DC fault detection device, control method for DC fault detection device, and program

Abstract

To provide a DC accident detection device capable of detecting a DC accident or an accident section with high reliability in a power transmission system, a control method for the DC accident detection device, and a program.SOLUTION: A DC accident detection device comprises an accident occurrence detection unit, and first, second and third accident section determination units. Based on comparing a first index value indicating a power change of a DC power transmission line connected to a power converter with a first threshold, the accident occurrence detection unit detects a DC accident that has occurred in the DC power transmission line. The first accident section determination unit determines a DC power transmission line in which a second index value indicating a power change of the DC power transmission line exceeds a second threshold, as an accident section. The second accident section determination unit determines whether or not a DC power transmission line is an accident section based on the current change of the DC power transmission line. The third accident section determination unit determines which DC power transmission line is an accident section by ANDing a determination result of the first accident section determination unit and a determination result of the second accident section determination unit.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] An embodiment of the present invention relates to a DC fault detection device, a control method for a DC fault detection device, and a program. [Background technology]

[0002] In a multi-terminal high-voltage direct current (HVDC) system, when an accident (DC system accident) occurs in the DC system, a protection system is required that selectively isolates the section where the DC system accident occurred (fault section) in order to minimize the extent to which the HVDC system will be shut down due to the DC system accident. The protection system maintains power transmission through the remaining healthy DC system by isolating only the DC transmission line where the DC system accident occurred using a DC circuit breaker. With the operation of this protection system, the HVDC system can reduce the impact of the DC system accident on the AC system to which it is connected.

[0003] In an HVDC system, from the viewpoint of overcurrent protection of semiconductor elements used in power converters, it is necessary to detect (identify) a fault section in a shorter time than in an AC system and to disconnect a DC transmission line where a DC system fault has occurred (remove the fault section). In general, in an HVDC system, it is considered desirable to complete detection and removal of the fault section within about 5 ms after the occurrence of a DC system fault.

[0004] In AC systems, faults are usually detected using a method that uses differential currents at both ends of the transmission line. This method is not suitable for detecting faults in HVDC systems because it takes time for communication between the terminals of the transmission line. For this reason, a method for detecting faults in HVDC systems faster than in AC systems is required.

[0005] As a technique for detecting a fault section in an HVDC system, for example, a method is known that utilizes the difference in the rate of change of DC voltage caused by reactors (series reactors) inserted at both ends of a DC transmission line so as to be in series with the DC transmission line (see, for example, Non-Patent Document 1). In this method, the fault section is detected by measuring the rate of change of DC voltage on the line side of the series reactor. When a DC system fault occurs in a transmission line (i.e., when an intra-section fault occurs), the DC voltage changes sharply due to a surge voltage caused by the DC system fault (the rate of change of DC voltage is high). On the other hand, when a DC system fault occurs in the line of another DC transmission line (i.e., when an out-of-section fault occurs), the surge voltage that has passed through the series reactor connected to the other DC transmission line (the surge voltage leveled by the series reactor) is measured, so the DC voltage changes relatively slowly (the rate of change of DC voltage is low). In a conventional method that utilizes the difference in the rate of change of DC voltage, the fault section is detected by determining the DC transmission line in which the DC fault occurred from the difference in the rate of change of the DC voltage.

[0006] However, in a conventional technique that detects a fault section only based on the measurement results of the rate of change of DC voltage, such as the method that uses the difference in the rate of change of DC voltage, if noise is superimposed on the measured DC voltage, it is possible that the technique may malfunction, for example by detecting a DC fault when no DC fault actually occurs. In this case, the reliability of the detection of the DC fault or the fault section will decrease. [Prior art documents] [Non-patent literature]

[0007] [Non-Patent Document 1] J. Sneath, ADRajapakse, “Fault Detection and Interruption in an Earthed HVDC Grid Using ROCOV and Hybrid DC Brakers”, IEEE TRANSACTIONS ON POWER DELIVERY, Vol. 31, No. 3, June 2016 Summary of the Invention [Problem to be solved by the invention]

[0008] The problem that the present invention aims to solve is to provide a DC fault detection device, a control method for a DC fault detection device, and a program that can detect DC faults and fault sections with high reliability in a power transmission system. [Means for solving the problem]

[0009] A DC fault detection device according to an embodiment includes a fault occurrence detection unit, a first fault section determination unit, a second fault section determination unit, and a third fault section determination unit. The DC fault detection device detects a fault section of a DC fault in a DC power transmission system including three or more power converters that convert AC to DC and vice versa, and a plurality of DC transmission lines connecting DC side terminals of the power converters. The fault occurrence detection unit detects the DC fault that has occurred in any of the plurality of first DC transmission lines connected to the first power converter based on a comparison between a first index value indicating a power change of the first DC transmission line, which is the DC transmission line connected to the corresponding first power converter, and a first threshold value indicating a power change of the DC transmission line caused by the DC fault. The first fault section determination unit compares a second index value indicating a power change of each of the first DC transmission lines with a second threshold value indicating a power change of the DC transmission line caused by the DC fault after a first predetermined time has elapsed since the occurrence of the DC fault was detected by the fault occurrence detection unit, and determines that the line of the first DC transmission line for which the second index value exceeds the second threshold value is the fault section. The second fault section determination unit determines whether or not each of the lines of the first DC transmission lines is the fault section based on a current change of each of the first DC transmission lines after a second predetermined time has elapsed since the occurrence of the DC fault was detected by the fault occurrence detection unit. The third fault section determination unit determines which of the lines of the first DC transmission lines is the fault section by performing a logical AND between the first determination result by the first fault section determination unit and the second determination result by the second fault section determination unit. [Brief description of the drawings]

[0010] [Figure 1] 1 is a diagram showing an example of the configuration of a multi-terminal power transmission system to which a DC fault detection device according to an embodiment is applied; [Diagram 2] 1 is a diagram showing an example of the configuration of a DC fault detection device according to a first embodiment. [Diagram 3] 10A and 10B are diagrams for explaining an example of detecting the occurrence of a DC fault based on the amount of change in DC voltage. [Figure 4]10 is a diagram for explaining an example of determining a DC transmission line in which a DC fault has occurred based on the absolute value of the amount of change in DC current. FIG. [Diagram 5] 4 is a flowchart showing an example of a process in the DC fault detection device. [Figure 6] FIG. 13 is a diagram showing an example of the configuration of a DC fault detection device according to a second embodiment. [Figure 7] FIG. 13 is a diagram for explaining an example of detecting the occurrence of a DC fault based on the absolute value of the amount of change in DC current. [Figure 8] 4 is a flowchart showing an example of a process in the DC fault detection device. [Figure 9] FIG. 13 is a diagram showing an example of the configuration of a DC fault detection device according to a third embodiment. [Figure 10] 4 is a flowchart showing an example of a process in the DC fault detection device. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A DC fault detection device, a control method for a DC fault detection device, and a program according to an embodiment will be described below with reference to the drawings.

[0012] [Power transmission system configuration] Fig. 1 is a diagram showing an example of the configuration of a multi-terminal power transmission system to which a DC fault detection device according to an embodiment is applied. Fig. 1 shows an example of a three-terminal high-voltage direct current (HVDC) system. The power transmission system 1 shown in Fig. 1 shows an example of a power transmission system to which the DC fault detection device according to an embodiment is applied, and does not show a state in which the DC fault detection device according to an embodiment is applied. A configuration in which the DC fault detection device according to an embodiment is applied to the power transmission system 1 will be described later.

[0013] The power transmission system 1 is a system that transmits power from an AC system on the power transmission side to an AC system on the power receiving side. The power transmission system 1 converts AC power from the AC system on the power transmission side into DC power before transmitting the power, and then converts it back to AC power on the power receiving side and transmits it to the AC system on the power receiving side. The method of converting AC power into DC power and transmitting it can suppress power loss compared to the method of transmitting AC power as it is, so it is a method that is particularly applicable to large-capacity, long-distance power transmission. The power transmission system 1 includes, for example, three AC systems 2 (AC systems 2-1 to 2-3), three power converters 3 (power converters 3-1 to 3-3), and a DC system 4. In the power transmission system 1, the AC system 2 and the DC system 4 are connected via the power converter 3. The power transmission system 1 is an example of a "DC transmission system" in the claims.

[0014] Each of the AC systems 2 is, for example, an AC power system configured by an AC power source, an AC transmission line, etc. Each of the AC systems 2 supplies AC power to the connected power converter 3, or receives AC power from the power converter 3. Each of the AC systems 2 may be, for example, a power generation facility that generates AC power, a facility that transmits AC power transmitted from another AC system on the power transmission side to another AC system on the power receiving side, or a facility that supplies AC power to each of the consumers present at the connected destinations.

[0015] Each of the power converters 3 is a converter that converts AC power and DC power mutually. Each of the power converters 3 is a type of AC-DC converter that converts AC power input to an AC side terminal into DC power and outputs it to a DC side terminal, or converts DC power input to a DC side terminal into AC power and outputs it to an AC side terminal. Each of the power converters 3 is, for example, a self-excited power converter. Each of the power converters 3 converts AC power supplied (input) from the AC system 2 to an AC side terminal into DC power and supplies (outputs) it from the DC side terminal to the DC system 4, or converts DC power supplied (input) from the DC system 4 to a DC side terminal into AC power and supplies (outputs) it from the AC side terminal to the AC system 2.

[0016] The DC system 4 is a DC power system that transmits DC power supplied by the power converter 3 to other power converters 3. The DC system 4 includes, for example, six DC buses 20 (DC buses 20-1 to 20-6), twelve reactors 30 (reactors 30-1 to 30-12), twelve DC circuit breakers 40 (DC circuit breakers 40-1 to 40-12), and six DC transmission lines 50 (DC transmission lines 50-1 to 50-6). In FIG. 1, the configuration of the DC system 4 is shown in which the reactor 30 is connected closer to the DC buses 20, and the DC circuit breaker 40 is connected further ahead, but the order in which the reactors 30 and the DC circuit breakers 40 are connected in the DC system 4 may be reversed.

[0017] Each of the DC busbars 20 is connected to a DC side terminal of the corresponding power converter 3. The DC busbar 20-1 is connected to one DC side terminal of the power converter 3-1, and a positive DC voltage is applied thereto. The DC busbar 20-2 is connected to the other DC side terminal of the power converter 3-1, and a negative DC voltage is applied thereto. The DC busbar 20-3 is connected to one DC side terminal of the power converter 3-2, and a positive DC voltage is applied thereto. The DC busbar 20-4 is connected to the other DC side terminal of the power converter 3-2, and a negative DC voltage is applied thereto. The DC busbar 20-5 is connected to one DC side terminal of the power converter 3-3, and a positive DC voltage is applied thereto. The DC busbar 20-6 is connected to the other DC side terminal of the power converter 3-3, and a negative DC voltage is applied thereto.

[0018] A plurality of DC transmission lines 50 are connected to each of the DC buses 20 via corresponding reactors 30 and DC circuit breakers 40. For example, the DC bus 20-1 is connected to a DC transmission line 50-1 via a reactor 30-1 and a DC circuit breaker 40-1.

[0019] Each of the reactors 30 is connected between the corresponding DC bus 20 and the DC transmission line 50. Each of the reactors 30 is, for example, a series reactor or a blocking coil. The reactor 30 is not limited to a series reactor or a blocking coil, and may be any type of reactor.

[0020] Each of the DC circuit breakers 40 is a circuit breaker for DC power. Each of the DC circuit breakers 40 is connected between the corresponding DC bus bar 20 and the DC transmission line 50. Each of the DC circuit breakers 40 electrically connects or disconnects the corresponding DC bus bar 20 and the DC transmission line 50 in response to control by a DC fault detection device (not shown) of an embodiment described later, for example. Each of the DC circuit breakers 40 transmits DC power via the connected DC transmission line 50 during steady operation (steady power transmission) in the power transmission system 1. When a DC system fault (hereinafter simply referred to as a "DC fault") occurs in the connected DC transmission line 50, each of the DC circuit breakers 40 cuts off a DC fault current (hereinafter simply referred to as a "fault current") flowing through the DC transmission line 50 in which the DC fault has occurred.

[0021] The DC transmission line 50 is a transmission line that transmits DC power. The DC transmission line 50 is, for example, a cable or an overhead transmission line. The DC transmission line 50 is not limited to a cable or an overhead transmission line, and may be any type of line.

[0022] (First embodiment) [Configuration of DC fault detection device] Fig. 2 is a diagram showing an example of the configuration of a DC fault detection device according to the first embodiment. Fig. 2 shows an example of a DC fault detection device 100 applied to a power converter 3-1 among the three power converters 3 included in the power transmission system 1 shown in Fig. 1. The DC fault detection device 100 is installed (connected) near the power converter 3-1 as a protection device for reducing the influence on the power transmission system 1 of a DC fault occurring in a DC transmission line 50 (DC transmission line 50-1, DC transmission line 50-2, DC transmission line 50-3, and DC transmission line 50-4) belonging to the power converter 3-1. The power converter 3-1 is an example of a "first power converter" in the claims. The DC transmission lines 50 (DC transmission lines 50-1, 50-2, 50-3, and 50-4) belonging to the power converter 3-1 are an example of a "first DC transmission line" in the claims.

[0023] The DC fault detection device 100 may be applied to each of the power converter 3-2 and the power converter 3-3. The DC fault detection device 100 applied to each of the power converter 3-2 and the power converter 3-3 has the same configuration as the DC fault detection device 100 applied to the power converter 3-1 shown in Fig. 2. That is, the DC fault detection device 100 applied to the power converter 3-2 is installed (connected) near the power converter 3-2 as a protection device for reducing the influence on the power transmission system 1 of a DC fault occurring in the DC transmission line 50 (DC transmission line 50-1, DC transmission line 50-2, DC transmission line 50-5, and DC transmission line 50-6) belonging to the power converter 3-2. The DC fault detection device 100 applied to the power converter 3-3 is installed (connected) near the power converter 3-3 as a protection device for reducing an influence on the power transmission system 1 of a DC fault occurring in the DC transmission lines 50 (DC transmission lines 50-3, 50-4, 50-5, and 50-6) belonging to the power converter 3-3. In the following description, the DC fault detection device 100 applied to the power converter 3-1 is also referred to as the "DC fault detection device 100-1," the DC fault detection device 100 not shown applied to the power converter 3-2 is also referred to as the "DC fault detection device 100-2," and the DC fault detection device 100 not shown applied to the power converter 3-3 is also referred to as the "DC fault detection device 100-3."

[0024] FIG. 2 also shows the configuration of the power transmission system 1 related to the DC fault detection device 100 (the AC system 2-1, the power converter 3-1, and a part of the DC system 4). In the power transmission system 1, a DC voltage detector and a DC current detector are provided when the DC fault detection device 100 is applied. In the part of the DC system 4 shown in FIG. 2, two DC voltage detectors 60 (DC voltage detectors 60-1 and 60-2) and four DC current detectors 70 (DC current detectors 70-1 to 70-4) are provided. The same applies when the DC fault detection device 100 is applied to each of the power converter 3-2 and the power converter 3-3. In the following description, the part of the DC system 4 in which the DC voltage detector 60 and the DC current detector 70 shown in FIG. 2 are provided is also referred to as "DC system 4-1".

[0025] Each of the DC voltage detectors 60 measures the DC voltage of the corresponding DC bus 20 in the vicinity of the power converter 3-1. Each of the DC voltage detectors 60 is formed of, for example, a voltage transformer (VT). Each of the DC voltage detectors 60 outputs information indicating the voltage value of the measured DC voltage of the DC bus 20 (hereinafter referred to as the "DC voltage value") to the DC fault detection device 100. The DC voltage detector 60-1 measures the DC voltage of the DC bus 20-1 and outputs the measured DC voltage value to the DC fault detection device 100. The DC voltage detector 60-2 measures the DC voltage of the DC bus 20-2 and outputs the measured DC voltage value to the DC fault detection device 100. The DC voltage detector 60-1 may measure the DC voltages of the DC transmission lines 50-1 and 50-3 connected to the DC bus 20-1 as the DC voltage of the DC bus 20-1, and output the measured DC voltage values ​​of each of the DC transmission lines 50 to the DC fault detection device 100. The DC voltage detector 60-2 may measure the DC voltages of the DC transmission lines 50-2 and 50-4 connected to the DC bus 20-2 as the DC voltage of the DC bus 20-2, and output the measured DC voltage values ​​of each of the DC transmission lines 50 to the DC fault detection device 100.

[0026] Each of the DC current detectors 70 measures a DC current flowing through the corresponding DC transmission line 50 near the power converter 3-1. Each of the DC current detectors 70 is composed of, for example, a current transformer (CT). Each of the DC current detectors 70 outputs information indicating a current value of the measured DC current flowing through the corresponding DC transmission line 50 (hereinafter referred to as a "DC current value") to the DC fault detection device 100. The DC current detector 70-1 measures a DC current flowing through the DC transmission line 50-1 connected to the DC bus 20-1, and outputs the measured DC current value to the DC fault detection device 100. The DC current detector 70-2 measures a DC current flowing through the DC transmission line 50-2 connected to the DC bus 20-2, and outputs the measured DC current value to the DC fault detection device 100. The DC current detector 70-3 measures the DC current flowing through the DC transmission line 50-3 connected to the DC bus 20-1, and outputs the measured DC current value to the DC fault detection device 100. The DC current detector 70-4 measures the DC current flowing through the DC transmission line 50-4 connected to the DC bus 20-2, and outputs the measured DC current value to the DC fault detection device 100.

[0027] The DC fault detection device 100 detects (senses) a DC fault that has occurred in any of the DC transmission lines 50 included in the DC system 4-1, based on the DC voltage value output by the DC voltage detector 60 and the DC current value output by the DC current detector 70, and detects (identifies) the line of the DC transmission line 50 in which the DC fault has occurred as a fault section. The DC fault detection device 100 controls the DC circuit breaker 40 belonging to the identified fault section, i.e., the DC circuit breaker 40 to which the DC transmission line 50 in which the DC fault has occurred is connected, to electrically disconnect the DC bus 20 and the DC transmission line 50. The DC fault detection device 100 includes, for example, a DC voltage measurement unit 101, a DC current measurement unit 102, an accident occurrence detection unit 103, a first fault section determination unit 104, a second fault section determination unit 105, a third fault section determination unit 106, and a cut-off command output unit 107.

[0028] The DC fault detection device 100 and the components included in the DC fault detection device 100 realize the following functions by, for example, a hardware processor such as a CPU (Central Processing Unit) executing a program (software). Some or all of the functions of the DC fault detection device 100 and the components included in the DC fault detection device 100 may be realized by hardware (including circuitry) such as an LSI (Large Scale Integration), an ASIC (Application Specific Integrated Circuit), an FPGA (Field-Programmable Gate Array), or a GPU (Graphics Processing Unit), or may be realized by a combination of software and hardware. Some or all of the functions of the DC fault detection device 100 and the components included in the DC fault detection device 100 may be realized by a dedicated LSI. The program may be stored in advance in a storage device (storage device with a non-transient storage medium) such as an HDD (Hard Disk Drive) or flash memory provided in the DC fault detection device 100 or the power transmission system 1, or may be stored in a removable storage medium (non-transient storage medium) such as a DVD or CD-ROM, and installed in a storage device provided in the DC fault detection device 100 by attaching the storage medium to a drive device provided in the DC fault detection device 100 or the power transmission system 1.

[0029] The DC voltage measurement unit 101 acquires the DC voltage values ​​output by the DC voltage detectors 60-1 and 60-2, and measures the amount of change in the voltage value of the DC voltage of the DC bus 20 (hereinafter referred to as the "DC voltage change amount") based on the acquired DC voltage values. More specifically, the DC voltage measurement unit 101 calculates the amount of DC voltage change between the steady power transmission state and the current power transmission state by taking the difference between the DC voltage value during steady operation in the power transmission system 1 and the DC voltage value acquired this time. In other words, the DC voltage measurement unit 101 calculates the difference between the current DC voltage value acquired from the positive pole side DC voltage detector 60-1 and the positive pole side DC voltage value during steady operation as the amount of DC voltage change of the DC transmission line 50 connected to the positive pole side DC bus 20-1. Furthermore, the DC voltage measurement unit 101 calculates the difference between the current DC voltage value acquired from the negative side DC voltage detector 60-2 and the negative side DC voltage value during steady operation as the DC voltage change amount of the DC transmission line 50 connected to the negative side DC bus 20-2.

[0030] Incidentally, in the power transmission in the power transmission system 1, the power transmission on the positive side and the power transmission on the negative side are paired. However, in the power transmission system 1, the polarities are different between the positive side and the negative side, but the voltage value of the DC voltage to be transmitted, that is, the absolute value, is the same. For this reason, the DC voltage measurement unit 101 may take the absolute value of the DC voltage change calculated by taking the difference between the absolute value of the DC voltage value during steady operation and the absolute values ​​of the DC voltage values ​​acquired from each of the DC voltage detectors 60-1 and 60-2 as the DC voltage change of the power converter 3-1 or the DC transmission line 50 belonging to the power converter 3-1. In this case, the DC voltage measurement unit 101 may calculate the DC voltage change from only one of the DC voltage values ​​having a larger absolute value out of the current DC voltage values ​​acquired from each of the DC voltage detectors 60-1 and 60-2. The DC voltage values ​​on the positive and negative sides during steady operation, or the absolute value of the DC voltage values ​​during steady operation, may be, for example, obtained by measuring DC voltage values ​​acquired during steady operation of the power transmission system 1 for a predetermined period of time, or may be set in advance by a control device (not shown) of the power transmission system 1. In the following description, the polarity of power transmission in the power transmission system 1 is not distinguished, that is, the voltage value of the DC voltage is taken to be an absolute value.

[0031] The DC voltage measurement unit 101 outputs the calculated DC voltage change amount to each of the fault occurrence detection unit 103 and the first fault section determination unit 104. The DC voltage change amount is an example of a "first index value" and a "second index value" in the claims.

[0032] The accident occurrence detection unit 103 detects a DC accident that has occurred in any of the DC transmission lines 50 (DC transmission lines 50-1 to 50-6) of the power transmission system 1 based on the DC voltage change amount output by the DC voltage measurement unit 101. At this time, the accident occurrence detection unit 103 compares the DC voltage change amount output by the DC voltage measurement unit 101 with a predetermined first threshold vth1, and detects that no DC accident has occurred in any of the DC transmission lines 50 when the DC voltage change amount does not exceed the first threshold vth1. On the other hand, the accident occurrence detection unit 103 detects that a DC accident has occurred in any of the DC transmission lines 50 when the DC voltage change amount exceeds the first threshold vth1. The accident occurrence detection unit 103 outputs information on the detection of a DC accident, i.e., information indicating whether or not a DC accident has occurred in any of the DC transmission lines 50, to each of the first fault section determination unit 104 and the second fault section determination unit 105.

[0033] When the fault occurrence detection unit 103 detects that a DC fault has occurred in any of the DC transmission lines 50, the first fault section determination unit 104 detects whether or not any of the DC transmission lines 50 (DC transmission lines 50-1, 50-2, 50-3, and 50-4) belonging to the power converter 3-1 is the fault section in which the DC fault has occurred, based on the DC voltage change output by the DC voltage measurement unit 101 after a first predetermined time has elapsed since the DC fault was detected. In other words, the first fault section determination unit 104 determines whether or not any of the DC transmission lines 50 belonging to the power converter 3-1 is included in the fault section in which the DC fault has occurred. At this time, the first fault section determination unit 104 compares the amount of DC voltage change output by the DC voltage measurement unit 101 after the first predetermined time has elapsed with a predetermined second threshold vth2, and determines whether any of the DC transmission lines 50 belonging to the power converter 3-1 is included in the fault section depending on whether the amount of DC voltage change exceeds the second threshold vth2. The second threshold vth2 is a value for detecting an amount of DC voltage change larger than the first threshold vth1 (first threshold vth1<second threshold vth2). The second threshold vth2 is determined by parameters such as the position of the point where the DC fault occurred (fault point), the rated voltage of the DC system 4-1, the inductance value of the reactor 30, the type of the DC transmission line 50, and the length of the DC transmission line 50. Considering the influence of the above parameters on the DC voltage change amount, the second threshold vth2 is set as follows: maximum value of the DC voltage change amount when the DC transmission line 50 is not included in the fault section (outside the fault section)<second threshold vth2<minimum value of the DC voltage change amount when the DC transmission line 50 is included in the fault section (inside the fault section). The first predetermined time from the detection of the occurrence of a DC fault, during which the first fault section determination unit 104 determines whether or not any of the DC transmission lines 50 belonging to the power converter 3-1 is included in the fault section, depends on the sampling time of the DC voltage in the DC voltage measurement unit 101 and the calculation time required for the determination by the first fault section determination unit 104, but is desirably as short as possible.This is because the first predetermined time in the first fault section determination unit 104 affects the time required to protect the power transmission system 1 from the effects of a DC fault that has occurred (the time between the detection of a DC fault and the interruption (disconnection) of the DC transmission line 50).

[0034] The first fault section determination unit 104 sets a fault section inside / outside flag indicating the determination result. The fault section inside / outside flag indicates whether or not any line of the DC transmission line 50 belonging to the power converter 3-1 is included in the fault section. When the DC voltage change amount exceeds the second threshold vth2, the first fault section determination unit 104 determines that any DC transmission line 50 belonging to the power converter 3-1 is included in the fault section, and sets the fault section inside / outside flag to a value indicating that it is included in the fault section (for example, "1"). On the other hand, when the DC voltage change amount does not exceed the second threshold vth2, the first fault section determination unit 104 determines that none of the DC transmission lines 50 belonging to the power converter 3-1 is included in the fault section, and sets the fault section inside / outside flag to a value indicating that it is not included in the fault section (for example, "0"). The first fault section determination unit 104 outputs the fault section inside / outside flag to the third fault section determination unit 106. The fault section inside / outside flag is an example of a "first determination result" in the claims.

[0035] Here, an example will be described in which the fault occurrence detection unit 103 and the first fault section determination unit 104 detect (detect) the occurrence of a DC fault in the power transmission system 1. In the following description, it is assumed that the DC fault occurring in the DC transmission line 50-1 is, for example, a ground fault that is assumed to occur due to a lightning strike. More specifically, as shown in FIG. 2, it is assumed that the DC fault X occurs at the end of the DC transmission line 50-1, that is, the DC fault X occurs in the DC transmission line 50-1 at a position close to the power converter 3-1 in the power transmission system 1. A ground fault is an accident in which the DC current (fault current) flowing in the DC transmission line 50-1 changes suddenly.

[0036] FIG. 3 is a diagram for explaining an example of detecting the occurrence of a DC accident X based on the amount of change in the DC voltage (amount of change in the DC voltage). FIG. 3(a) shows an example of a change in the DC voltage over time from 0.2 [ms] before the accident to 2 [ms] after the accident when the DC accident X occurs at time T0 (actually, a short time before time T0), with the horizontal axis being time [ms] and the vertical axis being DC voltage [pu]. FIG. 3(a) also shows an example of a change in the DC voltage over time in the power converters 3-2 and 3-3 included in the power transmission system 1 for comparison. The solid line a shown in FIG. 3(a) is an example of a change in the DC voltage value (here, the DC voltage of the DC transmission line 50-1) acquired by the DC voltage measuring unit 101 included in the DC accident detection device 100-1 applied to the power converter 3-1. The short dashed line b shown in FIG. 3(a) is an example of a change in the DC voltage of any of the DC transmission lines 50 (here, the DC transmission line 50-1) acquired by the DC voltage measurement unit 101 included in the DC fault detection device 100-2 (not shown) applied to the power converter 3-2. The long dashed line c shown in FIG. 3(a) is an example of a change in the DC voltage of any of the DC transmission lines 50 (here, for example, the DC transmission line 50-3) acquired by the DC voltage measurement unit 101 included in the DC fault detection device 100-3 (not shown) applied to the power converter 3-3. FIG. 3(b) shows an example of a temporal change in the fault section inside / outside flag from 0.2 [ms] before the occurrence of the fault to 2 [ms] after the occurrence of the fault when a DC fault X occurs at time T0 (actually, a short time before time T0), with the horizontal axis representing time [ms] and the vertical axis representing the value of the fault section inside / outside flag. For comparison, (b) of Fig. 3 also shows an example of a change over time in the fault section inside / outside flag in the power converter 3-2 and the power converter 3-3 provided in the power transmission system 1. The solid line a shown in (b) of Fig. 3 is an example of a change in the fault section inside / outside flag that indicates the determination result of the first fault section determination unit 104 provided in the DC fault detection device 100-1 determining whether or not any line of the DC transmission line 50 belonging to the power converter 3-1 is included in the fault section.The short dashed line b shown in (a) of Figure 3 is an example of a change in the fault section inside / outside flag representing the judgment result of the first fault section judgment unit 104 provided in the DC fault detection device 100-2 (not shown), and the long dashed line c is an example of a change in the fault section inside / outside flag representing the judgment result of the first fault section judgment unit 104 provided in the DC fault detection device 100-3 (not shown).

[0037] During steady operation of the power transmission system 1 (time before time T0), as shown in (a) of FIG. 3, the DC voltage of each DC transmission line 50 remains almost constant at the rated value (=1 [pu]). For this reason, the fault occurrence detection unit 103 does not detect a DC fault. Therefore, during steady operation of the power transmission system 1 (time before time T0), the first fault section determination unit 104 determines that none of the DC transmission lines 50 is included in the fault section, and sets the fault section inside / outside flag to a value (e.g., "0") that indicates that the DC transmission lines 50 are not included in the fault section (outside the fault section).

[0038] On the other hand, when a DC fault X occurs in the power transmission system 1, the DC voltage at the ground fault point (here, the end of the DC transmission line 50-1 on the power converter 3-1 side) at the DC fault X drops, and a short-circuit current (fault current) flows from the power converters 3-1 and 3-2 toward the ground fault point. As a result, the DC voltage of the power converter 3-1 changes as shown by the solid line a in FIG. 3(a). Then, the DC voltage measurement unit 101 outputs the amount of DC voltage change (here, the amount of DC voltage change of the DC transmission line 50-1) calculated based on the DC voltage of the power converter 3-1 to each of the fault occurrence detection unit 103 and the first fault section determination unit 104. As a result, the accident occurrence detection unit 103 detects a DC accident X that has occurred in any of the DC transmission lines 50 of the power transmission system 1 at time T0 when the DC voltage change amount of the power converter 3-1 output by the DC voltage measurement unit 101 exceeds the first threshold value vth1, and outputs information indicating the occurrence of the DC accident X to each of the first fault section determination unit 104 and the second fault section determination unit 105. As shown in FIG. 3(a), at time T1 when a predetermined time t2 has elapsed from time T0 when the accident occurrence detection unit 103 detected the DC accident X, the first fault section determination unit 104 compares the DC voltage change amount dv2a output by the DC voltage measurement unit 101 with the second threshold value vth2 to determine whether or not any of the DC transmission lines 50 belonging to the power converter 3-1 is included in the fault section. The time t2 is an example of the "first predetermined time" in the claims. In (a) of Fig. 3, because the DC voltage change amount dv2a exceeds the second threshold value vth2 at time T1, the first fault section determination unit 104 determines that any of the DC transmission lines 50 belonging to the power converter 3-1 is included in the fault section. Then, the first fault section determination unit 104 sets the fault section inside / outside flag to a value (for example, "1") representing that the line is included in the fault section (within the fault section), as shown by the solid line a in (b) of Fig. 3.

[0039] Furthermore, as time passes after the occurrence of the DC fault X, the DC voltage of the power converter 3-2 changes as shown by the short dashed line b in FIG. 3(a). The time at which the DC voltage of the power converter 3-2 changes is delayed from the time at which the DC voltage of the power converter 3-1 changes because the distance between the DC fault X and the power converter 3-2 is greater than the distance between the DC fault X and the power converter 3-1. In this way, when the DC fault X occurs at the end of the DC transmission line 50-1 close to the power converter 3-1 in the power transmission system 1, the change in the DC voltage appears with a delay time corresponding to the distance from the position of the DC fault X. In this case, the DC voltage measurement unit 101 included in the DC fault detection device 100-2 outputs the amount of DC voltage change (here, the amount of DC voltage change of the DC transmission line 50-1) calculated based on the DC voltage of the power converter 3-2 to the corresponding fault occurrence detection unit 103 and the first fault section determination unit 104. As a result, the accident occurrence detection unit 103 included in the DC fault detection device 100-2 detects a DC fault X that has occurred in any of the DC transmission lines 50 of the power transmission system 1 at time T2 when the DC voltage change amount of the power converter 3-2 output by the corresponding DC voltage measurement unit 101 exceeds the first threshold value vth1. Then, the accident occurrence detection unit 103 included in the DC fault detection device 100-2 also outputs information indicating that the DC fault X has occurred to each of the corresponding first fault section determination unit 104 and second fault section determination unit 105. At time T3 when a predetermined time t2 has elapsed since time T2 when the corresponding fault occurrence detection unit 103 detected the DC fault X, the first fault section determination unit 104 included in the DC fault detection device 100-2 compares the DC voltage change output by the corresponding DC voltage measurement unit 101 with the second threshold value vth2 to determine whether or not any of the DC transmission lines 50 belonging to the power converter 3-2 is included in the fault section. In (a) of Fig. 3, since the DC voltage change dv2b exceeds the second threshold value vth2 at time T3, the first fault section determination unit 104 included in the DC fault detection device 100-2 determines that any of the DC transmission lines 50 belonging to the power converter 3-2 is included in the fault section.Then, the first fault section determination unit 104 provided in the DC fault detection device 100-2 sets the fault section inside / outside flag to a value (e.g., "1") indicating that the fault section is included (within the fault section), as shown by the short dashed line b in (b) of Figure 3.

[0040] As time passes after the occurrence of the DC fault X, the DC voltage of the power converter 3-3 (for example, the DC voltage of the DC transmission line 50-3) changes as shown by the long dashed line c in Fig. 3(a). However, as shown by the long dashed line c in Fig. 3(a), the amount of change in the DC voltage of the power converter 3-3 is smaller than the amount of change in the DC voltage of the power converter 3-1 and the power converter 3-2. This is because, when a short-circuit current (fault current) flows toward the point (ground fault point) of the DC fault X that occurred in the DC transmission line 50-1, there are more reactors 30 in the path through which the short-circuit current passes through the DC transmission line 50-3. That is, since the number of reactors 30 existing between the point of the DC accident X and the DC voltage detector 60 in the line of the DC transmission line 50-1 not included in the fault section is greater than the number of reactors 30 existing between the point of the DC accident X and the DC voltage detector 60 in the line of the DC transmission line 50-1 included in the fault section, it is possible to obtain a greater effect of suppressing the change in the fault current due to the reactors 30. In this case as well, the DC voltage measurement unit 101 included in the DC fault detection device 100-3 outputs the amount of DC voltage change calculated based on the DC voltage of the power converter 3-3 (here, for example, the amount of DC voltage change of the DC transmission line 50-3) to the corresponding fault occurrence detection unit 103 and first fault section determination unit 104, respectively. As a result, the accident occurrence detection unit 103 included in the DC fault detection device 100-3 detects a DC fault X that has occurred in any of the DC transmission lines 50 of the power transmission system 1 at time T4 when the DC voltage change amount of the power converter 3-3 outputted by the corresponding DC voltage measurement unit 101 exceeds the first threshold value vth1. Then, the accident occurrence detection unit 103 included in the DC fault detection device 100-3 also outputs information indicating the occurrence of the DC fault X to each of the corresponding first fault section determination unit 104 and second fault section determination unit 105. The first fault section determination unit 104 provided in the DC fault detection device 100-3 compares the DC voltage change amount output by the corresponding DC voltage measurement unit 101 with a second threshold value vth2 at time T5 when a predetermined time t2 has elapsed since time T4 when the corresponding fault occurrence detection unit 103 detected the DC fault X, and determines whether any of the DC transmission lines 50 belonging to the power converter 3-3 is included in the fault section.In (a) of Fig. 3, the DC voltage change amount dv2c does not exceed the second threshold value vth2 at time T5. This is because, for example, although the DC transmission line 50-3 is affected by the DC fault X that occurred in the DC transmission line 50-1, the DC transmission line 50-3 itself does not have the DC fault X, and therefore the DC voltage drop is small. For this reason, the first fault section determination unit 104 provided in the DC fault detection device 100-3 determines that any of the DC transmission lines 50 belonging to the power converter 3-3 is not included in the fault section, and maintains the fault section inside / outside flag at a value (for example, "0") that indicates that the DC transmission line 50 is not included in the fault section (outside the fault section), as indicated by the long dashed line c shown in (b) of Fig. 3.

[0041] In this manner, the fault occurrence detection unit 103 included in the DC fault detection device 100 applied to each of the power converters 3 detects the DC fault X that has occurred in any of the DC transmission lines 50 (DC transmission lines 50-1 to 50-6) of the power transmission system 1 when the DC voltage change amount of the power converter 3 output by the corresponding DC voltage measurement unit 101 exceeds the first threshold value vth1. Then, the first fault section determination unit 104 included in the DC fault detection device 100 applied to each of the power converters 3 determines whether or not any of the DC transmission lines 50 belonging to the corresponding power converter 3 is included in the fault section when a predetermined time t2 has elapsed since the corresponding fault occurrence detection unit 103 detected the DC fault X. The first fault section determination unit 104 included in the DC fault detection device 100 applied to each of the power converters 3 outputs a fault section inside / outside flag indicating the determination result to the third fault section determination unit 106.

[0042] Although the configuration has been described in which the DC voltage change amount is calculated by the DC voltage measurement unit 101 and output to each of the accident occurrence detection unit 103 and the first fault section determination unit 104, the DC voltage change amount may be calculated by the accident occurrence detection unit 103 or the first fault section determination unit 104 instead of the DC voltage measurement unit 101. In this case, the DC voltage measurement unit 101 outputs the DC voltage values ​​acquired from each of the DC voltage detectors 60-1 and 60-2 as they are to the accident occurrence detection unit 103 and the first fault section determination unit 104. The accident occurrence detection unit 103 calculates the DC voltage change amount in the same manner as the above-mentioned DC voltage measurement unit 101, and compares the calculated DC voltage change amount with a predetermined first threshold value vth1 to detect the occurrence of a DC accident in the DC transmission line 50. The first fault section determination unit 104 calculates the DC voltage change amount in the same manner as the above-mentioned DC voltage measurement unit 101, and compares the calculated DC voltage change amount with a predetermined second threshold value vth2 to determine whether or not any of the DC transmission lines 50 belonging to the corresponding power converter 3 is included in the fault section.

[0043] Returning to FIG. 2, the DC current measuring unit 102 acquires the DC current values ​​output by each of the DC current detectors 70 (DC current detectors 70-1 to 70-4) and measures the absolute value of the change in the current value of the DC current flowing through each of the DC transmission lines 50 based on the acquired DC current values ​​(hereinafter referred to as "absolute value of DC current change"). More specifically, the DC current measuring unit 102 calculates the absolute value of the DC current change between the steady power transmission state and the current power transmission state by taking the difference between the absolute value of the DC current value during steady operation in the power transmission system 1 and the absolute value of the currently acquired DC current value. In other words, the DC current measuring unit 102 calculates the difference between the absolute value of the current DC current value acquired from the DC current detector 70-1 and the absolute value of the DC current value during steady operation on the positive pole side as the absolute value of the DC current change of the DC transmission line 50-1. Furthermore, the DC current measuring unit 102 calculates the difference between the absolute value of the current DC current value acquired from the DC current detector 70-2 and the absolute value of the DC current value during steady operation on the negative side as the absolute value of the DC current change of the DC transmission line 50-2. Furthermore, the DC current measuring unit 102 calculates the difference between the absolute value of the current DC current value acquired from the DC current detector 70-3 and the absolute value of the DC current value during steady operation on the positive side as the absolute value of the DC current change of the DC transmission line 50-3. Furthermore, the DC current measuring unit 102 calculates the difference between the absolute value of the current DC current value acquired from the DC current detector 70-4 and the absolute value of the DC current value during steady operation on the negative side as the absolute value of the DC current change of the DC transmission line 50-4.

[0044] The DC current measuring unit 102 outputs each calculated absolute value of the DC current change amount to the second fault section determination unit 105. The absolute value of the DC current change amount is an example of the "current change" in the claims.

[0045] When the fault occurrence detection unit 103 detects that a DC fault has occurred in any of the DC transmission lines 50, the second fault section determination unit 105 detects whether or not any of the DC transmission lines 50 (DC transmission lines 50-1, 50-2, 50-3, and 50-4) belonging to the power converter 3-1 is the fault section in which the DC fault has occurred, based on the absolute value of the DC current change output by the DC current measurement unit 102 after a second predetermined time has elapsed since the DC fault was detected. In other words, the second fault section determination unit 105 determines in which of the DC transmission lines 50 belonging to the power converter 3-1 the DC fault has occurred. At this time, the second fault section determination unit 105 compares the respective absolute values ​​of the DC current change output by the DC current measurement unit 102 after the second predetermined time has elapsed, and determines that the line of the DC transmission line 50 having the largest absolute value of the DC current change is the fault section where the DC fault has occurred. More specifically, the second fault section determination unit 105 focuses on each DC transmission line 50, and sequentially compares the absolute value of the DC current change of the focused DC transmission line 50 with the absolute values ​​of the DC current change of the other DC transmission lines 50, thereby determining (selecting) the line of the DC transmission line 50 having the largest absolute value of the DC current change after the second predetermined time has elapsed as the DC transmission line 50 of the fault section where the DC fault has occurred. The second predetermined time from the detection of the occurrence of a DC fault, during which the second fault section determination unit 105 determines which DC transmission line 50 belonging to the power converter 3-1 is included in the fault section, depends on the sampling time of the DC current in the DC current measurement unit 102 and the calculation time required for the determination by the second fault section determination unit 105, but is desirably as short as possible. This is because, like the first predetermined time (time t2) in the first fault section determination unit 104, the second predetermined time in the second fault section determination unit 105 also affects the time to protect the power transmission system 1 from the influence of the DC fault that has occurred (the time between the detection of the DC fault and the interruption (disconnection) of the DC transmission line 50).

[0046] The second fault section determination unit 105 outputs information representing the determination result to the third fault section determination unit 106. The information representing the determination result output by the second fault section determination unit 105 to the third fault section determination unit 106 may be, for example, flag information representing whether or not each line of the DC transmission line 50 is included in the fault section, similar to the fault section inside / outside flag output by the first fault section determination unit 104 to the second fault section determination unit 105. The information on the determination result output by the second fault section determination unit 105 to the third fault section determination unit 106 is an example of a "second determination result" in the claims.

[0047] Here, an example will be described in which the second fault section determination unit 105 detects (determines) in which DC transmission line 50 belonging to the power converter 3-1 a DC fault has occurred. In the following description, it is assumed that a DC fault X has occurred at the end of the DC transmission line 50-1 as shown in FIG.

[0048] Fig. 4 is a diagram for explaining an example of determining a DC transmission line 50 in which a DC fault X has occurred based on the absolute value of the amount of change in the DC current. Fig. 4 shows an example of a temporal change in the absolute value of the amount of change in the DC current when the DC fault X occurs at time T0 (actually, a short time before time T0), with the horizontal axis being time [ms] and the vertical axis being the absolute value of the amount of change in the DC current. The solid line d shown in Fig. 4 is an example of a change in the absolute value of the amount of change in the DC current of the DC transmission line 50-1 calculated based on the DC current value acquired by the DC current measuring unit 102, the short dashed line e is an example of a change in the absolute value of the amount of change in the DC current of the DC transmission line 50-2, the long dashed line f is an example of a change in the absolute value of the amount of change in the DC current of the DC transmission line 50-3, and the dashed-dotted line g is an example of a change in the absolute value of the amount of change in the DC current of the DC transmission line 50-4.

[0049] During steady-state operation of the power transmission system 1 (time before time T0), the DC current flowing through each DC transmission line 50 is constant, and therefore the absolute value of the DC current change amount of each DC transmission line 50 remains unchanged at "0" as shown in Fig. 4. For this reason, the second fault section determination unit 105 does not determine (select) any of the DC transmission lines 50 as the DC transmission line 50 in the fault section.

[0050] On the other hand, when a DC fault X occurs in the DC transmission line 50-1, a short-circuit current (fault current) flows from the power converters 3-1 and 3-2 toward a ground fault point in the DC fault X. Therefore, the absolute values ​​of the DC current variations in each of the DC transmission lines 50 at time T7 when a second predetermined time t3 has elapsed since time T6 when the fault occurrence detection unit 103 detected the DC fault X rise as shown in Fig. 4. At this time, the absolute values ​​of the DC current variations in each of the DC transmission lines 50 rise most significantly in the DC transmission line 50-1, which is within the fault section, as shown by the solid line a in Fig. 4, and the absolute values ​​of the DC current variations in the DC transmission lines 50-3 (long dashed line f shown in Fig. 4) and the DC transmission line 50-4 (dotted line g shown in Fig. 4), which are outside the fault section, rise little. This is because, as explained in Fig. 3, the lines of the DC transmission lines 50-3 and 50-4 outside the fault section have many reactors 30 compared to the line of the DC transmission line 50-1 inside the fault section, and therefore the reactors 30 have a greater effect of suppressing the change in the fault current (the change in the DC current becomes slower). On the other hand, as shown by the short dashed line e in Fig. 4, the absolute value of the DC current change in the DC transmission line 50-2 paired with the DC transmission line 50-1 where the DC fault X occurred also increases significantly. This is due to the influence of the DC fault X that occurred in the DC transmission line 50-1, but since no DC fault X actually occurred in the DC transmission line 50-2, the absolute value of the DC current change in the DC transmission line 50-2 does not increase significantly more than the absolute value of the DC current change in the DC transmission line 50-1. Even if a DC fault occurs in any of the DC transmission lines 50 belonging to the power converter 3-2, or if a DC fault occurs in any of the DC transmission lines 50 belonging to the power converter 3-3, the relationship of the absolute value of the DC current change amount shown in Fig. 4 remains the same. For this reason, the second fault section determination unit 105 determines (selects) the DC transmission line 50-1 having the maximum absolute value of the DC current change amount, that is, the line of the DC transmission line 50 in the fault section, as the DC transmission line 50 in the fault section where the DC fault X occurred. The time t3 is an example of a "second predetermined time" in the claims.

[0051] In this way, the second fault section determination unit 105 included in the DC fault detection devices 100 applied to each of the power converters 3 determines, when a predetermined time t3 has elapsed since the corresponding fault occurrence detection unit 103 detected the DC fault X (time T6), the DC transmission line 50 having the largest absolute value of the DC current change amount output by the corresponding DC current measurement unit 102 is the DC transmission line 50 in the fault section where the DC fault occurred. The second fault section determination unit 105 included in the DC fault detection devices 100 applied to each of the power converters 3 outputs information indicating the determination result (e.g., flag information) to the third fault section determination unit 106.

[0052] Although the configuration has been described in which the DC current measurement unit 102 calculates the absolute value of the DC current change amount and outputs it to the second fault section determination unit 105, the DC current change amount absolute value may be calculated by the second fault section determination unit 105 instead of the DC current measurement unit 102. In this case, the DC current measurement unit 102 outputs the DC current values ​​acquired from each DC current detector 70 directly to the second fault section determination unit 105. The second fault section determination unit 105 calculates the DC current change amount absolute value in the same manner as the above-mentioned DC current measurement unit 102, and determines the DC transmission line 50 having the largest DC current change amount absolute value among the calculated absolute values ​​of the DC current change amount of each DC transmission line 50 as the DC transmission line 50 in the fault section where the DC fault occurred.

[0053] 2, the third fault section determination unit 106 determines that any one of the DC transmission lines 50 (DC transmission line 50-1, DC transmission line 50-2, DC transmission line 50-3, and DC transmission line 50-4) belonging to the power converter 3-1 is a fault section, based on the determination result by the first fault section determination unit 104 and the determination result by the second fault section determination unit 105. In other words, the third fault section determination unit 106 determines (specifies) the final fault section as the DC fault detection device 100, based on the fault section inside / outside flag output by the first fault section determination unit 104 and the information indicating the determination result output by the second fault section determination unit 105. More specifically, the third fault section determination unit 106 performs a logical AND between the fault section inside / outside flag output by the first fault section determination unit 104 and the DC transmission line 50 in which it is determined that a DC fault has occurred output by the second fault section determination unit 105, and identifies whether or not a DC fault has occurred in the DC transmission line 50 belonging to the power converter 3-1. The third fault section determination unit 106 outputs information indicating the identified DC transmission line 50 as information of the determination result to the shutdown command output unit 107. The information of the determination result output by the third fault section determination unit 106 to the shutdown command output unit 107 may be, for example, instruction information indicating to shut off the DC circuit breaker 40 connected to the DC transmission line 50 in which it is determined that a DC fault has occurred (to disconnect the DC transmission line 50).

[0054] The tripping command output unit 107 generates a tripping command value for controlling the tripping (disconnection) of the DC transmission line 50 belonging to the power converter 3-1 in which the DC fault has occurred, based on the information of the determination result output by the third fault section determination unit 106, and outputs the generated tripping command value to the corresponding DC circuit breaker 40. As a result, the DC circuit breaker 40 to which the tripping command value has been input electrically disconnects the connected DC bus 20 and DC transmission line 50 in accordance with the input tripping command value, and removes the DC transmission line 50 in which the DC fault has occurred from the DC system 4-1.

[0055] [Processing of DC fault detection device] 5 is a flowchart showing an example of processing in the DC fault detection device 100. The processing of this flowchart is repeatedly executed while the power transmission system 1 is in operation.

[0056] When the power transmission system 1 is in operation and the DC fault detection device 100 is operating, the DC voltage measurement unit 101 acquires the DC voltage values ​​measured and output by each DC voltage detector 60, and calculates the DC voltage change amount based on the acquired DC voltage values. Then, the DC voltage measurement unit 101 outputs the calculated DC voltage change amount to each of the fault occurrence detection unit 103 and the first fault section determination unit 104. When the power transmission system 1 is in operation and the DC fault detection device 100 is operating, the DC current measurement unit 102 acquires the DC current values ​​measured and output by each DC current detector 70, and calculates the DC current change amount absolute value based on the acquired DC current values. Then, the DC current measurement unit 102 outputs the calculated DC current change amount absolute value to the second fault section determination unit 105.

[0057] The accident occurrence detection unit 103 compares the DC voltage change amount output by the DC voltage measurement unit 101 with a first threshold vth1, and determines whether or not the DC voltage change amount exceeds the first threshold vth1 (step S100). That is, the accident occurrence detection unit 103 detects whether or not a DC accident has occurred in any of the DC transmission lines 50 of the power transmission system 1. When it is determined in step S100 that the DC voltage change amount does not exceed the first threshold vth1, the accident occurrence detection unit 103 outputs information indicating that no DC accident has occurred in any of the DC transmission lines 50 to each of the first fault section determination unit 104 and the second fault section determination unit 105, and ends the current process.

[0058] On the other hand, if it is determined in step S100 that the DC voltage change amount exceeds the first threshold value vth1, the accident occurrence detection unit 103 detects that a DC accident has occurred in one of the DC transmission lines 50, and outputs information indicating that a DC accident has occurred in one of the DC transmission lines 50 to each of the first fault section determination unit 104 and the second fault section determination unit 105.

[0059] When information indicating that a DC fault has occurred in any of the DC transmission lines 50 is input from the fault occurrence detection unit 103, the first fault section determination unit 104 compares the DC voltage change output by the DC voltage measurement unit 101 with a second threshold value vth2 after a first predetermined time (for example, time t2 shown in FIG. 3(a)) has elapsed, and determines whether the DC voltage change exceeds the second threshold value vth2 (step S110). That is, the first fault section determination unit 104 determines whether any of the DC transmission lines 50 belonging to the power converter 3-1 is included in the fault section.

[0060] If it is determined in step S110 that the DC voltage change amount does not exceed the second threshold vth2, the first fault section determination unit 104 determines that none of the DC transmission lines 50 belonging to the power converter 3-1 are included in the fault section, that is, that the DC fault has occurred outside the section (detection section) of the line of the DC transmission line 50 detected by the DC fault detection device 100 (step S150). Then, the first fault section determination unit 104 maintains the fault section inside / outside flag at a value (for example, "0") representing that the fault section is not included (outside the fault section), and ends the current process.

[0061] On the other hand, if it is determined in step S110 that the DC voltage change amount exceeds the second threshold value vth2, the first fault section determination unit 104 determines that any of the DC transmission lines 50 belonging to the power converter 3-1 is included in the fault section, that is, that a DC fault has occurred within the detection section, and changes the fault section inside / outside flag to a value (e.g., "1") indicating that it is included in the fault section (within the fault section), and outputs it to the third fault section determination unit 106.

[0062] When the second fault section determination unit 105 receives information indicating that a DC fault has occurred in any of the DC transmission lines 50 from the fault occurrence detection unit 103, the second fault section determination unit 105 determines whether or not the absolute value of the DC current change amount of the focused DC transmission line 50 is larger than the absolute values ​​of the DC current change amount of the other DC transmission lines 50 after a second predetermined time (for example, time t3 shown in FIG. 4) has elapsed (step S120). The process of step S120 in the second fault section determination unit 105 is performed for all DC transmission lines 50 belonging to the power converter 3-1. As a result, the second fault section determination unit 105 determines (selects) the line of the DC transmission line 50 having the largest absolute value of the DC current change amount output by the DC current measurement unit 102 as the DC transmission line 50 in the fault section where the DC fault has occurred.

[0063] If it is determined in step S120 that the absolute value of the DC current change amount of any of the focused DC transmission lines 50 is not larger than the absolute value of the DC current change amount of the other DC transmission lines 50, the second fault section determination unit 105 determines that none of the DC transmission lines 50 belonging to the power converter 3-1 is included in the fault section, that is, that the DC fault has occurred outside the detection section (step S150). Then, the second fault section determination unit 105 ends this processing.

[0064] On the other hand, if it is determined in step S120 that the absolute value of the DC current change in the DC transmission line 50 of interest is larger than the absolute values ​​of the DC current change in the other DC transmission lines 50, the second fault section determination unit 105 determines that a DC fault has occurred within the detection section, selects the DC transmission line 50 belonging to the power converter 3-1 and having the largest absolute value of the DC current change, and outputs it to the third fault section determination unit 106 as information representing the determination result.

[0065] The third fault section determination unit 106 determines (specifies) any line of the DC transmission line 50 belonging to the power converter 3-1 as a fault section by logical ANDing the determination result (fault section inside / outside flag) output by the first fault section determination unit 104 and the determination result (DC transmission line 50 with the maximum DC current change absolute value) output by the second fault section determination unit 105 (step S130). That is, when the first fault section determination unit 104 determines that the DC voltage change amount exceeds the second threshold vth2 and changes the fault section inside / outside flag, the third fault section determination unit 106 finally specifies the DC transmission line 50 determined by the second fault section determination unit 105 to have the maximum DC current change absolute value as the DC transmission line 50 in the fault section. The third fault section determination unit 106 outputs information representing the finally specified DC transmission line 50 to the shutdown command output unit 107.

[0066] The trip command output unit 107 generates a trip command value for controlling the tripping of the DC transmission line 50 in the fault section identified by the third fault section determination unit 106, and outputs the generated trip command value to the corresponding DC circuit breaker 40 (step S140). Then, the trip command output unit 107 ends the current process.

[0067] With such a configuration and processing, the DC fault detection device 100 finally identifies the DC transmission lines 50 included in the fault section among the DC transmission lines 50 belonging to the applied power converter 3, based on the judgment by the fault occurrence detection unit 103 and the first fault section judgment unit 104 based on the DC voltage change amount and the judgment by the second fault section judgment unit 105 based on the DC current change amount absolute value. The flowchart shown in FIG. 5 shows a case where the judgment by the first fault section judgment unit 104 (step S110) is followed by the judgment by the second fault section judgment unit 105 (step S120), but the processing of step S110 and the processing of step S120 may be performed in parallel, that is, at the same time.

[0068] As described above, the DC fault detection device 100 of the first embodiment finally identifies the DC transmission line 50 included in the fault section among the DC transmission lines 50 belonging to the applied power converter 3 based on the determination result based on the DC voltage change amount by the fault occurrence detection unit 103 and the first fault section determination unit 104 and the determination result based on the DC current change amount absolute value by the second fault section determination unit 105. In other words, in the multi-terminal (here, three-terminal) power transmission system 1, the DC fault detection device 100 of the first embodiment alone can detect and determine a DC fault that has occurred in any of the DC transmission lines 50 (here, the DC transmission line 50-1, the DC transmission line 50-2, the DC transmission line 50-3, and the DC transmission line 50-4) belonging to one applied terminal (here, the DC side terminal of the power converter 3-1) and finally identify the DC transmission line 50 included in the fault section. Therefore, in the DC fault detection device 100 of the first embodiment, the time required for information communication between terminals of the transmission line, which was required in a conventional protection system applied to an AC system, is reduced, and the DC transmission line 50 where a DC fault has occurred can be detected as the DC transmission line 50 included in the fault section in a short time. Moreover, in the DC fault detection device 100 of the first embodiment, the DC transmission line 50 included in the fault section is finally identified by the logical product of two determination results (determination methods), namely, a determination based on a DC voltage value (amount of DC voltage change) and a determination based on a DC current value (absolute value of amount of DC current change). Therefore, in the DC fault detection device 100 of the first embodiment, the probability of malfunction is reduced and a DC fault or fault section can be detected with high reliability, compared with a case where the DC transmission line 50 included in the fault section is identified by one determination method using a differential current at both ends of the transmission line, as in a conventional protection system applied to an AC system.

[0069] The DC fault detection device 100 of the first embodiment described above has a configuration including the tripping command output unit 107, and controls the DC circuit breaker 40 to which the DC transmission line 50 finally identified as being included in the fault section is connected, thereby isolating (removing) the DC transmission line 50 in which a DC fault has occurred from the DC system 4-1. However, the control of the isolation (removal) of the DC transmission line 50 in which a DC fault has occurred may be performed, for example, by a control device other than the DC fault detection device 100 of the first embodiment, based on information of the DC transmission line 50 finally identified by the DC fault detection device 100 of the first embodiment as being included in the fault section. In other words, another control device instead of the DC fault detection device 100 of the first embodiment may be provided with the tripping command output unit 107. In this case, the third fault section determination unit 106 provided in the DC fault detection device 100 of the first embodiment outputs information on the determination result indicating the DC transmission line 50 in which it has been determined that a DC fault has occurred (which may be, for example, instruction information indicating to shut off the DC circuit breaker 40 connected to the DC transmission line 50 in which it has been determined that a DC fault has occurred), to a shutoff command output unit 107 provided in another control device.

[0070] In the above-described DC fault detection device 100 of the first embodiment, the calculation in the DC fault detection device 100 has been described as being performed based on the DC voltage value acquired from the DC voltage detector 60 or the DC current value acquired from the DC current detector 70. In other words, the calculation of the DC voltage change amount in the DC voltage measurement unit 101 and the calculation of the absolute value of the DC current change amount by the DC current measurement unit 102 have been described as being performed using the acquired values ​​themselves. However, it is considered that the DC voltage value output by the DC voltage detector 60 and the DC current value output by the DC current detector 70 contain noise components. For this reason, the DC fault detection device 100 of the first embodiment (more specifically, the DC voltage measurement unit 101 and the DC current measurement unit 102) may be configured to perform a process for removing noise, such as moving average or filter adaptation, before or after performing the calculation process.

[0071] Second Embodiment [Configuration of DC fault detection device] The second embodiment will be described below. FIG. 6 is a diagram showing an example of the configuration of a DC fault detection device according to the second embodiment. FIG. 6 shows an example of a DC fault detection device 200 applied to the power converter 3-1 among the three power converters 3 included in the power transmission system 1 shown in FIG. 1, similarly to the DC fault detection device 100 of the first embodiment. In FIG. 6, components having a common function to the DC fault detection device 100 are given the same reference numerals, and detailed description is omitted. Similarly to the DC fault detection device 100, the DC fault detection device 200 is installed (connected) near the power converter 3-1 as a protection device for reducing the influence on the power transmission system 1 of a DC fault occurring in the DC transmission line 50 (DC transmission line 50-1, DC transmission line 50-2, DC transmission line 50-3, and DC transmission line 50-4) belonging to the power converter 3-1.

[0072] The DC fault detection device 200 may be applied to each of the power converter 3-2 and the power converter 3-3, similarly to the DC fault detection device 100. The DC fault detection device 200 applied to each of the power converter 3-2 and the power converter 3-3 has the same configuration as the DC fault detection device 200 applied to the power converter 3-1 shown in Fig. 6. That is, the DC fault detection device 200 applied to the power converter 3-2 is also installed (connected) near the power converter 3-2 as a protection device for reducing the influence on the power transmission system 1 of a DC fault occurring in the DC transmission line 50 (DC transmission line 50-1, DC transmission line 50-2, DC transmission line 50-5, and DC transmission line 50-6) belonging to the power converter 3-2. The DC fault detection device 200 applied to the power converter 3-3 is also installed (connected) near the power converter 3-3 as a protection device for reducing an influence on the power transmission system 1 of a DC fault occurring in the DC transmission lines 50 (DC transmission lines 50-3, 50-4, 50-5, and 50-6) belonging to the power converter 3-3. In the following description, the DC fault detection device 200 applied to the power converter 3-1 is also referred to as the "DC fault detection device 200-1," the DC fault detection device 200 (not shown) applied to the power converter 3-2 is also referred to as the "DC fault detection device 200-2," and the DC fault detection device 200 (not shown) applied to the power converter 3-3 is also referred to as the "DC fault detection device 200-3."

[0073] The DC fault detection device 200 is configured to detect and determine a DC fault that has occurred in any of the DC transmission lines 50 belonging to the power converter 3-1 based on the absolute value of the DC current change amount, and to finally identify the DC transmission line 50 included in the fault section. FIG. 6 also shows the configuration of the power transmission system 1 related to the DC fault detection device 200 (the AC system 2-1, the power converter 3-1, and a part of the DC system 4). In the power transmission system 1, a DC current detector is provided when the DC fault detection device 200 is applied. In the part of the DC system 4 shown in FIG. 6, four DC current detectors 70 (DC current detectors 70-1 to 70-4) are provided. In other words, in the power transmission system 1 to which the DC fault detection device 200 is applied, the two DC voltage detectors 60 (DC voltage detectors 60-1 and 60-2) provided in the DC system 4-1 when the DC fault detection device 100 is applied are omitted. The same applies to the case where DC fault detection device 200 is applied to each of power converter 3-2 and power converter 3-3. In the following description, a part of DC system 4 having DC current detector 70 shown in Fig. 6 is also referred to as "DC system 4-2".

[0074] The DC fault detection device 200 detects (detects) a DC fault that has occurred in any of the DC transmission lines 50 included in the DC system 4-2 based on the DC current value output by the DC current detector 70, and detects (specifies) the line of the DC transmission line 50 in which the DC fault has occurred as a fault section. The DC fault detection device 200 controls the DC circuit breaker 40 belonging to the specified fault section, that is, the DC circuit breaker 40 to which the DC transmission line 50 in which the DC fault has occurred is connected, to electrically cut off between the DC bus 20 and the DC transmission line 50. The DC fault detection device 200 includes, for example, a DC current measurement unit 102, an fault occurrence detection unit 203, a first fault section determination unit 204, a second fault section determination unit 105, a third fault section determination unit 106, and a cutoff command output unit 107.

[0075] The DC fault detection device 200 and the components included in the DC fault detection device 200 realize the following functions by, for example, a hardware processor such as a CPU executing a program (software). Some or all of the functions of the DC fault detection device 200 and the components included in the DC fault detection device 200 may be realized by hardware (including circuitry) such as an LSI, ASIC, FPGA, or GPU, or may be realized by a combination of software and hardware. Some or all of the functions of the DC fault detection device 200 and the components included in the DC fault detection device 200 may be realized by a dedicated LSI. The program may be stored in advance in a storage device (storage device with a non-transient storage medium) such as an HDD or flash memory provided in the DC fault detection device 200 or the power transmission system 1, or may be stored in a removable storage medium (non-transient storage medium) such as a DVD or CD-ROM, and installed in a storage device provided in the DC fault detection device 200 by attaching the storage medium to a drive device provided in the DC fault detection device 200 or the power transmission system 1.

[0076] The DC current measuring unit 102 outputs each DC current change absolute value calculated based on the DC current value output by each of the DC current detectors 70 (DC current detectors 70-1 to 70-4) to each of the fault occurrence detection unit 203, the first fault section determination unit 204, and the second fault section determination unit 105. The DC current change absolute value is another example of the "first index value," "second index value," and "current change" in the claims.

[0077] The accident occurrence detection unit 203 detects a DC accident that has occurred in any of the DC transmission lines 50 (DC transmission lines 50-1 to 50-6) of the power transmission system 1 based on the DC current change absolute value output by the DC current measurement unit 102. At this time, the accident occurrence detection unit 203 compares the DC current change absolute value output by the DC current measurement unit 102 with a predetermined first threshold value ith1, and detects that no DC accident has occurred in any of the DC transmission lines 50 when the DC current change absolute value does not exceed the first threshold value ith1. On the other hand, the accident occurrence detection unit 203 detects that a DC accident has occurred in any of the DC transmission lines 50 when the DC current change absolute value exceeds the first threshold value ith1. The accident occurrence detection unit 203 outputs information on the detection of a DC accident, i.e., information indicating whether or not a DC accident has occurred in any of the DC transmission lines 50, to each of the first fault section determination unit 204 and the second fault section determination unit 105. The accident occurrence detection unit 203 is another example of an "accident occurrence detection unit" in the claims. The first threshold value ith1 is another example of a "first threshold value" in the claims.

[0078] When the fault occurrence detection unit 203 detects that a DC fault has occurred in any of the DC transmission lines 50, the first fault section determination unit 204 detects whether or not any of the DC transmission lines 50 (DC transmission lines 50-1, 50-2, 50-3, and 50-4) belonging to the power converter 3-1 is the fault section in which the DC fault has occurred, based on the absolute value of the DC current change output by the DC current measurement unit 102 after a first predetermined time has elapsed since the DC fault was detected. In other words, the first fault section determination unit 204 determines whether or not any of the DC transmission lines 50 belonging to the power converter 3-1 is included in the fault section in which the DC fault has occurred. At this time, the first fault section determination unit 204 compares the absolute value of the DC current change amount output by the DC current measurement unit 102 after the first predetermined time has elapsed with a predetermined second threshold value ith2, and determines whether any of the DC transmission lines 50 belonging to the power converter 3-1 is included in the fault section depending on whether the absolute value of the DC current change amount exceeds the second threshold value ith2. The second threshold value ith2 is a value for detecting an absolute value of the DC current change amount larger than the first threshold value ith1 (first threshold value ith1<second threshold value ith2). The second threshold value ith2 is determined by parameters such as the position of the point where the DC fault occurred (fault point), the rated voltage of the DC system 4-2, the inductance value of the reactor 30, the type of the DC transmission line 50, and the length of the DC transmission line 50, for example. Considering the influence of the above parameters on the absolute value of the DC current change amount, the second threshold value ith2 is set as follows: maximum value of the absolute value of the DC current change amount when the DC transmission line 50 is not included in the fault section (outside the fault section)<second threshold value ith2<minimum value of the absolute value of the DC current change amount when the DC transmission line 50 is included in the fault section (inside the fault section). The first predetermined time from the detection of the occurrence of a DC fault, during which the first fault section determination unit 204 determines whether or not any of the DC transmission lines 50 belonging to the power converter 3-1 is included in the fault section, depends on the sampling time of the DC current in the DC current measurement unit 102 and the calculation time required for the determination by the first fault section determination unit 204, but is desirably as short as possible.This is because the first predetermined time in the first fault section determination unit 204 affects the time required to protect the power transmission system 1 from the effects of a DC fault that has occurred (the time between the detection of a DC fault and the interruption (disconnection) of the DC transmission line 50).

[0079] The first fault section determination unit 204 sets a fault section inside / outside flag indicating the determination result. The fault section inside / outside flag indicates whether or not any line of the DC transmission line 50 belonging to the power converter 3-1 is included in the fault section, similarly to the fault section inside / outside flag set by the first fault section determination unit 104 included in the DC fault detection device 100. When the DC current change amount absolute value exceeds the second threshold value ith2, the first fault section determination unit 204 determines that any DC transmission line 50 belonging to the power converter 3-1 is included in the fault section, and sets the fault section inside / outside flag to a value indicating that it is included in the fault section (for example, "1"). On the other hand, when the DC current change amount absolute value does not exceed the second threshold value ith2, the first fault section determination unit 204 determines that none of the DC transmission lines 50 belonging to the power converter 3-1 is included in the fault section, and sets the fault section inside / outside flag to a value indicating that it is not included in the fault section (for example, "0"). The first fault section determining unit 204 outputs the fault section inside / outside flag to the third fault section determining unit 106 .

[0080] The first fault section determination unit 204 is another example of a "first fault section determination unit" in the claims. The second threshold ith2 is another example of a "second threshold" in the claims. The fault section inside / outside flag output by the first fault section determination unit 204 to the third fault section determination unit 106 is another example of a "first determination result" in the claims.

[0081] Here, an example will be described in which the fault occurrence detection unit 203 and the first fault section determination unit 204 detect (detect) the occurrence of a DC fault in the power transmission system 1. In the following description, it is assumed that the DC fault occurring in the DC transmission line 50-1 is a ground fault. More specifically, it is assumed that the DC fault X occurs at the end of the DC transmission line 50-1 as shown in Fig. 2, that is, the DC fault X occurs in the DC transmission line 50-1 at a position close to the power converter 3-1 in the power transmission system 1.

[0082] FIG. 7 is a diagram for explaining an example of detecting the occurrence of a DC accident X based on the absolute value of the change in the DC current (absolute value of the DC current change). FIG. 7(a) shows an example of a change in the absolute value of the DC current change over time from 0.2 [ms] before the accident to 2 [ms] after the accident when the DC accident X occurs at time T10 (actually, a short time before time T10), with the horizontal axis being time [ms] and the vertical axis being the absolute value of the DC current change. FIG. 7(a) also shows an example of a change in the absolute value of the DC current change over time in the power converters 3-2 and 3-3 included in the power transmission system 1 for comparison. Each absolute value of the DC current change shown in FIG. 7(a) is the absolute value of the DC current change in the DC transmission line 50 with the largest change among the DC current values ​​acquired by the DC current measuring unit 102. The solid line j shown in Fig. 7(a) is an example of a change in the absolute value of the DC current change amount of the DC transmission line 50 (here, the DC transmission line 50-1) with the largest amount of change, calculated and output by the DC current measurement unit 102 included in the DC fault detection device 200-1 applied to the power converter 3-1. The short-dashed line k shown in Fig. 7(a) is an example of a change in the absolute value of the DC current change amount of the DC transmission line 50 (here, the DC transmission line 50-1) with the largest amount of change, calculated and output by the DC current measurement unit 102 included in the DC fault detection device 200-2 (not shown) applied to the power converter 3-2. The long dashed line l shown in (a) of FIG. 7 is an example of a change in the absolute value of the DC current change amount of the DC transmission line 50 (here, for example, the DC transmission line 50-3) with the largest change amount, which is calculated and output by the DC current measurement unit 102 included in the DC fault detection device 200-3 (not shown) applied to the power converter 3-3. (b) of FIG. 7 shows an example of a temporal change in the fault section inside / outside flag from 0.2 [ms] before the occurrence of the accident to 2 [ms] after the occurrence of the accident when the DC fault X occurs at time T10 (actually, a short time before time T10), with the horizontal axis being time [ms] and the vertical axis being the value of the fault section inside / outside flag. (b) of FIG. 7 also shows an example of a temporal change in the fault section inside / outside flag in the power converters 3-2 and 3-3 included in the power transmission system 1 for comparison.The solid line j shown in (b) of Fig. 7 is an example of a change in the fault section inside / outside flag indicating the determination result of the first fault section determination unit 204 provided in the DC fault detection device 200-1 determining whether or not any line of the DC transmission line 50 belonging to the power converter 3-1 is included in the fault section. The short-dashed line k shown in (a) of Fig. 7 is an example of a change in the fault section inside / outside flag indicating the determination result of the first fault section determination unit 204 provided in the DC fault detection device 200-2 (not shown), and the long-dashed line l is an example of a change in the fault section inside / outside flag indicating the determination result of the first fault section determination unit 204 provided in the DC fault detection device 200-3 (not shown).

[0083] During steady operation of the power transmission system 1 (time before time T10), the DC current of each DC transmission line 50 is stable, and therefore, as shown in FIG. 7(a), the absolute value of the DC current change amount of each DC transmission line 50 remains unchanged at "0". For this reason, the fault occurrence detection unit 203 does not detect a DC fault. Therefore, during steady operation of the power transmission system 1 (time before time T10), the first fault section determination unit 204 determines that none of the DC transmission lines 50 is included in the fault section, and sets the fault section inside / outside flag to a value (e.g., "0") that indicates that the DC transmission lines 50 are not included in the fault section (outside the fault section).

[0084] On the other hand, when a DC fault X occurs in the power transmission system 1, the DC voltage at the ground fault point (here, the end of the DC transmission line 50-1 on the power converter 3-1 side) at the DC fault X drops, and a short-circuit current (fault current) flows from the power converters 3-1 and 3-2 toward the ground fault point. Therefore, the DC current measurement unit 102 outputs the absolute value of the DC current change amount of the power converter 3-1 (here, the absolute value of the DC current change amount of the DC transmission line 50-1) that changes as shown by the solid line j in FIG. 7(a), calculated based on the acquired DC current value, to each of the fault occurrence detection unit 203, the first fault section determination unit 204, and the second fault section determination unit 105. As a result, the fault occurrence detection unit 203 detects a DC fault X that has occurred in any of the DC transmission lines 50 of the power transmission system 1 at time T10 when the DC current change absolute value of the power converter 3-1 output by the DC current measurement unit 102 exceeds the first threshold value ith1, and outputs information indicating the occurrence of the DC fault X to each of the first fault section determination unit 204 and the second fault section determination unit 105. As shown in Fig. 7(a) , at time T11 when a predetermined time t2a has elapsed from time T10 when the fault occurrence detection unit 203 detected the DC fault X, the first fault section determination unit 204 compares the DC current change di2a represented by the DC current change absolute value output by the DC current measurement unit 102 with the second threshold value ith2 to determine whether or not any of the DC transmission lines 50 belonging to the power converter 3-1 is included in the fault section. Time t2a is another example of a "first predetermined time" in the claims. In (a) of Fig. 7, since the DC current change amount di2a exceeds the second threshold value ith2 at time T11, the first fault section determination unit 204 determines that any of the DC transmission lines 50 belonging to the power converter 3-1 is included in the fault section. Then, the first fault section determination unit 204 sets the fault section inside / outside flag to a value (e.g., "1") representing that the line is included in the fault section (within the fault section), as indicated by the solid line j shown in (b) of Fig. 7.

[0085] Furthermore, when time has passed since the occurrence of the DC fault X, the DC current measurement unit 102 included in the DC fault detection device 200-2 outputs the absolute value of the DC current change amount of the power converter 3-2 (here, the absolute value of the DC current change amount of the DC transmission line 50-1) which changes as indicated by the short-dashed line k shown in (a) of Fig. 7, calculated based on the acquired DC current value, to the corresponding fault occurrence detection unit 203, the first fault section determination unit 204, and the second fault section determination unit 105. The reason that the time at which the DC current measurement unit 102 outputs the absolute value of the DC current change amount of the power converter 3-2 is later than the time at which it outputs the absolute value of the DC current change amount of the power converter 3-1 is that the change in the DC current also appears shifted by a delay time according to the distance from the position of the DC fault X, similar to the DC voltage. As a result, the accident occurrence detection unit 203 included in the DC fault detection device 200-2 detects the DC fault X that has occurred in any of the DC transmission lines 50 of the power transmission system 1 at time T12 when the DC current change absolute value of the power converter 3-2 output by the corresponding DC current measurement unit 102 exceeds the first threshold value ith1, and outputs information indicating the occurrence of the DC fault X to each of the corresponding first fault section determination unit 204 and the second fault section determination unit 105. The first fault section determination unit 204 included in the DC fault detection device 200-2 compares the DC current change di2b represented by the DC current change absolute value output by the corresponding DC current measurement unit 102 with the second threshold value ith2 at time T13 when a predetermined time t2a has elapsed from time T12 when the corresponding accident occurrence detection unit 203 detected the DC fault X, and determines whether any of the DC transmission lines 50 belonging to the power converter 3-2 is included in the fault section. 7(a), because the DC current change amount di2b exceeds the second threshold value ith2 at time T13, the first fault section determination unit 204 included in the DC fault detection device 200-2 determines that any of the DC transmission lines 50 belonging to the power converter 3-2 is included in the fault section. Then, the first fault section determination unit 204 included in the DC fault detection device 200-2 sets the fault section inside / outside flag to a value (for example, "1") representing that the line is included in the fault section (within the fault section), as indicated by the short-dashed line k shown in FIG. 7(b).

[0086] Further, when time has passed since the occurrence of the DC fault X, the DC current measurement unit 102 included in the DC fault detection device 200-3 outputs the absolute value of the DC current change amount of the power converter 3-3 (here, for example, the absolute value of the DC current change amount of the DC transmission line 50-3) calculated based on the acquired DC current value and changing as shown by the long dashed line l in Fig. 7(a) to the corresponding fault occurrence detection unit 203, the first fault section determination unit 204, and the second fault section determination unit 105. The change amount of the DC current change amount absolute value output by the DC current measurement unit 102 included in the DC fault detection device 200-3 is smaller than the change amount of the DC current change amount absolute value output by the DC current measurement unit 102 included in the power converter 3-1 or the power converter 3-2 as shown by the long dashed line l in Fig. 7(a). This is because there are more reactors 30 in the path passing through the DC transmission line 50-3 when a short-circuit current (fault current) flows toward the point (ground fault point) of the DC fault X that occurred in the DC transmission line 50-1, and therefore it is possible to obtain a greater effect of suppressing the change in the fault current by the reactor 30. As a result, the fault occurrence detection unit 203 provided in the DC fault detection device 200-3 detects the DC fault X that occurred in any of the DC transmission lines 50 of the power transmission system 1 at time T14 when the absolute value of the DC current change amount of the power converter 3-3 outputted by the corresponding DC current measurement unit 102 exceeds the first threshold value ith1, and outputs information indicating the occurrence of the DC fault X to each of the corresponding first fault section determination unit 204 and second fault section determination unit 105. The first fault section determination unit 204 included in the DC fault detection device 200-3 compares the DC current change amount di2c represented by the DC current change amount absolute value output by the corresponding DC current measurement unit 102 with the second threshold value ith2 at time T15 when a predetermined time t2a has elapsed since time T14 when the corresponding fault occurrence detection unit 203 detected the DC fault X, and determines whether any of the DC transmission lines 50 belonging to the power converter 3-3 is included in the fault section. In (a) of Fig. 7, the DC current change amount di2c does not exceed the second threshold value ith2 at time T15.This is because, for example, although the change in the DC current of the DC transmission line 50-3 is affected by the DC fault X that occurred in the DC transmission line 50-1, like the DC voltage, the amount of change in the DC current is small because the DC fault X has not occurred in the DC transmission line 50-3 itself. Therefore, the first fault section determination unit 204 provided in the DC fault detection device 200-3 determines that any of the DC transmission lines 50 belonging to the power converter 3-3 is not included in the fault section, and maintains the fault section inside / outside flag at a value (for example, "0") that indicates that the DC transmission line 50 is not included in the fault section (outside the fault section), as shown by the long dashed line l in (b) of Fig. 7.

[0087] In this manner, the fault occurrence detection unit 203 included in the DC fault detection device 200 applied to each of the power converters 3 detects the DC fault X that has occurred in any of the DC transmission lines 50 (DC transmission lines 50-1 to 50-6) of the power transmission system 1 when the DC current change absolute value output by the corresponding DC current measurement unit 102 exceeds the first threshold value ith1. Then, the first fault section determination unit 204 included in the DC fault detection device 200 applied to each of the power converters 3 determines whether or not any of the DC transmission lines 50 belonging to the corresponding power converter 3 is included in the fault section when a predetermined time t2a has elapsed since the corresponding fault occurrence detection unit 203 detected the DC fault X. The first fault section determination unit 204 included in the DC fault detection device 200 applied to each of the power converters 3 outputs a fault section inside / outside flag indicating the determination result to the third fault section determination unit 106.

[0088] Although the configuration has been described in which the DC current measurement unit 102 calculates the DC current change absolute value and outputs it to the accident occurrence detection unit 203, the first fault section determination unit 204, and the second fault section determination unit 105, respectively, the DC current change absolute value may be calculated by the accident occurrence detection unit 203, the first fault section determination unit 204, and the second fault section determination unit 105, respectively, instead of the DC current measurement unit 102. In this case, the DC current measurement unit 102 outputs the DC current values ​​acquired from each of the DC current detectors 70 (DC current detectors 70-1 to 70-4) as they are to the accident occurrence detection unit 203, the first fault section determination unit 204, and the second fault section determination unit 105. The fault occurrence detection unit 203 calculates an absolute value of a DC current change amount in the same manner as the above-mentioned DC current measurement unit 102, and compares the calculated absolute value of a DC current change amount with a predetermined first threshold value ith1 to detect the occurrence of a DC fault in the DC transmission line 50. The first fault section determination unit 204 calculates an absolute value of a DC current change amount in the same manner as the above-mentioned DC current measurement unit 102, and compares the calculated absolute value of a DC current change amount with a predetermined second threshold value ith2 to determine whether or not any of the DC transmission lines 50 belonging to the corresponding power converter 3 is included in the fault section.

[0089] 6 , when the fault occurrence detection unit 203 detects that a DC fault has occurred in any of the DC transmission lines 50, the second fault section determination unit 105 detects whether or not any of the lines of the DC transmission lines 50 belonging to the power converter 3-1 is the fault section in which the DC fault has occurred, based on the absolute value of the DC current change amount output by the DC current measurement unit 102 after a second predetermined time has elapsed since the detection of the DC fault. That is, in the DC fault detection device 200, the second fault section determination unit 105 is triggered to determine which of the DC transmission lines 50 belonging to the power converter 3-1 has a DC fault when the fault occurrence detection unit 203 detects a DC fault that has occurred in any of the DC transmission lines 50, instead of when the fault occurrence detection unit 103 detects a DC fault that has occurred in any of the DC transmission lines 50 in the DC fault detection device 100. The second fault section determining unit 105 outputs information indicating the determination result to the third fault section determining unit 106.

[0090] The operation of the second fault section determination unit 105 to detect (determine) in which DC transmission line 50 belonging to the power converter 3-1 a DC fault has occurred is similar to that of the second fault section determination unit 105 provided in the DC fault detection apparatus 100 (see FIG. 4). Therefore, the operation of the third fault section determination unit 106 and the shutoff command output unit 107 provided in the DC fault detection apparatus 200 is also similar to that of the third fault section determination unit 106 and the shutoff command output unit 107 provided in the DC fault detection apparatus 100.

[0091] [Processing of DC fault detection device] Fig. 8 is a flowchart showing an example of processing in the DC fault detection device 200. The processing in this flowchart is also repeatedly executed while the power transmission system 1 is in operation. In the flowchart shown in Fig. 8, the same processing as that in the DC fault detection device 100 of the first embodiment is given the same step number, and detailed description thereof will be omitted.

[0092] When the power transmission system 1 is in operation and the DC fault detection device 200 is in operation, the DC current measurement unit 102 acquires the DC current values ​​measured and output by each of the DC current detectors 70, and calculates an absolute value of the DC current change amount based on the acquired DC current values. Then, the DC current measurement unit 102 outputs the calculated absolute value of the DC current change amount to each of the fault occurrence detection unit 203, the first fault section determination unit 204, and the second fault section determination unit 105.

[0093] The accident occurrence detection unit 203 compares the absolute value of the DC current change amount output by the DC current measurement unit 102 with the first threshold value ith1, and determines whether or not the absolute value of the DC current change amount exceeds the first threshold value ith1 (step S200). That is, the accident occurrence detection unit 203 detects whether or not a DC accident has occurred in any of the DC transmission lines 50 of the power transmission system 1. When it is determined in step S200 that the absolute value of the DC current change amount does not exceed the first threshold value ith1, the accident occurrence detection unit 203 outputs information indicating that no DC accident has occurred in any of the DC transmission lines 50 to each of the first fault section determination unit 204 and the second fault section determination unit 105, and ends the current process.

[0094] On the other hand, if it is determined in step S200 that the absolute value of the DC current change amount exceeds the first threshold value ith1, the accident occurrence detection unit 203 detects that a DC accident has occurred in one of the DC transmission lines 50, and outputs information indicating that a DC accident has occurred in one of the DC transmission lines 50 to each of the first fault section determination unit 204 and the second fault section determination unit 105.

[0095] When information indicating that a DC fault has occurred in any of the DC transmission lines 50 is input from the fault occurrence detection unit 203, the first fault section determination unit 204 compares the absolute value of the DC current change output by the DC current measurement unit 102 with the second threshold value ith2 after a first predetermined time (for example, time t2a shown in FIG. 7(a)) has elapsed, and determines whether or not the absolute value of the DC current change exceeds the second threshold value ith2 (step S210). That is, the first fault section determination unit 204 determines whether or not any of the DC transmission lines 50 belonging to the power converter 3-1 is included in the fault section.

[0096] If it is determined in step S210 that the DC current change amount absolute value does not exceed the second threshold value ith2, the first fault section determination unit 204, like the first fault section determination unit 104 provided in the DC fault detection device 100, determines that none of the DC transmission lines 50 belonging to the power converter 3-1 are included in the fault section, that is, that the DC fault has occurred outside the section (detection section) of the line of the DC transmission line 50 detected by the DC fault detection device 200 (step S150). Then, the first fault section determination unit 204 maintains the fault section inside / outside flag at a value (for example, "0") indicating that the fault section is not included (outside the fault section), and ends the current process.

[0097] On the other hand, if it is determined in step S210 that the absolute value of the DC current change amount exceeds the second threshold value ith2, the first fault section determination unit 204 determines that any of the DC transmission lines 50 belonging to the power converter 3-1 is included in the fault section, that is, that a DC accident has occurred within the detection section, and changes the fault section inside / outside flag to a value (e.g., “1”) indicating that it is included in the fault section (within the fault section), and outputs it to the third fault section determination unit 106.

[0098] When information indicating that a DC fault has occurred in any of the DC transmission lines 50 is input from the fault occurrence detection unit 203, the second fault section determination unit 105 determines, similarly to the DC fault detection device 100, whether or not the absolute value of the DC current change in the focused DC transmission line 50 is larger than the absolute values ​​of the DC current change in the other DC transmission lines 50 after a second predetermined time (e.g., time t3 shown in FIG. 4) has elapsed (step S120).

[0099] If it is determined in step S120 that the absolute value of the DC current change of any of the focused DC transmission lines 50 is not greater than the absolute value of the DC current change of the other DC transmission lines 50, the second fault section determination unit 105, like the DC fault detection device 100, determines that none of the DC transmission lines 50 belonging to the power converter 3-1 are included in the fault section, that is, the DC fault has occurred outside the detection section (step S150), and terminates the current processing.

[0100] On the other hand, if it is determined in step S120 that the absolute value of the DC current change in the focused DC transmission line 50 is larger than the absolute values ​​of the DC current change in the other DC transmission lines 50, the second fault section determination unit 105, like the DC fault detection device 100, determines that a DC fault has occurred within the detection section, selects the DC transmission line 50 belonging to the power converter 3-1 and having the largest absolute value of the DC current change, and outputs it to the third fault section determination unit 106 as information representing the determination result.

[0101] The third fault section determination unit 106 determines (specifies) any line of the DC transmission line 50 belonging to the power converter 3-1 as a fault section by logical ANDing the determination result (fault section inside / outside flag) output by the first fault section determination unit 204 and the determination result (DC transmission line 50 having the largest absolute value of the DC current change amount) output by the second fault section determination unit 105 (step S130). That is, when the first fault section determination unit 204 determines that the absolute value of the DC current change amount exceeds the second threshold value ith2 and the fault section inside / outside flag is changed, the third fault section determination unit 106 finally specifies, as in the DC fault detection device 100, the DC transmission line 50 determined by the second fault section determination unit 105 to have the largest absolute value of the DC current change amount as the DC transmission line 50 in the fault section. The third fault section determination unit 106 outputs information indicating the finally identified DC transmission line 50 to the shutdown command output unit 107.

[0102] The trip command output unit 107, like the DC fault detection device 100, generates a trip command value for controlling the tripping of the DC transmission line 50 in the fault section identified by the third fault section determination unit 106, and outputs the generated trip command value to the corresponding DC circuit breaker 40 (step S140). Then, the trip command output unit 107 ends the current process.

[0103] With such a configuration and processing, the DC fault detection device 200 finally identifies the DC transmission lines 50 included in the fault section among the DC transmission lines 50 belonging to the applied power converter 3, based on the determination by the fault occurrence detection unit 203 and the first fault section determination unit 204 based on the absolute value of the DC current change amount and the determination by the second fault section determination unit 105 based on the absolute value of the DC current change amount. The flowchart shown in FIG. 8 illustrates a case where the determination by the first fault section determination unit 204 (step S210) is followed by the determination by the second fault section determination unit 105 (step S120), but the processing of step S210 and the processing of step S120 may be performed in parallel, that is, at the same time.

[0104] As described above, the DC fault detection device 200 of the second embodiment finally identifies the DC transmission line 50 included in the fault section among the DC transmission lines 50 belonging to the applied power converter 3 based on the determination result based on the DC current change amount absolute value by the fault occurrence detection unit 203 and the first fault section determination unit 204 and the determination result based on the DC current change amount absolute value by the second fault section determination unit 105. That is, in the DC fault detection device 200 of the second embodiment, similarly to the DC fault detection device 100 of the first embodiment, in the multi-terminal power transmission system 1, the DC fault detection device 200 of the second embodiment alone can detect and determine a DC fault that has occurred in any of the DC transmission lines 50 belonging to one applied terminal and finally identify the DC transmission line 50 included in the fault section. Therefore, in the DC fault detection device 200 of the second embodiment, as in the DC fault detection device 100 of the first embodiment, the time required for information communication required in a protection system applied to a conventional AC system is reduced, and the DC transmission line 50 in which a DC fault has occurred can be detected as the DC transmission line 50 included in the fault section in a short time. Moreover, the DC fault detection device 200 of the second embodiment can detect and determine a DC fault occurring in any of the DC transmission lines 50 without using the DC voltage value (DC voltage change amount) of the DC transmission line 50 used in the DC fault detection device 100 of the first embodiment for detecting and determining a DC fault occurring in any of the DC transmission lines 50. Therefore, in the power transmission system 1 to which the DC fault detection device 200 of the second embodiment is applied, it is not necessary to provide a DC voltage detector 60 in the DC system 4 as in the power transmission system 1 to which the DC fault detection device 100 of the first embodiment is applied. The DC fault detection device 200 of the second embodiment can also provide the same effects as those of the DC fault detection device 100 of the first embodiment.

[0105] Like the DC fault detection device 100 of the first embodiment, the DC fault detection device 200 of the second embodiment described above may also be configured so that the control of disconnection (removal) of the DC transmission line 50 in which a DC fault has occurred is performed by a control device other than (separate from) the DC fault detection device 200 of the second embodiment.

[0106] In the DC fault detection device 200 of the second embodiment described above, similarly to the DC fault detection device 100 of the first embodiment, the calculation in the DC fault detection device 200, i.e., the calculation of the absolute value of the DC current change amount by the DC current measuring unit 102, is not performed using the DC current value itself acquired from the DC current detector 70, but rather, before or after the calculation processing, processing for the purpose of noise removal, such as moving average or filter adaptation, may be performed.

[0107] (Third embodiment) [Configuration of DC fault detection device] The third embodiment will be described below. Fig. 9 is a diagram showing an example of the configuration of a DC fault detection device according to the third embodiment. Fig. 9 shows an example of a DC fault detection device 300 applied to the power converter 3-1 among the three power converters 3 included in the power transmission system 1 shown in Fig. 1, similar to the DC fault detection device 100 of the first embodiment and the DC fault detection device 200 of the second embodiment. In Fig. 9, components having functions common to the DC fault detection device 100 and the DC fault detection device 200 are given the same reference numerals, and detailed description thereof will be omitted. Like the DC fault detection device 100 and the DC fault detection device 200, the DC fault detection device 300 is installed (connected) in the vicinity of the power converter 3-1 as a protection device for reducing the impact on the power transmission system 1 of a DC fault that occurs in the DC transmission lines 50 (DC transmission line 50-1, DC transmission line 50-2, DC transmission line 50-3, and DC transmission line 50-4) belonging to the power converter 3-1.

[0108] DC fault detection apparatus 300 may be applied to each of power converter 3-2 and power converter 3-3, similar to DC fault detection apparatus 100 and DC fault detection apparatus 200. DC fault detection apparatus 300 applied to each of power converter 3-2 and power converter 3-3 has the same configuration as DC fault detection apparatus 300 applied to power converter 3-1 shown in FIG.

[0109] The DC fault detection device 300, like the DC fault detection device 100, is configured to detect and determine a DC fault that has occurred in any of the DC transmission lines 50 belonging to the power converter 3-1, and to finally identify the DC transmission line 50 included in the fault section, based on the DC voltage change amount and the absolute value of the DC current change amount. FIG. 9 also shows the configuration of the power transmission system 1 related to the DC fault detection device 300 (the configuration of the AC system 2-1, the power converter 3-1, and a part of the DC system 4). In the power transmission system 1, a DC voltage detector and a DC current detector are provided when the DC fault detection device 300 is applied. The configuration of the DC system 4 provided with the DC voltage detector and the DC current detector is the same as the configuration of the DC system 4 to which the DC fault detection device 100 is applied. For this reason, in Figure 9, a DC system 4-1 is shown as a portion of the configuration of the DC system 4 to which the DC fault detection device 300 is applied, which is the same as the portion of the configuration of the DC system 4 to which the DC fault detection device 100 is applied (see Figure 2), and detailed explanations are omitted.

[0110] The DC fault detection device 300 detects (detects) a DC fault that has occurred in any of the DC transmission lines 50 included in the DC system 4-1, based on the DC voltage value output by the DC voltage detector 60 and the DC current value output by the DC current detector 70, and detects (specifies) the line of the DC transmission line 50 in which the DC fault has occurred as the fault section. The DC fault detection device 300 controls the DC circuit breaker 40 belonging to the specified fault section, i.e., the DC circuit breaker 40 to which the DC transmission line 50 in which the DC fault has occurred is connected, to electrically cut off the connection between the DC bus 20 and the DC transmission line 50. The DC fault detection device 300 includes, for example, a DC voltage measurement unit 101, a DC current measurement unit 102, an accident occurrence detection unit 103, an accident occurrence detection unit 203, a first fault section determination unit 104, a second fault section determination unit 105, a third fault section determination unit 106, and a shutoff command output unit 107. The DC fault detection device 300 has a configuration in which the accident occurrence detection unit 203 included in the DC fault detection device 200 of the second embodiment is added to the DC fault detection device 100 of the first embodiment.

[0111] The DC fault detection device 300 and the components included in the DC fault detection device 300 realize the following functions by, for example, a hardware processor such as a CPU executing a program (software). Some or all of the functions of the DC fault detection device 300 and the components included in the DC fault detection device 300 may be realized by hardware (including circuitry) such as an LSI, ASIC, FPGA, or GPU, or may be realized by a combination of software and hardware. Some or all of the functions of the DC fault detection device 300 and the components included in the DC fault detection device 300 may be realized by a dedicated LSI. The program may be stored in advance in a storage device (storage device with a non-transient storage medium) such as an HDD or flash memory provided in the DC fault detection device 300 or the power transmission system 1, or may be stored in a removable storage medium (non-transient storage medium) such as a DVD or CD-ROM, and installed in the storage device provided in the DC fault detection device 300 by attaching the storage medium to a drive device provided in the DC fault detection device 300 or the power transmission system 1.

[0112] The DC voltage measurement unit 101 outputs the respective DC voltage change amounts calculated based on the DC voltage values ​​output by the DC voltage detector 60-1 and the DC voltage detector 60-2 to the accident occurrence detection unit 103 and the first fault section determination unit 104, respectively.

[0113] The accident occurrence detection unit 103 compares the amount of DC voltage change output by the DC voltage measurement unit 101 with a predetermined first threshold value vth1, and outputs information indicating that a DC accident has occurred in any of the DC transmission lines 50 (DC transmission lines 50-1 to 50-6) of the power transmission system 1 (information indicating whether a DC accident has occurred in any of the DC transmission lines 50) to each of the first fault section determination unit 104 and the second fault section determination unit 105. The accident occurrence detection unit 103 is an example of a "first accident occurrence detection unit" in the claims.

[0114] When the fault occurrence detection unit 103 detects that a DC fault has occurred in any of the DC transmission lines 50, the first fault section determination unit 104 compares the DC voltage change output by the DC voltage measurement unit 101 after a first predetermined time has elapsed since the detection of the DC fault with a predetermined second threshold value vth2 to determine whether or not any of the DC transmission lines 50 belonging to the power converter 3-1 is included in the fault section in which the DC fault has occurred. The first fault section determination unit 104 outputs a fault section inside / outside flag indicating the determination result to the third fault section determination unit 106.

[0115] The operation of the accident occurrence detection unit 103 to detect (detect) that a DC accident has occurred in the power transmission system 1, and the operation of the first fault section determination unit 104 to detect (determine) whether the DC transmission line 50 belonging to the power converter 3-1 is included in the fault section in which the DC accident has occurred are similar to those of the DC fault detection device 100 of the first embodiment (see Figure 3).

[0116] The DC current measurement unit 102 outputs each absolute value of DC current change calculated based on the DC current value output by each of the DC current detectors 70 (DC current detectors 70-1 to 70-4) to the accident occurrence detection unit 203 and the second fault section determination unit 105.

[0117] The accident occurrence detection unit 203 compares the absolute value of the DC current change amount output by the DC current measurement unit 102 with a predetermined first threshold value ith1, and detects a DC accident that has occurred in any of the DC transmission lines 50 (DC transmission lines 50-1 to 50-6) of the power transmission system 1 (information indicating whether a DC accident has occurred in any of the DC transmission lines 50) and outputs the result to the second fault section determination unit 105. The accident occurrence detection unit 203 is an example of a "second accident occurrence detection unit" in the claims. The first threshold value ith1 is an example of a "third threshold value" in the claims.

[0118] When the fault occurrence detection unit 103 detects that a DC fault has occurred in any of the DC transmission lines 50, or when the fault occurrence detection unit 203 detects that a DC fault has occurred in any of the DC transmission lines 50, the second fault section determination unit 105 determines which of the DC transmission lines 50 belonging to the power converter 3-1 has a DC fault, based on the absolute value of the DC current change amount output by the DC current measurement unit 102 after a second predetermined time has elapsed since the detection of the DC fault. That is, in the DC fault detection device 300, the second fault section determination unit 105 determines which of the DC transmission lines 50 belonging to the power converter 3-1 has a DC fault, when the fault occurrence detection unit 103 detects a DC fault that has occurred in any of the DC transmission lines 50, or when the fault occurrence detection unit 203 detects a DC fault that has occurred in any of the DC transmission lines 50. The second fault section determining unit 105 outputs information indicating the determination result to the third fault section determining unit 106.

[0119] The operation of the second fault section determination unit 105 to detect (determine) in which DC transmission line 50 belonging to the power converter 3-1 a DC fault has occurred is similar to that of the second fault section determination unit 105 provided in the DC fault detection apparatus 100 and the DC fault detection apparatus 200 (see FIG. 4). Therefore, the operation of the third fault section determination unit 106 and the shutoff command output unit 107 provided in the DC fault detection apparatus 300 is also similar to that of the third fault section determination unit 106 and the shutoff command output unit 107 provided in the DC fault detection apparatus 100 and the DC fault detection apparatus 200.

[0120] [Processing of DC fault detection device] Fig. 10 is a flowchart showing an example of processing in the DC fault detection device 300. The processing in this flowchart is also repeatedly executed while the power transmission system 1 is in operation. In the flowchart shown in Fig. 10, the same processing as that in the DC fault detection device 100 of the first embodiment or the DC fault detection device 200 of the second embodiment is given the same step number, and detailed description thereof will be omitted.

[0121] When the power transmission system 1 is in operation and the DC fault detection device 300 is operating, the DC voltage measurement unit 101 acquires the DC voltage values ​​measured and output by each DC voltage detector 60, and calculates the DC voltage change amount based on the acquired DC voltage values. Then, the DC voltage measurement unit 101 outputs the calculated DC voltage change amount to each of the accident occurrence detection unit 103 and the first fault section determination unit 104. When the power transmission system 1 is in operation and the DC fault detection device 300 is operating, the DC current measurement unit 102 acquires the DC current values ​​measured and output by each DC current detector 70, and calculates the DC current change amount absolute value based on the acquired DC current value. Then, the DC current measurement unit 102 outputs the calculated DC current change amount absolute value to each of the accident occurrence detection unit 203 and the second fault section determination unit 105.

[0122] The accident occurrence detection unit 103 compares the amount of DC voltage change output by the DC voltage measurement unit 101 with a first threshold value vth1, and determines whether or not the amount of DC voltage change exceeds the first threshold value vth1 (step S100). That is, the accident occurrence detection unit 103 detects whether or not a DC accident has occurred in any of the DC transmission lines 50 of the power transmission system 1. When it is determined in step S100 that the amount of DC voltage change does not exceed the first threshold value vth1, the accident occurrence detection unit 103 outputs information indicating that no DC accident has occurred in any of the DC transmission lines 50 to each of the first fault section determination unit 104 and the second fault section determination unit 105, and the process proceeds to step S200.

[0123] On the other hand, if it is determined in step S100 that the DC voltage change amount exceeds the first threshold value vth1, the accident occurrence detection unit 103 detects that a DC accident has occurred in one of the DC transmission lines 50, and outputs information indicating that a DC accident has occurred in one of the DC transmission lines 50 to each of the first fault section determination unit 104 and the second fault section determination unit 105.

[0124] When information indicating that a DC fault has occurred in any of the DC transmission lines 50 is input from the fault occurrence detection unit 103, the first fault section determination unit 104 compares the DC voltage change output by the DC voltage measurement unit 101 with a second threshold value vth2 after a first predetermined time (for example, time t2 shown in FIG. 3(a)) has elapsed, and determines whether the DC voltage change exceeds the second threshold value vth2 (step S110). That is, the first fault section determination unit 104 determines whether any of the DC transmission lines 50 belonging to the power converter 3-1 is included in the fault section.

[0125] If it is determined in step S110 that the DC voltage change amount does not exceed the second threshold value vth2, the first fault section determination unit 104, like the DC fault detection device 100, determines that none of the DC transmission lines 50 belonging to the power converter 3-1 are included in the fault section, that is, that the DC fault has occurred outside the section (detection section) of the line of the DC transmission line 50 detected by the DC fault detection device 300 (step S150). Then, the first fault section determination unit 104 maintains the fault section inside / outside flag at a value (for example, "0") representing that the fault section is not included (outside the fault section), and ends the current process.

[0126] On the other hand, if it is determined in step S110 that the DC voltage change amount exceeds the second threshold value vth2, the first fault section determination unit 104 determines that any of the DC transmission lines 50 belonging to the power converter 3-1 is included in the fault section, that is, that a DC fault has occurred within the detection section, and changes the fault section inside / outside flag to a value (e.g., "1") indicating that it is included in the fault section (within the fault section), and outputs it to the third fault section determination unit 106.

[0127] In step S100, when it is determined that the DC voltage change amount does not exceed the first threshold vth1, the accident occurrence detection unit 203 compares the DC current change amount absolute value output by the DC current measurement unit 102 with the first threshold ith1, and determines whether or not the DC current change amount absolute value exceeds the first threshold ith1 (step S200). That is, the accident occurrence detection unit 203 detects whether or not a DC accident has occurred in any of the DC transmission lines 50 of the power transmission system 1. In step S200, when it is determined that the DC current change amount absolute value does not exceed the first threshold ith1, the accident occurrence detection unit 203 outputs information indicating that a DC accident has not occurred in any of the DC transmission lines 50 to the second fault section determination unit 105, and ends the current process.

[0128] On the other hand, if it is determined in step S200 that the absolute value of the DC current change amount exceeds the first threshold value ith1, the accident occurrence detection unit 203 detects that a DC accident has occurred in one of the DC transmission lines 50, and outputs information indicating that a DC accident has occurred in one of the DC transmission lines 50 to the second fault section determination unit 105.

[0129] When the second fault section determination unit 105 receives information indicating that a DC fault has occurred in any of the DC transmission lines 50 from the fault occurrence detection unit 103 or receives information indicating that a DC fault has occurred in any of the DC transmission lines 50 from the fault occurrence detection unit 203, the second fault section determination unit 105 determines whether or not the absolute value of the DC current change amount of the focused DC transmission line 50 is larger than the absolute values ​​of the DC current change amount of the other DC transmission lines 50 after a second predetermined time (for example, time t3 shown in FIG. 4) has elapsed (step S120). The process of step S120 in the second fault section determination unit 105 is similar to that in the DC fault detection device 100 or the DC fault detection device 200.

[0130] If it is determined in step S120 that the absolute value of the DC current change of any of the focused DC transmission lines 50 is not greater than the absolute value of the DC current change of the other DC transmission lines 50, the second fault section determination unit 105, like the DC fault detection device 100 and the DC fault detection device 200, determines that none of the DC transmission lines 50 belonging to the power converter 3-1 are included in the fault section, that is, the DC fault has occurred outside the detection section (step S150), and terminates the current processing.

[0131] On the other hand, if it is determined in step S120 that the absolute value of the DC current change in the DC transmission line 50 of interest is greater than the absolute values ​​of the DC current change in the other DC transmission lines 50, the second fault section determination unit 105, like the DC fault detection device 100 and the DC fault detection device 200, determines that a DC fault has occurred within the detection section, selects the DC transmission line 50 belonging to the power converter 3-1 and having the largest absolute value of the DC current change, and outputs it to the third fault section determination unit 106 as information representing the determination result.

[0132] The third fault section determination unit 106 determines (specifies) any line of the DC transmission line 50 belonging to the power converter 3-1 as a fault section by logical ANDing the determination result (fault section inside / outside flag) output by the first fault section determination unit 104 and the determination result (DC transmission line 50 having the maximum DC current change absolute value) output by the second fault section determination unit 105 (step S130). That is, when the first fault section determination unit 104 determines that the DC voltage change amount exceeds the second threshold value vth2 and changes the fault section inside / outside flag, the third fault section determination unit 106 finally specifies the DC transmission line 50 determined by the second fault section determination unit 105 to have the maximum DC current change absolute value as the DC transmission line 50 in the fault section, similarly to the DC fault detection device 100 and the DC fault detection device 200. The third fault section determination unit 106 outputs information indicating the finally identified DC transmission line 50 to the shutdown command output unit 107.

[0133] The trip command output unit 107, like the DC fault detection device 100 and the DC fault detection device 200, generates a trip command value for controlling the tripping of the DC transmission line 50 in the fault section identified by the third fault section determination unit 106, and outputs the generated trip command value to the corresponding DC circuit breaker 40 (step S140). Then, the trip command output unit 107 ends this process.

[0134] With such a configuration and processing, the DC fault detection device 300 finally identifies the DC transmission lines 50 included in the fault section among the DC transmission lines 50 belonging to the applied power converter 3, based on the determination based on the DC voltage change amount by the fault occurrence detection unit 103 and the first fault section determination unit 104, and the determination based on the DC current change amount absolute value by the fault occurrence detection unit 203 and the second fault section determination unit 105. The flowchart shown in Fig. 10 illustrates a case where the determination by the fault occurrence detection unit 103 (step S100) is followed by the determination by the fault occurrence detection unit 203 (step S200), but the processing of step S100 and the processing of step S200 may be performed in parallel, that is, at the same time.

[0135] As described above, the DC fault detection device 300 of the third embodiment finally identifies the DC transmission line 50 included in the fault section among the DC transmission lines 50 belonging to the applied power converter 3 based on the determination result based on the DC voltage change amount by the fault occurrence detection unit 103 and the first fault section determination unit 104 and the determination result based on the DC current change amount absolute value by the fault occurrence detection unit 203 and the second fault section determination unit 105. That is, like the DC fault detection device 100 of the first embodiment and the DC fault detection device 200 of the second embodiment, in the multi-terminal power transmission system 1, the DC fault detection device 300 of the third embodiment alone can detect and determine a DC fault that has occurred in any of the DC transmission lines 50 belonging to one applied terminal and finally identify the DC transmission line 50 included in the fault section. Therefore, in the DC fault detection device 300 of the third embodiment, similarly to the DC fault detection device 100 of the first embodiment and the DC fault detection device 200 of the second embodiment, the time required for information communication required in a protection system applied to a conventional AC system is reduced, and the DC transmission line 50 where a DC fault has occurred can be detected as the DC transmission line 50 included in the fault section in a short time. Moreover, in the DC fault detection device 300 of the third embodiment, similarly to the DC fault detection device 100 of the first embodiment, detection and determination of a DC fault occurring in any of the DC transmission lines 50 can be performed by logical AND of two determination results (determination methods), namely, a determination based on a DC voltage value (amount of change in DC voltage) and a determination based on a DC current value (absolute value of amount of change in DC current). Therefore, in the power transmission system 1 to which the DC fault detection device 300 of the third embodiment is applied, the same effect as that of the power transmission system 1 to which the DC fault detection device 100 of the first embodiment is applied can be obtained.

[0136] Like the DC fault detection device 100 of the first embodiment and the DC fault detection device 200 of the second embodiment, the DC fault detection device 300 of the third embodiment described above may be configured so that the control of disconnection (removal) of the DC transmission line 50 in which a DC fault has occurred is performed by a control device other than (separate from) the DC fault detection device 300 of the third embodiment.

[0137] In the DC fault detection device 300 of the third embodiment described above, similarly to the DC fault detection device 100 of the first embodiment and the DC fault detection device 200 of the second embodiment, the calculations in the DC fault detection device 300 are not performed using the DC voltage value acquired from the DC voltage detector 60 or the DC current value acquired from the DC current detector 70 itself, but rather the calculations may be performed before or after the calculations using processing aimed at noise removal, such as moving average or filter adaptation.

[0138] In the above-described DC fault detection device 300 of the third embodiment, the configuration in which the accident occurrence detection unit 203 included in the DC fault detection device 200 of the second embodiment is added to the DC fault detection device 100 of the first embodiment has been described. However, the DC fault detection device 300 of the third embodiment may be configured to add the DC voltage measurement unit 101 and the accident occurrence detection unit 103 included in the DC fault detection device 100 of the first embodiment to the DC fault detection device 200 of the second embodiment. In other words, instead of or in addition to the first fault section determination unit 104 included in the DC fault detection device 300 of the third embodiment, the first fault section determination unit 204 may be included. In this case, the operation and processing of each component may be equivalent to the operation and processing of each component included in the DC fault detection device 100 of the first embodiment or the DC fault detection device 200 of the second embodiment. For example, when the first fault section determination unit 204 is provided instead of the first fault section determination unit 104 provided in the DC fault detection device 300 of the third embodiment, the process of step S210 in the flowchart of the DC fault detection device 200 of the second embodiment shown in FIG. 8 can be performed instead of the process of step S110 in the flowchart shown in FIG. 10. In this case, the DC voltage change amount is an example of the "first index value" in the claims, and the DC current change amount absolute value is an example of the "second index value" in the claims. For example, when the first fault section determination unit 204 is provided in addition to the first fault section determination unit 104 provided in the DC fault detection device 300 of the third embodiment, the process of step S210 in the flowchart of the DC fault detection device 200 of the second embodiment shown in FIG. 8 can be performed after the process of step S200 in the flowchart shown in FIG. 10, and then the process of step S120 can be proceeded to. In this case, the process of step S110 in the flowchart shown in FIG. 10 may be omitted.

[0139] As described above, in the DC fault detection device of each embodiment, the DC fault detection device applied to each terminal of a multi-terminal power transmission system can detect and determine a DC fault that has occurred in any of the DC transmission lines belonging to one of the applied terminals, and can finally identify the DC transmission lines included in the fault section. Furthermore, the DC fault detection device of each embodiment can detect a DC fault or whether the DC transmission line in which the DC fault has occurred is included in the fault section in a short time with high reliability.

[0140] According to at least one of the embodiments described above, a DC fault detection device (100) detects a fault section of a DC fault in a DC power transmission system (1) including three or more power converters (3) that convert AC to DC and vice versa and a plurality of DC transmission lines (50) connecting between DC side terminals of each of the power converters, the ... a first DC transmission line (DC transmission line 50-1, DC transmission line 50-2, DC transmission line 50-3) that is a DC transmission line connected to a first power converter (3-1) that is a corresponding power converter. a fault occurrence detection unit (103) for detecting a DC fault occurring in any of a plurality of first DC transmission lines connected to the first power converter based on a comparison between a first index value (amount of DC voltage change) indicating a power change in the power lines (50-3 and 50-4) and a first threshold value (vth1) indicating a power change in the DC transmission lines caused by a DC fault; and a first fault section determination unit (104) for comparing a second index value (amount of change in DC voltage) indicating a change in power of the power line with a second threshold (vth2) indicating a change in power of the DC transmission line caused by a DC fault, and determining that a section of the first DC transmission line for which the second index value exceeds the second threshold is a fault section; and By providing a second fault section determination unit (105) that determines whether or not each of the lines of the first DC transmission lines is a fault section, and a third fault section determination unit (106) that determines which of the lines of the first DC transmission lines is a fault section by logically multiplying the first determination result (fault section inside / outside flag) by the first fault section determination unit and the second determination result by the second fault section determination unit, it is possible to detect DC faults and fault sections with high reliability in the power transmission system.

[0141] Although some embodiments of the present invention have been described, these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their modifications are included in the scope of the invention and its equivalents described in the claims, as well as in the scope and spirit of the invention. [Explanation of symbols]

[0142] 1 Power transmission system, 2, 2-1, 2-2, 2-3 AC system, 3, 3-1, 3-2, 3-3 Power converter, 4, 4-1, 4-2 DC system, 20, 20-1, 20-2, 20-3, 20-4, 20-5, 20-6 DC bus, 30, 30-1, 30-2, 30-3, 30-4, 30 -5, 30-6, 30-7, 30-8, 30-9, 30-10, 30-11, 30-12···Reactor, 40, 40-1, 40-2, 40-3, 40-4, 40-5, 40-6, 40-7, 40-8, 40-9, 40-10, 40-11, 40-12···DC circuit breaker, 50, 50-1, 50-2, 50- 3, 50-4, 50-5, 50-6... DC transmission line, 60, 60-1, 60-2... DC voltage detector, 70, 70-1, 70-2, 70-3, 70-4... DC current detector, 100, 100-1, 100-2, 100-3, 200, 200-1, 200-2, 200-3, 300... DC fault detection device, 101... DC voltage measurement unit, 102... DC current measurement unit, 103... fault occurrence detection unit, 104... first fault section determination unit, 105... second fault section determination unit, 106... third fault section determination unit, 107... shutdown command output unit, 203... fault occurrence detection unit, 204... first fault section determination unit

Claims

1. A DC fault detection device for detecting a fault section of a DC fault in a DC power transmission system including three or more power converters for converting AC to DC and vice versa, and a plurality of DC transmission lines connecting DC side terminals of each of the power converters, comprising: a fault occurrence detection unit that detects the DC fault that has occurred in any of the plurality of first DC transmission lines connected to the first power converter based on a comparison between a first index value indicating a power change in a first DC transmission line that is the DC transmission line connected to the first power converter that is the corresponding power converter and a first threshold value indicating a power change in the DC transmission line caused by the DC fault; a first fault section determination unit that, after a first predetermined time has elapsed since the occurrence of the DC fault is detected by the fault occurrence detection unit, compares a second index value indicating a power change in each of the first DC transmission lines with a second threshold value indicating a power change in the DC transmission line caused by the DC fault, and determines that a line of the first DC transmission line for which the second index value exceeds the second threshold value is the fault section; a second fault section determination unit that determines whether or not a line of each of the first DC transmission lines is the fault section based on a current change in each of the first DC transmission lines after a second predetermined time has elapsed since the occurrence of the DC fault was detected by the fault occurrence detection unit; and a third fault section determination unit that determines which line of the first DC transmission line is the fault section by performing a logical AND operation on a first determination result by the first fault section determination unit and a second determination result by the second fault section determination unit; A DC fault detection device comprising:

2. A DC fault detection device for detecting a fault section of a DC fault in a DC power transmission system including three or more power converters for converting AC to DC and vice versa, and a plurality of DC transmission lines connecting DC side terminals of each of the power converters, comprising: a first fault occurrence detection unit that detects a DC fault that has occurred in any of the first DC transmission lines connected to the first power converter based on a comparison between a first index value indicating a power change in a first DC transmission line that is the DC transmission line connected to the first power converter that is a corresponding one of the power converters and a first threshold value indicating a power change in the DC transmission line caused by the DC fault; a first fault section determination unit that, after a first predetermined time has elapsed since the occurrence of the DC fault is detected by the first fault occurrence detection unit, compares a second index value indicating a power change in each of the first DC transmission lines with a second threshold value indicating a power change in the DC transmission line caused by the DC fault, and determines that a line of the first DC transmission line for which the second index value exceeds the second threshold value is the fault section; a second fault occurrence detection unit that detects the DC fault that has occurred in any one of the first DC transmission lines connected to the first power converter based on a comparison between a current change in the first DC transmission line and a third threshold value that indicates a current change in the DC transmission line caused by the DC fault; a second fault section determination unit that determines whether or not a line of each of the first DC transmission lines is the fault section based on a current change in each of the first DC transmission lines after a second predetermined time has elapsed since the first fault occurrence detection unit detected the occurrence of the DC fault or since the second fault occurrence detection unit detected the occurrence of the DC fault; a third fault section determination unit that determines which line of the first DC transmission line is the fault section by performing a logical AND operation on a first determination result by the first fault section determination unit and a second determination result by the second fault section determination unit; A DC fault detection device comprising:

3. The current change is an absolute value of a change in a current value, the second fault section determination unit focuses on each of the multiple first DC transmission lines connected to the first power converter, compares an absolute value of an amount of change in a current value of the first DC transmission line of interest with absolute values ​​of an amount of change in a current value of the other first DC transmission lines, and determines that a line of the first DC transmission line having a maximum absolute value of an amount of change in the current value after the second predetermined time has elapsed is the fault section.

3. The DC fault detection device according to claim 1 or 2.

4. the first index value and the second index value are amounts of change in voltage values ​​of the first DC transmission lines measured in the vicinity of the first power converter, The DC fault detection device according to any one of claims 1 to 3.

5. the first index value and the second index value are absolute values ​​of amounts of change in current values ​​of the first DC transmission lines measured in the vicinity of the first power converter; The DC fault detection device according to any one of claims 1 to 3.

6. the first index value is an amount of change in a voltage value of each of the first DC transmission lines measured in the vicinity of the first power converter; the second index value is an absolute value of an amount of change in a current value of each of the first DC transmission lines measured in the vicinity of the first power converter; 4. The DC fault detection device according to claim 2 or 3.

7. A method for controlling a DC fault detection device that detects a fault section of a DC fault in a DC power transmission system including three or more power converters that convert AC to DC and vice versa, and a plurality of DC transmission lines that connect DC side terminals of each of the power converters, comprising: The computer detecting the DC fault occurring in any one of the plurality of first DC transmission lines connected to the first power converter based on a comparison between a first index value indicating a power change in the first DC transmission line, which is the DC transmission line connected to the first power converter, which is the corresponding power converter, and a first threshold value indicating a power change in the DC transmission line caused by the DC fault; after a first predetermined time has elapsed since the occurrence of the DC fault is detected, a second index value indicating a power change of each of the first DC transmission lines is compared with a second threshold value indicating a power change of the DC transmission line caused by the DC fault, and a line of the first DC transmission line for which the second index value exceeds the second threshold value is determined to be the fault section; determining whether or not each of the first DC transmission lines is the fault section based on a current change in each of the first DC transmission lines after a second predetermined time has elapsed since the occurrence of the DC fault was detected; determining which of the lines of the first DC transmission lines is the fault section by performing a logical AND between a first determination result, in which the line of the first DC transmission line in which the second index value has exceeded the second threshold value is the fault section, and a second determination result, in which it is determined whether or not each of the lines of the first DC transmission lines is the fault section, based on a current change in each of the first DC transmission lines; A method for controlling a DC fault detection device.

8. A method for controlling a DC fault detection device that detects a fault section of a DC fault in a DC power transmission system including three or more power converters that convert AC to DC and vice versa, and a plurality of DC transmission lines that connect DC side terminals of each of the power converters, comprising: The computer detecting the DC fault occurring in any one of the plurality of first DC transmission lines connected to the first power converter based on a comparison between a first index value indicating a power change in the first DC transmission line, which is the DC transmission line connected to the first power converter, which is the corresponding power converter, and a first threshold value indicating a power change in the DC transmission line caused by the DC fault; after a first predetermined time has elapsed from the timing of detecting the occurrence of the DC fault based on the comparison between the first index value and the first threshold value, a second index value indicating a power change in each of the first DC transmission lines is compared with a second threshold value indicating a power change in the DC transmission line caused by the DC fault, and a line of the first DC transmission line for which the second index value exceeds the second threshold value is determined to be the fault section; detecting a DC fault occurring in any one of a plurality of first DC transmission lines connected to the first power converter based on a comparison between a current change in the first DC transmission line and a third threshold value indicating a current change in the DC transmission line caused by the DC fault; determining whether or not each of the first DC transmission lines is the fault section based on a timing at which the occurrence of the DC fault is detected based on a comparison of the second index value with the second threshold value or a current change in each of the first DC transmission lines after a second predetermined time has elapsed from a timing at which the occurrence of the DC fault is detected based on a comparison of a current change in the first DC transmission line with the third threshold value; determining which of the lines of the first DC transmission lines is the fault section by performing a logical AND between a first determination result, in which the line of the first DC transmission line in which the second index value has exceeded the second threshold value is the fault section, and a second determination result, in which it is determined whether or not each of the lines of the first DC transmission lines is the fault section, based on a current change in each of the first DC transmission lines; A method for controlling a DC fault detection device.

9. A program for controlling a DC fault detection device that detects a fault section of a DC fault in a DC power transmission system including three or more power converters that convert AC to DC and vice versa, and a plurality of DC transmission lines that connect between DC side terminals of each of the power converters, comprising: On the computer, detecting a DC fault occurring in any one of a plurality of first DC transmission lines connected to the first power converter based on a comparison between a first index value indicating a power change in a first DC transmission line that is the DC transmission line connected to the first power converter that is the corresponding power converter and a first threshold value indicating a power change in the DC transmission line caused by the DC fault; after a first predetermined time has elapsed since the occurrence of the DC fault is detected, a second index value indicating a power change in each of the first DC transmission lines is compared with a second threshold value indicating a power change in the DC transmission line caused by the DC fault, and a line of the first DC transmission line for which the second index value exceeds the second threshold value is determined to be the fault section; determining whether or not each of the first DC transmission lines is the fault section based on a current change in each of the first DC transmission lines after a second predetermined time has elapsed since the occurrence of the DC fault was detected; determining which of the lines of the first DC transmission lines is the fault section by performing a logical AND between a first determination result, in which the line of the first DC transmission line in which the second index value has exceeded the second threshold value is determined to be the fault section, and a second determination result, in which it is determined whether or not each of the lines of the first DC transmission lines is the fault section, based on a current change in each of the first DC transmission lines; program.

10. A program for controlling a DC fault detection device that detects a fault section of a DC fault in a DC power transmission system including three or more power converters that convert AC to DC and vice versa, and a plurality of DC transmission lines that connect between DC side terminals of each of the power converters, comprising: On the computer, detecting a DC fault occurring in any one of a plurality of first DC transmission lines connected to the first power converter based on a comparison between a first index value indicating a power change in a first DC transmission line that is the DC transmission line connected to the first power converter that is the corresponding power converter and a first threshold value indicating a power change in the DC transmission line caused by the DC fault; a second index value indicating a power change in each of the first DC transmission lines is compared with a second threshold value indicating a power change in the DC transmission lines caused by the DC fault, after a first predetermined time has elapsed since the occurrence of the DC fault was detected based on a comparison between the first index value and the first threshold value, and a line of the first DC transmission line for which the second index value exceeds the second threshold value is determined to be the fault section; detecting the DC fault occurring in any one of the first DC transmission lines connected to the first power converter based on a comparison between a current change in the first DC transmission line and a third threshold value indicating a current change in the DC transmission line caused by the DC fault; determining whether or not each of the first DC transmission lines is the fault section based on a timing at which the occurrence of the DC fault is detected based on a comparison of the second index value with the second threshold value or a current change in each of the first DC transmission lines after a second predetermined time has elapsed from a timing at which the occurrence of the DC fault is detected based on a comparison of a current change in the first DC transmission line with the third threshold value; determining which of the lines of the first DC transmission lines is the fault section by performing a logical AND between a first determination result, in which the line of the first DC transmission line in which the second index value has exceeded the second threshold value is determined to be the fault section, and a second determination result, in which it is determined whether or not each of the lines of the first DC transmission lines is the fault section, based on a current change in each of the first DC transmission lines; program.

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