DC power transmission system and control device for DC power transmission system

Abstract

The present invention provides a DC power transmission system and a control device for the DC power transmission system that enable the interruption of DC fault currents while maintaining the operation of power generation equipment. [Solution] The system includes a series multiple transformer 10, a positive-side power converter 20 and a negative-side power converter 30 connected to the series multiple transformer 10 that convert AC power generated by a renewable energy power generation system 100 into DC power, a positive-side DC transmission line LN-P that transmits the DC power converted by the positive-side power converter 20, a negative-side DC transmission line LN-N that transmits the DC power converted by the negative-side power converter 30, a second positive-side power converter 60 connected to the positive-side DC transmission line LN-P that converts DC power into AC power, and a negative-side power converter 70 connected to the negative-side DC transmission line LN-N that converts DC power into AC power, wherein the AC terminals of the positive-side power converter 20 and the AC terminals of the negative-side power converter 30 are connected to the windings of the series multiple transformer 10.

Description

[Technical Field]

[0001] This embodiment relates to a DC power transmission system and a control device for a DC power transmission system. [Background technology]

[0002] In recent years, there has been active consideration and implementation of renewable energy power generation systems, such as offshore wind power generation, which generates electricity using wind turbines installed at sea. Renewable energy power generation systems are installed in locations suitable for power generation, far from areas with high electricity demand that consume large amounts of power. For this reason, the development of high-voltage direct current (HVDC) transmission systems to efficiently transmit electricity from renewable energy power generation systems over long distances has become important. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2021-141704 [Overview of the project] [Problems that the invention aims to solve]

[0004] As a method for connecting offshore wind power generation facilities to onshore AC power grids, a multi-terminal DC transmission system is being considered that uses multi-terminal DC transmission lines for protection control and power flow control. Conventionally, expensive DC circuit breakers were necessary to quickly restore power in the event of a DC fault. Another protection method for DC transmission line faults is the non-selective fault interruption method, which uses AC circuit breakers to interrupt the fault current without using DC circuit breakers, and its effectiveness when used in interconnection lines between AC systems has been reported.

[0005] However, when the non-selective fault interrupting method is applied to the power line of an offshore wind power generation facility, the power supply voltage of the power generation facility separated by the opening of the AC circuit breaker is lost, which causes the power generation facility to stop. Therefore, the fault ride-through requirement (FRT requirement) for the power generation facility cannot be satisfied, making it difficult to continue the operation of the power generation facility, and there has been a risk that a long-term shutdown of the power generation facility becomes unavoidable.

[0006] The present embodiment is intended to solve the above problem, and an object of the present invention is to provide a DC power transmission system and a control device for the DC power transmission system that can cut off a DC fault current while maintaining the operation of a power generation facility.

Means for Solving the Problem

[0007] A bipolar DC power transmission system connected to the renewable energy power generation system according to an embodiment has the following configuration. (1) A series multiple transformer connected to said renewable energy power generation system. (2) A first positive-side power converter and a first negative-side power converter respectively connected to said series multiple transformer and configured to convert AC power generated by said renewable energy power generation system into DC power. (3) A positive-side DC transmission line configured to transmit said DC power converted by said first positive-side power converter. (4) A negative-side DC transmission line configured to transmit said DC power converted by said first negative-side power converter. (5) A second positive-side power converter connected to said positive-side DC transmission line and configured to convert the transmitted said DC power into said AC power. (6) A second negative-side power converter connected to said negative-side DC transmission line and configured to convert the transmitted said DC power into said AC power. (7) An AC terminal of said first positive-side power converter and an AC terminal of said first negative-side power converter are respectively connected to windings of said series multiple transformer. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] [Figure 1]It is a diagram illustrating an example configuration of the DC power transmission system according to the first embodiment. [Figure 2] It is a diagram illustrating an example configuration of a power converter of the DC power transmission system according to the first embodiment. [Figure 3] It is a diagram illustrating an example configuration of a series multiplex transformer of the DC power transmission system according to the first embodiment. [Figure 4] It is a block diagram illustrating an example functional configuration of a control device of the DC power transmission system according to the first embodiment. [Figure 5] It is a flowchart illustrating an operation of the DC power transmission system according to the first embodiment. [Figure 6] It is a flowchart illustrating an operation of the DC power transmission system according to the first embodiment. [Figure 7] It is a graph showing simulation results of the DC power transmission system according to the first embodiment, where (a) shows waveforms at respective terminals and (b) shows voltage amplitudes of an offshore wind turbine generator. [Figure 8] It is a diagram showing a system configuration for simulation. [Figure 9] It is a diagram illustrating an example of fault ride-through requirements for wind power generation facilities of the DC power transmission system according to the first embodiment. [Figure 10] It is a diagram illustrating an example configuration of a DC power transmission system according to another embodiment. DETAILED DESCRIPTION OF EMBODIMENTS

[0009] [1. First Embodiment] [1-1. Configuration of DC Power Transmission System] The following describes an example of the configuration of a bipolar high-voltage DC power transmission system connected to a renewable energy power generation system. Figure 1 is a diagram showing an example of the configuration of a DC power transmission system 1 according to the first embodiment. The DC power transmission system 1 is a bipolar system having a positive electrode and a negative electrode, with a neutral wire shared by the positive and negative electrodes. In the system configuration example in Figure 1, it is assumed that the transmitting side of the DC power transmission system 1 is a renewable energy power generation system 100 and the receiving side is a land-based grid 300. The renewable energy power generation system 100 and the land-based grid 300 are connected by DC power transmission equipment 200.

[0010] The renewable energy power generation system 100 includes, for example, a wind turbine installed offshore that generates alternating current (AC) power by rotating according to the wind speed. Figure 1 shows the configuration of a DC power transmission system 1 connected to one renewable energy power generation system 100, but the number of renewable energy power generation systems 100 connected to the DC power transmission system 1 is not limited to one. Furthermore, the renewable energy power generation system 100 is not limited to a wind turbine, but may be other renewable energy power generation devices such as solar power generators.

[0011] The DC power transmission equipment 200 is a DC system that transmits electricity generated by the renewable energy power generation system 100 to the land-based power system 300. The land-based power system 300 is an AC system installed on land. Although Figure 1 illustrates the case where the land-based power system 300 has one terminal, the land-based power system 300 may have two or more terminals.

[0012] The DC power transmission system 1 described above will now be explained, starting with the AC power transmission system on the transmission side connected to the renewable energy power generation system 100. The renewable energy power generation system 100 is connected to a bipolar power converter via a series multiple transformer 10. The bipolar power converter includes a positive-side power converter 20 and a negative-side power converter 30.

[0013] Figure 2 shows an example of the configuration of a power converter. The power converter operates based on the control of a control unit (not shown) and converts AC power from an AC system to DC power and supplies it to a DC system, or converts DC power from a DC system to AC power and supplies it to an AC system. The power converter includes a first positive-side power converter, the positive-side power converter 20, and a first negative-side power converter, the negative-side power converter 30, as well as a second positive-side power converter, the positive-side power converter 60, and a second negative-side power converter, the negative-side power converter 70, which will be described later. When the positive-side power converters 20, 60 and the negative-side power converters 30, 70 are not distinguished, they are referred to as a power converter.

[0014] The positive-side power converter 20 and the negative-side power converter 30 convert the AC power generated by the renewable energy power generation system 100 and transmitted via the AC transmission line into DC power and output it to the DC transmission equipment 200. The positive-side power converter 20 and the negative-side power converter 30 are modular multilevel converters (MMCs), and a configuration using two sets of star-connected MMCs (DSCC: Double-Star Chopper-Cells) may be adopted.

[0015] The positive-side power converter 20 and the negative-side power converter 30 are provided with multiple legs between the positive terminal of the DC terminal and the negative terminal of the DC terminal. The number of legs corresponds, for example, to the number of phases of the AC power supplied by the AC system. In this embodiment, the AC system supplies three-phase three-wire AC power consisting of a first phase (R phase), a second phase (S phase), and a third phase (T phase). Therefore, the positive-side power converter 20 and the negative-side power converter 30 each include legs corresponding to the first phase (R phase), the second phase (S phase), and the third phase (T phase).

[0016] Each leg includes an upper arm and a lower arm, and each arm comprises n cells connected in series, forming a half-bridge circuit. In each leg, the terminals of the reactor are connected to the corresponding phase AC terminals of the AC system. The upper part of the multi-stage connected half-bridge circuit is called the upper arm, and the lower part is called the lower arm. The positive-side power converter 20 and the negative-side power converter 30 each include three-phase AC terminals connected to the windings of the series multiplexer 10.

[0017] The positive-side power converter 20 and the negative-side power converter 30 output an AC voltage of constant amplitude and constant frequency to their AC terminals during steady-state power transmission. An AC current, according to the grid voltage established by voltage control, is supplied from the renewable energy power generation system 100 to the AC terminals of the positive-side power converter 20 and the negative-side power converter 30. The positive-side power converter 20 and the negative-side power converter 30 then convert the AC power of the AC current supplied by the renewable energy power generation system 100 into DC power using the established grid voltage and output it from their DC terminals.

[0018] Figure 3 shows the equivalent circuit of the series multiplexer 10. The series multiplexer 10 is configured such that the voltages generated in the secondary and tertiary windings are in series on the primary winding side. The primary side of the series multiplexer 10 is connected to the renewable energy power generation system 100. The AC terminals of the positive-side power converters 20 are connected to the secondary side of the series multiplexer 10. The AC terminals of the negative-side power converters 30 are connected to the tertiary side of the series multiplexer 10. By connecting the series multiplexer 10 as described above, the secondary and tertiary windings share the series primary winding. Therefore, the sum of the voltages of the positive-side power converters 20 and the negative-side power converters 30 can be applied to the renewable energy power generation system 100 connected to the primary side.

[0019] With the above configuration, the AC power generated by the renewable energy power generation system 100, which is connected as an AC grid, is input to the positive-side power converter 20 and the negative-side power converter 30 via the series multiple transformer 10, and converted into DC power.

[0020] As shown in Figure 1, a positive-side short-circuit switch 41 is provided on the AC system side of the positive-side power converter 20, configured to short-circuit the AC output terminals of the positive-side power converter 20. The positive-side short-circuit switch 41 is connected, for example, to the AC line between the series multiple transformer 10 and the positive-side power converter 20. When the positive-side short-circuit switch 41 is in the closed state, the renewable energy power generation system 100 and the positive-side power converter 20 are connected. The positive-side short-circuit switch 41 is open during steady-state power transmission. When the positive-side short-circuit switch 41 is closed, the AC output terminals of the positive-side power converter 20 are short-circuited. As a result, the AC voltage and DC voltage of the positive-side power converter 20 become 0.

[0021] Furthermore, a negative-side short-circuit switch 42 is provided on the AC system side of the negative-side power converter 30, configured to short-circuit the AC output terminals of the negative-side power converter 30. The negative-side short-circuit switch 42 is connected, for example, to the AC line between the series multiple transformer 10 and the negative-side power converter 30. When the negative-side short-circuit switch 42 is in the closed state, the renewable energy power generation system 100 and the negative-side power converter 30 are connected. The negative-side short-circuit switch 42 is open during steady-state power transmission. When the negative-side short-circuit switch 42 is closed, the AC output terminals of the negative-side power converter 30 are short-circuited. As a result, the AC voltage and DC voltage of the negative-side power converter 30 become 0. When the positive-side short-circuit switch 41 and the negative-side short-circuit switch 42 are not distinguished, they are referred to as short-circuit switches 41 and 42.

[0022] In other words, the short-circuit switches 41 and 42 are switches that can short-circuit the AC output terminal of the faulty pole, for example, when a fault occurs on the corresponding pole side. The short-circuit switches 41 and 42 may be configured to short-circuit the terminals of the windings to create a zero voltage, or they may be configured to create a zero voltage by switching the positive-side power converter 20 and the negative-side power converter 30.

[0023] As shown in Figure 1, the DC power converted by the positive-side power converter 20 and the negative-side power converter 30 is transmitted to the land-based power system 300 via the DC power transmission equipment 200. The DC power transmission equipment 200 includes a positive-side DC transmission line LN-P, a neutral line 50, and a negative-side DC transmission line LN-N. The positive-side power converter 20 is connected to the positive-side DC transmission line LN-P and the neutral line 50. The negative-side power converter 30 is connected to the positive-side DC transmission line LN-N and the neutral line 50. The neutral line 50 is shared by the positive and negative poles to form a bipolar system. DC power transmission is performed in the DC power transmission equipment 200 based on this bipolar configuration. A grounding electrode is connected to the neutral line 50. However, it is also possible to adopt an earth return circuit method that eliminates the need for a neutral wire by connecting a grounding electrode not only to the connection points of the positive-side power converter 20 and the negative-side power converter 30, but also to the connection points of the positive-side power converter 60 and the negative-side power converter 70.

[0024] The positive-side power converter 20 is connected to the positive-side power converter 60 via the positive-side DC transmission line LN-P and the neutral line 50. The negative-side power converter 30 is connected to the negative-side power converter 70 via the negative-side DC transmission line LN-N and the neutral line 50. The configuration of the positive-side power converter 60 and the negative-side power converter 70 is basically the same as that of the positive-side power converter 20 and the negative-side power converter 30. However, they differ from the positive-side power converter 20 and the negative-side power converter 30 in that they convert the DC power transmitted from the DC transmission equipment 200 into AC power and output it to the land-based power system 300.

[0025] A first positive electrode isolation switch, NCB1P, is provided between the positive electrode power converter 20 and the neutral line 50. Similarly, a second positive electrode isolation switch, NCB2P, is provided between the neutral line 50 and the positive electrode power converter 60. Both NCB1P and NCB2P are closed during steady-state power transmission, but are opened to disconnect the positive electrode in the event of a main line fault on the positive side.

[0026] A first negative-side polarity isolation switch, NCB1N, is provided between the negative-side power converter 30 and the neutral line 50. Similarly, a second negative-side polarity isolation switch, NCB2N, is provided between the neutral line 50 and the negative-side power converter 70. Negative-side polarity isolation switches NCB1N and NCB2N are closed during steady-state power transmission. Negative-side polarity isolation switches NCB1N and NCB2N are opened to disconnect the negative electrode in the event of a main line fault on the negative side. When the positive-side polarity isolation switches NCB1P and NCB2P and the negative-side polarity isolation switches NCB1N and NCB2N are not distinguished, they are referred to as polarity isolation switches NCB.

[0027] Furthermore, a first positive-side disconnector, positive-side disconnector SP1, is installed in series at one end of the positive-side DC transmission line LN-P, and a second positive-side disconnector, positive-side disconnector SP2, is installed in series at the other end. Positive-side disconnectors SP1 and SP2 are high-speed disconnectors that do not have current interruption capability and are closed during steady-state power transmission. When positive-side disconnectors SP1 and SP2 are conducting, DC coupling is performed between the positive-side power converter 20, the positive-side DC transmission line LN-P, and the positive-side power converter 60. In the event of a fault in the positive-side DC transmission line LN-P, positive-side disconnectors SP1 and SP2 are opened after the operation of the positive-side polarity isolation switches NCB1P and NCB2P to isolate the faulted section. The positive-side disconnectors SP1 and SP2 are, for example, one of the following: a DC circuit breaker, a DC switch, or a DC disconnector.

[0028] At one end of the negative-side DC transmission line LN-N, a first negative-side disconnector, negative-side disconnector SN1, is installed in series, and at the other end, a second negative-side disconnector, negative-side disconnector SN2, is installed in series. Negative-side disconnectors SN1 and SN2 are high-speed disconnectors that do not have current interruption capability and are closed during steady-state power transmission. When negative-side disconnectors SN1 and SN2 are conducting, DC coupling is performed between the negative-side power converter 30, the negative-side DC transmission line LN-N, and the negative-side power converter 70. Negative-side disconnectors SN1 and SN2 are opened in the event of a fault in the negative-side DC transmission line LN-N after the operation of the negative-side polarity isolation switches NCB1N and NCB2N in order to isolate the faulted section. The negative-side disconnectors SN1 and SN2 are, for example, DC circuit breakers, DC switches, and DC disconnectors. When the positive-side disconnectors SP1 and SP2, the negative-side disconnectors SN1 and SN2 are not distinguished, they are referred to as disconnector S.

[0029] On the AC system side of the positive-side power converter 60, a positive-side AC circuit breaker 81 is connected, which is capable of interrupting the connection between the positive-side power converter 60 and the AC system connected to the positive-side power converter 60. The positive-side AC circuit breaker 81 is an AC circuit breaker that connects and interrupts the connection between the positive-side power converter 60 and the land-based power system 300. When the positive-side AC circuit breaker 81 is closed, it sends the AC power converted by the positive-side power converter 60 to the land-based power system 300. When the positive-side AC circuit breaker 81 is open, it interrupts the AC power that the positive-side power converter 60 sends to the land-based power system 300.

[0030] On the AC system side of the negative-side power converter 70, a negative-side AC circuit breaker 82 is connected, which is provided to be able to interrupt the connection between the negative-side power converter 70 and the AC system connected to the negative-side power converter 70. The negative-side AC circuit breaker 82 is an AC circuit breaker that connects and interrupts the connection between the negative-side power converter 70 and the land-based system 300. When the negative-side AC circuit breaker 82 is closed, it sends the AC power converted by the negative-side power converter 70 to the land-based system 300. When the negative-side AC circuit breaker 81 and the negative-side AC circuit breaker 82 are not distinguished, they are referred to as AC circuit breakers.

[0031] As described above, in the DC power transmission system 1, a control unit (not shown) performs grid forming (GFM) operation to operate the positive-side power converter 20 and the negative-side power converter 30 as voltage sources. That is, it controls the voltage of the positive-side power converter 20 and the negative-side power converter 30 to establish the AC grid voltage for the renewable energy system 100 to operate. Then, in the DC power transmission system 1, the renewable energy power generation system 100 generates an AC current according to the established AC grid voltage and supplies it to the positive-side power converter 20 and the negative-side power converter 30. As a result, in the DC power transmission system 1, the positive-side power converter 20 and the negative-side power converter 30 convert the AC power of the AC current supplied by the renewable energy power generation system 100 into DC power at the established grid voltage and transmits power as DC.

[0032] [1-2. Control device configuration] Figure 4 shows a block diagram of the control device 400 that controls the DC power transmission system 1. The control device 400 will be specifically described in relation to system switching. Specifically, the configuration for controlling the operation of the DC power transmission system 1 in response to an accident that occurs while the DC power transmission system 1 is performing steady-state power transmission will be described below. The configuration for controlling other operations of the DC power transmission system 1, including steady-state power transmission, will not be described.

[0033] Some or all of the components of the control device 400 are realized by, for example, a hardware processor executing a program (software) stored in a memory (storage unit) not shown. The memory not shown is realized by, for example, semiconductor memory elements such as ROM (Read Only Memory), RAM (Random Access Memory), and flash memory, as well as a hard disk drive (HDD) and an optical disc.

[0034] A hardware processor refers to circuits such as CPUs (Central Processing Units), GPUs (Graphics Processing Units), LSIs (Large Scale Integration), SOCs (System on Chips), Application Specific Integrated Circuits (ASICs), and programmable logic devices (e.g., Simple Programmable Logic Devices (SPLDs), Complex Programmable Logic Devices (CPLDs), and Field Programmable Gate Arrays (FPGAs)). Instead of storing the program in memory (not shown), the program may be directly embedded within the hardware processor's circuitry.

[0035] In this case, the hardware processor implements each function by reading and executing programs embedded within the circuit. The hardware processor is not limited to being configured as a single circuit; it may also be configured as a single hardware processor by combining multiple independent circuits to implement each function. Multiple components may be integrated into a single hardware processor to implement each function. Multiple components may be incorporated into a single dedicated LSI to implement each function.

[0036] Here, the program (software) may be stored in advance in a storage device that constitutes a storage device such as a semiconductor memory element like ROM, RAM, or flash memory, or a hard disk drive (a storage device equipped with a non-transient storage medium), or it may be stored in a removable storage medium such as a DVD or CD-ROM (a non-transient storage medium), and when the storage medium is mounted in a drive device provided in the power converter or control device 400, it may be installed in a storage device (not shown) provided in the power converter or control device 400. The program (software) may also be downloaded in advance from another computer device via a network (not shown) and installed in a storage device included in the DC power transmission system 1 or control device 400. The program (software) installed in the storage device included in the DC power transmission system 1 or control device 400 may be transferred to a processing circuit included in the DC power transmission system 1 or control device 400 and executed.

[0037] The control device 400 includes a fault detection unit 401, a gate block unit 402, a short-circuit unit 403, a fault current interruption unit 404, a residual current interruption unit 405, a fault section disconnection unit 406, and a power transmission restoration unit 407. The fault detection unit 401 detects a ground fault by detecting a voltage drop at the DC terminal of the power converter. The power converter includes a positive-side power converter 20, a negative-side power converter 30, a positive-side power converter 60, and a negative-side power converter 70. For example, as shown in Figure 1, if a ground fault occurs at the positive terminal, the control device detects the ground fault at the positive terminal by detecting a power drop in the positive-side power converter 20 and the positive-side power converter 60.

[0038] The gate block unit 402 is a processing unit that controls the semiconductor switches of a power converter in order to disconnect the power converter on the pole where a fault has occurred from the DC power transmission equipment 200. For example, if the fault detection unit 401 detects a ground fault at the positive pole, the gate block unit 402 controls the semiconductor switches of the positive-side power converter 20 and the positive-side power converter 60 to gate block all cells. The gate block unit 402 may also be configured to gate block when a fault is detected, such as by an overcurrent in the power converter.

[0039] The short-circuit section 403 is a processing unit that closes the short-circuit switch on the faulty pole, thereby short-circuiting the AC terminal of the power converter on the faulty pole. For example, if the fault detection section 401 detects a ground fault on the positive pole, the short-circuit section 403 closes the positive-side short-circuit switch 41, short-circuiting the AC output terminal of the positive-side power converter 20. This operation of the positive-side short-circuit switch 41 causes the AC voltage and DC voltage of the positive-side power converter 20 to become zero, reducing the fault current flowing from the positive-side power converter 20 to the fault point.

[0040] The fault current interruption unit 404 is a processing unit that interrupts the AC circuit breaker of the pole where the fault has occurred. For example, if the fault detection unit 401 detects a ground fault at the positive pole, the fault current interruption unit 404 opens the positive-side AC circuit breaker 81 and interrupts the AC power that the positive-side power converter 60 sends to the land-based power system 300. This interrupts the fault current flowing from the positive-side power converter 60 towards the fault point.

[0041] The residual current interruption unit 405 is a processing unit that interrupts the pole isolation switch of the pole where the fault occurred. For example, if the fault detection unit 401 detects a ground fault at the positive pole, it opens the positive pole side pole isolation switch NCB1P and the positive pole side pole isolation switch NCB2P, disconnecting the DC power transmission equipment 200 of the pole where the fault occurred. This interrupts the residual current that continues to flow due to the inductance of the DC circuit.

[0042] The fault section disconnection unit 406 is a processing unit that shuts off the disconnector of the pole where the fault occurred. For example, if the fault detection unit 401 detects a ground fault at the positive pole, the fault section disconnection unit 406 opens the positive pole side disconnector SP1 and the positive pole side disconnector SP2, disconnecting the fault section of the cable of the pole where the fault occurred from the DC power transmission equipment 200.

[0043] The power transmission restoration unit 407 is a processing unit that resumes power transmission to the pole where a fault occurred after the fault has been resolved. For example, if a ground fault at the positive pole is resolved, the power transmission restoration unit 407 controls the healthy section of the cable to recharge after the positive pole power converter 20 and the positive pole power converter 60 have deblocked. The power transmission restoration unit 407 then opens the positive pole short-circuit switch 41. The power transmission restoration unit 407 also closes the positive pole isolation switches NCB1P and NCB2P, the positive pole AC circuit breaker 81, the positive pole disconnector SP1 and the positive pole disconnector SP2. Finally, it controls the output of the positive pole power converter 20 and the positive pole power converter 60 to be resumed.

[0044] Furthermore, the configuration of the power transmission restoration unit 407 described above can be interpreted as including a configuration that performs each process. Specifically, the power transmission restoration unit 407 may include a deblocking unit that deblocks the power converter that was gate-blocked after the fault occurred, a short-circuit switch interruption unit that interrupts the short-circuit switch that was turned on after the fault occurred, and a switch turning-on unit that turns on the various switches that were interrupted after the fault occurred.

[0045] [1-2. Operation of the First Embodiment] The system switching process by the control device 400 of this embodiment will be described with reference to Figures 5 and 6. In the following description, the flow of the system switching process will be described in detail, and the control during steady-state power transmission before system switching and the details of the actual accident will be omitted.

[0046] (Disconnection flow for accident intervals) Figure 5 is a flowchart showing the system switching process implemented by the DC power transmission system 1. The fault detection unit 401 determines whether or not it has detected a ground fault (step S01). The fault detection unit 401 determines that it has detected a ground fault when it detects a voltage drop at the DC terminal of the power converter. If the fault detection unit 401 determines that it has detected a ground fault (step S01, YES), the control device 400 executes the process in step S20. On the other hand, if the fault detection unit 401 determines that it has not detected a ground fault (step S01, NO), the fault detection unit 401 executes the process in step S01 again. In the following explanation, as an example, we will describe the case when a unipolar ground fault occurs at the positive terminal.

[0047] When the fault detection unit 401 detects a ground fault, the gate block unit 402 immediately gate blocks the power converter on the pole where the fault occurred (step S02). In the case of a ground fault on the positive pole side, the gate block unit 402 gate blocks the positive pole power converter 20 and the positive pole power converter 60.

[0048] Once the gate block section 402 completes the gate blocking, the short-circuit section 403 closes the short-circuit switch of the faulty pole (step S03). In the case of a ground fault on the positive side, on the offshore side, the gate block section 402 closes the positive side short-circuit switch 41, causing the AC voltage and DC voltage of the positive side power converter 20 to become 0.

[0049] Furthermore, the fault current interruption unit 404 trips the AC circuit breaker of the pole where the fault occurred (step S04). In the case of a ground fault on the positive pole side, on the land side, the fault current interruption unit 404 opens the positive pole AC circuit breaker 81, thereby interrupting the fault current flowing from the positive pole power converter 60 towards the fault point. Note that steps S03 and S04 may be performed simultaneously, or in reverse order.

[0050] The operation of steps S03 and S04 reduces the current flowing to the DC power transmission equipment 200. Then, when it is confirmed that the DC current at the DC terminal of the power converter on the faulty pole has decreased to below the rated value, the residual current interruption unit 405 shuts off the pole isolation switch on the faulty pole (step S05). In the case of a ground fault on the positive pole side, the residual current interruption unit 405 shuts off the residual current by opening the positive pole isolation switch NCB1P and the positive pole isolation switch NCB2P.

[0051] The fault section disconnection unit 406 disconnects the disconnector of the pole where the fault occurred (step S06). In the case of a ground fault on the positive pole side, the fault section disconnection unit 406 disconnects the faulted section from the DC power transmission equipment 200 by opening the positive pole side disconnector SP1 and the positive pole side disconnector SP2. During the period from the occurrence of a fault on the positive pole until power transmission is resumed, power transmission continues on the negative pole side without interruption.

[0052] (Power transmission restoration flow) Figure 6 is a flowchart showing the system switching process implemented by the DC power transmission system 1. After the fault section is disconnected, the fault is resolved (step S11). Once the fault is resolved, the power restoration unit 407 deblocks the power converters that were gate-blocked after the fault occurred (step S12). In the case of a ground fault on the positive electrode side, the power restoration unit 407 deblocks the positive electrode side power converter 20 and the positive electrode side power converter 60.

[0053] After the power converter on the faulty pole is deblocked, the healthy section of the cable is recharged (step S13). Once the healthy section is fully charged, the power restoration unit 407 shuts off the short-circuit switch that was turned on after the fault occurred (step S14). In the case of a ground fault on the positive pole side, the power restoration unit 407 opens the positive pole side short-circuit switch 41.

[0054] Further, the power transmission restoration unit 407 turns on various switches that were interrupted after the accident occurred (step S15). In the case of a ground fault on the positive electrode side, the power transmission restoration unit 407 closes the positive electrode side pole separation switch NCB1P, the positive electrode side pole separation switch NCB2P, the positive electrode side AC circuit breaker 81, the positive electrode side disconnector SP1, and the positive electrode side disconnector SP2. Then, the output from the positive electrode side power converter 20 and the positive electrode side power converter 60 is resumed by a control unit (not shown) (step S16).

[0055] [1-3. Simulation of the First Embodiment] FIG. 7 shows simulation results of the DC power transmission system 1 according to the present embodiment. FIG. 7(a) shows the waveform of each terminal, and FIG. 7(b) shows the voltage amplitude of an offshore wind turbine. Note that the simulation results in FIG. 7 are obtained through simulation in the system configuration shown in FIG. 8. In the simulation system configuration, two onshore grids 300 are connected, and a fault occurs on the positive electrode side in one of the onshore grids 300.

[0056] Each waveform is shown in a per-unit system based on the self-capacitance of the converter. Before the accident occurs, the CVCF Z transmits 1 pu, and the DCAVR Z and APR Z each receive 0.5 pu of power. A unipolar ground fault occurs at time 0 s (t1). Thereafter, the CVCF P detects the accident and performs gate blocking, and when the SCB P is closed, the DC current i2 P gradually decreases. The DCAVR P and APR P perform gate blocking immediately after the accident, and when the CB 1P and CB 3P are opened at 0.051 s (t3), the DC currents i 1p , i 3p decrease. At 0.111 s (t4), the NCB 1P and NCB 3P are opened, and at 0.126 s (t5), the NCB 2P is opened to interrupt the residual current. After all pole separation circuit breakers are opened, an opening command is given to the high-speed disconnector, and at 0.179 s (t6) the SB1P , SB 2P The circuit opens, and the faulty cable is disconnected from the DC system.

[0057] Then, at 0.23s(t7), NCB 1P NCB 2P NCB 3P The circuit is reclosed and CVCF is executed in 0.24s (t8). P DCAVR P APR P Restart it. At this time, the positive terminal cable of Line A was discharged due to an accident, so CVCF P and DCAVR P is a direct current i 2P , i 1P While controlling the voltage, the cable is charged, and the DC voltage v 1P It restores the voltage. And after the DC voltage reaches the rated value, 0.312s(t 10 ) CB1 P By reclosing, DCAVR P It will reconnect to land-based system #1. CVCF P SCB at 0.29s(t9) P The circuit is restarted, and then the AC voltage is increased over 150ms. This creates a CVCF P DC current i 2p and DCAVR P DC current i 1p It has recovered. Also, the CVCF on the negative electrode side during the period from the positive electrode accident to the resumption of power transmission N DCAVR N APR N The system will not shut down and will continue to transmit power.

[0058] Figure 7(b) shows the voltage amplitude V of an offshore wind power generation facility. AC And the voltage amplitude V specified in the FRT (Fault Relay) requirements for wind power generation equipment FTR Therefore, the negative electrode continues to operate and outputs the rated AC voltage, while a drop in AC voltage occurs at the positive electrode, resulting in V AC The voltage drops to 0.5 pu. After power is restored, it recovers to 1 pu. Since this voltage drop lasts for less than 0.9 seconds, V AC、 V FTRThe wind power generation facilities will continue to operate as the demand exceeds the limit.

[0059] [1-4. Effects of the First Embodiment] (1) The DC power transmission system 1 of this embodiment is a bipolar DC power transmission system 1 connected to a renewable energy power generation system 100, comprising: a series multiple transformer 10 connected to the renewable energy power generation system 100; a positive-side power converter 20 and a negative-side power converter 30 connected to the series multiple transformer 10 respectively, which convert AC power generated by the renewable energy power generation system 100 into DC power; and a positive-side power converter that transmits the DC power converted by the positive-side power converter 20. The series multiple transformer 10 includes a DC transmission line LN-P, a negative-side DC transmission line LN-N that transmits DC power converted by the negative-side power converter 30, a positive-side power converter 60 connected to the positive-side DC transmission line LN-P and converting the transmitted DC power into AC power, and a negative-side power converter 70 connected to the negative-side DC transmission line LN-N and converting the transmitted DC power into AC power. The AC terminals of the positive-side power converter 20 and the AC terminals of the negative-side power converter 30 are connected to the windings of the series multiple transformer 10, respectively.

[0060] (2) The series multiplex transformer 10 is configured such that the voltages generated in the secondary and tertiary windings are multiplexed in series on the primary winding side, the renewable energy generation system 100 is connected to the primary winding, the AC terminals of the positive-side power converter 20 are connected to the secondary winding, and the AC terminals of the negative-side power converter 30 are connected to the tertiary winding.

[0061] As described above, in the DC power transmission system 1, the renewable energy generation system 100 is connected to a bipolar power converter via a series multiple transformer 10. On the renewable energy generation system 100 side of the series multiple transformer 10, the AC voltage of the bipolar power converter is added. For example, if a ground fault occurs on the positive side, the faulted section on the positive side is disconnected from the DC power transmission equipment 200. However, since power transmission continues on the negative side, the AC voltage of the negative side power converter 30 is maintained at the rated voltage. Therefore, the operation of the renewable energy generation system 100 can be maintained.

[0062] Here, the relationship between the operating time of the renewable energy power generation system 100 and the control of the operation of the power converter by the control device 400 will be explained. Figure 9 is a diagram showing an example of the voltage drop tolerance specifications for the renewable energy power generation system 100 that constitutes the DC power transmission system 1. In the example shown in Figure 9, even if the voltage established by the power converter drops to 0V due to an accident occurring in the DC power transmission system 1, the renewable energy power generation system 100 can continue to operate for an operating time of 150ms.

[0063] On the other hand, in the DC power transmission system 1, the series multiple transformer 10 is configured such that the sum of the voltages of the positive-side power converter 20 and the negative-side power converter 30 is applied to the renewable energy power generation system 100 connected to the primary side. Therefore, for example, if a ground fault occurs on the positive side, the faulted section on the positive side is disconnected from the DC power transmission equipment 200. However, since power transmission continues on the negative side, the AC voltage of the negative-side power converter 30 is maintained at the rated voltage. Therefore, the power supply voltage of the renewable energy power generation system 100 appears as a 50% voltage drop, and the voltage does not drop to 0V. If the voltage drop is 50%, the renewable energy power generation system 100 can continue operating in accordance with the FRT requirements of the grid connection regulations as long as the duration of the voltage drop is within 0.9 seconds.

[0064] As described above, the DC power transmission system 1 can interrupt DC fault currents while maintaining the operation of the renewable energy power generation system 100. This prevents long-term shutdown of the renewable energy power generation system 100 and avoids the loss of opportunities for power generation and DC power transmission by the renewable energy power generation system 100.

[0065] (3) A positive-side short-circuit switch 41 is provided on the AC system side of the positive-side power converter 20, which is capable of short-circuiting the AC terminals of the positive-side power converter 20, and a negative-side short-circuit switch 42 is provided on the AC system side of the negative-side power converter 30, which is capable of short-circuiting the AC terminals of the negative-side power converter 30.

[0066] Short-circuit switches 41 and 42 allow the current flowing from the renewable energy generation system 100 to the fault point to be bypassed. For example, if a ground fault occurs on the positive side, the faulted section on the positive side is disconnected from the DC power transmission equipment 200. At this time, the AC voltage of the positive-side power converter 20 can be reduced to zero by closing the positive-side short-circuit switch 41.

[0067] (4) On the AC system side of the positive-side power converter 60, a positive-side AC circuit breaker 81 is provided that can interrupt the positive-side power converter 60 and the land system 300 connected to the positive-side power converter 60. On the AC system side of the negative-side power converter 70, a negative-side AC circuit breaker 82 is provided that can interrupt the negative-side power converter 70 and the land system 300 connected to the negative-side power converter 70.

[0068] The AC circuit breakers 81 and 82 make it possible to interrupt short-circuit currents to the AC land-based power system 300. For example, if a ground fault occurs on the positive terminal side, the faulted section on the positive terminal side is disconnected from the DC power transmission equipment 200. At this time, the fault current flowing from the land-based power system 300 to the positive terminal power converter 60 can be interrupted by the positive terminal AC circuit breaker 81.

[0069] (5) The positive electrode power converter 20 is provided with a positive electrode polarity isolation switch NCB1P, the positive electrode power converter 60 is provided with a positive electrode polarity isolation switch NCB2P, the negative electrode power converter 3 is provided with a negative electrode polarity isolation switch NCB1N, and the negative electrode power converter 70 is provided with a negative electrode polarity isolation switch NCB1N.

[0070] The pole isolation switch NCB can interrupt residual current. For example, if a ground fault occurs on the positive side, the faulted section on the positive side is disconnected from the DC power transmission equipment 200. At this time, by switching off the positive side pole isolation switch NCB1P and the positive side pole isolation switch NCB2P, it is possible to interrupt residual current and disconnect the positive side DC power transmission line LN-P including the fault point.

[0071] (6) A positive-side disconnector SP1 is provided at one end of the positive-side DC transmission line LN-P, and a positive-side disconnector SP2 is provided at the other end. A negative-side disconnector SN1 is provided at one end of the negative-side DC transmission line LN-N, and a negative-side disconnector SN2 is provided at the other end.

[0072] The disconnector S can disconnect the DC transmission line including the fault point. For example, if a ground fault occurs on the positive terminal side, the faulted section on the positive terminal side is disconnected from the DC transmission equipment 200. At this time, by shutting off the positive terminal side disconnectors SP1 and SP2, the positive terminal side DC transmission line LN-P can be disconnected, making it possible to restart the faulted terminal side.

[0073] (7) A control device 400 for the DC power transmission system 1 described in (1) to (6) above, wherein during steady-state operation, the DC power transmission system 1 has the positive side short-circuit switch 41 and the negative side short-circuit switch 42 open, and the positive side AC circuit breaker 81, the negative side AC circuit breaker 82, the positive side polarity separation switch NCB1P, the positive side polarity separation switch NCB2P, the negative side polarity separation switch NCB1N, the negative side polarity separation switch NCB2N, the positive side disconnector SP1, the positive side disconnector SP2, the negative side disconnector SN1, and the negative side disconnector SN2 closed, and the control device 400 detects a ground fault by detecting a voltage drop at the DC terminals of the positive side power converter 20 and the positive side power converter 60, or the negative side power converter 30 and the negative side power converter 70. The system includes: a fault detection unit 401; a gate blocking unit 402 that gate blocks the power converter of the pole where a fault detected by the fault detection unit 401 has occurred; a short-circuit unit 403 that closes the short-circuit switch of the pole where the fault occurred after the gate blocking is complete, thereby short-circuiting the AC terminal of the power converter of the pole where the fault occurred; a fault current interruption unit 404 that interrupts the AC circuit breaker of the pole where the fault occurred after the gate blocking is complete; a residual current interruption unit 405 that interrupts the pole separation switch of the pole where the fault occurred after the DC current at the DC terminal of the power converter has fallen below a predetermined level due to the short-circuit unit 403 and the fault current interruption unit 404; and a fault section disconnection unit 406 that interrupts the disconnector SN of the pole where the fault occurred after the residual current has been interrupted by the residual current interruption unit 405.

[0074] The configuration of the control device 400 allows for the sequential operation of the gate block of the power converter, the closing of the short-circuit switch, the tripping of the AC circuit breaker, the tripping of the station isolation switch, and the tripping of the disconnector at the faulty pole when a fault occurs. By performing the appropriate operations at the faulty pole in the correct order, it becomes possible to disconnect the DC transmission line including the fault point and restart it.

[0075] (8) The power converter is further comprising a deblocking unit for deblocking the power converter that has been gate-blocked by the gate block unit 402, a short-circuit switch interruption unit for interrupting the short-circuit switch that was turned on after the fault occurred, and a switch turning unit for turning on the AC circuit breaker, pole-separating switch, and disconnector that were interrupted after the fault occurred.

[0076] The configuration of the power transmission restoration unit 407 allows for the resumption of power transmission in a DC transmission line, including the fault location, after the fault has been resolved.

[0077] [2. Other Embodiments] While several embodiments of the present invention have been described herein, these embodiments are presented as examples and are not intended to limit the scope of the invention. The embodiments described above can be implemented in various other forms, and various omissions, substitutions, and modifications can be made without departing from the scope of the invention. These embodiments and their variations are included within the scope and essence of the invention, as well as within the scope of the claims and its equivalents.

[0078] As described above, the land grid 300 connected to the renewable energy power generation system 100 may have two or more terminals. Figure 10 shows a DC power transmission system 1 including two-terminal land grids 301 and 302. In the DC power transmission system 1 of Figure 10, the positive-side power converter 20 is connected to the positive-side DC transmission line LN-P1, the positive-side DC transmission line LN-P2, and the neutral lines 51 and 52. The negative-side power converter 30 is connected to the positive-side DC transmission line LN-N1, the positive-side DC transmission line LN-N2, and the neutral lines 51 and 52.

[0079] The positive-side power converter 20 is connected to the positive-side power converter 61 via the positive-side DC transmission line LN-P1 and the neutral line 51, and the positive-side power converter 62 is connected via the positive-side DC transmission line LN-P2 and the neutral line 52. The negative-side power converter 30 is connected to the negative-side power converter 71 via the negative-side DC transmission line LN-N1 and the neutral line 51, and the negative-side power converter 72 is connected via the negative-side DC transmission line LN-N2 and the neutral line 52.

[0080] A positive electrode isolation switch NCB1P is provided between the positive electrode power converter 20 and the neutral wires 51 and 52, a positive electrode isolation switch NCB2P is provided between the neutral wire 51 and the positive electrode power converter 61, and a positive electrode isolation switch NCB3P is provided between the neutral wire 52 and the positive electrode power converter 62. A negative electrode isolation switch NCB1N is provided between the negative electrode power converter 30 and the neutral wires 51 and 52, a negative electrode isolation switch NCB2N is provided between the neutral wire 51 and the negative electrode power converter 71, and a negative electrode isolation switch NCB3N is provided between the neutral wire 52 and the negative electrode power converter 72.

[0081] Positive side DC power line LN-P1 is equipped with positive side disconnectors SP1 and SP2 at both ends, and positive side disconnectors SP3 and SP4 at both ends of positive side DC power line LN-P2. Negative side DC power line LN-N1 is equipped with negative side disconnectors SN1 and SN2 at both ends, and negative side disconnectors SN3 and SN4 at both ends of negative side DC power line LN-N2.

[0082] A positive AC circuit breaker 81a is connected to the AC system side of the positive power converter 61, and a positive AC circuit breaker 82a is connected to the AC system side of the negative power converter 71. A positive AC circuit breaker 81b is connected to the AC system side of the positive power converter 62, and a positive AC circuit breaker 82b is connected to the AC system side of the negative power converter 72.

[0083] The system switching flow for a DC power transmission system including two-terminal land-based systems 301 and 302 having the above configuration is as described in [1-3. Simulation of the First Embodiment]. If there are two or more land-based systems, power can also be transmitted to the land-based system 301 side after disconnecting the faulty cable (positive-side DC transmission line LN-P2 in the example of Figure 10) from the DC power transmission equipment 200. [Explanation of Symbols]

[0084] 1: DC power transmission system 100: Renewable energy power generation systems 200: DC power transmission equipment 300, 301, 302: Terrestrial systems 10: Series multiplex transformer 20: Positive-side power converter 30: Negative-side power converter 41: Positive side short-circuit switch 42: Negative side short-circuit switch 50, 51, 52: Neutral line LN-P, LN-P1, LN-P2: Positive side DC transmission line LN-N, LN-N1, LN-N2: Negative side DC transmission line 60, 61, 62: Positive side power converter 70, 71, 72: Negative-side power converter 81, 81a, 81b: AC circuit breaker on the positive side 82, 82a, 82b: Negative-side AC circuit breaker SN1, SN2, SN3, SN4: Negative side disconnector SP1, SP2, SP5, SP4: Positive side disconnector NCB1N, NCB2N, NCB3N: Negative side polarity isolation switch NCB1P, NCB2P, NCB3P: Positive side polarity isolation switch 400: Control device 401: Accident Detection Department 402: Gate block section 403: Short circuit 404: Fault current interruption unit 405: Residual current interruption section 406: Accident section derailment 407: Power transmission restoration section

Claims

1. In a bipolar DC power transmission system connected to a renewable energy power generation system, A series multiplex transformer connected to the aforementioned renewable energy power generation system, A first positive-side power converter and a first negative-side power converter are connected to the series multiple transformer respectively and convert the AC power generated by the renewable energy power generation system into DC power, A positive-side DC power transmission line that transmits the DC power converted by the first positive-side power converter, A negative-side DC power transmission line that transmits the DC power converted by the first negative-side power converter, A second positive-side power converter connected to the positive-side DC power transmission line, which converts the transmitted DC power into AC power, It includes a second negative-side power converter connected to the negative-side DC power transmission line, which converts the transmitted DC power into AC power, The AC terminals of the first positive-side power converter and the AC terminals of the first negative-side power converter are respectively connected to the windings of the series multiplex transformer. DC power transmission system.

2. The aforementioned series multiplexer is configured such that the voltages generated in the secondary and tertiary windings are multiplexed in series on the primary winding side. The renewable energy generation system is connected to the primary winding. The AC terminal of the first positive-side power converter is connected to the secondary winding. The AC terminal of the first negative-side power converter is connected to the tertiary winding. The DC power transmission system according to claim 1.

3. A positive-side short-circuit switch is provided on the AC system side of the first positive-side power converter, which is capable of short-circuiting the AC terminals of the first positive-side power converter. A negative-side short-circuit switch is provided on the AC system side of the first negative-side power converter, which is configured to short-circuit the AC terminals of the first negative-side power converter. The DC power transmission system according to claim 1 or 2.

4. On the AC system side of the second positive-side power converter, a positive-side AC circuit breaker is provided that is capable of interrupting the second positive-side power converter and the AC system connected to the second positive-side power converter. On the AC system side of the second negative-side power converter, a negative-side AC circuit breaker is provided, which is capable of interrupting the second negative-side power converter and the AC system connected to the second negative-side power converter. The DC power transmission system according to claim 3.

5. The first positive-side power converter is provided with a first positive-side polarity isolation switch, and the second positive-side power converter is provided with a second positive-side polarity isolation switch. The first negative-side power converter is provided with a first negative-side polarity isolation switch, and the second negative-side power converter is provided with a second negative-side polarity isolation switch. The DC power transmission system according to claim 4.

6. A first positive-side disconnector is provided at one end of the positive-side DC transmission line, and a second positive-side disconnector is provided at the other end. A first negative-side disconnector is provided at one end of the negative-side DC transmission line, and a second negative-side disconnector is provided at the other end. The DC power transmission system according to claim 5.

7. A control device for a DC power transmission system according to claim 6, The aforementioned DC power transmission system, during steady-state operation, The positive side short-circuit switch and the negative side short-circuit switch are opened. The positive-side AC circuit breaker, the negative-side AC circuit breaker, the first positive-side polarity separation switch, the second positive-side polarity separation switch, the first negative-side polarity separation switch, the second negative-side polarity separation switch, the first positive-side disconnector, the second positive-side disconnector, the first negative-side disconnector, and the second negative-side disconnector are all closed. The control device is An fault detection unit that detects a ground fault by detecting a voltage drop at the DC terminals of the first positive-side power converter and the second positive-side power converter, or the first negative-side power converter and the second negative-side power converter, The aforementioned fault detection unit includes a gate block unit that gate blocks the power converter of the pole where the fault detected by the fault detection unit occurred, After the completion of the gate block, the short-circuit switch of the faulty pole is turned on to short-circuit the AC terminal of the power converter on the faulty pole, After the completion of the gate block, a fault current interruption unit is provided to interrupt the AC circuit breaker of the pole where the fault occurred, After the DC current at the DC terminal of the power converter falls below a predetermined level due to the short-circuit section and the fault current interruption section, a residual current interruption section interrupts the pole separation switch of the pole where the fault occurred, The system includes a fault section disconnection unit that disconnects the disconnector of the pole where the fault occurred after the residual current has been interrupted by the residual current interruption unit. Control device for a DC power transmission system.

8. A deblocking unit that deblocks the power converter that has been gate-blocked by the gate-blocking unit, A short-circuit switch interruption unit that interrupts the short-circuit switch that was turned on after the accident occurred, A switch-on unit for closing the AC circuit breaker, the polarity separation switch, and the disconnector that were shut off after the accident occurred, A control device for a DC power transmission system according to claim 7, further comprising:

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