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

Abstract

To detect a DC accident and an accident section with higher reliability.SOLUTION: A DC accident detection device of an embodiment, includes first to third accident section determination parts. The DC accident detection device detects an accident section of a DC accident in a DC power transmission system containing three or more power converters that convert an AC and a DC each other and a plurality of DC power transmission lines connecting each DC side terminal of the three or more power converters. The first accident section determination part determines that each DC power transmission line connected to the DC side terminal of the power converter of which a first instruction value indicating the change of the DC power in the power converter is larger than a threshold value of the three or more power converters is the DC power transmission line in the accident section. The second accident section determination part determines that the DC power transmission line in the accident section of the DC power transmission line of which a second index value indicating the change of the DC power in each DC power transmission line becomes the maximum is the DC power transmission line in the accident section. The third accident section determination part determines the DC power transmission line in the accident section of the plurality of DC power transmission lines on the basis of a logic product of each determination result.SELECTED DRAWING: Figure 2

Description

[Technical Field]

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

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

[0003] In HVDC systems, from the perspective of overcurrent protection for semiconductor elements used in power converters, it is necessary to detect (identify) fault sections in a shorter time than in AC systems and disconnect DC transmission lines where a DC system fault has occurred (remove the fault section). In AC systems, fault sections are usually detected using a method that uses differential currents at both ends of the transmission line. This method requires time for communication between the terminals of the transmission line, making it unsuitable as a method for detecting fault sections in HVDC systems. For this reason, a method for detecting fault sections faster than in AC systems is required for HVDC systems.

[0004] One known technique for detecting fault sections in HVDC systems is to use the difference in the rate of change of DC voltage due to reactors (series reactors) inserted in series with the DC transmission line at both ends. This method detects the fault section by measuring the rate of change of DC voltage on the line side of the series reactor. When a DC system fault occurs within a transmission line (an intra-section fault), the DC voltage changes sharply due to the surge voltage generated by the DC system fault (a high rate of change of DC voltage). On the other hand, when a DC system fault occurs within another DC transmission line (an extra-section fault), the surge voltage leveled by the series reactor connected to the other DC transmission line is measured, resulting in a relatively gradual change in DC voltage (a low rate of change of DC voltage). Conventional methods that use the difference in the rate of change of DC voltage detect the fault section by identifying the DC transmission line where the DC fault occurred based on this difference in the rate of change of DC voltage.

[0005] However, in conventional techniques that detect fault sections solely based on the measurement results of the rate of change of DC voltage, such as the method that utilizes the difference in the rate of change of DC voltage, it is possible that malfunctions may occur, such as detecting a DC fault even though no DC fault actually occurred, if noise is superimposed on the measured DC voltage. In this case, the reliability of the detection of DC faults and fault sections is reduced, leaving room for improvement in the reliability of detection. [Prior art documents] [Non-patent literature]

[0006] [Non-Patent Document 1] J. Sneath, ADRajapakse, “Fault Detection and Interruptionin 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]

[0007] The problem to be solved by the present invention is to provide a DC fault detection device, a DC fault detection method, and a program that can detect DC faults and fault sections with higher reliability in a power transmission system. [Means for solving the problem]

[0008] A DC fault detection device according to an embodiment includes 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 multiple DC transmission lines connecting the DC side terminals of the three or more power converters. The first fault section determination unit determines that a DC transmission line connected to a DC side terminal of a power converter having a first index value indicating a change in DC power in the power converter that is greater than a threshold value is the DC transmission line in the fault section. The second fault section determination unit determines that a DC transmission line having a second index value indicating a change in DC power in each DC transmission line that is the largest is the DC transmission line in the fault section. The third fault section determination unit determines which DC transmission lines are within the fault section among the multiple DC transmission lines based on the logical product of the determination result by the first fault section determination unit and the determination result by the second fault section determination unit. [Brief explanation of the drawings]

[0009] [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; [Figure 2] 1 is a diagram showing an example of the configuration of a DC fault detection device 100 according to a first embodiment. [Figure 3] FIG. 3 is a diagram for explaining the determination content by the first fault section determination unit 102 of the first embodiment. [Figure 4]FIG. 4 is a diagram for explaining the determination content by the second fault section determination unit 103 of the first embodiment. [Figure 5] 3 is a flowchart showing an example of processing in the DC fault detection device 100 of the first embodiment. [Figure 6] FIG. 10 is a diagram showing an example of the configuration of a DC fault detection device 100A according to a second embodiment. [Figure 7] FIG. 10 is a diagram for explaining the determination content by a first fault section determination unit 102A of the second embodiment. [Figure 8] 10 is a flowchart showing an example of processing in a DC fault detection device 100A according to the second embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, a DC fault detection device, a DC fault detection method, and a program according to an embodiment will be described with reference to the drawings.

[0011] [Power transmission system configuration] FIG. 1 is a diagram illustrating 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 illustrates a three-terminal HVDC system 1 as an example of a multi-terminal power transmission system. The HVDC 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 HVDC 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 to DC power before transmitting it can reduce power loss compared to a method of transmitting AC power directly, and is therefore particularly applicable to large-capacity, long-distance power transmission. The HVDC 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 HVDC system 1, the AC system 2 and the DC system 4 are connected via the power converter 3. The HVDC system 1 is an example of a "direct current transmission system."

[0012] Each of the AC systems 2 is an AC power system configured, for example, 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, or 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 located at the connected ends.

[0013] Each of the power converters 3 is a converter that converts AC power to DC power and vice versa. 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 the AC side terminal. Each of the power converters 3 is, for example, a self-commutated 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.

[0014] The DC system 4 is a DC power system that transmits DC power supplied by the power converters 3 to other power converters 3. The DC system 4 shown in Fig. 1 includes, for example, DC buses 20-1 to 20-6, DC circuit breakers 30-1 to 30-12, reactors 40-1 to 40-12, and DC transmission lines 50-1 to 50-6. Hereinafter, the DC buses 20-1 to 20-6 will be simply referred to as "DC buses 20" unless they are to be distinguished from one another. Similarly, the DC circuit breakers 30-1 to 30-12, reactors 40-1 to 40-12, and DC transmission lines 50-1 to 50-6 will be simply referred to as "DC circuit breakers 30," "reactors 40," and "DC transmission lines 50."

[0015] FIG. 1 shows a configuration of the DC system 4 in which the DC circuit breaker 30 is connected closer to the DC bus 20 and the reactor 40 is connected further ahead, but the order in which the DC circuit breaker 30 and the reactor 40 are connected in the DC system 4 may be reversed.

[0016] The DC bus 20 is connected to the DC side terminals of the power converter 3. The DC bus 20-1 is connected to one DC side terminal of the power converter 3-1 and has a positive DC voltage applied thereto. The DC bus 20-2 is connected to the other DC side terminal of the power converter 3-1 and has a negative DC voltage applied thereto. The DC bus 20-3 is connected to one DC side terminal of the power converter 3-2 and has a positive DC voltage applied thereto. The DC bus 20-4 is connected to the other DC side terminal of the power converter 3-2 and has a negative DC voltage applied thereto. The DC bus 20-5 is connected to one DC side terminal of the power converter 3-3 and has a positive DC voltage applied thereto. The DC bus 20-6 is connected to the other DC side terminal of the power converter 3-3 and has a negative DC voltage applied thereto.

[0017] A plurality of DC transmission lines 50 are connected to the DC bus 20 via DC circuit breakers 30 and reactors 40. For example, the DC bus 20-1 is connected to a DC transmission line 50-1 via a DC circuit breaker 30-1 and a reactor 40-1.

[0018] The DC circuit breaker 30 is a circuit breaker for DC power. The DC circuit breaker 30 is connected between a DC bus 20 and a DC transmission line 50. The DC circuit breaker 30 electrically connects or disconnects the target DC bus 20 and the DC transmission line 50, for example, in accordance with control by a DC fault detection device of an embodiment described later. The DC circuit breaker 30 transmits DC power via the connected DC transmission line 50 during steady-state operation (steady-state power transmission) in the HVDC system 1. When a DC system fault (hereinafter simply referred to as a "DC fault") occurs in the connected DC transmission line 50, the DC circuit breaker 30 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.

[0019] The reactor 40 is connected between the DC bus 20 and the DC transmission line 50. The reactor 40 is, for example, a series reactor or a blocking coil. The reactor 40 is not limited to a series reactor or a blocking coil, and may be any type of reactor.

[0020] 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.

[0021] (First embodiment) [Configuration of DC fault detection device] FIG. 2 is a diagram illustrating an example of the configuration of a DC fault detection device 100 according to the first embodiment. The DC fault detection device 100 detects a fault section of a DC fault in a DC transmission system including, for example, three or more power converters that convert AC to DC and vice versa and a plurality of DC transmission lines connecting the DC side terminals of the three or more power converters. FIG. 2 illustrates an example of the DC fault detection device 100 applied to the power converter 3-1, one of the three power converters 3-1 to 3-3 included in the HVDC system 1 shown in FIG. 1. The DC fault detection device 100 serves as a protection device for reducing the impact on the HVDC system 1 of a DC fault that occurs 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) connected to the DC side terminal of the power converter 3-1. The DC fault detection device 100 is installed (connected) near (for example, within a predetermined distance from) the power converter 3-1.

[0022] The DC fault detection device 100 may be applied to each of the power converter 3-2 and the power converter 3-3 in the HVDC system 1. 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. 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 that reduces the impact on the HVDC system 1 of a DC fault that occurs in a 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) connected to the DC side terminal of 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 the impact on the HVDC system 1 of a DC fault that occurs in the DC transmission lines 50 (DC transmission lines 50-3, 50-4, 50-5, and 50-6) connected to the DC side terminals of the power converter 3-3. The DC fault detection device 100 applied to the power converter 3-1 will be described below.

[0023] FIG. 2 also shows the configuration of the HVDC system 1 related to the DC fault detection device 100 (the configuration of the AC system 2-1, the power converter 3-1, and part of the DC system 4). In the HVDC system 1, a DC current detector is provided when the DC fault detection device 100 is applied. This is also the case 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 configuration of part of the DC system 4 shown in FIG. 2, DC current detectors 60-1 to 60-4 are provided. In the following description, the DC current detectors 60-1 to 60-4 will be simply referred to as "DC current detector 60" except when they are to be distinguished from one another.

[0024] The DC current detector 60 continuously measures the DC current flowing through the corresponding DC transmission line 50 near the power converter 3-1 at a predetermined period. The DC current detector 60 is configured with, for example, a current transformer (CT). The DC current detector 60 outputs information indicating the current value of the measured DC current flowing through the corresponding DC transmission line 50 (hereinafter referred to as the "DC current value") to the DC fault detection device 100. Specifically, the DC current detector 60-1 measures the DC current flowing through the DC transmission line 50-1 connected to the DC bus 20-1 at a predetermined sampling period and outputs the measured DC current value to the DC fault detection device 100. The DC current detector 60-2 measures the DC current flowing through the DC transmission line 50-2 connected to the DC bus 20-2 at a predetermined sampling period and outputs the measured DC current value to the DC fault detection device 100. The DC current detector 60-3 measures the DC current flowing through the DC transmission line 50-3 connected to the DC bus 20-1 at a predetermined sampling period and outputs the measured DC current value to the DC fault detection device 100. The DC current detector 60-4 measures the DC current flowing through the DC transmission line 50-4 connected to the DC bus 20-2 at a predetermined sampling period and outputs the measured DC current value to the DC fault detection device 100.

[0025] The DC fault detection device 100 detects a DC fault that has occurred in any of the DC transmission lines 50 included in the DC system 4 based on the DC current value output by the DC current detector 60, and detects (identifies) 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 30 (the DC circuit breaker 30 to which the DC transmission line 50 in which the DC fault has occurred is connected) that belongs to the identified fault section, and electrically disconnects the DC bus 20 and the DC transmission line 50. The DC fault detection device 100 will be described in detail below.

[0026] The DC fault detection device 100 includes, for example, a DC current measurement unit 101, a first fault section determination unit 102, a second fault section determination unit 103, a third fault section determination unit 104, and a shutdown command output unit 105. The DC fault detection device 100 and the components included in the DC fault detection device 100 realize the following functions by 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. The DC fault detection apparatus 100 and some or all of the functions of the components included in the DC fault detection apparatus 100 may be realized by a dedicated LSI. The program may be stored in advance in a storage device (a storage device including a non-transitory storage medium) such as an HDD (Hard Disk Drive) or flash memory included in the DC fault detection apparatus 100 or the HVDC system 1, or may be stored in a removable storage medium (a non-transitory storage medium) such as a DVD or CD-ROM, and installed in the storage device included in the DC fault detection apparatus 100 by loading the storage medium into a drive device included in the DC fault detection apparatus 100 or the HVDC system 1.

[0027] The DC current measuring unit 101 acquires the DC current value output by each of the DC current detectors 60 (DC current detectors 60-1 to 60-4) and measures a first index value and a second index value based on the acquired DC current value. The first index value is used by the first fault section determination unit 102, and the second index value is used by the second fault section determination unit 103. The first index value and the second index value are values ​​indicating a change in DC power in each of the multiple DC transmission lines 50. The first index value and the second index value are, for example, absolute values ​​of the change in the current value of the DC current flowing through each DC transmission line 50 (hereinafter referred to as "DC current change absolute value"). The DC current measuring unit 101 measures, as the DC current change absolute value, the absolute value of the difference in the DC current of the DC transmission line measured at different times in the time-series data of the DC current value. The difference between the DC currents of a DC transmission line measured at different times is, for example, the difference between the DC current measured at a first reference time (the present time) and the DC current measured at a second reference time that is a predetermined time before the first time. The predetermined time may be, for example, a time based on a sampling period or a time based on a target fault detection time. The time based on a sampling period includes, for example, the interval between successive samples or the interval between several samples. For example, when the average value for each certain interval in time-series data of DC current values ​​is calculated by shifting the interval using a moving average, the predetermined time may be set based on the number of intervals (number of samples).

[0028] For example, the DC current measuring unit 101 measures the absolute value of the DC current change in the DC transmission line 50-1 based on the DC current value acquired from the DC current detector 60-1 as the absolute value of the DC current change on the positive side. The DC current measuring unit 101 also measures the absolute value of the DC current change in the DC transmission line 50-2 based on the DC current value acquired from the DC current detector 60-2 as the absolute value of the DC current change on the negative side. The DC current measuring unit 101 also measures the absolute value of the DC current change in the DC transmission line 50-3 based on the DC current value acquired from the DC current detector 60-3 as the absolute value of the DC current change on the positive side. The DC current measuring unit 101 also measures the absolute value of the DC current change in the DC transmission line 50-4 based on the DC current value acquired from the DC current detector 60-4 as the absolute value of the DC current change on the negative side.

[0029] When measuring the absolute value of the DC current change amount, the DC current measuring unit 101 may perform processing for the purpose of noise removal, such as moving average or filter adaptation. In this case, for example, the DC current measuring unit 101 measures the absolute value of the DC current change amount based on the moving average result of three points of the difference between the DC current value at the time of determination (current time) and the DC current value four samples before the time of determination. In addition, the DC current measuring unit 101 performs filtering to remove DC current values ​​that do not fall within a predetermined range.

[0030] Alternatively, instead of the absolute value of the difference between the DC currents of the DC transmission line measured by the DC current detector 60 at different times, the DC current measuring unit 101 may time-differentiate the DC current values ​​of the DC transmission line 50 measured at a predetermined period and measure the absolute value of the resulting value (change rate of the DC current) (hereinafter referred to as the "absolute value of DC current change rate") as the first index value or the second index value. Therefore, both the first index value and the second index value may be the "absolute value of DC current change amount" or the "absolute value of DC current change rate," or one of the first index value and the second index value may be the "absolute value of DC current change amount" and the other may be the "absolute value of DC current change rate." In the first embodiment, the description will be given assuming that both the first index value and the second index value are the absolute value of DC current change amount.

[0031] The DC current measuring unit 101 outputs the measured absolute value of the DC current change amount to the first fault section determining unit 102 and the second fault section determining unit 103. The DC current change amount absolute value is an example of a "current change".

[0032] The first fault section determination unit 102 determines whether or not any of the DC transmission lines 50 belonging to the power converter 3 (the DC transmission lines 50 connected to the DC side terminals of the power converter 3) is included in the fault section in which the DC fault has occurred, based on the absolute value of the DC current change amount measured by the DC current measurement unit 101. For example, the first fault section determination unit 102 determines whether or not the DC transmission line 50 to be determined is a DC transmission line in the fault section.

[0033] FIG. 3 is a diagram for explaining the determination content by the first fault section determination unit 102 of the first embodiment. In the example of FIG. 3, the horizontal axis represents time, and the vertical axis represents the DC current value and the absolute value of the DC current change amount of each of the power converters 3-1 to 3-3. The example of FIG. 3 also shows how the fault section detection flag switches over time. Note that in the first embodiment, multiple DC transmission lines are connected to each of the power converters 3-1 to 3-3. However, for convenience of explanation, the example of FIG. 3 only shows the case where the absolute value of the DC current change amount is the largest among the power converters 3-1 to 3-3 as a representative example. In the following, as an example, a description will be given assuming that a DC fault X has occurred at the end of the DC transmission line 50-1 as shown in FIG. 2. The DC fault X is, for example, a ground fault that is expected to occur due to a lightning strike. The ground fault is, for example, an accident in which the DC current (fault current) flowing through the DC transmission line 50-1 suddenly changes. In addition, in FIG. 3, it is assumed that time T1 is the earliest, followed by times T2, T3, T4, T5, and T6 in that order.

[0034] The first fault section determination unit 102 determines whether the absolute value of the DC current change amount for each DC transmission line 50 measured by the DC current measurement unit 101 in each of the power converters 3-1 to 3-3 is greater than a predetermined first threshold value ith. The first threshold value ith is an example of a "threshold value." The first threshold value ith is determined based on parameters such as the location of the fault point, the rated voltage of the DC system, the inductance value of the reactor, the type of DC transmission line, and the length of the DC transmission line. The first threshold value ith is set to be greater than the maximum absolute value of the DC current change amount outside the fault section and smaller than the minimum absolute value of the DC current change amount within the fault section, taking into account the influence of the above parameters on the DC current change rate. A different value of the first threshold value ith may be set for each DC transmission line 50.

[0035] For example, if the absolute value of the DC current change amount is greater than the first threshold value ith, the first fault section determination unit 102 determines that the target DC transmission line 50 for which the absolute value of the DC current change amount is measured is in the fault section. This is because, outside the fault section, there are more reactors between the current measurement point and the fault point than in the fault section, which reduces the current change rate (the amount of DC current change over a certain period of time). Therefore, if the absolute value of the DC current change amount is equal to or less than the first threshold value ith, the first fault section determination unit 102 determines that the target DC transmission line 50 is outside the fault section (not in the fault section).

[0036] In the example of FIG. 3 , when a fault occurs at time T1, the DC current in the DC transmission line 50 belonging to the power converter 3-1 increases, causing the absolute value of the DC current change to increase. At time T2, the absolute value of the DC current change exceeds the first threshold value ith. This state continues until time T3. Therefore, the first fault section determination unit 102 determines that the DC transmission line 50 belonging to the power converter 3-1, whose absolute value of the DC current change has been measured, is in the fault section during the period from time T2 to T3. Also, the DC current in the power converter 3-2 increases at time T4, causing the absolute value of the DC current change to increase. At time T5, the absolute value of the DC current change exceeds the first threshold value ith. This state continues until time T6. Therefore, the first fault section determination unit 102 determines that the DC transmission line 50 belonging to the power converter 3-2, whose absolute value of the DC current change has been measured, is in the fault section during the period from time T5 to T6. For the power converter 3-3, there is no section where the absolute value of the DC current change amount is greater than the first threshold value ith. Therefore, the first fault section determination unit 102 may determine that the DC transmission line 50 belonging to the power converter 3-3 is outside the fault section.

[0037] For example, the first fault section determination unit 102 outputs the determination result of whether or not each of the multiple DC transmission lines 50 connected to the power converters 3-1 to 3-3 is within a fault section to the third fault section determination unit 104. Note that the first fault section determination unit 102 may set a preset fault section detection flag to a value (e.g., "1") indicating that the line is within the fault section or a value (e.g., "0") indicating that the line is not within the fault section (not detected or outside the fault section), and output the fault section detection flag for each of the power converters 3-1 to 3-3 (or for each DC transmission line 50 belonging to the power converter 3) over time to the third fault section determination unit 104.

[0038] The second fault section determination unit 103 determines that the DC transmission line with the largest absolute value of the DC current change amount among the DC transmission lines 50 belonging to the power converter 3 is the DC transmission line within the fault section, based on the absolute value of the DC current change amount measured by the DC current measurement unit 101. This is because, outside the fault section, there are more reactors between the current measurement points and the fault point than within the fault section, which reduces the current change rate, i.e., the DC current change amount over a certain period of time.

[0039] Fig. 4 is a diagram for explaining the determination content by the second fault section determination unit 103 of the first embodiment. In the example of Fig. 4, the horizontal axis represents time, and the vertical axis represents the DC current value of the DC transmission line connecting the power converters and the absolute value of the DC current change amount. Note that in the example of Fig. 4, for convenience of explanation, the DC transmission lines 50-1 to 50-4 connected to the power converter 3-1 will be described, but similar processing is also performed for all other DC transmission lines included in the HVDC system 1. In Fig. 4, time T11 is assumed to be earlier than time T12.

[0040] In the example of FIG. 4, when a fault occurs at time T11 (corresponding to time T1 in FIG. 3), a short-circuit current larger than the current value during normal operation flows from the power converters 3-1 and 3-2 toward the fault point. Therefore, the absolute value of the DC current change amount after the fault occurs is the largest in the DC transmission line 50-1 in the fault section at time T12, a predetermined time after the fault occurs. This absolute value of the DC current change amount is larger than the absolute values ​​of the DC current changes in the other DC transmission lines 50-2 to 50-4, including at other times. Therefore, the second fault section determination unit 103 determines that the DC transmission line 50-1 is in the fault section, and determines that the DC transmission lines 50-2 to 50-4 other than the DC transmission line 50-1 are not in the fault section (outside the fault section).

[0041] The reason why the first fault section determination unit 102 and the second fault section determination unit 103 use the absolute value of the DC current change amount for determination is to be able to compare the magnitude of the DC current change amount even under conditions where the current direction differs depending on the location of the fault. To improve detection accuracy, the second fault section determination unit 103 may determine the fault section using multiple time samples and determine the fault section by taking the logical product of the multiple determination results. The second fault section determination unit 103 outputs information about the DC transmission line 50 determined to be within the fault section to the third fault section determination unit 104. At this time, information about the time when it was determined to be within the fault section may also be output.

[0042] The third fault section determination unit 104 ultimately determines which DC transmission lines are in the fault section based on the respective determination results of the first fault section determination unit 102 and the second fault section determination unit 103. For example, the third fault section determination unit 104 determines which DC transmission lines connected to which power converters 3 are in the fault section by calculating the logical product of the determination result of the first fault section determination unit 102 and the determination result of the third fault section determination unit 104. For example, in the situation shown in the example of FIG. 3, the first fault section determination unit 102 determines that the DC transmission lines 50 of the power converters 3-1 and 3-2 are in the fault section, and in the situation shown in the example of FIG. 4, the second fault section determination unit 103 determines that the DC transmission line 50-1 of the power converter 3-1 is in the fault section. Therefore, the third fault section determination unit 104 finally determines that the DC transmission line 50-1 of the power converter 3-1 is in the fault section by performing a logical AND of these values. The third fault section determination unit 104 outputs the determination result to the shutdown command output unit 105.

[0043] The tripping command output unit 105 performs control to trip (disconnect) the DC transmission line 50, which is determined to be in the fault section, from the DC system based on the determination result by the third fault section determination unit 104. For example, when it is determined that the DC transmission line 50-1 is in the fault section, the tripping command output unit 105 generates a tripping command value that causes the DC circuit breaker 30-1 to electrically disconnect the DC bus 20-1 and the DC transmission line 50-1, and outputs the generated tripping command value to the DC circuit breaker 30-1. As a result, the DC circuit breaker 30-1 to which the tripping command value has been input electrically disconnects the DC bus 20 and the DC transmission line 50 connected to it in accordance with the input tripping command value, and removes the DC transmission line 50-1, in which the DC fault has occurred, from the DC system 4.

[0044] In the above example, the determination process of the first fault section determination unit 102 and the determination process of the second fault section determination unit 103 are performed in parallel (at the same time). However, instead, one determination process may be performed first and the other determination process may be performed later. For example, the determination process of the first fault section determination unit 102 may be performed first, and if it is determined that the DC transmission line 50 is located in the fault section, the determination process of the second fault section determination unit 103 may be executed. In this case, the third fault section determination unit 104 ultimately determines which DC transmission lines are located in the fault section based on the determination result of the second fault section determination unit 103.

[0045] [Processing of DC fault detection device] 5 is a flowchart showing an example of processing in the DC fault detection device 100 of the first embodiment. The processing of this flowchart is repeatedly executed while the HVDC system 1 is in operation.

[0046] When the HVDC system 1 is operational and the DC fault detection device 100 operates, the DC current measurement unit 101 acquires the DC currents flowing in the DC transmission lines 50 measured by the DC current detector 60 (step S100). Next, the DC current measurement unit 101 measures the absolute value of the DC current change amount based on the acquired DC currents (step S102).

[0047] Next, the first fault section determination unit 102 determines whether the measured absolute value of the DC current variation is greater than the first threshold value ith (step S104). If it is determined that the absolute value of the DC current variation is greater than the first threshold value ith, the second fault section determination unit 103 determines whether the absolute value of the DC current variation of the target DC transmission line is greater than the absolute values ​​of the DC current variation of the other DC transmission lines (step S106).

[0048] If it is determined that the absolute value of the DC current change amount of the DC transmission line to be determined is larger than the absolute values ​​of the DC current change amount of the other DC transmission lines, the third fault section determination unit 104 determines that the target DC transmission line is a DC transmission line in the fault section (step S108). Next, the shutdown command output unit 105 electrically shuts off the DC bus 20 connected to the target section and the target DC transmission line 50 so that electricity does not flow through the fault section, and removes the DC transmission line 50 where the DC fault has occurred from the DC system 4 (step S110).

[0049] Furthermore, if the process of step S104 determines that the absolute value of the DC current change amount is not greater than the first threshold value ith (is equal to or less than the first threshold value ith), or if the process of step S106 determines that the absolute value of the DC current change amount of the DC transmission line being determined is not greater than those of the other DC transmission lines, the third fault section determination unit 104 determines that the DC transmission line being determined is not in the fault section (step S112), and the process of this flowchart ends.

[0050] In the flowchart shown in Figure 5, a judgment is made in the first fault section judgment unit 102 (step S104) followed by a judgment in the second fault section judgment unit 103 (step S106). However, the processing in step S104 and the processing in step S106 may be performed in parallel (at the same time) and a judgment may be made based on the logical product thereof, or they may be performed in the reverse order.

[0051] As described above, according to the first embodiment, it is possible to detect a DC fault or a fault section with higher reliability. For example, according to the first embodiment, in a multi-terminal DC power transmission system (a DC power transmission system using multiple DC transmission lines), it is possible to detect a fault section using only local-end information (information managed on the side of one power converter in the DC system 4). This eliminates the need for information communication between the terminals (power converters), and thus DC faults can be detected in a shorter time due to the saved communication time. Furthermore, according to the first embodiment, it is possible to detect a DC fault or a fault section with higher reliability, with a lower probability of malfunction, compared to a conventional protection system applied to an AC system, which identifies a DC transmission line 50 included in a fault section using a single determination method using a differential current between both ends of the transmission line.

[0052] (Second embodiment) Next, a DC fault detection device according to a second embodiment will be described. The DC fault detection device according to the second embodiment differs from the DC fault detection device 100 according to the first embodiment in that it includes a DC voltage measurement unit that measures the DC voltage of the DC system 4, and determines the fault section using the DC voltage measured by the DC voltage measurement unit and the DC current measured by the DC current measurement unit. Therefore, the following description will mainly focus on the above-mentioned differences. Furthermore, components having functions common to those of the HVDC system 1 according to the first embodiment described above will be assigned the same reference numerals, and detailed description thereof will be omitted here.

[0053] [Configuration of DC fault detection device] Fig. 6 is a diagram showing an example of the configuration of a DC fault detection device 100A of the second embodiment. Similar to the DC fault detection device 100 of the first embodiment, the DC fault detection device 100A shown in Fig. 6 shows an example of a DC fault detection device applied to the power converter 3-1 of the three power converters 3 included in the HVDC system 1 shown in Fig. 1. The example of Fig. 6 also shows the configuration of the HVDC system 1 related to the DC fault detection device 100A (the configuration of the AC system 2-1, the power converter 3-1, and part of the DC system 4).

[0054] In the example of FIG. 6, when the DC fault detection device 100A is applied, a DC current detector and a DC voltage detector are provided in the HVDC system 1. In the configuration of a portion of the DC system 4 shown in FIG. 6, four DC current detectors 60 (DC current detectors 60-1 to 60-4) and two DC voltage detectors 70 (DC voltage detectors 70-1 and 70-2) are provided. The same applies when the DC fault detection device 100A is applied to each of the power converters 3-2 and 3-3. The DC fault detection device 100A may be applied to each of the power converters 3-2 and 3-3, just like the DC fault detection device 100. The DC fault detection device 100A applied to each of the power converters 3-2 and 3-3 also has the same configuration as the DC fault detection device 100A applied to the power converter 3-1 shown in FIG. 6.

[0055] The DC voltage detectors 70 are located near the power converters 3-1 and continuously measure the DC voltage of the corresponding DC buses 20 at predetermined intervals. Each DC voltage detector 70 is configured, for example, with a voltage transformer (VT). Each DC voltage detector 70 outputs information indicating the voltage value of the measured DC voltage of the DC buses 20 (hereinafter referred to as "DC voltage value") to the DC fault detection device 100A.

[0056] The DC voltage detector 70-1 measures the DC voltage of the DC bus 20-1 at a predetermined sampling period and outputs the measured DC voltage value to the DC fault detection apparatus 100A. The DC voltage detector 70-1 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 the DC transmission lines 50 to the DC fault detection apparatus 100A. The DC voltage detector 70-2 measures the DC voltage of the DC bus 20-2 at a predetermined sampling period and outputs the measured DC voltage values ​​to the DC fault detection apparatus 100A. The DC voltage detector 70-2 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 the DC transmission lines 50-2 and 50-4 to the DC fault detection device 100A.

[0057] The DC fault detection device 100A includes, for example, a DC current measurement unit 101A, a DC voltage measurement unit 110, a first fault section determination unit 102A, a second fault section determination unit 103, a third fault section determination unit 104, and a shutdown command output unit 105. The DC fault detection device 100A and the components included in the DC fault detection device 100A realize the following functions when a hardware processor such as a CPU executes a program (software). Some or all of the functions of the components included in the DC fault detection device 100A and the DC fault detection device 100A 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 components included in the DC fault detection device 100A and the DC fault detection device 100A may be realized by a dedicated LSI. The program may be stored in advance in a storage device (storage device with a non-transitory storage medium) such as an HDD or flash memory provided in the DC fault detection device 100A or the HVDC system 1, or may be stored in a removable storage medium (non-transitory storage medium) such as a DVD or CD-ROM, and installed in a storage device provided in the DC fault detection device 100A by inserting the storage medium into a drive device provided in the DC fault detection device 100A or the HVDC system 1.

[0058] The DC fault detection device 100A differs from the DC fault detection device 100 of the first embodiment described above in that it has a DC voltage measurement unit 110, and has a DC current measurement unit 101A and a first fault section determination unit 102A instead of the DC current measurement unit 101 and the first fault section determination unit 102 of the first embodiment. The following description will mainly focus on these differences.

[0059] The DC current measuring unit 101A acquires the DC current values ​​output by each of the DC current detectors 60 (DC current detectors 60-1 to 60-4), and measures the above-mentioned second index value based on the acquired DC current values.

[0060] The DC voltage measurement unit 110 acquires DC voltage values ​​output by each of the DC voltage detectors 70 (DC voltage detectors 70-1 and 70-2) and measures a first index value based on the acquired DC voltage values. The first index value in the second embodiment is, for example, a difference value of DC voltages at the DC side terminals of the power converter 3, and more specifically, an amount of change in the voltage value of the DC voltage of the DC bus 20 (hereinafter referred to as "DC voltage change amount"). The DC voltage measurement unit 110 measures, as the DC voltage change amount, a difference between the DC voltages of the DC bus 20 measured at different times in time-series data of DC voltage values. The difference between the DC voltages of the DC bus 20 measured at different times is, for example, a difference between the DC voltage measured at a first reference time (the present time) and the DC voltage measured at a second time that is a predetermined time before the first time. The predetermined time may be, for example, a time based on a sampling period or a time based on a target fault detection time. The time based on a sampling period includes, for example, the interval between successive samples or the interval between several samples. For example, when the average value for each certain interval in time-series data of DC voltage values ​​is calculated by using a moving average while shifting the interval, the predetermined time may be set based on the number of intervals (number of samples).

[0061] For example, the DC voltage measurement unit 110 may measure a difference based on the DC voltage value acquired from the positive DC voltage detector 70-1 as the amount of DC voltage change in the DC transmission line 50 connected to the positive DC bus 20-1. Furthermore, the DC voltage measurement unit 110 may measure a difference based on the DC voltage value acquired from the negative DC voltage detector 70-2 as the amount of DC voltage change in the DC transmission line 50 connected to the negative DC bus 20-2.

[0062] When measuring the amount of change in DC voltage, the DC voltage measuring unit 110 may perform processing for the purpose of noise removal, such as moving average or filter adaptation. In this case, for example, the DC voltage measuring unit 110 measures the amount of change in DC voltage based on the moving average result of three points of the difference between the DC voltage value at the time of determination (current time) and the DC voltage value four samples before the time of determination. In addition, the DC voltage measuring unit 110 performs filtering to remove DC voltage values ​​that do not fall within a predetermined range.

[0063] Furthermore, instead of the difference between the DC voltages of the DC bus 20 measured at different times by the DC voltage detector 70, the DC voltage measurement unit 110 may time-differentiate the DC voltage values ​​measured at a predetermined period and measure the absolute value of the resulting value (change rate of the DC voltage) (hereinafter referred to as "DC voltage change rate") as the first index value. In the second embodiment, the first index value will be described as the amount of DC voltage change, and the second index value will be described as the absolute value of the amount of DC current change. The DC voltage measurement unit 110 outputs the calculated amount of DC voltage change to the first fault section determination unit 102A.

[0064] The first fault section determination unit 102A determines whether any line of the DC transmission line 50 belonging to the power converter 3 is in the fault section, based on the DC voltage change amount output by the DC voltage measurement unit 110. For example, the first fault section determination unit 102A determines whether the DC bus 20 or DC transmission line 50 to be determined is a DC bus or DC transmission line in the fault section.

[0065] FIG. 7 is a diagram illustrating the determination content by the first fault section determination unit 102A of the second embodiment. In the example of FIG. 7, the horizontal axis represents time, and the vertical axis represents the DC voltage value and DC voltage change amount of each of the power converters 3-1 to 3-3. The DC voltage change amount shown in the example of FIG. 7 is the positive-side DC voltage change amount based on the difference between the DC voltage values ​​measured by the DC voltage measurement unit 110 and acquired from the positive-side DC voltage detector 70-1. The example of FIG. 7 also shows how the fault section detection flag switches over time. In the following, as an example, a DC fault X is assumed to have occurred at the end of the DC transmission line 50-1 as shown in FIG. 6. The DC fault X is, for example, a ground fault that is expected to occur due to a lightning strike. In FIG. 7, time T21 is the earliest, followed by times T22, T23, T24, T25, and T26 in that order.

[0066] The first fault section determination unit 102A determines whether the amount of DC voltage change in the direction in which the DC voltage value approaches 0 (zero) is greater than a second threshold value vth in each of the power converters 3-1 to 3-3 based on the measurement results by the DC voltage measurement unit 110, and determines whether the DC transmission line is a DC transmission line in the fault section based on the determination result. The second threshold value vth is an example of a "threshold value." The second threshold value vth is determined by parameters such as the location of the fault point, the rated voltage, the inductance value of the reactor, the type of DC transmission line, and the length of the DC transmission line. Taking into account the influence of the above parameters on the voltage change rate, the second threshold value vth needs to be greater than the maximum value of the DC voltage change amount outside the fault section and smaller than the minimum value of the DC voltage change amount in the fault section. A different value of the second threshold value vth may be set for each power converter.

[0067] When the first fault section determination unit 102A determines that the amount of DC voltage change approaching zero is greater than the second threshold value vth, it determines that the DC transmission line 50 belonging to the power converter 3 being determined (more specifically, the DC transmission line 50 connected to the DC bus 20 belonging to the power converter 3) is in the fault section. This is because, outside the fault section, there are more reactors between the current measurement point and the fault point than in the fault section, resulting in a smaller voltage change rate. Therefore, when the first fault section determination unit 102A determines that the amount of DC voltage change approaching zero is not greater than the second threshold value vth (is equal to or less than the second threshold value vth), it determines that the DC transmission line 50 belonging to the power converter being determined is outside the fault section (is not in the fault section). In addition, in order to improve detection accuracy, the first fault section determination unit 102A may perform the determination using multiple time samples and determine the fault section by taking the logical product of the multiple determination results.

[0068] 7 is the DC voltage change amount on the positive pole side. In this case, the DC voltage change amount in the fault section increases in the negative direction from the positive side to approach 0 (the DC voltage value tends to decrease). Therefore, when using the DC voltage change amount on the positive pole side, the first fault section determination unit 102A may determine that the DC transmission line 50 belonging to the power converter 3 being determined is within the fault section if the DC voltage change amount is smaller than the second threshold value vth, and may determine that the DC transmission line 50 belonging to the power converter 3 being determined is outside the fault section if the DC voltage change amount is not smaller than the second threshold value vth (is equal to or greater than the second threshold value vth).

[0069] Conversely, when the determination is made using the negative DC voltage change amount based on the DC voltage value acquired by the DC voltage measurement unit 110 from the negative DC voltage detector 70-2, the DC voltage change amount in the fault section increases in the positive direction from the negative side to approach 0 (the DC voltage value tends to increase). Therefore, when the negative DC voltage change amount is used, the first fault section determination unit 102A may determine that the DC transmission line 50 belonging to the power converter 3 being determined is within the fault section if the DC voltage change amount is greater than the second threshold value vth, and may determine that the DC transmission line 50 belonging to the power converter 3 being determined is outside the fault section if the DC voltage change amount is not greater than the second threshold value vth (equal to or less than the second threshold value vth).

[0070] In this way, by using the DC voltage change amount rather than the absolute value of the DC voltage change amount in the determination by the first fault section determination unit 102A, it is possible to accurately grasp differences in the change trend of the DC voltage change amount due to a ground fault on the positive or negative side (whether it is increasing in the positive or negative direction), etc., and therefore it is possible to detect a DC fault or a fault section with higher reliability and accuracy. Also, in the second embodiment, because it is determined based on the DC voltage whether the power converter is located in the fault section, even if there is noise in the current, it is possible to make a more accurate determination without being affected by the noise.

[0071] In the example of FIG. 7 , when a fault occurs at time T21, the positive DC voltage value of power converter 3-1 decreases, and the DC voltage change amount increases in the negative direction (approaching zero), and at time T22, the DC voltage change amount becomes smaller than the second threshold value vth, and this state continues until time T23. Therefore, the first fault section determination unit 102A determines that the DC transmission line 50 belonging to power converter 3-1 is in the fault section during the period from time T22 to T23. Furthermore, at time T24, the DC voltage of power converter 3-2 decreases, and the DC voltage change amount increases in the negative direction (approaching zero), and at time T25, the DC voltage change amount becomes smaller than the second threshold value vth, and this state continues until time T26. Therefore, the first fault section determination unit 102A determines that the DC transmission line 50 belonging to power converter 3-2 is in the fault section during the period from time T25 to T26. For the power converter 3-3, there is no section where the amount of change in DC voltage approaching 0 is greater than the second threshold value vth. Therefore, the first fault section determination unit 102A determines that the power converter 3-3 is inside or outside the fault section.

[0072] Similar to the first fault section determination unit 102 shown in the first embodiment, the first fault section determination unit 102A outputs the determination result of whether or not each of the power converters 3-1 to 3-3 (or each of the DC transmission lines 50 or DC buses 20 belonging to the power converters 3-1 to 3-3) is in a fault section to the third fault section determination unit 104. Note that the first fault section determination unit 102A may set the fault section detection flag to a value (e.g., "1") indicating that the power converter is in the fault section, or a value (e.g., "0") indicating that the power converter is not in the fault section (not detected or outside the fault section), and output the fault section detection flag with the passage of time to the third fault section determination unit 104.

[0073] The operation of the second fault section determination unit 103 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 103 provided in the DC fault detection apparatus 100. In addition, the operation of the third fault section determination unit 104 and the shutdown command output unit 105 provided in the DC fault detection apparatus 100A is also similar to that of the third fault section determination unit 104 and the shutdown command output unit 105 provided in the DC fault detection apparatus 100.

[0074] In the above example, the determination process of the first fault section determination unit 102A and the determination process of the second fault section determination unit 103 are performed in parallel (at the same time). However, instead, one determination process may be performed first and the other determination process may be performed later. For example, the determination process of the first fault section determination unit 102A may be performed first, and if it is determined that the DC transmission line 50 is located in the fault section of the power converter, the determination process of the second fault section determination unit 103 may be executed. In this case, the third fault section determination unit 104 ultimately determines which DC transmission line is located in the fault section based on the determination result of the second fault section determination unit 103.

[0075] [Processing of DC fault detection device] Fig. 8 is a flowchart showing an example of processing in the DC fault detection device 100A of the second embodiment. The processing of this flowchart is also repeatedly executed while the HVDC system 1 is in operation. Note that the flowchart of Fig. 8 differs from the processing of steps S100 to S112 in the DC fault detection device 100A of the first embodiment described above in that it does not include processing of step S104 but includes processing of steps S200 to S204. The following description will focus on these differences.

[0076] When the HVDC system 1 is operational and the DC fault detection device 100 operates, the DC voltage measurement unit 110 acquires the DC voltages output by the DC voltage detectors 70-1 and 70-2 (step S200). Next, the DC voltage measurement unit 110 calculates the DC voltage change amount of the DC bus 20 based on the acquired DC voltages (step S202). Next, the first fault section determination unit 102A determines whether the DC voltage change amount when the DC voltage approaches 0 is greater than a second threshold value vth (step S204). In the process of step S204, if the DC voltage change amount on the positive pole side is used, the first fault section determination unit 102A may determine whether the DC voltage change amount is smaller than the second threshold value vth, or if the DC voltage change amount on the negative pole side is used, may determine whether the DC voltage change amount is greater than the second threshold value vth.

[0077] If it is determined that the amount of DC voltage change when the DC voltage approaches 0 is greater than the second threshold value vth, the DC current measurement unit 101 acquires the DC current flowing in each of the DC transmission lines 50 measured by the DC current detector 60 (step S100). Next, the DC current measurement unit 101 calculates the amount of DC current change based on the acquired DC current (step S102).

[0078] Next, the second fault section determination unit 103 determines whether the absolute value of the DC current variation amount of the DC transmission line to be determined is larger than the absolute values ​​of the DC current variation amount of the other DC transmission lines (step S106).

[0079] If it is determined that the absolute value of the DC current change amount of the DC transmission line to be determined is larger than the absolute values ​​of the DC current change amount of the other DC transmission lines, the third fault section determination unit 104 determines that the DC transmission line to be determined is a fault section (step S108). Next, the shutdown command output unit 105 electrically shuts off the DC bus 20 connected to the target section and the DC transmission line 50 so that electricity does not flow through the fault section, and removes the DC transmission line 50 where the DC fault has occurred from the DC system 4 (step S110).

[0080] Furthermore, if the process of step S204 determines that the amount of DC voltage change when the DC voltage approaches 0 is not greater than the second threshold ith (is equal to or less than the first threshold ith), or if the process of step S106 determines that the absolute value of the amount of DC current change of the DC transmission line being determined is not greater than those of the other DC transmission lines, the third fault section determination unit 104 determines that the DC transmission line being determined is not in the fault section (step S112). This ends the process of this flowchart.

[0081] In the flowchart shown in Figure 5, the processing related to the judgment in the first fault section judgment unit 102A (steps S200 to S204) is shown followed by the processing related to the judgment in the second fault section judgment unit 103 (steps S100, S102, S106), but the processing of steps S200 to S204 and the processing of steps S100, S102, and S106 may be performed in parallel (at the same time) or in the reverse order.

[0082] As described above, the second embodiment not only achieves the same effects as the first embodiment, but also uses two types of information, DC current and DC voltage, to determine the fault section. Therefore, even if one of the information is affected by a disturbance such as noise, the other is not affected, and the fault section can be detected with higher accuracy.

[0083] Each of the first and second embodiments may be combined with at least a part of the other embodiments. Furthermore, the DC fault detection devices 100 and 100A shown in the first and second embodiments may have a separate control device control the disconnection (removal) of the DC transmission line 50 where a DC fault has occurred. In this case, the DC fault detection devices 100 and 100A may not be provided with the shutdown command output unit 105.

[0084] Furthermore, the processing executed by the DC current measurement unit 101 in the first embodiment may be executed by the first fault section determination unit 102 and the second fault section determination unit 103. In this case, the DC fault detection apparatus 100 does not need to be provided with the configuration of the DC current measurement unit 101. Furthermore, in the second embodiment, the processing executed by the DC current measurement unit 101 may be executed by the second fault section determination unit 103, and the processing executed by the DC voltage measurement unit 110 may be executed by the first fault section determination unit 102A. In this case, the DC fault detection apparatus 100A does not need to be provided with the configurations of the DC current measurement unit 101 and the DC voltage measurement unit 110.

[0085] According to at least one embodiment described above, the DC fault detection device 100, 100A detects a fault section of a DC fault in an HVDC system 1 including three or more power converters 3 that convert AC and DC mutually and a plurality of DC transmission lines 50 connecting between the DC side terminals of the three or more power converters 3, and determines that the DC transmission line 50 connected to the DC side terminal of a power converter, among the three or more power converters 3, whose first index value indicating a change in DC power in the power converter is greater than a threshold value is a DC transmission line in the fault section. By having a fault section determination unit 102, a second fault section determination unit 103 that determines that the DC transmission line among the multiple DC transmission lines 50 for which the second index value indicating the change in DC power in each DC transmission line is the largest is a DC transmission line within the fault section, and a third fault section determination unit 104 that determines which DC transmission line among the multiple DC transmission lines is within the fault section based on the logical product of the determination result by the first fault section determination unit 102 and the determination result by the second fault section determination unit 103, DC faults and fault sections can be detected with higher reliability.

[0086] Furthermore, according to the embodiment, in a multi-terminal DC power transmission system, it is possible to detect a fault section using only local-end information, so that information communication between power converters is not required, and communication time is saved, so that it is possible to detect a DC fault in a short time. Furthermore, according to the embodiment, instead of a two-stage process of detecting a fault and then detecting the fault section, only the fault section is determined, so that the fault section can be identified more quickly.

[0087] Although several 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 within the scope and spirit of the invention, as well as within the scope of the invention described in the claims and their equivalents. [Explanation of symbols]

[0088] 1...HVDC system, 2...AC system, 3...power converter, 4...DC system, 20...DC busbar, 30...DC circuit breaker, 40...reactor, 50...DC transmission line, 60...DC current detector, 70...DC voltage detector, 100, 100A...DC fault detection device, 101, 101A...DC current measurement unit, 102, 102A...first fault section determination unit, 103...second fault section determination unit, 104...third fault section determination unit, 105...shutdown command output unit, 110...DC voltage measurement 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 that convert AC to DC and vice versa, and a plurality of DC transmission lines connecting DC side terminals of the three or more power converters, a first fault section determination unit that determines that a DC transmission line connected to a DC side terminal of a power converter among the three or more power converters, the DC transmission line being connected to the DC side terminal of the power converter having a first index value indicating a change in DC power in the power converter that is greater than a threshold value, is a DC transmission line in the fault section; a second fault section determination unit that determines, among the plurality of DC transmission lines, a DC transmission line having a maximum second index value indicating a change in DC power in each of the DC transmission lines, as the DC transmission line in the fault section; and a third fault section determination unit that determines which of the plurality of DC transmission lines is in the fault section based on a logical product of a determination result by the first fault section determination unit and a determination result by the second fault section determination unit; and A DC fault detection device comprising:

2. the first index value is an absolute value of a difference between DC currents of the DC transmission line measured at different times; 2. The DC fault detection device according to claim 1.

3. the first index value is an absolute value of a time-differentiated value of the DC current of the DC transmission line measured at a predetermined period; 2. The DC fault detection device according to claim 1.

4. the first index value is a difference value between DC voltages at DC side terminals of the power converter measured at different times, the first fault section determination unit determines, based on the DC voltage difference value, that a DC transmission line in which a change amount of the DC voltage in a direction approaching zero is greater than the threshold value is a DC transmission line in the fault section.

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

5. the first index value is a value obtained by time-differentiating a DC voltage at a DC side terminal of the power converter measured at a predetermined period; 2. The DC fault detection device according to claim 1.

6. the second index value is an absolute value of a difference between DC currents of the DC transmission line measured at different times; 2. The DC fault detection device according to claim 1.

7. the second index value is an absolute value of a time-differentiated value of the DC current of the DC transmission line measured at a predetermined period; 2. The DC fault detection device according to claim 1.

8. 1. A DC fault detection method for detecting 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 three or more power converters, comprising: The computer determining that a DC transmission line connected to a DC side terminal of a power converter, among the three or more power converters, for which a first index value indicating a change in DC power in the power converter is greater than a threshold value, is a DC transmission line in a fault section; determining, among the plurality of DC transmission lines, a DC transmission line having a maximum second index value indicating a change in DC power in each of the DC transmission lines, as the DC transmission line in the fault section; determining which of the plurality of DC transmission lines is in the fault section based on a logical product of a result of determining that the DC transmission line is in the fault section using the first index value and a result of determining that the DC transmission line is in the fault section using the second index value; DC fault detection method.

9. 1. A program for detecting 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 three or more power converters, On the computer, determining that a DC transmission line connected to a DC side terminal of a power converter, among the three or more power converters, having a first index value indicating a change in DC power in the power converter that is greater than a threshold value, is a DC transmission line in a fault section; determining, among the plurality of DC transmission lines, a DC transmission line for which a second index value indicating a change in DC power in each of the DC transmission lines is the largest, as the DC transmission line in the fault section; determining which of the plurality of DC transmission lines is in the fault section based on a logical product of a result of determining that the DC transmission line is in the fault section using the first index value and a result of determining that the DC transmission line is in the fault section using the second index value; program.

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