than the magnitude of the other phase current vectors. Second, that even in the case of backward power flow, the phase difference between the phase current vector after phase shift and the corresponding phase voltage vector is closer to 0° (or 180°) in the case of phase A, which is the earth fault phase, than the phase difference in the case of the other phases.

[0156] <<3. When there is a single-phase ground fault on phase A, there is backward power flow, and the ground fault resistance is high>> Figure 25 is a vector diagram for the case where there is a backward load current due to a single-phase ground fault on phase A and the ground fault resistance is high. The ground fault resistance is the fault point resistance Rf in Figure 20.

[0157] The direction of the A-phase fault current Iaf is different between the vector diagram of Figure 25 and the vector diagram of Figure 23. That is, in the case of Figure 23 where there is no earth fault resistance (i.e., Rf = 0), the angle between the A-phase voltage vector Va and the A-phase fault current vector Iaf during the earth fault is close to the line angle, whereas in the case of Figure 25 where the earth fault resistance is relatively high, the angle between the A-phase voltage vector Va and the A-phase fault current vector Iaf during the earth fault is smaller due to the presence of earth fault resistance. In particular, in the case of Figure 25, the direction of the A-phase fault current vector Iaf is almost opposite to the direction of the A-phase load current vector Ia" of the backward flow. In this case, the direction of the A-phase current vector Ia (= Ia" + Iaf) during the fault is also opposite to the direction of the A-phase load current vector Ia".

[0158] Furthermore, in the vector diagram of Figure 25, the direction of the positive-sequence current I1 is different from that in the vector diagram of Figure 23 because the fault point resistance Rf is high. Here, since Iaf = I0 + I1 + I2 and I0 = I1 = I2, Iaf = 3 · I1 holds. Therefore, the direction of the positive-sequence current I1, i.e., the direction of the A-phase fault current Iaf, is different between Figure 25 and Figure 23. Furthermore, according to equations (8A) to (8C), the directions of the voltage vectors Va, Vb, and Vc during the fault in Figure 25 are also different from those in Figure 23. Since Figure 25 is otherwise similar to Figure 23, the same or corresponding parts are given the same reference symbols and descriptions will not be repeated.

[0159] In FIG. 25 , when the line-to-line current vectors Iab″=Ia″-Ib″, Ibc″=Ib″-Ic″, and Ica″=Ic″-Ia″ before the fault are compared with the line-to-line current vectors Iab=Ia-Ib, Ibc=Ib-Ic, and Ica=Ic-Ia during the fault, the magnitudes of the line-to-line current vectors Iab”, Ibc”, and Ica″ before the fault are approximately equal to the magnitudes of the line-to-line current vectors Iab, Ibc, and Ica during the fault. For this reason, in the case of the first embodiment, there is almost no difference in the amount of change in the effective value of the line-to-line current vectors before and during the fault. For example, in the case of such a load current and ground fault, the faulted phase determination unit 70 in the first embodiment may erroneously determine that a one-phase ground fault is not present.

[0160] Figure 26 shows the relationship between the projection values ​​of the phase current vectors after three phase shifts in the vector diagram of Figure 25. The vector diagram of Figure 26 corresponds to Figure 24, but the direction of the A-phase voltage vector Va relative to the direction of the A-phase current vector Ia during a fault differs from that in Figure 24. The other current vectors and voltage vectors in Figure 26 are the same as those in Figure 24.

[0161] 26, the above-described formulas (10A) and (10B) hold for the projection values ​​Ma, Mb, and Mc when the threshold value β is set to 1.2 to 1.5. Therefore, the second projection value determination unit 75 in FIG. 19 can detect with high sensitivity that a single-line-to-ground fault has occurred in the A phase corresponding to the maximum projection value Ma.

[0162] [Summary of operation of fault phase determination unit] Figures 27 and 28 are flowcharts showing the procedure for determining a faulty phase in the distance relay 10 of embodiment 2. The flowcharts of Figures 27 and 28 are partially modified versions of the flowchart for embodiment 1 shown in Figure 18. In the following description of Figures 27 and 28, steps common to the flowchart of Figure 18 are given the same reference numerals and their description will not be repeated.

[0163] In the flowchart of Fig. 27, steps S2, S4, and S6 are added before step S10. Specifically, the current change amount determination unit 71A of Fig. 19 detects the amount of change in the instantaneous value of the phase current in step S2, and determines whether the effective value of the amount of change in the instantaneous value of the phase current is equal to or greater than threshold value A2 in step S4. More specifically, the effective value of the amount of change in phase current ΔIx(t) is calculated using the above-mentioned equation (5) or (6), and compared with threshold value A2.

[0164] If the result of the determination in step S4 above shows that the effective value of the amount of change in the instantaneous value of the phase current for all three phases is less than threshold value A2 (NO for all three phases in step S4), the fault phase determination unit 70A determines that there is no fault in the transmission line 2 (step S6) and ends the process. On the other hand, if the effective value of the amount of change in the instantaneous value of the phase current for at least one phase is equal to or greater than threshold value A2 (YES for at least one phase in step S4), the fault phase determination unit 70A proceeds to step S10. Note that although the amount of change in the instantaneous value of the phase current is used in the above, the amount of change in the instantaneous value of the line-to-line current may also be used.

[0165] In the next step S10, current change amount determination unit 71A detects the amount of change in the effective value of the line current, and in the next step S20, determines whether the amount of change in the effective value of the line current is equal to or greater than threshold value A1. Specifically, this is the same as what has been described in the first embodiment with reference to equations (4A) to (4C), and therefore description thereof will not be repeated here.

[0166] If the determination result in step S20 above shows that the absolute values ​​of the changes in the effective values ​​of only two line currents are equal to or greater than threshold value A1, and if the absolute values ​​of the changes in the effective values ​​of all three line currents are equal to or greater than threshold value A1, the processing is the same as in the first embodiment shown in Fig. 18. Therefore, the same or corresponding steps are denoted by the same reference numerals, and description thereof will not be repeated.

[0167] On the other hand, if the result of the determination in step S20 is that the absolute value of the change in the effective value of only one line current is equal to or greater than the threshold value A1, or if the absolute value of the change in the effective value of all line currents is less than the threshold value A1, the processing proceeds to step S40 in FIG. 28.

[0168] In step S40, the second projection value calculation unit 74 of the fault phase determination unit 70A shifts the phase current vectors of the A, B, and C phases by the line angle θ. Then, the second projection value calculation unit 74 orthogonally projects each of the phase current vectors after the phase shift in the direction of the phase voltage vector of the same phase, thereby calculating the length of the orthogonal projection vector as a projection value. As a result, a projection value is calculated for each of the A, B, and C phases.

[0169] In the next step S42, the second projection value determination unit 75 of the fault phase determination unit 70A determines whether the maximum projection value of the three calculated projection values ​​is β times (for example, β=1.2 to 1.5) or more of any of the other projection values. If the maximum projection value is β times or more of any of the other projection values ​​(YES in step S42), the second projection value determination unit 75 determines that the phase corresponding to the maximum projection value is the faulty phase of the one-phase-to-ground fault (step S44).

[0170] On the other hand, if the maximum projection value is not β times or more the other projection values ​​(NO in step S42), that is, if the three projection values ​​are approximately the same in magnitude, the second projection value determination unit 75 determines that there is no fault in the power transmission line 2 (step S46). This completes the fault phase determination process.

[0171] [Effects of the second embodiment] As described above, the fault phase determination unit 70A of the distance relay 10 of embodiment 2 can achieve fault phase determination without erroneous determination even when CT saturation occurs, as in embodiment 1, and can correctly determine the fault phase even in the case of a distant fault or a high-resistance fault where the voltage change is small. Furthermore, the fault phase determination unit 70A in embodiment 2 can detect faults with high sensitivity based on the amount of change in the instantaneous value of the phase current or line current, and can further determine a one-phase-to-ground fault phase with high sensitivity by using the projection value of the phase current vector.

[0172] Embodiment 3 In the first and second embodiments, the first projection value calculation units 72, 72A and the second projection value calculation unit 74 perform a process of shifting the phase of the line-to-line current vector or the phase current vector by the line angle. In the third embodiment, the phase shift angle θ can be set according to the power flow direction before the fault occurs, thereby enabling more reliable determination. Furthermore, the calculation can be simplified.

[0173] Specifically, the line angle is typically about 85° to 89° for overhead lines and about 70° to 80° for cable lines. Therefore, in the case of overhead line transmission lines, if the load current is a forward flow, it is desirable to set the phase angle θ to, for example, about 70° to 80°, and if it is a backward flow, it is desirable to set the phase angle θ to, for example, about 90° to 100°. In the case of cable transmission lines, if the load current is a forward flow, it is desirable to set the phase angle θ to, for example, about 60° to 70°, and if it is a backward flow, it is desirable to set the phase angle θ to, for example, about 80° to 90°.

[0174] By setting the phase shift angle as described above, the phase shift angle can be set to a value closer to the phase difference between the faulted phase current and the faulted phase voltage when a load current is present, thereby widening the margin when comparing projection values.

[0175] Whether the current is forward or backward can be determined by measuring the direction (phase) of the current relative to the voltage before the fault. Therefore, the setting value of the phase angle θ can be automatically switched.

[0176] Embodiment 4 The faulty phase determination unit of the distance relay of embodiment 4 is an improvement of the faulty phase determination unit in embodiment 1, as in embodiment 2, and is designed to be able to determine a one-phase ground fault phase with higher sensitivity.

[0177] In the event of a single-phase-to-ground fault, the vectors of the line-to-line currents of the two healthy phases do not change. Therefore, a method for detecting changes in these vectors provides highly sensitive fault detection. To achieve this, for example, one method is to calculate the effective value of the difference between the current instantaneous value and the instantaneous value an integer number of cycles before the current time and then observe the amount of change. However, because the frequency may gradually change when a fault occurs, the integer number of cycles must be two or three. Considering an evolving fault, this method is difficult to apply. For this reason, in the first embodiment, the presence or absence of a fault is determined based on changes in the effective value of the line-to-line current, rather than changes in the line-to-line current vector. However, as described in the second embodiment, depending on the phase relationship between the load current and the fault current, there may be a difference between the line-to-line current vector before the fault and the line-to-line current vector at the fault, but no difference between the effective value of the line-to-line current vector before the fault and the line-to-line current vector at the fault.

[0178] Therefore, in this embodiment, the faulty phase is determined by observing changes in the line-to-line current vector for a short period of time (for example, up to two or three cycles) after the occurrence of a fault, and thereafter observing changes in the effective value of the line-to-line current. This will be described in detail below with reference to the drawings.

[0179] [Functional configuration of the calculation processing unit] Fig. 29 is a block diagram showing the functional configuration of the arithmetic processing unit 40 of Fig. 2 in the distance relay of embodiment 4. In the arithmetic processing unit 40 of Fig. 29, the configuration of the faulty phase determination unit 70B is different from the configuration of the faulty phase determination unit 70 of Fig. 3.

[0180] 29, the fault phase determination unit 70B includes a first current change amount determination unit 71, a first projection value calculation unit 72, a first projection value determination unit 73, a second current change amount determination unit 76, a second projection value calculation unit 77, a second projection value determination unit 78, and an output selection unit 79. Note that the first current change amount determination unit 71, the first projection value calculation unit 72, and the first projection value determination unit 73 perform the same processes as the current change amount determination unit 71, the projection value calculation unit 72, and the projection value determination unit 73 in FIG. 3 according to the first embodiment, respectively.

[0181] The second current change amount determination unit 76 calculates the effective value of the difference between the instantaneous value Iab(t) of the AB phase line current at the current time t and the instantaneous value Iab(tn cycles) of the AB phase line current n cycles before the current time t as the effective value of the change amount ΔIab(t) of the instantaneous value of the AB phase line current. That is, the second current change amount determination unit 76 calculates |ΔIab(t)|rms = |Iab(t) - Iab(t-n cycle)|rms ... (11A) The second current change amount determination unit 76 similarly calculates the effective value of the amount of change in the instantaneous value of the line current for the BC phase and the CA phase, that is, the effective value of the amount of change in the instantaneous value of the line current for the BC phase and the CA phase, that is, |ΔIbc(t)|rms = |Ibc(t) - Ibc(t-n cycle)|rms ... (11B) |ΔIca(t)|rms = |Ica(t) - Ica(t-n cycle)|rms …(11C) The n cycles is set to 2 or 3 cycles, which is a period that will not cause a change in the frequency of the power system when a fault occurs.

[0182] When a fault occurs, the above processes (11A) to (11C) can detect the true fault current, eliminating the influence of the load current before the fault occurred, enabling highly sensitive fault detection. However, once n cycles have passed since the fault occurred, the processes (11A) to (11C) result in the difference between the waveforms after the fault occurred, and the difference is meaningless. Therefore, the calculation results of the above processes (11A) to (11C) are valid only for a period shorter than n cycles from the fault occurrence and are limited to use within that period. Therefore, the output selection unit 79 of the fault phase determination unit 70B is configured to preferentially output the determination result of the second current change amount determination unit 76 for a period shorter than n cycles (e.g., 1 to 1.5 cycles when n = 2 cycles). Thereafter, the fault phase determination unit 70B is configured to output the determination result of the first current change amount determination unit 71, which has the same configuration as in the first embodiment.

[0183] The second current change amount determination unit 76 compares the effective value of the amount of change in the instantaneous value of the line current calculated according to the above (11A) to (11C) with a threshold value A3. For example, about 10 to 20% of the rated current of the CT is selected as the threshold value A3.

[0184] If the result of the above comparison shows that only two line currents are equal to or greater than threshold value A3, second current change amount determination unit 76 determines that a one-phase ground fault has occurred. In this case, second current change amount determination unit 76 determines that the two phases related to the line currents that are less than threshold value A3 are healthy phases, and determines that the remaining phase is a ground fault phase.

[0185] As a result of the above comparison, the second current change amount determination unit 76 determines that there is no fault when the effective value of the change amount in the instantaneous value of any of the line currents calculated according to (11A) to (11C) above is less than threshold value A3.

[0186] On the other hand, the second current change amount determination unit 76 determines that a two-phase or three-phase fault has occurred when the effective values ​​of the changes in the instantaneous values ​​of all the line currents calculated according to the above (11A) to (11C) are equal to or greater than the threshold value A3. In this case, the second projection value calculation unit 77 and the second projection value determination unit 78 determine the faulty phase.

[0187] The second projection value calculation unit 77 calculates line-to-line current vectors based on the pure fault current from which the influence of the load current has been removed, which is obtained by subtracting the instantaneous value of the line-to-line current n cycles (n=2 or 3) before the current time t from the instantaneous value of the line-to-line current at the current time t. The second projection value calculation unit 77 shifts the phase of each of the three line-to-line current vectors due to the pure fault current by the line angle θ (or the phase shift angle θ in the third embodiment). The second projection value calculation unit 77 orthogonally projects each of the phase-shifted line-to-line current vectors in the direction of the line-to-line voltage vectors that are in phase with each other, and calculates the length of the orthogonal projection vector as the projection value.

[0188] The second projection value determination unit 78 compares the maximum projection value of the three projection values ​​calculated by the second projection value calculation unit 77 with each of the other two projection values. If the ratio of the maximum projection value to each of the other projection values ​​is equal to or greater than a threshold value α, the second projection value determination unit 78 determines that the two phases related to the line-to-line current vector used to calculate the maximum projection value are short-circuit fault phases. α is selected to be, for example, 1.2 to 1.5.

[0189] The output selection unit 79 also limits the output of the determination result of the second projection value determination unit 78 so that it is output preferentially only for a period shorter than n cycles from the occurrence of the fault (for example, 1 to 1.5 cycles when n=2 cycles). Thereafter, the output selection unit 79 of the fault phase determination unit 70B is configured to output the determination result of the first projection value determination unit 73 having the same configuration as in the first embodiment.

[0190] [Summary of operation of fault phase determination unit] Fig. 30 is a flowchart showing the procedure for determining a faulty phase in the distance relay 10 of the fourth embodiment. The determination of a faulty phase according to the flowchart in Fig. 30 is executed in combination with the determination of a faulty phase according to the flowchart in Fig. 18 of the first embodiment. The explanation so far will be summarized below with reference to Fig. 30 and Fig. 18.

[0191] The flowchart of FIG. 30 illustrates the operations of the second current change amount determination unit 76, the second projection value calculation unit 77, the second projection value determination unit 78, and the output selection unit 79 of FIG. 29. The determination results of the flowchart of FIG. 30 are output only within a time limit. The time limit is set to a period shorter than the n cycles of the above-described equations (11A) to (11C). As described above, the flowchart of FIG. 30 uses the instantaneous change amount of the line-to-line current, so there is a time limit restriction. On the other hand, the flowchart of FIG. 18 does not have a time limit restriction. However, because the flowchart of FIG. 30 uses a pure fault current without a load current, the determination results are more reliable than those of the flowchart of FIG. 18. Therefore, as shown in steps S123, S125, S135, and S137, within the time limit, the determination results of the flowchart of FIG. 30 are output preferentially over the determination results of the flowchart of FIG. 18.

[0192] Specifically, in step S110 of FIG. 30, the second current change amount determination unit 76 sequentially calculates the difference between the instantaneous value of the line current at the current time and the instantaneous value of the line current n cycles prior to the current time for each of the three line currents as the amount of change in the instantaneous value of the line current over time.

[0193] In the next step S120, the second current change amount determination unit 76 compares the effective value of the amount of change in the instantaneous value of the line current with a threshold value A3. As a result, if the effective value of the amount of change in only one line current is equal to or greater than the threshold value A3, or if the effective value of the amount of change in all line currents is less than the threshold value A3, the second current change amount determination unit 76 determines that there is no fault (step S122). In the next step S123, if the time limit has not yet passed, the output selection unit 79 outputs the determination result of step S122 with priority. On the other hand, if the time limit has passed, the output selection unit 79 outputs the determination result according to the flowchart of FIG. 18.

[0194] If the comparison result in step S120 shows that the effective values ​​of the changes in only two line currents are equal to or greater than threshold value A3, second current change amount determination unit 76 determines that a one-phase ground fault has occurred, that the two phases related to the line current whose absolute value of the effective value of the change is less than threshold value A3 are healthy phases, and that the remaining phase is faulty (step S124). In the next step S125, output selection unit 79 outputs the determination result of step S124 preferentially if it is within the time limit. On the other hand, if the time limit has elapsed, output selection unit 79 outputs the determination result according to the flowchart of FIG. 18.

[0195] If the comparison result in step S120 shows that the effective values ​​of the changes in all three line currents are equal to or greater than threshold value A3, second current change amount determination unit 76 determines that a two-phase fault (short circuit or ground fault) or a three-phase fault (short circuit or ground fault) has occurred (step S126). In this case, faulty phase determination unit 70B proceeds to the next step S130.

[0196] In step S130, the second projection value calculation unit 77 calculates line-to-line current vectors based on the pure fault current from which the influence of the load current has been removed, which is obtained by subtracting the instantaneous value of the line-to-line current n cycles (n=2 to 3) before the current time t from the instantaneous value of the line-to-line current at the current time t. The second projection value calculation unit 77 shifts the phase of each of the three line-to-line current vectors due to the pure fault current by the line angle θ (or the phase shift angle θ in the third embodiment). The second projection value calculation unit 77 orthogonally projects each of the phase-shifted line-to-line current vectors in the direction of the line-to-line voltage vectors that are in phase with each other, and calculates the length of the orthogonal projection vector as a projection value.

[0197] In the next step S132, the second projection value determination unit 78 compares the maximum projection value of the three projection values ​​calculated by the second projection value calculation unit 77 with each of the other two projection values. Then, the second projection value determination unit 78 determines whether the maximum projection value is α times or more the other projection values. α is selected to be, for example, 1.2 to 1.5.

[0198] If the maximum projection value is α times or more the other projection values ​​(YES in step S132), the second projection value determination unit 78 determines that the two phases corresponding to the line currents used to calculate the maximum projection value are faulty phases (step S134). In the next step S135, the output selection unit 79 outputs the determination result of step S134 preferentially if it is within the time limit. If the time limit has elapsed, the output selection unit 79 outputs the determination result according to the flowchart of FIG.

[0199] On the other hand, if the maximum projection value is not α times or more the other projection values ​​(NO in step S132), that is, if the three projection values ​​are approximately the same magnitude, the second projection value determination unit 78 determines that a three-phase fault has occurred (step S136). In the next step S137, the output selection unit 79 outputs the determination result of step S136 preferentially if it is within the time limit. If the time limit has elapsed, the output selection unit 79 outputs the determination result according to the flowchart of FIG.

[0200] [Effects of the Fourth Embodiment] According to the fault phase determination unit 70B of the distance relay 10 of the fourth embodiment, the fault phase can be determined with higher sensitivity by using the line-to-line current due to the pure fault current. As a result, it is possible to prevent erroneous fault phase determination even in the case of CT saturation. Note that fault phase determination using the pure fault current is only valid for n cycles from the occurrence of the fault, so in the case of an evolving fault, it may not be possible to determine the fault phase. However, in such a case, the fault phase determination according to the first embodiment can be used, so this is not a problem. In other words, according to the fourth embodiment, the reliability of the fault phase determination in the case of the first embodiment can be further improved.

[0201] Embodiment 5 [overview] In the fifth embodiment, the method of calculating the projection value in the projection value calculation unit (72 in FIG. 3, 72 and 74 in FIG. 19, and 72 and 77 in FIG. 29) is changed in the distance relay 10 of the first, second, and fourth embodiments.

[0202] In the first, second, and fourth embodiments, when AC electrical quantities P(t) and Q(t) (where t: time) are displayed as vectors on a complex plane, the projection value calculation unit calculates the length of the orthogonally projected vector when vector P is orthogonally projected in the direction of vector Q as the projection value of vector P. For example, in the first embodiment, the line current vector Iab∠θ (or Ibc∠θ, Ica∠θ) between the AB phases (or the BC phase or the CA phase) after a phase shift by the line angle θ of the power transmission line corresponds to vector P, and the line voltage vector Vab (or Vbc, Vca) between the in-phase AB phases (or the BC phase or the CA phase) corresponds to vector Q.

[0203] In the case of the distance relay 10 of the fifth embodiment, the projection value calculation unit calculates the projection value by dividing the length of the orthogonal projection vector when the vector P is orthogonally projected in the direction of the vector Q, instead of the length of the orthogonal projection vector. Hereinafter, the projection value in the case of the fifth embodiment will be referred to as the "weighted projection value M'", but may also be simply referred to as the "projection value M'". Similarly, the "projection value calculation unit" and the "projection value determination unit" in the first, second, and fourth embodiments will be referred to as the "weighted projection value calculation unit" and the "weighted projection value determination unit", respectively, in the fifth embodiment, but may also be simply referred to as the "projection value calculation unit" and the "projection value determination unit".

[0204] Specifically, if the amplitude values ​​of the AC electrical quantities P(t) and Q(t) are |P| and |Q|, respectively, the phase difference between them is φ, and the angular frequency of the power system is ω, the instantaneous values ​​of the AC electrical quantities P(t) and Q(t) are as follows: P(t) = |P| cos(ωt) …(12A) Q(t)=|Q|·cos(ωt+φ) …(12B) It is expressed as:

[0205] The inner product (P, Q) of vector P and vector Q is calculated by using the values ​​P(t-90°) and Q(t-90°) of the current AC electrical quantities P(t) and Q(t) 90° earlier in electrical angle. (P,Q)=P(t)·Q(t)+P(t-90°)·Q(t-90°) …(13) By substituting the above equations (12A) and (12B) into the above equation (13), (P,Q)=|P|·|Q|·cosφ It can be confirmed that the following occurs. When AC electric quantity P(t) and Q(t) are acquired at a sampling period of every 30° of electrical angle, the current AC electric quantity P m ,Q m and AC electrical quantity P three sampling periods before the present time m-3 ,Q m-3 Using and, P m Q m +P m-3 Q m-3 The inner product (P,Q) is calculated by

[0206] The square of the magnitude |Q| of vector Q is |Q| 2 teeth, |Q| 2 =Q(t)·Q(t)+Q(t-90°)·Q(t-90°) …(14) By substituting the above equations (12A) and (12B) into the above equation (14), it can be confirmed that the right-hand side of the above equation (14) is equal to the left-hand side. When the AC electrical quantity Q(t) is obtained at a sampling period of every 30° electrical angle, the current AC electrical quantity Q m and the AC electrical quantity Q three sampling periods before the present time m-3 Using and, Q m 2 +Q m-3 2 The square of the magnitude of the AC electric quantity Q(t) |Q| 2 is calculated.

[0207] By using the inner product (P, Q) of vectors P and Q in the above equation (13) and the magnitude |Q| of vector Q based on the above equation (14), the length of the orthogonal projection vector of vector P in the direction of vector Q, that is, the projection value M in the first, second, and fourth embodiments, is given by M = (P, Q) / |Q| …(15) On the other hand, the value obtained by further dividing the length of the orthogonal projection vector in the above equation (15) by the magnitude |Q| of the vector Q, that is, the weighted projection value M′ in the case of the fifth embodiment, is given by M' = M / |Q| = (P, Q) / |Q| 2 …(16) is given by

[0208] Therefore, according to the above equations (13) to (16), when actually calculating the projection value M in the first, second, and fourth embodiments shown in equation (15) using the sampled values ​​of the AC electricity quantities P(t) and Q(t), the square root of the value on the right side of equation (14) must be calculated as the value of the magnitude |Q| of the vector Q. In contrast, when actually calculating the weighted projection value M' in the fifth embodiment shown in equation (16) above, it is sufficient to simply divide the right side of equation (13) by the right side of equation (14), and there is no need to calculate the square root. In this way, the calculation of the weighted projection value M' in the fifth embodiment is simpler than the calculation of the projection value M in the first, second, and fourth embodiments.

[0209] In the above description, the amplitude values ​​|P| and |Q| of the AC electric quantities P(t) and Q(t) are used as shown in equations (12A) and (12B), but it goes without saying that the magnitudes of the finally derived equations (13) to (16) and vectors P and Q may be effective values ​​instead of amplitude values. In the following, how the first, second, and fourth embodiments are specifically modified in the case of the fifth embodiment will be described in more detail with reference to the drawings.

[0210] [Modification of the first embodiment] Fig. 31 is a block diagram showing the functional configuration of the calculation processing unit 40 of Fig. 2 in the distance relay 10 according to a modification of the first embodiment. In the fault-phase determination unit 70 according to the modification of the first embodiment shown in Fig. 31, the projection value calculation unit 72 and the projection value determination unit 73 according to the first embodiment shown in Fig. 3 are changed to a weighted projection value calculation unit 72A and a weighted projection value determination unit 73A, respectively. Since other points in Fig. 31 are the same as those in Fig. 3, the same or corresponding parts are designated by the same reference numerals and description thereof will not be repeated.

[0211] 31, the weighted projection value calculation unit 72A of the fault phase determination unit 70 calculates vectors Iab∠θ, Ibc∠θ, and Ica∠θ by shifting the phases of the three line current vectors Iab, Ibc, and Ica by the line angle θ of the power transmission line. Then, the weighted projection value calculation unit 72A orthogonally projects the phase-shifted line current vectors Iab∠θ, Ibc∠θ, and Ica∠θ in the directions of the in-phase line voltage vectors Vab, Vbc, and Vca, respectively, and calculates weighted projection values ​​Mab', Mbc', and Mca' by dividing the lengths of the orthogonal projection vectors by the magnitudes of the corresponding in-phase line voltage vectors.

[0212] Furthermore, the weighted projection value determination unit 73A of the fault phase determination unit 70 compares the maximum weighted projection value among the three weighted projection values ​​Mab', Mbc', Mc' calculated by the weighted projection value calculation unit 72A with each of the other two weighted projection values. If the ratio of the maximum weighted projection value to each of the other weighted projection values ​​is equal to or greater than the threshold value α', the weighted projection value determination unit 73A determines that the two phases related to the line-to-line current vector used in calculating the maximum weighted projection value are short-circuit fault phases.

[0213] For example, in the case of a short circuit fault in the BC phase, the weighted projection values ​​Mab', Mbc', and Mc' are as follows: Mbc' ≥ α' · Mca' … (17A) Mbc' ≥ α' · Mab' … (17B) The following relational expression holds. In the above expressions (17A) and (17B), α'>1, and is selected, for example, in the range of α'=1.2 to 1.5 with a margin. Here, the magnitude of the line voltage Vbc of the BC phase, which is the short-circuit fault phase, is smaller than the magnitudes of the line voltages Vca and Vab of the other two phases. Therefore, by calculating a weighted projection value obtained by dividing the length of the orthogonal projection vector by the magnitude of the corresponding line voltage vector, the difference between the faulty phase and the non-faulty phase becomes more pronounced. As a result, the reliability of the determination of the short-circuit fault phase can be improved compared to the first embodiment. For example, a larger margin can be obtained between the weighted projection value of the faulty phase and the weighted projection value of the non-faulty phase.

[0214] FIG. 32 is a flowchart showing a faulty phase determination procedure in a distance relay according to a modification of the first embodiment. In the flowchart of FIG. 32, steps S30, S32, and S34 in the flowchart of FIG. 18 are replaced with steps S30A, S32A, and S34A, respectively. Since the other steps in FIG. 32 are the same as those in FIG. 18, the same or corresponding steps are designated by the same reference numerals and will not be described again. Note that, although the descriptions of FIGS. 3 and 31 and the descriptions of FIGS. 18 and 32 refer to currents and voltages as "effective values," it goes without saying that "amplitude values" may be used instead of "effective values."

[0215] In step S30A of Fig. 32, the weighted projection value calculation unit 72A of the fault phase determination unit 70 shifts the phases of the line current vectors of each of the AB phase, BC phase, and CA phase by the line angle θ. Then, the weighted projection value calculation unit 72A calculates weighted projection values ​​by dividing the lengths of three orthogonal projection vectors obtained by orthogonally projecting the line current vectors of the three phases after the phase shift in the direction of the line voltage vectors of the same phase by the effective values ​​(or amplitude values) of the line voltages of the same phase. As a result, weighted projection values ​​are calculated for each of the AB phase, BC phase, and CA phase.

[0216] In the next step S32A, the weighted projection value determination unit 73A of the fault phase determination unit 70 determines whether the maximum weighted projection value among the three weighted projection values ​​calculated by the weighted projection value calculation unit 72A is equal to or greater than α' times (for example, α' = 1.2 to 1.5) any of the other weighted projection values. If the maximum weighted projection value is equal to or greater than α' times any of the other weighted projection values ​​(YES in step S32A), the weighted projection value determination unit 73A determines that the two phases corresponding to the line-to-line currents used in calculating the maximum weighted projection value are short-circuit fault phases (step S34A).

[0217] On the other hand, if the maximum weighted projection value is not α' times or more the other weighted projection values ​​(NO in step S32A), that is, if the three weighted projection values ​​are approximately the same in magnitude, the weighted projection value determining unit 73A determines that a three-phase fault has occurred (step S36). This completes the fault phase determination procedure.

[0218] [Modification of the second embodiment] Fig. 33 is a block diagram showing the functional configuration of the arithmetic processing unit 40 of Fig. 2 in a distance relay according to a modification of the second embodiment. In a fault-phase determination unit 70A of the distance relay 10 according to the modification of the second embodiment shown in Fig. 33, the first projection value calculation unit 72, the first projection value determination unit 73, the second projection value calculation unit 74, and the second projection value determination unit 75 of the second embodiment shown in Fig. 19 are changed to a first weighted projection value calculation unit 72A, a first weighted projection value determination unit 73A, a second weighted projection value calculation unit 74A, and a second weighted projection value determination unit 75A, respectively. Since other points in Fig. 33 are the same as those in Fig. 19, the same or corresponding parts are designated by the same reference numerals and detailed description thereof will not be repeated.

[0219] First, a brief description will be given of the operation of the current change amount determination unit 71A of the fault phase determination unit 70A in Fig. 33, which is the premise for the operations of the first weighted projection value calculation unit 72A and the second weighted projection value calculation unit 74A. The operation of the fault phase determination unit 70A is the same as in the second embodiment.

[0220] Specifically, the current change amount determination unit 71A determines whether the effective value of the amount of change in the instantaneous value of the phase current of each phase is equal to or greater than the threshold value A2. If the effective value of the amount of change in the phase current of at least one phase is equal to or greater than the threshold value A2, the current change amount determination unit 71A then determines whether the absolute value of the amount of change in the effective value (or amplitude value) of each line current is equal to or greater than the threshold value A1. In this case, the subsequent processing differs depending on the number of line currents whose absolute values ​​of the amount of change in the effective value are equal to or greater than the threshold value A1.

[0221] In a first case, when the absolute value of the change in the effective value of each of the three line currents is equal to or greater than threshold value A1, current change amount determination unit 71A in Fig. 33 determines that a two-phase or three-phase fault has occurred. In this case, first weighted projection value calculation unit 72A calculates vectors obtained by shifting the phase of each of the three line current vectors by the line angle of the power transmission line, similar to weighted projection value calculation unit 72A in the modification of Embodiment 1 shown in Fig. 31. Then, first weighted projection value calculation unit 72A orthogonally projects each of the phase-shifted line current vectors in the direction of the line voltage vector of the same phase, and calculates a value obtained by dividing the length of the orthogonal projection vector by the magnitude of the corresponding line voltage vector, as a weighted projection value.

[0222] Furthermore, the first weighted projection value determining unit 73A in Fig. 33 compares the maximum weighted projection value among the three weighted projection values ​​calculated by the first weighted projection value calculating unit 72A with each of the other two weighted projection values, similar to the weighted projection value determining unit 73A in the modification of the first embodiment shown in Fig. 31. Then, when the ratio of the maximum weighted projection value to each of the other weighted projection values ​​is equal to or greater than the threshold value α', the first weighted projection value determining unit 73A determines that the two phases related to the line-to-line current vector used in calculating the maximum weighted projection value are short-circuit fault phases.

[0223] In a second case, when the absolute values ​​of the changes in the effective values ​​of only two line currents are equal to or greater than threshold value A1, current change amount determination unit 71A determines that a one-phase ground fault has occurred, as in the modified example of embodiment 1. In this case, fault phase determination unit 70A determines that the two phases related to the line currents that are less than threshold value A1 are healthy phases, and determines the other phase as a ground fault phase.

[0224] In the third case, when the absolute value of the change in the effective value of only one line current is equal to or greater than the threshold value A1, or when the absolute value of the change in the effective value of all line currents is less than the threshold value A1, the second weighted projection value calculation unit 74A calculates new second weighted projection values ​​for the phase current vectors of each phase, and the second weighted projection value determination unit 75A determines whether or not there is a single-phase ground fault and, in the case of a single-phase ground fault, the faulty phase based on the calculated second weighted projection values.

[0225] Specifically, the second weighted projection value calculation unit 74A shifts the phase of each phase current vector Ia, Ib, and Ic by the line angle θ of the power transmission line 2. Then, the second weighted projection value calculation unit 74A orthogonally projects each of the phase-shifted phase current vectors Ia∠θ, Ib∠θ, and Ic∠θ in the direction of the in-phase phase voltage vectors Va, Vb, and Vc, respectively, and calculates the second weighted projection values ​​Ma', Mb', and Mc' by dividing the length of the orthogonal projection vector by the magnitude of the in-phase phase voltage vector (i.e., the effective value or amplitude of the in-phase phase voltage). Thus, the second weighted projection values ​​Ma', Mb', and Mc' are calculated for each of the three phase current vectors Ia, Ib, and Ic.

[0226] The second weighted projection value determination unit 75A compares the maximum second weighted projection value among the three second weighted projection values ​​Ma', Mb', and Mc' calculated by the second weighted projection value calculation unit 74A with each of the other two second weighted projection values. If the ratio of the maximum second weighted projection value to each of the other second weighted projection values ​​is equal to or greater than a threshold value β', the second weighted projection value determination unit 75A determines that the phase of the phase current vector corresponding to the maximum second weighted projection value is a one-phase-to-ground fault phase. On the other hand, if the ratio of the maximum second weighted projection value to each of the other second weighted projection values ​​is less than the threshold value β', the second weighted projection value determination unit 75A determines that no fault has occurred in the power transmission line 2. The threshold value β' is selected to be, for example, 1.2 to 1.5.

[0227] Here, the magnitude (effective value or amplitude value) of the phase voltage of the ground fault phase is smaller than the magnitudes of the phase voltages of the other two phases. Therefore, by calculating the second weighted projection value by dividing the length of the orthogonal projection vector by the magnitude of the corresponding in-phase phase voltage vector, the difference between the faulted phase and the non-faulted phase becomes more pronounced. As a result, the reliability of the determination of the ground fault phase can be improved compared to the second embodiment. For example, a larger margin can be obtained between the weighted projection value of the faulted phase and the weighted projection value of the non-faulted phase.

[0228] 34 and 35 are flowcharts showing a faulty phase determination procedure in the distance relay 10 of a modification of the second embodiment. The flowchart of FIG. 34 is the flowchart of FIG. 27 in which steps S30, S32, and S34 are replaced with steps S30A, S32A, and S34A, respectively. The flowchart of FIG. 35 is the flowchart of FIG. 28 in which steps S40, S42, and S44 are replaced with steps S40A, S42A, and S44A. Steps S30A, S32A, and S34A in FIG. 34 are the same as steps S30A, S32A, and S34A in the modification of the first embodiment shown in FIG. 32, and therefore their description will not be repeated. Furthermore, the other steps in FIGS. 34 and 35 are the same as those in FIGS. 27 and 28, and therefore the same or corresponding steps are designated by the same reference numerals, and their description will not be repeated. In the explanations of Figures 19 and 33 and Figures 27, 28, 34, and 35, the current and voltage are described as "effective values," but it goes without saying that "amplitude values" may be used instead of "effective values."

[0229] 35, the second weighted projection value calculation unit 74A of the fault phase determination unit 70A shifts the phase current vectors of the A, B, and C phases by the line angle θ. Then, the second weighted projection value calculation unit 74A orthogonally projects each of the phase current vectors after the phase shift in the direction of the in-phase phase voltage vector, and calculates a weighted projection value by dividing the length of the orthogonal projection vector by the effective value (or amplitude value) of the in-phase phase voltage. As a result, a weighted projection value is calculated for each of the A, B, and C phases.

[0230] In the next step S42A, the second weighted projection value determination unit 75A of the fault phase determination unit 70A determines whether the maximum weighted projection value among the three weighted projection values ​​calculated in step S40A is greater than or equal to β' times (for example, β' = 1.2 to 1.5) any of the other weighted projection values. If the maximum weighted projection value is greater than or equal to β' times any of the other weighted projection values ​​(YES in step S42A), the second weighted projection value determination unit 75A determines that the phase corresponding to the maximum weighted projection value is the faulted phase of a single-phase-to-ground fault (step S44A).

[0231] On the other hand, if the maximum weighted projection value is not β' times or more the other weighted projection values ​​(NO in step S42A), that is, if the three weighted projection values ​​are approximately the same in magnitude, the second weighted projection value determination unit 75A determines that there is no fault in the power transmission line 2 (step S46). This completes the fault phase determination process.

[0232] [Modification of the fourth embodiment] Fig. 36 is a block diagram showing the functional configuration of the arithmetic processing unit 40 of Fig. 2 in a distance relay according to a modification of the embodiment 4. In a fault phase determination unit 70B of the distance relay 10 according to the modification of the embodiment 4 shown in Fig. 36, the first projection value calculation unit 72, the first projection value determination unit 73, the second projection value calculation unit 77, and the second projection value determination unit 78 in the case of the embodiment 4 shown in Fig. 29 are changed to a first weighted projection value calculation unit 72A, a first weighted projection value determination unit 73A, a second weighted projection value calculation unit 77A, and a second weighted projection value determination unit 78A, respectively.

[0233] Among the above-described new components, the operations of first weighted projection value calculation unit 72A and first weighted projection value determination unit 73A are the same as those of weighted projection value calculation unit 72A and weighted projection value determination unit 73A in the modification of embodiment 1 shown in Fig. 31. Therefore, description of these components will not be repeated.

[0234] The second weighted projection value calculation unit 77A and the second weighted projection value determination unit 78A operate when the second current change amount determination unit 76 determines that a two-phase or three-phase fault has occurred. Below, we will briefly explain the operation of the second current change amount determination unit 76, which is a prerequisite for the operation of the second weighted projection value calculation unit 77A and the second weighted projection value determination unit 78A. The operation of the second current change amount determination unit 76 is the same as in the fourth embodiment.

[0235] Specifically, the second current change amount determination unit 76 calculates the effective value of the difference between the instantaneous value of each of the line currents of the AB phase, BC phase, and CA phase at the current time and the instantaneous value of the line current of the same phase n cycles prior (n=2 or 3) from the current time as the effective value of the amount of change in the instantaneous value of the line current of each phase. The second current change amount determination unit 76 compares the calculated effective value of the amount of change in the instantaneous value of the line current of each phase with a threshold value A3. For example, a value approximately 10 to 20% of the rated current of the CT is selected as the threshold value A3.

[0236] If the comparison result shows that the effective value of the amount of change in the instantaneous value of the line current for only two line currents is equal to or greater than threshold value A3, the second current change amount determination unit 76 determines that there is a one-phase ground fault. If the effective value of the amount of change in the instantaneous value of the line current for only one line current is equal to or greater than threshold value A3, or if the effective value of the amount of change in the instantaneous value of the line current for both line currents is less than threshold value A3, the second current change amount determination unit 76 determines that there is no fault.

[0237] On the other hand, if the effective values ​​of the changes in the instantaneous values ​​of all three line currents are equal to or greater than threshold value A3, second current change amount determination unit 76 determines that a two-phase or three-phase fault has occurred. In this case, second weighted projection value calculation unit 77A calculates line-to-line current vectors based on the pure fault current from which the influence of the load current has been removed, which is obtained by subtracting the instantaneous value of the line current n cycles (n = 2 or 3) before the current time t from the instantaneous value of the line current at the current time t. Second weighted projection value calculation unit 77A shifts the phase of each of the three line-to-line current vectors due to the pure fault current by line angle θ (or phase shift angle θ in the third embodiment). Second weighted projection value calculation unit 77A orthogonally projects each of the phase-shifted line-to-line current vectors in the direction of the in-phase line voltage vector, and calculates a weighted projection value by dividing the length of the orthogonal projection vector by the magnitude of the in-phase line voltage vector (i.e., the effective value or amplitude value of the line voltage).

[0238] Furthermore, the second weighted projection value determination unit 78A compares the maximum weighted projection value among the three weighted projection values ​​calculated by the second weighted projection value calculation unit 77A with each of the other two weighted projection values. If the ratio of the maximum weighted projection value to each of the other weighted projection values ​​is equal to or greater than a threshold value α', the second projection value determination unit 78 determines that the two phases related to the line-to-line current vector used to calculate the maximum weighted projection value are short-circuit fault phases. α' is selected to be, for example, 1.2 to 1.5.

[0239] Here, the magnitude (effective value or amplitude value) of the line voltage of the short-circuited fault phase is smaller than the magnitude of the line voltages of the other phases. Therefore, by calculating a weighted projection value by dividing the length of the orthogonal projection vector by the magnitude of the corresponding line voltage vector, the difference between the faulted phase and the non-faulted phase becomes more pronounced. As a result, the reliability of the determination of the short-circuited fault phase can be improved compared to the first embodiment. For example, a larger margin can be obtained between the weighted projection value of the faulted phase and the weighted projection value of the non-faulted phase.

[0240] The output selection unit 79 of the fault phase determination unit 70B preferentially outputs the determination result of the second weighted projection value determination unit 78A for a period shorter than n cycles from the occurrence of the fault (for example, 1 to 1.5 cycles when n=2 cycles). Thereafter, the output selection unit 79 outputs the determination result of the first weighted projection value determination unit 73A. Since other points in Figure 36 are the same as those in Figure 29, the same or corresponding parts are designated with the same reference numerals and description thereof will not be repeated.

[0241] FIG. 37 is a flowchart showing a fault determination procedure in a distance relay according to a modification of the fourth embodiment. The flowchart in FIG. 37 illustrates the operations of second current change amount determination unit 76, second weighted projection value calculation unit 77A, second weighted projection value determination unit 78A, and output selection unit 79 in FIG. 36. The flowchart in FIG. 37 is similar to the flowchart in FIG. 30 for the fourth embodiment, except that steps S123, S125, S130, S132, S134, S135, and S137 are replaced with steps S123A, S125A, S130A, S132A, S134A, S135A, and S137A. The other steps in FIG. 37 are similar to those in FIG. 30, and therefore the same or corresponding steps are designated by the same reference numerals and will not be described in detail again. In the explanations of Figures 29 and 36 and Figures 30 and 37, the current and voltage are described as "effective values", but it goes without saying that "amplitude values" may be used instead of "effective values".

[0242] The determination result according to the flowchart in FIG. 37 is output only within the time limit. The time limit is set to a period shorter than n cycles from the occurrence of the fault (for example, 1 to 1.5 cycles if n=2 cycles). The reason for this is that the line-to-line current vector is calculated based on the pure fault current from which the influence of the load current has been removed, which is obtained by subtracting the instantaneous value of the line-to-line current n cycles (n=2 or 3) before the current time from the instantaneous value of the line-to-line current at the current time, and the calculation becomes meaningless after the time limit has elapsed. Therefore, as shown in steps S123A, S125A, S135A, and S137A in FIG. 37, the determination result according to the modification of the first embodiment shown in FIG. 32 is output after the time limit has elapsed. In the modification of the first embodiment, the fault phase is determined based on the change in the effective value of the line-to-line current, so it is not restricted by the time limit.

[0243] As in the case of embodiment 4 in Fig. 30, in step S110 in Fig. 37, the second current change amount determination unit 76 sequentially calculates, over time, the difference between the instantaneous value of the line current at the current time and the instantaneous value of the line current n cycles prior to the current time (n = 2 or 3), as the amount of change in the instantaneous value of the line current. In the next step S120, the second current change amount determination unit 76 compares the effective value of the amount of change in the instantaneous value of the line current calculated in step S110 with threshold value A3.

[0244] If the comparison result shows that the effective values ​​of the changes in all three line currents are equal to or greater than threshold value A3, second current change amount determination unit 76 determines that a two-phase fault (short circuit or ground fault) or a three-phase fault (short circuit or ground fault) has occurred (step S126). In this case, faulty phase determination unit 70B proceeds to step S130A.

[0245] In step S130A, the second weighted projection value calculation unit 77A calculates line-to-line current vectors based on the pure fault current from which the influence of the load current has been removed, the pure fault current being obtained by subtracting the instantaneous value of the line-to-line current n cycles (n=2 to 3) before the current time from the instantaneous value of the line-to-line current at the current time. The second weighted projection value calculation unit 77A shifts the phase of each of the three line-to-line current vectors due to the pure fault current by the line angle θ (or the phase shift angle θ in the third embodiment). The second weighted projection value calculation unit 77A orthogonally projects each of the phase-shifted line-to-line current vectors in the direction of the in-phase line voltage vector, and calculates a weighted projection value by dividing the length of the orthogonal projection vector by the magnitude of the in-phase line voltage vector (i.e., the effective value or amplitude value of the line voltage vector).

[0246] In the next step S132A, second weighted projection value determination unit 78A compares the maximum weighted projection value among the three weighted projection values ​​calculated by second weighted projection value calculation unit 77A with each of the other two weighted projection values. Then, second weighted projection value determination unit 78A determines whether the maximum weighted projection value is α' times or more the other weighted projection values. α' is selected to be, for example, 1.2 to 1.5.

[0247] If the maximum weighted projection value is equal to or greater than α' times the other weighted projection values ​​(YES in step S132A), the second weighted projection value determination unit 78A determines that the two phases corresponding to the line-to-line currents used to calculate the maximum weighted projection value are faulty phases (step S134A). In the next step S135A, if the time limit has not passed, the output selection unit 79 outputs the determination result of step S134A with priority. If the time limit has passed, the output selection unit 79 outputs the determination result according to the flowchart of the modification of the first embodiment shown in FIG.

[0248] On the other hand, if the maximum weighted projection value is not α' times or more the other weighted projection values ​​(NO in step S132A), that is, if the three weighted projection values ​​are approximately the same, the second weighted projection value determining unit 78A determines that a three-phase fault has occurred (step S136). In the next step S137A, if the time limit has not yet been reached, the output selecting unit 79 outputs the determination result of step S136 with priority. If the time limit has elapsed, the output selecting unit 79 outputs the determination result according to the flowchart of the modified example of the first embodiment shown in FIG.

[0249] [Effects of the fifth embodiment] As described above, in the fifth embodiment, the method of calculating the projection value in the first, second, and fourth embodiments is modified. Specifically, when determining the presence or absence of a short-circuit fault, a phase-shifted line-to-line current vector, which has been phase-shifted by the line angle of the transmission line, is orthogonally projected in the direction of a corresponding in-phase line voltage vector, and the length of the orthogonal projection vector is divided by the magnitude of the in-phase line voltage vector (i.e., the effective value or amplitude value of the line voltage) to calculate a projection value (also referred to as a weighted projection value). Similarly, when determining the presence or absence of a ground fault, a phase current vector, which has been phase-shifted by the line angle of the transmission line, is orthogonally projected in the direction of a corresponding in-phase phase voltage vector, and the length of the orthogonal projection vector is divided by the magnitude of the in-phase phase voltage vector (i.e., the effective value or amplitude value of the phase voltage) to calculate a projection value (also referred to as a weighted projection value).

[0250] As described above, by using the weighted projection values, the difference between the weighted projection values ​​of the faulted phase and the weighted projection values ​​of the non-faulted phase becomes more significant, thereby improving the reliability of fault determination. Furthermore, when actually calculating the weighted projection values ​​using the instantaneous value data of the electrical quantities, there is no need to calculate square roots, which reduces the amount of calculation.

[0251] The embodiments disclosed herein should be considered to be illustrative in all respects and not restrictive. The scope of this application is defined by the claims, not the above description, and is intended to include all modifications within the meaning and scope of the claims. [Explanation of symbols]

[0252] REFERENCE SIGNS LIST 1 generator, 2 transmission line, 3 local substation, 4 remote substation, 5 current transformer, 6 voltage transformer, 7 circuit breaker, 10 distance relay, 20 input conversion unit, 21 auxiliary transformer, 30 A / D conversion unit, 31 analog filter, 32 sample-and-hold circuit, 33 multiplexer, 34 A / D converter, 40 arithmetic processing unit, 41 CPU, 42 RAM, 43 ROM, 44 non-volatile memory, 45 bus, 50 I / O unit, 51 digital input circuit, 52 digital output circuit, 60 input data holding unit, 70, 70A, 70B fault phase determination unit, 71, 71A first current change amount determination unit, 72, 72A first projection value calculation unit, 73 first projection value determination unit, 74, 77 second projection value calculation unit, 75, 78 second projection value determination unit, 76 Second current change amount determination unit, 80 distance relay element, 81 short circuit distance relay element, 82 ground fault distance relay element, 90 logic operation unit.

Claims

1. a first projection value calculation unit that shifts the phases of three line-to-line current vectors by a phase angle corresponding to a line angle based on time-series data of currents and voltages detected from the three-phase transmission line, orthogonally projects the three phase-shifted line current vectors in the directions of the corresponding line voltage vectors, and calculates lengths of the orthogonal projection vectors as first projection values; a first projection value determination unit that determines whether a ratio of a maximum first projection value among the three calculated first projection values ​​to the other first projection values ​​is equal to or greater than a first threshold value; the first projection value determination unit determines that two phases related to the line-to-line current vector used in calculating the maximum first projection value are short-circuit fault phases when a ratio of the maximum first projection value is equal to or greater than the first threshold.

2. The power transmission line protection device includes: a first current change amount determination unit that determines whether or not a change amount in the effective value of each line current is equal to or greater than a second threshold value; the first current change amount determination unit, when the change amounts of the effective values ​​of only two line currents are equal to or greater than the second threshold, determines two phases related to the line currents whose change amounts of the effective values ​​are less than the second threshold as healthy phases, and determines the remaining one phase as a ground fault phase; 2. The power transmission line protection device according to claim 1, wherein the first current change amount determination unit determines that a two-phase fault or a three-phase fault has occurred when changes in the effective values ​​of all line currents are equal to or greater than the second threshold value.

3. 3. The power transmission line protection device according to claim 2, wherein the first projection value determination unit determines a three-phase fault when the ratio of the maximum first projection value is less than the first threshold value in a case where the first current change amount determination unit determines a two-phase fault or a three-phase fault.

4. the first current change amount determination unit further determines whether or not the amount of change in the instantaneous value of each line current or the amount of change in the instantaneous value of each phase current is equal to or greater than a third threshold; 4. The power transmission line protection device according to claim 2, wherein the first current change amount determination unit determines that there is no fault in the three-phase power transmission line when amounts of change in instantaneous values ​​of all line currents or amounts of change in instantaneous values ​​of all phase currents are less than the third threshold value.

5. The power transmission line protection device includes: a second projection value calculation unit that shifts the phases of three phase current vectors by the phase shift angle corresponding to the line angle based on time-series data of currents and voltages detected from the three-phase transmission line, orthogonally projects the three phase current vectors after the phase shift in the directions of the corresponding phase voltage vectors, and calculates lengths of the orthogonal projection vectors as second projection values; a second projection value determination unit that determines whether a ratio of a maximum second projection value among the three calculated second projection values ​​to the other second projection values ​​is equal to or greater than a fourth threshold value, 5. The power transmission line protection device according to claim 4, wherein the second projection value determination unit determines that a phase of a phase current vector used in calculating the maximum second projection value is a ground fault phase when a proportion of the maximum second projection value is equal to or greater than the fourth threshold value.

6. The power transmission line protection device includes: a second projection value calculation unit that calculates, based on time series data of currents and voltages detected from the three-phase transmission line, a difference between an instantaneous value of each of three line currents and an instantaneous value n cycles earlier, shifts a line current vector based on the time series data of the difference amount by the phase shift angle corresponding to the line angle, orthogonally projects the line current vectors based on the time series data of the difference amount after the phase shift in directions of the corresponding line voltage vectors, and calculates lengths of the orthogonal projection vectors as second projection values; a second projection value determination unit that determines whether a ratio of a maximum second projection value among the three calculated second projection values ​​to the other second projection values ​​is equal to or greater than the first threshold value, the second projection value determination unit determines, when a ratio of the maximum second projection value is equal to or greater than the first threshold, that two phases related to the line-to-line current vector used in calculating the maximum second projection value are short-circuit fault phases; The power transmission line protection device further comprises:

4. The power transmission line protection device according to claim 3, further comprising an output selection unit that outputs the determination result of the second projection value determination unit in preference to the determination result of the first projection value determination unit when the time limit shorter than n cycles has not elapsed since the occurrence of the fault in the three-phase power transmission line, and that outputs the determination result of the first projection value determination unit in preference to the determination result of the second projection value determination unit when the time limit has elapsed since the occurrence of the fault.

7. The power transmission line protection device includes: a second current change amount determination unit that determines whether an effective value of the difference amount based on time-series data of the difference amount is equal to or greater than the second threshold value; when the effective value of the difference amount for each of two line currents is equal to or greater than the second threshold and the effective value of the difference amount for the other line current is less than the second threshold, the second current change amount determination unit determines two phases related to the line current whose effective value of the difference amount is less than the second threshold as healthy phases and determines the remaining one phase as having a ground fault; the second current change amount determination unit determines that a two-phase fault or a three-phase fault has occurred when an effective value of the difference amount for all line currents is equal to or greater than the second threshold value; 7. The power transmission line protection device according to claim 6, wherein the output selection unit outputs the determination result of the second current change amount determination unit in preference to the determination result of the first current change amount determination unit when the time limit has not elapsed since the occurrence of the fault, and outputs the determination result of the first current change amount determination unit in preference to the determination result of the second current change amount determination unit when the time limit has elapsed since the occurrence of the fault.

8. 8. The power transmission line protection device according to claim 7, wherein the second projection value determination unit determines that a three-phase fault has occurred when the second current change amount determination unit determines that a two-phase fault or a three-phase fault has occurred and when the proportion of the maximum second projection value is less than the first threshold value.

9. When the three-phase transmission line is an overhead line transmission line and the load current is a forward current, the phase shift angle is set between 70° and 80°; The power transmission line protection device according to any one of claims 1 to 3, wherein when the three-phase transmission line is an overhead line transmission line and the load current is a backward flow, the phase shift angle is set between 90° and 100°.

10. When the three-phase transmission line is a cable transmission line and the load current is a forward current, the phase shift angle is set between 60° and 70°; The power transmission line protection device according to any one of claims 1 to 3, wherein when the three-phase transmission line is a cable transmission line and the load current is a backward flow, the phase shift angle is set between 80° and 90°.

11. 4. The power transmission line protection device according to claim 1, wherein, instead of lengths of orthogonal projection vectors obtained by orthogonally projecting the three line-to-line current vectors after the phase shift in directions of the corresponding line voltage vectors, the first projection value calculation unit calculates, as the first projection value, values ​​obtained by dividing the lengths of the orthogonal projection vectors by the magnitudes of the corresponding line voltage vectors.

12. the first projection value calculation unit calculates, as the first projection value, a value obtained by dividing the length of an orthogonal projection vector obtained by orthogonally projecting the three line-to-line current vectors after the phase shift in the directions of the corresponding line voltage vectors, by the magnitude of the corresponding line voltage vector; 6. The power transmission line protection device according to claim 5, wherein, instead of lengths of orthogonal projection vectors obtained by orthogonally projecting the three phase current vectors after the phase shift in the directions of the corresponding phase voltage vectors, the second projection value calculation unit calculates values ​​obtained by dividing the lengths of the orthogonal projection vectors by the magnitudes of the corresponding phase voltage vectors as the second projection values.

13. the first projection value calculation unit calculates, as the first projection value, a value obtained by dividing the length of an orthogonal projection vector obtained by orthogonally projecting the three line-to-line current vectors after the phase shift in the directions of the corresponding line voltage vectors, by the magnitude of the corresponding line voltage vector; 9. The power line protection device according to claim 6, wherein, instead of a length of an orthogonal projection vector obtained by orthogonally projecting the line-to-line current vector based on the time-series data of the difference amount after the phase shift in a direction of the corresponding line-to-line voltage vector, the second projection value determination unit calculates, as the second projection value, a value obtained by dividing the length of the orthogonal projection vector by a magnitude of the corresponding line-to-line voltage vector.

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