Method of detecting fault in transmission line of power transmission system

Abstract

A method of detecting a fault in a protected line, the protected line constituting at least part of a transmission line in a power transmission system, the transmission line having at least one phase. The method comprises:-determining a positive sequence power, a negative sequence power and a zero sequence power for the positive sequence, the negative sequence and the zero sequence of the power of the protected line, respectively; determining whether a criterion is satisfied, where the criterion is based on a composite sequence power expression according to SCmp = k1 * S1-(k2 * S2 + k0 * S0), where SCmp is a composite sequence power, S1 is a positive sequence power, S2 is a negative sequence power, S0 is a zero sequence power, and k1, k2, k0 are settable non-negative weighting factors; and-if it is determined that the criterion is met, determining that a fault has been detected on the protected line. The method further includes determining an increment value ([delta] SCmp) of the composite sequence power as a difference between a current composite sequence power value (SCmp (current)) and a previous composite sequence power value (SCmp (previous)). The difference is determined as a difference between the active power portions and a difference between the reactive power portions.

Description

Technical Field

[0001] The present disclosure relates generally to power transmission systems, and more particularly to a method and system for detecting a fault in a protected line constituting at least a portion of a transmission line in a power transmission system. Background Art

[0002] In power transmission systems, such as high voltage (HV) or medium voltage (MV) power transmission systems, there is a risk of major short circuit faults (e.g. phase to ground or phase to phase) occurring. Such faults must be detected and remedied as quickly as possible. Therefore, transmission lines are monitored. Prior art solutions typically utilize measurements of current or impedance. They have significant disadvantages. For example, in order to identify the location of a fault along a transmission line, it may be necessary to send measurements between devices at different ends of the transmission line. Prior art methods have very high requirements for time synchronization of the devices in order to produce correct results. Additionally, with respect to impedance measurements, they have disadvantages regarding, for example, high impedance faults, signal volume effects due to remote end feeding, and even impedance measurements for healthy phases. Summary of the invention

[0003] In view of the above, the present disclosure focuses on how to reduce the shortcomings of the prior art.

[0004] To better address this concern, in a first aspect, the present disclosure provides a method for detecting a fault in a protected line, the protected line constituting at least a portion of a transmission line in a power transmission system, the transmission line having at least one phase. The method comprises:

[0005] Determine positive sequence power, negative sequence power and zero sequence power for the positive sequence, negative sequence and zero sequence of the power of the protected line respectively; determine whether a criterion is satisfied, wherein the criterion is based on the following composite sequence power expression:

[0006] S Cmp =k1*S1-(k2*S2+k0*S0)

[0007] Where S Cmp is the composite sequence power, S1 is the positive sequence power, S2 is the negative sequence power, S0 is the zero sequence power, and k1, k2, k0 are settable non-negative weighting factors; and if it is determined that the criterion is satisfied, it is determined that a fault has been detected on the protected line. The method further includes: determining the incremental value of the composite sequence power as the difference between the current composite sequence power value and the previous composite sequence power value.

[0008] By using incremental values ​​of the composite sequence power, a simpler way of detecting different fault indicators can be obtained and some requirements can be relaxed.

[0009] The operation of determining the incremental value of the composite sequence power may include: determining the incremental value of active power as the difference between the active power portion of the current composite sequence power value and the active power portion of the previous composite sequence power value, and determining the incremental value of reactive power as the difference between the reactive power portion of the current composite sequence power value and the reactive power portion of the previous composite sequence power value.

[0010] The method may further include: determining a first incremental value of the composite sequence power at a first measuring point at the end of the protected line, including determining a first incremental value of active power and a first incremental value of reactive power at the first measuring point. The criterion is as follows: the first incremental value of active power and the first incremental value of reactive power are positive and above a predetermined threshold. The determination that a fault has been detected includes: determining that a fault has been detected in a forward direction from the first measuring point, wherein the forward direction is from the first measuring point toward the protected line. In other words, the forward direction points along the protected line in a direction away from the measuring point. The process of determining in which direction along the transmission line a fault has occurred is called transient directionality. During steady-state conditions, the incremental values ​​of active power and reactive power are typically zero, which facilitates the setting of appropriate thresholds. As a result, the detection of faults will be fast and reliable.

[0011] The method may further include: determining a second incremental value of the composite sequence power at a second measuring point at another end of the protected line and at a distance from the first measuring point, wherein the second incremental value ΔS of the composite sequence power is Cmp2 The first active power increment value and the second reactive power increment value are included. The criterion is constituted as follows: the first active power increment value and the second active power increment value and the first reactive power increment value and the second reactive power increment value are all positive and above a threshold value. The determination that a fault has been detected on the protected line comprises: determining that a fault has occurred between the first measuring point and the second measuring point. Thus, a directional comparison is obtained, by which it can be determined whether a fault has occurred between the measuring points on the protected line, and by which the properties of the transmission line (such as line impedance) do not need to be involved.

[0012] For the case where the protected line comprises at least two ends, the method may comprise: determining a composite sequence power value at each end of the protected line, determining the differential sequence power as the sum of the composite sequence powers at these ends. The criterion is constituted as follows: the differential sequence power exceeds a threshold value. Thus, a differential protection is obtained, wherein due to the use of the composite sequence power (which in this respect can be regarded as time-invariant), the synchronization requirements between the ends are relaxed compared to prior art methods based on measuring the current / impedance of the transmission line.

[0013] For the case where the protected line includes at least two ends, the method may include: determining a composite sequence power value at each end of the protected line, and determining the differential sequence power as the difference between the sum of the composite sequence powers at the ends of the protected line and the power loss along the transmission line, wherein the criterion is constructed as follows: the differential sequence power value exceeds a threshold value.

[0014] By taking the power loss into account, the determination is more suitable for long transmission lines.

[0015] In addition, the differential sequence power can be determined as follows:

[0016]

[0017] in is the composite sequence power at the mth end, is the active power part of the composite sequence power, is the reactive power part of the composite sequence power, S 损耗 is the power loss along the transmission line, P 损耗 is the active power loss along the transmission line, Q 损耗 is the reactive power loss along the transmission line, and n is the number of ends of the protected line.

[0018] The method may comprise: determining a sequence power value at an arrival point located along the transmission line at a distance from a measuring point, wherein the measuring point is located at the end of the protected line; and determining incremental power values ​​at the arrival point by means of the sequence power values ​​at the arrival point, wherein each incremental sequence power value is the difference between a current sequence power value and a previous sequence power value. The criterion is based on the incremental sequence values. Determining that a fault has been detected comprises: determining that a fault has occurred between the measuring point and the arrival point. Thus, distance protection is obtained, wherein an actual measurement of the impedance of the transmission line is not required.

[0019] The incremental power value may include an incremental power value of the positive sequence power at the arrival point, and an incremental power value of a combination of the negative sequence power at the arrival point and the zero sequence power at the arrival point.

[0020] Determining the incremental sequence power value may include determining according to the following equation:

[0021] ΔS1@RP<ΔS 20 @RP

[0022] where ΔS1@RP is the incremental sequence power value of the positive sequence power at the arrival point, and ΔS 20 @RP is the incremental sequence power value of the combination of the negative sequence power at the arrival point and the zero sequence power at the arrival point.

[0023] The method may further include: determining a first voltage phasor and a first current phasor of different sequences at a measurement point; determining a second current phasor at an arrival point with the aid of the first current phasor; determining the second voltage phasor at the arrival point with the aid of the first voltage phasor, the first current phasor, the line impedance of the transmission line, and an arrival factor related to the distance; determining a sequence power value at the arrival point with the aid of the second voltage phasor and the second current phasor; and determining an incremental sequence power value at the arrival point with the aid of the sequence power value at the arrival point.

[0024] Determining that a fault has occurred between the measurement point and the arrival point may include determining that the fault is an asymmetric fault.

[0025] The power transmission system may be a three-phase system, but the method is also applicable to power transmission systems with other numbers of phases (more than three phases or less than three phases).

[0026] Determining the incremental sequence power value at the arrival point may include: using a distance-dependent arrival factor and a predetermined transmission line impedance. The method may further include: recursively estimating a value of the arrival factor, for which, according to ΔQ1@RP=ΔQ 20 The expression for @RP is valid, where ΔQ1@RP is the reactive power portion of the positive sequence incremental power, and ΔQ 20 @RP is the reactive power portion of the combined incremental power value. From this, in addition, the location of the fault is also determined, since the expression is only valid at the fault point.

[0027] According to another aspect, a fault detection system for a power transmission system is provided, the power transmission system comprising a transmission line, the transmission line further comprising a protected line constituting at least a part of the transmission line, wherein the transmission line has at least one phase. The fault detection system comprises an intelligent electronic device (111, 112, 113) which is connectable to the transmission line and configured to detect a fault on the protected line, wherein the intelligent electronic device is configured to:

[0028] -Determine the positive sequence power, negative sequence power and zero sequence power of the protected line respectively;

[0029] - Determine whether a criterion is satisfied, wherein the criterion is based on a composite sequence power expression according to:

[0030] S Cmp =k1*S1-(k2*S2+k0*S0)

[0031] Where S Cmpis the composite sequence power, S1 is the positive sequence power, S2 is the negative sequence power, S0 is the zero sequence power, and k1, k2, k0 are non-negative and configurable weighting factors; and

[0032] - If it is determined that the criterion is satisfied, it is determined that a fault has been detected on the protected line. The intelligent electronic device is further configured to: determine the incremental value of the composite sequence power as the difference between the current composite sequence power value and the previous composite sequence power value, including: determining the incremental value of active power as the difference between the active power part of the current composite sequence power value and the active power part of the previous composite sequence power value, and determining the incremental value of reactive power as the difference between the reactive power part of the current composite sequence power value and the reactive power part of the previous composite sequence power value.

[0033] The fault detection system provides corresponding advantages to the above method.

[0034] The intelligent electronic device may be connected to the transmission line at a measuring point, the measuring point being located at the end of the protected line. The intelligent electronic device may be configured to: determine a sequence power value at an arrival point located along the transmission line and at a distance from the measuring point; and determine an incremental sequence power value at the arrival point by means of the sequence power value at the arrival point. Each incremental sequence power value is a difference between a current sequence power value and a previous sequence power value. The criterion is based on the incremental sequence value. Determining that a fault has been detected includes: determining that a fault has occurred between the measuring point and the arrival point.

[0035] The incremental power value may include an incremental power value of the positive sequence power at the arrival point, and an incremental power value of a combination of the negative sequence power at the arrival point and the zero sequence power at the arrival point.

[0036] According to another aspect, a computer program product is provided that includes instructions that, when downloaded into an intelligent electronic device, cause the intelligent electronic device to implement the method described above. The computer program product may be implemented as a carrier containing code for these instructions (such as a portable memory device), a computer program stored on a computer-readable medium (such as a server, a hard disk), a downloadable computer program code, etc., as understood by those skilled in the art. For the purposes of this application, it will be noted that the term power transmission system refers to any suitable power transmission system, such as alternating current (AC) and alternating current / direct current (AC / DC) transmission systems. The term transmission line refers to any suitable AC transmission line, such as HVAC transmission lines, MVAC transmission lines (also known as distribution lines), long-distance and short-distance transmission lines, etc. The term protected line refers to a portion of a transmission line that is being monitored for faults. The term forward direction refers to a reference direction that is generally directed toward the protected line from a measurement point. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Exemplary embodiments will now be described in more detail and with reference to the accompanying drawings, in which:

[0038] Figure 1 A block diagram showing an embodiment of the present fault detection system arranged in a transmission system;

[0039] Figure 2 A flow chart showing an embodiment of the present fault detection method; and

[0040] Figure 3 is a graphical representation of the power conditions in a transmission line subject to a fault. DETAILED DESCRIPTION

[0041] Figure 1 The most general example of a power transmission system 100 is shown, which includes a transmission line 101 having one or more phases (e.g., three phases) and at least two ends. In the illustrated example, there are three ends, one of which is shown as an alternative to a two-end type transmission line. Electrical devices 102, 103, 104 are connected to the transmission line 101 at each of its ends. The electrical devices 102-104 may be power sources or electrical loads. The transmission line 101 is provided with one or more switching devices 105, 106, 107, 108, 109, 110, allowing the circuit to be disconnected to limit the current flow in the power transmission system 100. The fault detection system for the power transmission system 100 includes one or more intelligent electrical devices (IEDs) 111, 112, 113. Each IED 111-113 is typically connected to the transmission line 101 at its end, either directly or via some other device, and is arranged to monitor electrical activity in the transmission line 101, for example by measuring the current and voltage in the transmission line 101. Therefore, for the purpose of this disclosure, the portion of the transmission line 101 to which the IEDs 111-113 are connected is referred to as a measuring point. Furthermore, the IEDs 111-113 are arranged to detect faults in the transmission line 101 and are arranged to switch one or more of the switch devices 105-110. It should be noted that the transmission line 101 or a portion thereof monitored by the IEDs 111-113 constitutes a protected line 114.

[0042] As will be appreciated by those skilled in the art, Figure 1The overall structure of the power transmission system shown in is only an example among an almost infinite number of different variants. In order to give only a brief comment on the diversity, and in addition to what has been described above, the power transmission system may be part of a power grid or may be connected to a power grid, the power sources may be different kinds of power sources (such as wind power plants, solar power plants, nuclear power plants, transformers, etc.), and the loads may be different kinds of loads (such as production sites, etc.). The power transmission system may include a plurality of transmission lines, etc. The transmission lines may be HV AC lines or MV AC lines, which may extend between power sources, between power sources and loads, etc., and thus, as recognized above, they may constitute lines also referred to as distribution lines. The length of the transmission lines may vary greatly, from a few kilometers or even shorter up to hundreds of kilometers.

[0043] According to an exemplary embodiment of the present method of detecting a fault in the protected line 114, the method includes: Figure 2 The operation illustrated in the flowchart of . In box 201, the sequence power of the transmission line 101 is determined. That is, the positive sequence power S1 for the positive sequence, the negative sequence power S2 for the negative sequence, and the zero sequence power S0 for the zero sequence of the power in the transmission line 101 are determined. The method further includes: in box 202, determining whether a criterion is satisfied. The criterion is determined based on at least one of the following: a composite sequence power S determined based on the positive sequence power S1, the negative sequence power S2, and the zero sequence power S0 Cmp and at least one expression determined based on at least two of the positive sequence power S1, the negative sequence power S2 and the zero sequence power S0. If the criterion is satisfied, it is determined that a fault has been detected on the protected line 114 (block 203).

[0044] The criterion may be based on an expression determined based on at least one of the positive sequence power S1 , the negative sequence power S2 , and the zero sequence power S0 , and additionally a threshold value.

[0045] The criterion can be based on the composite sequence power expression according to the following:

[0046] S Cmp =k1*S1-(k2*S2+k0*S0)

[0047] Where S Cmp is the composite sequence power, S1 is the positive sequence power, S2 is the negative sequence power, S0 is the zero sequence power, and k1, k2, k0 are settable non-negative weighting factors.

[0048] Based on the composite sequence power S CmpAnd / or determining the criterion based on an expression of at least two of the sequence powers includes different possible alternatives, such as using one or more of the sequence powers in the determination, using the resulting composite sequence power in a different way, determining an instantaneous value, determining an incremental value, predicting a future value, determining a value relative to a threshold, etc. This will be apparent from and elaborated in detail in the following examples.

[0049] For the sake of clarity, an example of how to determine the sequence power will now be given for a three-phase transmission line. The IEDs 111-113 are arranged to measure phase current signals and phase voltage signals of the three phases. The reference direction Ref is determined to point away from the end to which the IEDs 111-113 are connected towards the other end of the transmission line 101. In order to clearly identify the phase and sequence components, the following nomenclature will be used throughout this application:

[0050] 1. The phasors will be indicated by the indices A, B, and C (e.g., I A ,I B and I C )

[0051] 2. The sequence quantities will be indicated by indices 1, 2, and 0 for the positive, negative, and zero sequence components, respectively (eg, I1, I2, and I0).

[0052] 3. All values ​​given in the examples will be presented in per unit or percentage.

[0053] The fundamental frequency phasors of the three-phase voltage and current signals should be derived. These signals are sampled and a full cycle discrete Fourier transform (DFT) filter or some other suitable type of filtering may be applied. The phasors of the sequence components of the current and voltage are then calculated by using known matrix equations as shown below for the current signal: The voltage phasors are calculated similarly.

[0054]

[0055] The character "α" represents a complex operator; a = e i*120° =e j*120° ,and Therefore, the positive sequence current phasor I1, the negative sequence current phasor I2 and the zero sequence current phasor I0 are all composed of the phase current I A ,I B and I C to be determined by different combinations of

[0056] Then, the sequence powers S1, S2, S0 (which are complex values) are calculated using the following known formulas:

[0057]

[0058] in" <c>” indicates that the current phasor value should be complex conjugated before performing the multiplication, P is the active power, and Q is the reactive power.

[0059] After the sequence power is determined, the composite sequence power is calculated according to the following expression:

[0060] S Cmp =k1*S1-(k2*S2+k0*S0)

[0061] Where S Cmp is the composite sequence power, S1 is the positive sequence power, S2 is the negative sequence power, S0 is the zero sequence power, and k1, k2, k0 are non-negative and settable weighting factors.

[0062] This expression for the composite sequence power can be expanded as follows:

[0063] S Cmp =k1*S1-k2*S2-k0*S0=P Cmp +j*Q Cmp

[0064] As mentioned above, the weighting factors k1, k2, k0 are non-negative and settable. For the following considerations, in order to simplify the description, the weighting factors are all selected to be 1.0, i.e. k1=k2=k0=1.0. However, the weighting factors can be given other values ​​in order to increase the influence of some sequence power components in the total value of the composite sequence power.

[0065] Negative sequence power S2 and zero sequence power S0 in composite sequence power S Cmp The negative sign is taken in the expression of , because they actually flow in the direction opposite to the direction of the positive sequence power S1. Therefore, they will add up to the positive sequence power and will increase the sensitivity of the method.

[0066] In addition, the composite sequence power S Cmp The expression of is useful for detecting different kinds of faults in the transmission line 101 , as will become apparent below.

[0067] The method of detecting a fault in a protected line in a power transmission system according to the present disclosure results from novel findings based on the understanding of how power becomes distributed in a power transmission line in the presence of a short circuit, such as Figure 3 The situation shown is for an ideal case where there is no fault resistance at the fault point and there is no load on the power transmission system before the fault occurs. However, Figure 3 Some basic principles illustrated in are generally valid. The relationship between the sequence power at the fault point (FP) can be expressed as:

[0068] S1@FP=-(S2@FP+S0@FP)

[0069] This can be expressed and understood in words as follows: at the fault point FP, the positive sequence power will be equal to the sum of the negative sequence power and the zero sequence power, but with opposite signs. In a physical sense, the negative sign means that the negative sequence power and the zero sequence power are reflected back from the fault point and flow from the fault point towards the source 301, which is indicated as Figure 3 This is true even for a symmetrical three-phase fault, but at the fault point FP and in the entire power transmission system, the zero-sequence power S0 and the negative-sequence power S2 are then equal to zero.

[0070] It should also be noted that at any arbitrary point T between the source 201 and the fault point FP (e.g. Figure 3 ) and therefore also at the points where the IED is connected, the following relationship will always be valid:

[0071] S1@T>-(S2@T+S0@T)

[0072] It will be noted that in order to obtain a completely adequate result, the weighting factors k should here all be equal to 1. In a practical power transmission system 100, the following will almost always be valid:

[0073] 1. The transmission line will be loaded before the fault occurs and provide some power to the load.

[0074] 2. The fault will be fed from both sides of the protected line, or from all sides (in case of multi-ended transmission lines).

[0075] 3. There will be some fault resistance (variable or constant) at the fault point.

[0076] Therefore, the above expression can be generalized so that it is valid under all operating conditions of the protected line. Based on the superposition theorem of the circuit, a general equation can be written for the sequence power at the fault point:

[0077] ΔS1@FP=P RF -(ΔS2@FP+ΔS0@FP)

[0078] in:

[0079] ΔS1@FP is the change in the positive sequence power at the fault point FP due to the fault. By determining the incremental value, the influence of the load before the fault occurs is eliminated. The incremental value of the sequence power is the difference between the current sequence power value and the previous sequence power value. For example, the incremental value of the sequence power can be calculated by the time difference between two power transmission system cycles. ΔS2@FP is the change in the negative sequence power at the fault point FP due to the fault. Note that before the fault, the negative sequence power is usually zero, that is, it does not exist. However, by using its incremental value, it is possible to eliminate any asymmetric or nonlinear load that existed before the fault.

[0080] ΔS0@FP is the change in zero-sequence power at the fault point due to the fault. Note that before the fault, the zero-sequence power is usually zero, i.e., non-existent. However, by using its incremental value, it is possible to eliminate any asymmetric or nonlinear load before the fault.

[0081] P RF It is the active power consumed by the fault point itself, that is, the power given to the fault resistor or arc at the fault point. Note that this quantity cannot be measured directly. It cannot be estimated in a simple way either. However, for almost every actual fault in the power transmission system, this active power will be consumed. This can be expressed and understood in words as follows: at the fault point, the change in positive sequence power S1 will be partially given to the fault resistor (that is, most of it is consumed by the fault point / resistance itself as active power), and part of it is reflected back as negative sequence power S2 and zero sequence power S0. This is true even for symmetrical three-phase faults, but at the fault point FP and in the entire power system, then the negative sequence power S2 and the zero sequence power S0 are equal to zero.

[0082] As explained above, throughout the power transmission system, the flow of positive sequence power S1 or its increment ΔS1 will be in the opposite direction to the flow of negative sequence power S2 and zero sequence power S0 or their increments ΔS2, ΔS0. Therefore, as mentioned above, the composite sequence power is defined as the difference between them, that is:

[0083] S Cmp =S1-S2-S0=P Cmp +j*Q Cmp

[0084] will result in all sequence power values ​​adding up to a combined actual sum. This can be considered as using all three sequence powers S1, S2, S0 in the same direction. Thus, the total value will be larger than if these sequence power values ​​were added with their respective signs, and thus the comparison with such values ​​will be more sensitive. Furthermore, such composite sequence power value will have a more or less constant value throughout the entire protected line if the losses along the protected line are neglected.

[0085] This will therefore make the relative position of the IEDs 111 - 113 with respect to the fault point FP less important than when the three sequence components and their respective powers are used on a one-by-one basis for protection purposes.

[0086] Furthermore, at any arbitrary point T between the source and the fault point (which may be a location where an IED is connected), the following relationship will always be valid:

[0087] ΔS1@T>-(ΔS2@T+ΔS0@T)

[0088] The use of this relationship between incremental values ​​of sequence power will be exemplified below.

[0089] According to an embodiment of the method, determining the comparison value may include: converting the composite sequence power S into Cmp The increment value ΔS Cmp Determined as the difference between the current composite sequence power value and the previous composite sequence power value:

[0090] ΔS Cmp =S Cmp (Current)-S Cmp (Previous) = ΔP Cmp +j*ΔQ Cmp

[0091] =(P Cmp (Current)-P Cmp (previous))+j*(Q Cmp (current)

[0092] -Q Cmp (previously))

[0093] Therefore, the incremental value of the composite sequence power ΔS Cmp The determination includes: the active power increment value ΔP Cmp Determined as the current composite sequence power value ΔS Cmp The active power part P of the current Cmp (Current) and previous composite sequence power value S Cmp The active power part P Cmp The difference between (previous) and the reactive power increment ΔQ Cmp Determined as the current composite sequence power value S Cmp The reactive power part Q Cmp (Current) and previous composite sequence power value S Cmp The reactive power component Q Cmp The time difference between the current value and the previous value may be, for example, two cycles of the power transmission system 100, wherein the cycle is related to the fundamental frequency.

[0094] The incremental value of the composite sequence power ΔS Cmp Several different approaches to determining a fault on the protected line 114 are available.

[0095] According to an embodiment of the method, the method may include determining a first incremental value ΔS of the composite sequence power at a first measurement point at the end of the protected line 114 (eg, where the first IED 111 is connected to the transmission line 101) Cmp1 . For ΔS Cmp1 The determination may include: determining a first active power increment value ΔP at a first measurement point Cmp1 and the first reactive power increment ΔQ Cmp1 In this case, the criterion to be met in order to determine that a fault has been detected is as follows: the first active power increment value ΔP Cmp1 and the first reactive power increment ΔQ Cmp1 is positive and above a predetermined threshold. The predetermined threshold may be, for example, 5% or some other suitable value. Furthermore, it is determined that a fault has been detected in a forward direction from the first measuring point, wherein the forward direction is from the first measuring point towards the protected line, ie, the reference direction Ref1 of the first IED 111.

[0096] According to an embodiment of the present method, the method may further include: determining a second incremental value ΔS of the composite sequence power at a second measuring point at the other end of the protected line 114 and at a certain distance from the first measuring point. Cmp2 The second measurement point may be a portion of the transmission line 101 to which the second IED 112 is connected. The second incremental value ΔS of the composite sequence power Cmp2 Including the second active power increment value ΔP Cmp2 and the second reactive power increment value ΔQ Cmp2 Here, the criterion is as follows: the first active power increment value ΔP Cmp1 and the second active power increment value ΔP Cmp2 And the first reactive power increment value ΔQ Cmp1 and the second reactive power increment value ΔQ Cmp2 All are positive and above the threshold. The positive direction from the first measurement point (e.g., the reference direction Ref1 of the first IED 111) is toward the second measurement point. The positive direction from the second measurement point (e.g., the reference direction Ref2 of the second IED 112) is also toward the protected line, which means toward the first measurement point. Thus, if the first active power increment value ΔP Cmp1 and the second active power increment value ΔP Cmp2 And the first reactive power increment value ΔQ Cmp1 and the second reactive power increment value are all positive and above the threshold, then the criterion is satisfied resulting in a determination that a fault has occurred between the first measuring point and the second measuring point. The result of these operations may be referred to as a direction comparison, which is a method of determining that a fault has occurred on a protected line 114, which extends between a first IED 111 and a second IED 112. After thus concluding that the detected fault is somewhere on the protected line, the first IED 111 and / or the second IED 112 may control one or more of the switch devices 105-110 to disconnect the current in the protected line. As will be appreciated, in order to ensure correct fault detection, there must be communication between the IEDs 111, 112. However, the communication may be limited to sending a binary number indicating a direction or indicating whether the direction is positive. When the fault detection system comprises two or more 1EDs 111-113, the fault detection may be implemented in one or several or all of the 1EDs 111-113. Each IED 111-113 (which is used to determine whether a fault has occurred) will receive a binary value from the other IED(s) 111-113. By the above use of sequence power, various kinds of faults will be detected. However, it may not be possible to determine the faulted phase. The directional comparison method does not require any data about the protected line, for example, does not require positive sequence and / or zero sequence line impedance as setting parameters.

[0097] According to an embodiment of the method, the method may include: determining a composite sequence power value S at each end of the protected line 114 Cmp , and the differential power S Diff Determined as the composite sequence power S at at least these ends Cmp The criterion to be met in order to determine that a fault has been detected is as follows: The differential power S Diff This method is called differential fault detection or differential protection. For example, there are three ends (e.g., Figure 1 In the case of one of the options shown in Cmp is determined by all three IEDs 111-113, and its sum is determined by each of the IEDs 111-113 receiving the composite sequence power values ​​from the other two IEDs. For an ideal line (i.e., without any P and Q losses), if there is no fault on the transmission line 101, the composite sequence value S from the IEDs 111-113 is Cmp The sum of should be zero. By setting a threshold slightly away from zero (such as, for example, 5%) and comparing the sum recalculated as a percentage of the nominal sum value with the threshold, it is concluded that if the threshold is exceeded, a fault has occurred on the transmission line 101. The determination of the differential sequence power can be expressed mathematically as:

[0098]

[0099] Where n is the number of ends of the transmission line 101, that is, n>2. Therefore, the composite sequence power value The sum of is used as a comparison value and is compared with a threshold value. Therefore, the criterion can be expressed as "the difference power value exceeds the threshold value".

[0100] As an alternative, incremental values ​​of differential sequence power can be used, namely:

[0101]

[0102] In this differential fault detection method, as in the above directional comparison, the time invariance of the complex power value is used to relax the time synchronization requirements for communication between IEDs. In other words, since sequence power is used, and the composite sequence power changes very little and slowly during normal conditions compared to the current, the timing requirements for synchronization can be relaxed compared to the timing requirements of the prior art method based directly on current measurement. As an example, for a 50 Hz power transmission system, the time synchronization requirements for power measurement may be approximately 1 to 3 milliseconds. This is to be compared with the prior art using current-based differential fault detection, which requires time synchronization of approximately 1 microsecond. Therefore, for power-based differential fault detection, the time synchronization requirements are significantly relaxed. At the same time, the data bandwidth required for communication is also greatly reduced, because there are only two actual quantities (i.e., P and R) that are used. Cmp , and Q Cmp ) are transferred between the IEDs 111-113. Additionally, these two quantities are nearly constant during steady-state conditions, which can further be used to reduce the required communication bandwidth.

[0103] However, it should be noted that, in practice, there are P and Q power losses along the transmission line 101, which have a significant impact at least on longer transmission lines. Therefore, for longer transmission lines 101, the following modification can be used to determine the differential power S Diff The expression for this is to include the power loss on the transmission line 101:

[0104]

[0105] in:

[0106] P 损耗 is the active power loss along the transmission line 101. It can be estimated as R1*|I1| 2 .

[0107] Q 损耗 is the reactive power loss along the transmission line 101. It can be estimated as X1*|I1| 2 -B1*|U1| 2 .

[0108] R1 is the positive sequence resistance of the transmission line 101,

[0109] X1 is the positive sequence reactance of transmission line 101,

[0110] B1 is the positive sequence susceptance of the transmission line 101 .

[0111] For which the incremental differential power ΔS is determined Diff Alternatively, power losses may be ignored because, in the event of an internal fault (ie, a fault on the protected line), the power changes at all ends of the transmission line 101 will be positive (indicating an internal fault) regardless of any power losses as explained above.

[0112] Another example of fault detection in the transmission line 101 is called distance fault detection or distance protection. The purpose of distance protection is to detect whether a fault occurs on the protected line based only on local measurements. This can be done by defining one or more distance protection zones. For example, two protection zones may be defined, wherein a first protection zone covers 80% of the protected line 114 and a second protection zone covers 120% of the protected line 114. A position at the far end of the protected line 114 (i.e., at, for example, 80% or 120% of the protected line, which in turn may be substantially the total length of the transmission line 101) relative to the IED 111-113 positions is denoted as a reach point RP. In order to perform distance protection, some transmission line properties will be taken into account. They include, but are not limited to, the line positive sequence impedance Z 1L and line zero-sequence impedance Z 0L If there are parallel transmission lines (not shown in the figure), the line mutual zero-sequence impedance Z can also be used ML Based on those properties and the selected protection zone (represented by the arrival factor r, which is 0.8 for the 80% case and 1.2 for the 120% case), and the sequence current phasors and sequence voltage phasors measured at the IED location (i.e., at the measurement point), it is possible to determine the sequence power at the set arrival point RP. In other words, the arrival factor r is related to the distance from the measurement point (i.e., where the IEDs 111-113 are connected to the transmission line 101) to the arrival point RP.

[0113] Therefore, according to an embodiment of the method, the method may include: determining sequence power values ​​S1, S2, S0 at an arrival point RP located along the transmission line 101 and at a certain distance from the measurement point and therefore from the IEDs 111-113; and determining incremental sequence power values ​​ΔS1, ΔS2, ΔS0 at the arrival point RP by means of the sequence power values ​​at the arrival point. Based on these incremental sequence power values, it can be determined that a fault has occurred between the measurement point and the arrival point. The method still uses the composite sequence power S Cmp The expression defined above is based on , but the various factors of the expression will be used instead of the composite sequence power itself, and these various factors will be determined for the arrival point (which will be denoted as @RP).

[0114] For example, the incremental sequence power value may include the incremental sequence power value ΔS1@RP of the positive sequence power at the arrival point, and the incremental sequence power value ΔS1@RP of the combination of the negative sequence power at the arrival point and the zero sequence power at the arrival point. 20 @RP. For example, the comparison value can then be defined as:

[0115] ΔS1@RP<ΔS 20 @RP

[0116] Therefore, when the criterion is met, that is, when the incremental value ΔS1@RP of the positive sequence power at the arrival point is less than the incremental value ΔS1@RP of the combination of the negative sequence power at the arrival point and the zero sequence power at the arrival point 20 @RP, then a fault has been detected, wherein the fault is located between the measurement point (i.e., the IED 111-113) and the arrival point RP. For example, the combination may be the entire part of the expression involving negative sequence power and zero sequence power and valid for the arrival point, which is the negative sum of those powers, i.e., S 20 @RP=-(S2@RP+S0@RP).

[0117] The incremental value of the sequence power can be determined by the following steps: determining the first voltage phasor and the first current phasor of different sequences (represented as U1@IED, U2@IED, U0@IED, I1@IED, I2@IED, I0@IED) at the measurement point (for example, at the first IED 111); determining the second current phasor at the arrival point (represented as I1@RP, I2@RP, I0@RP) by means of the first current phasor; determining the line impedance Z of the transmission line 101 by means of the first voltage phasor, the first current phasor, and the line impedance Z of the transmission line 101; determining the incremental value of the sequence power by the following steps: determining the first voltage phasor and the first current phasor of different sequences (represented as U1@IED, U2@IED, U0@IED, I1@IED, I2@IED, I0@IED) at the measurement point (represented as I1@RP, I2@RP, I0@RP) by means of the first current phasor; determining the second current phasor at the arrival point (represented as I1@RP, I2@RP, I0@RP) by means of the first voltage phasor, the first current phasor, and the line impedance Z of the transmission line 101; determining the incremental value of the sequence power by the following steps: determining the first voltage phasor and the first current phasor 1L , Z 0L , and arrival factor r to determine the second voltage phasor at the arrival point (expressed here as U1@RP, U2@RP, U0@RP); determine the sequence power value at the arrival point with the help of the second voltage phasor and the second current phasor; and determine the incremental sequence power value at the arrival point with the help of the sequence power value at the arrival point.

[0118] A more detailed example of the calculations performed to enable detection of distance faults is as follows. First, the current phasor and the voltage phasor at the arrival point RP are determined according to:

[0119] U1@RP=U1@IED-r*Z 1L *l1@IED

[0120] U2@RP=U2@IED-r*Z 1L *I2@IED

[0121] U0@RP=U0@IED-r*Z 0L *I0@IED

[0122] I1@RP=I1@IED

[0123] I2@RP=I2@IED

[0124] I0@RP=I0@IED

[0125] This example is simplified because, among other things, no shunt capacitance of the transmission line 101 / protected line 114 is taken into account. However, this is not absolutely necessary for detecting faults within the set protection zone. Furthermore, it should be noted that other more complex methods can also be used to estimate the sequence voltage components and sequence current components at the arrival point, for example by using a two-port equivalent network, line telegraph equations, etc., also taking the shunt capacitance into account. Once the current sequence phasors and the voltage sequence phasors at the set arrival point RP are known, the following sequence power at the arrival point RP will be calculated:

[0126] S1@RP=U1@RP*I1@RP <c>< / c> =P1@RP+j*Q1@RP

[0127] S2@RP=U2@RP*I2@RP <c>< / c> =P2@RP+j*Q2@RP

[0128] S0@RP=U0@RP*I0@RP <c>< / c> =P0@RP+j*Q0@RP

[0129] S 20 @RP=-(S2@RP+S0@RP)=P 20 @RP+j*Q 20 @RP

[0130] Note that, as described above, the combined sequence power S at the arrival point 20 @RP is considered as the negative sum of S2@RP and S0@RP. Finally, the incremental S1@RP value and incremental S 20 @RP value. This can be achieved, for example, by subtracting the old value of two power transmission system cycles from the currently calculated value, i.e. the term "previous" in the following expression corresponds to "the old value two cycles ago".

[0131] ΔS1@RP=S1@RP(current)-S1@RP(previous ΔP1@RP+j*ΔQ1@RP

[0132] ΔS 20 @RP=S 20 @RP(current)-S 20 @RP(previously=

[0133] =ΔP 20 @RP+j*ΔQ 20 @RP

[0134] It is also noted that when the fault point is within the setting range of the distance protection, the above calculation will exceed the actual fault point FP. As a result, the incremental value ΔS1 of the positive sequence power becomes smaller than the incremental value ΔS1 of the combination of the negative sequence power and the zero sequence power. 20 Note that the relationship between these two incremental powers is not physically possible at any point in the actual power transmission system, but arises in the calculations due to the fact that the actual fault point is located between the measurement point and the set arrival point RP (where the above calculations are done).

[0135] Distance fault detection based on the incremental value of sequence power can be performed as follows:

[0136] ΔS1@RP<ΔS 20 @RP+P RF

[0137] Since this is a complex expression, it can be broken down into two real expressions as given below:

[0138] ΔP1@RP<ΔP 20 @RP+P RF

[0139] ΔQ1@RP<ΔQ 20 @RP

[0140] Therefore, ΔP1@RP, ΔP 20 @RP+PRF, ΔQ1@RP and ΔQ 20 @RP is used as comparison value and they are compared with each other in pairs. The criterion to be satisfied in order to determine that a fault has been detected is then to satisfy those two expressions. The first expression has the active power P consumed by the fault resistor RF This unknown term. Therefore, this expression is more difficult to satisfy for real faults when the fault resistance is present. This is a known problem for distance fault detection when larger fault resistance values ​​can affect fault determination. The second expression for reactive power is easier to use and offers a clear advantage over traditional impedance-based distance protection. As shown in this expression, the fault resistance has no effect on the reactive power portion of the equation.

[0141] It should be noted that the above two expressions are only valid for all types of asymmetrical faults. If a symmetrical three-phase fault occurs, there is no negative sequence power and zero sequence power in the system. If such a three-phase fault occurs, the two expressions can be modified as follows:

[0142] ΔP1@RP<0+P FP ~0

[0143] ΔQ1@RP<0

[0144] By neglecting the fault resistance for a three-phase fault in the first of the two expressions above, the safety of the determination is increased, since the trip condition (i.e., the condition of the switching devices 105-110) is already satisfied for some small positive values ​​of ΔP1@RP, but the conclusion that a fault has occurred is slightly delayed until ΔP1@RP becomes negative. To confirm that the fault is in the forward direction, the transient directionality described above can be employed. Thus, for a three-phase fault, the following holds:

[0145] ΔP1@RP<0 and ΔP1@IED>0

[0146] ΔQ1@RP<0 and ΔQ1@IED>0

[0147] Because sequence power is used, the distance protection works in a three-phase manner (ie it is not phase separated) for all types of faults.

[0148] However, its main advantage will be speed of operation.Again, due to the presence of fault resistance (ie, active power dissipation) at the fault point, the second expression will usually be satisfied first for most practical faults.

[0149] As a further example embodiment, the method may be used to locate a fault by recursively finding the value of the arrival factor r for a set line impedance for which the relationship ΔQ1@RP = ΔQ20@RP is valid, since such a relationship is valid only at the fault point. More specifically, determining the delta-sequence power value at the arrival point may include using the distance-dependent arrival factor r and a predetermined transmission line impedance (e.g., Z 1L , and Z 0L ). From the above determination of the voltage phasor including the arrival factor r, it can be appreciated that it will be possible to recursively estimate the value of the arrival factor r for which, according to ΔQ1@RP=ΔQ 20 @RP is valid. ΔQ1@RP is the reactive power portion of the positive sequence incremental power, and ΔQ 20 @RP is the reactive power portion of the combined incremental power value.

[0150] While the disclosure has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive.

[0151] Although features and elements are described above in particular combinations, each feature or element may be used alone without the other features and elements, or in various combinations with or without the other features and elements.

[0152] Other variations of the disclosed embodiments may be understood and effected by those skilled in the art in practicing the claimed invention from a study of the drawings, the disclosure and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single processor or other unit may perform the functions of several items recited in a claim. The mere fact that certain features are recited in mutually different dependent claims does not indicate that a combination of these features cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.< / c>

Claims

1. A method for detecting a fault in a protected line, the protected line constituting at least a portion of a transmission line in a power transmission system, the transmission line having at least one phase, the method comprising: - determining the positive sequence power, negative sequence power and zero sequence power for the positive sequence, negative sequence and zero sequence of the power of the protected line respectively; - Determine whether a criterion is satisfied, wherein the criterion is based on a composite sequence power expression according to: S Cmp =k1*S1-( k 2*S2+k0*S0) Where S Cmp is the composite sequence power, S1 is the positive sequence power, S2 is the negative sequence power, S0 is the zero sequence power, and k1, k2, k0 are settable non-negative weighting factors; and - If it is determined that the criterion is satisfied, it is determined that a fault has been detected on the protected line, wherein the method further comprises: Cmp ) is determined as the current composite sequence power value (S Cmp (current)) and the previous composite sequence power value (S Cmp The difference between (previous)).

2. The method according to claim 1, wherein the step of determining the incremental value (ΔS Cmp )include: The active power increment (ΔP Cmp ) is determined as the active power part of the current composite sequence power value (P Cmp The active power part (P Cmp (previous)) and the reactive power increment (ΔQ Cmp ) is determined as the reactive power part (Q Cmp (current)) and the reactive power part (Q Cmp The difference between (previous)).

3. The method according to claim 2, comprising: Determine a first incremental value (ΔS) of the composite sequence power at a first measuring point at the end of the protected line (114) Cmp1 ), including determining a first active power increment value (ΔP Cmp1 ) and the first reactive power increment value (ΔQ Cmp1 ), wherein the criterion is constituted as follows: the first active power increment value and the first reactive power increment value are positive and above a predetermined threshold, and wherein the determination that a fault has been detected comprises: determining that a fault has been detected in a forward direction from the first measuring point, wherein the forward direction is from the first measuring point toward the protected line.

4. The method according to claim 3, further comprising: Determine a second incremental value (ΔS) of the composite sequence power at a second measuring point at the other end of the protected line and at a distance from the first measuring point. Cmp2 ), the second incremental value includes a second active power incremental value (ΔP Cmp2 ) and the second reactive power increment (ΔQ Cmp2 ), wherein the criterion is as follows: the first active power increment value and the second active power increment value and the first reactive power increment value and the second reactive power increment value are all positive and higher than the threshold value, and the determination that a fault has been detected on the protected line includes: determining that a fault has occurred between the first measuring point and the second measuring point.

5. A method according to any one of the preceding claims, wherein: The protected line (114) comprises at least two ends, and the method comprises: determining the composite sequence power value (S) at each end of the protected line Cmp ), and the differential power (S Diff ) is determined as the sum of the composite sequence powers at the end, wherein the criterion is constructed as follows: the differential sequence power exceeds a threshold value.

6. The method according to any one of claims 1 to 4, wherein: The protected line (114) comprises at least two ends, and the method comprises: determining the composite sequence power value (S) at each end of the protected line Cmp ), and the differential power (S Diff ) is determined as the sum of the composite sequence power at the end and the power loss (S) along the transmission line (101) 损耗 ), wherein the criterion is constructed as follows: the difference power value exceeds a threshold.

7. The method according to claim 6, wherein the differential sequence power is determined according to: in is the composite sequence power at the mth end, is the active power part of the composite sequence power, is the reactive power part of the composite sequence power, S 损耗 is the power loss along the transmission line, P 损耗 is the active power loss along the transmission line, Q 损耗 is the reactive power loss along the transmission line, and n is the number of ends of the protected line.

8. A method according to any one of the preceding claims, comprising: Determine a sequence power value at an arrival point located along the transmission line and at a certain distance from a measurement point, wherein the measurement point is located at the end of the protected line; and determine an incremental sequence power value at the arrival point with the aid of the sequence power value at the arrival point, wherein each incremental sequence power value is the difference between a current sequence power value and a previous sequence power value, and wherein the criterion is based on the incremental sequence value, and the determination that a fault has been detected includes: determining that a fault has occurred between the measurement point and the arrival point.

9. The method according to claim 8, wherein: The incremental power value includes an incremental power value of positive sequence power at the arrival point and an incremental power value of a combination of negative sequence power at the arrival point and zero sequence power at the arrival point.

10. The method of claim 9, wherein determining the incremental sequence power value comprises determining according to the following equation: ΔS1@RP<ΔS 20 @RP where ΔS1@RP is the incremental sequence power value of the positive sequence power at the arrival point, and ΔS 20@RP is the incremental sequence power value of the combination of the negative sequence power at the arrival point and the zero sequence power at the arrival point.

11. The method according to claim 9, wherein determining the incremental sequence power value at the arrival point comprises: Using the arrival factor associated with the distance and a predetermined transmission line impedance, the method further comprises: recursively estimating a value of the arrival factor, for which according to ΔQ1@RP=ΔQ 20 @RP is valid, where ΔQ1@RP is the reactive power portion of the positive sequence incremental power, and ΔQ 20 @RP is the reactive power portion of the combined incremental power value.

12. The method according to any one of claims 8 to 11, comprising: determining different sequences of first voltage phasors and first current phasors at the measuring points; determining a second current phasor at the arrival point by means of the first current phasor; determining a second voltage phasor at the arrival point by means of the first voltage phasor, the first current phasor, a line impedance of the transmission line, and an arrival factor related to the distance; determining a sequence power value at the arrival point by means of the second voltage phasor and the second current phasor; And determining an incremental sequence power value at the arrival point by means of the sequence power value at the arrival point.

13. A fault detection system for a power transmission system (100), the power transmission system comprising a transmission line (101), the transmission line further comprising a protected line constituting at least a part of the transmission line, wherein: The transmission line has at least one phase, and the fault detection system comprises an intelligent electronic device (111, 112, 113), which can be connected to the transmission line and is configured to detect a fault on the protected line, wherein the intelligent electronic device is configured to: - determining the positive sequence power, negative sequence power and zero sequence power for the positive sequence, negative sequence and zero sequence of the power of the protected line respectively; - Determine whether a criterion is satisfied, wherein the criterion is based on a composite sequence power expression according to: S Cmp =k1*S1-(k2*S2+k0*S0) Where S Cmp is the composite sequence power, S1 is the positive sequence power, S2 is the negative sequence power, S0 is the zero sequence power, and k1, k2, k0 are non-negative and settable weighting factors; and - If it is determined that the criterion is met, it is determined that a fault has been detected on the protected line, wherein the intelligent electronic device is configured to: Cmp ) is determined as the current composite sequence power value (S Cmp (current)) and the previous composite sequence power value (S Cmp The difference between (previous)).

14. The fault detection system according to claim 13, wherein: The intelligent electronic device (111, 112, 113) is connectable to the transmission line at a measuring point, the measuring point being located at the end of the protected line, wherein the intelligent electronic device is configured to: determine a sequence power value at an arrival point located along the transmission line and at a certain distance from the measuring point; and determine an incremental sequence power value at the arrival point with the aid of the sequence power value at the arrival point, wherein each incremental sequence power value is the difference between a current sequence power value and a previous sequence power value, and wherein the criterion is based on the incremental sequence value, wherein determining that a fault has been detected includes: determining that a fault has occurred between the measuring point and the arrival point.

15. The fault detection system according to claim 14, wherein: The incremental power value includes an incremental power value of positive sequence power at the arrival point and an incremental power value of a combination of negative sequence power at the arrival point and zero sequence power at the arrival point.

16. A computer program product comprising instructions which, when downloaded into an intelligent electronic device connectable to a transmission line of an electric power transmission system, cause the intelligent electronic device to implement the method according to any one of claims 1 to 12.

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