DC power transmission system and DC power transmission method

Abstract

The objective is to provide a DC power transmission system and a DC power transmission method that can suitably absorb surplus electricity from renewable energy sources generated as a result of an accident. [Solution] The DC power transmission system of this embodiment comprises a DC power transmission line, a transmitting-side power converter, a receiving-side power converter, and an energy absorption device. The DC power transmission line transmits DC power. The transmitting-side power converter converts AC power supplied by a renewable energy source connected to the AC power transmission system on the transmitting side into DC power and outputs it to the DC power transmission line. The receiving-side power converter converts the DC power transmitted by the DC power transmission line into AC power and supplies it to the AC power transmission system on the receiving side, including a load connected to the receiving side. The energy absorption device absorbs surplus power, at least when the AC power supplied by the renewable energy source becomes surplus power due to the transmission-side power converter stopping operation as a result of an accident.

Description

【Technical Field】 【0001】 Embodiments of the present invention relate to a DC power transmission system and a DC power transmission method. 【Background Art】 【0002】 In recent years, for example, the active consideration and introduction of renewable energy power sources, represented by offshore wind power generation in which power is generated by wind turbines installed at sea, have been promoted. Renewable energy power sources are unevenly distributed in locations suitable for power generation and are located far from large power demand areas that consume large amounts of power. Therefore, the development of a high-voltage direct current (HVDC) power transmission system for efficiently transmitting power from renewable energy power sources over long distances has become important and has attracted attention. High-voltage direct current power transmission systems (hereinafter referred to as "DC power transmission systems") include various system configurations such as multi-terminal configurations, configurations operating with two terminals among multi-terminals, and originally two-terminal configurations. 【0003】 Incidentally, in a DC power transmission system, for example, if an accident occurs in a DC transmission line or in another converter station connected to a multi-terminal DC power transmission system, the AC-DC converter in a converter station connected to the AC grid of a renewable energy power source may stop working due to the effects of this accident. In this case, the renewable energy power source is disconnected from the transmission system of the DC power transmission system and becomes a power source system that operates independently. At this time, the generators (for example, wind turbines) of the renewable energy power source system that have been disconnected from the DC power transmission system will continue to operate until it detects that they are operating independently, and the power generated during this period of continued operation becomes surplus power that has no destination (hereinafter referred to as "surplus power"). This surplus power can cause overvoltage in the AC grid of the renewable energy power source. And when overvoltage occurs, it becomes impossible to restart the AC-DC converter that has stopped working, and it becomes impossible to resume power transmission in the DC power transmission system. In other words, the entire system of renewable energy power sources and DC power transmission systems will experience a prolonged period of power transmission being stopped. 【0004】 In this regard, for example, Patent Document 1 proposes a DC power transmission system that consumes surplus power generated by accidents, etc., through an energy consumption device. However, the configuration proposed in Patent Document 1, which uses a so-called DC chopper circuit as the energy consumption device, is a countermeasure for DC systems in DC power transmission systems, and is therefore not suitable as a countermeasure for surplus power in AC systems, such as when the AC / DC converter at a converter station connected to an AC system has stopped operating, as described above. Furthermore, Patent Document 1 also proposes a configuration in which a so-called AC chopper circuit is connected to an AC system as an energy consumption device, and it is shown that the control procedure in the event of an accident is equivalent to the control procedure when an energy consumption device is connected to a DC system (control by a higher-level control device of the DC power transmission system). However, when surplus power is generated due to an accident, etc., the time until overvoltage occurs is short, that is, it reaches overvoltage very quickly. For this reason, the control procedure by a higher-level control device shown in Patent Document 1 may not be able to suppress the overvoltage to a voltage below the voltage that would cause problems when restarting the AC / DC converter. [Prior art documents] [Patent Documents] 【0005】 [Patent Document 1] International Publication No. 2024 / 042595 [Overview of the project] [Problems that the invention aims to solve] 【0006】 The problem that the present invention aims to solve is to provide a DC power transmission system and a DC power transmission method that can suitably absorb surplus power from renewable energy sources generated due to the effects of an accident in a multi-terminal DC power transmission system. [Means for solving the problem] 【0007】 The DC power transmission system of the embodiment comprises at least one DC transmission line, a transmitting-side power converter, a receiving-side power converter, and an energy absorption device. The DC transmission line transmits DC power. The transmitting-side power converter converts AC power supplied by a renewable energy source connected to the AC power transmission system on the transmitting side into DC power and outputs it to the DC transmission line. The receiving-side power converter converts the DC power transmitted by the DC transmission line into AC power and supplies it to the AC power transmission system on the receiving side, including a load connected to the receiving side. The energy absorption device is connected between the AC power transmission system on the transmitting side and the transmitting-side power converter and absorbs the AC power supplied by the renewable energy source. The energy absorption device absorbs surplus power when the AC power supplied by the renewable energy source becomes surplus power due to the transmission-side power converter stopping operation as a result of an accident occurring in any of the components that transmit power. The energy absorption device begins absorbing the surplus power when it detects either or both an overvoltage generated in the AC power transmission system on the transmission side due to the surplus power, or the shutdown of the power converter on the transmission side, and ends absorbing the surplus power after the shutdown of the renewable energy power source or the reduction of the AC power supplied has been completed, and before the power converter on the transmission side restarts. [Brief explanation of the drawing] 【0008】 [Figure 1] A diagram showing an example of the configuration of a DC power transmission system according to the first embodiment. [Figure 2] A sequence diagram showing an example of the flow of operations for controlling each component when an accident occurs in a DC power transmission system according to the first embodiment. [Figure 3] A sequence diagram showing an example of another operational flow for controlling each component in a DC power transmission system according to the first embodiment. [Figure 4] A diagram showing an example of the configuration of a DC power transmission system according to the second embodiment. [Figure 5]A sequence diagram showing an example of the flow of operations for controlling each component when an accident occurs in a DC power transmission system according to the second embodiment. [Modes for carrying out the invention] 【0009】 The DC power transmission system and DC power transmission method of the embodiment will be described below with reference to the drawings. 【0010】 (First embodiment) [Configuration of a DC power transmission system] The following describes an example of the configuration of a DC power transmission system. Figure 1 is a diagram showing an example of the configuration of a DC power transmission system according to the first embodiment. Figure 1 shows an example of a multi-terminal (two-terminal-one-terminal) DC power transmission system (High Voltage Direct Current (HVDC) system) that converts three-phase AC power supplied from each of two renewable energy sources (renewable energy sources RE-A and RE-B) into DC power for transmission, converts it back into three-phase AC power for supply to a single AC power transmission system (AC power transmission system TS-C). In Figure 1, renewable energy source RE-A and renewable energy source RE-B are, for example, renewable energy sources equipped with wind turbines installed offshore that supply generated power (AC power). 【0011】 The DC power transmission system 1 includes, for example, three AC current interruption devices 10 (AC current interruption devices 10-A, 10-B, and 10-C), three power converters 20 (power converters 20-A, 20-B, and 20-C), two DC power transmission lines LN (DC power transmission lines LN-AB and LN-AC), four DC current interruption devices 30 (DC current interruption devices 30-1 to 30-4), and two AC energy consumption devices 40 (AC energy consumption devices 40-A and 40-B). The DC power transmission system 1 shown in Figure 1 shows an example configuration in which an AC power transmission system TS-A is located between a renewable energy power source RE-A and an AC current interruption device 10-A, and an AC power transmission system TS-B is located between a renewable energy power source RE-B and an AC current interruption device 10-B. Figure 1 shows a higher-level control device 100 as an example of a control device that controls each component of the DC power transmission system 1. 【0012】 The higher-level control device 100 is a management device that controls the transmission of power in the DC power transmission system 1 by controlling the AC current interruption device 10, the DC current interruption device 30, and the AC energy consumption device 40, which are all provided in the DC power transmission system 1. The higher-level control device 100 may also control the transmission of power in the DC power transmission system 1 by controlling the renewable energy power source RE, the power converter 20, and the AC power transmission system TS, which are all provided in the DC power transmission system 1. 【0013】 In DC power transmission system 1, power is transmitted from the transmitting sides of renewable energy power sources RE-A and RE-B to the receiving side of AC power transmission system TS-C via DC transmission line LN-AC. Figure 1 shows the power flow direction of the power transmitted (supplied) from the transmitting side to the receiving side. More specifically, power from the transmitting side renewable energy power source RE-A is supplied to AC power transmission system TS-A, and from AC power transmission system TS-A, it is supplied to the DC transmission line LN-AC side via AC current interruption device 10-A and power converter 20-A, and then transmitted to the receiving side by DC transmission line LN-AC. Power from the renewable energy source RE-B on the transmission side is supplied to the AC transmission system TS-B, and from the AC transmission system TS-B, it is supplied to the DC transmission line LN-AB side via the AC current interruption device 10-B and the power converter 20-B, and further supplied to the DC transmission line LN-AC, and transmitted to the receiving side by the DC transmission line LN-AC. The power transmitted by the DC transmission line LN-AC is supplied to the AC transmission system TS-C on the receiving side via the power converter 20-C and the AC current interruption device 10-C. 【0014】 In the following explanation, the renewable energy power source RE-A on the transmission side will also be referred to as "transmission side system A," and the renewable energy power source RE-B on the transmission side will also be referred to as "transmission side system B." Furthermore, the AC transmission system TS-C on the receiving side will also be referred to as "receiving side system C." 【0015】 Each of the renewable energy sources RE is a power generation facility that generates alternating current electricity using a wind turbine that rotates according to the wind speed. Each of the renewable energy sources RE is, for example, an offshore power generation facility installed at sea. Renewable energy source RE-A supplies the generated alternating current electricity to the alternating current transmission system TS-A. Renewable energy source RE-B supplies the generated alternating current electricity to the alternating current transmission system TS-B. 【0016】 Renewable energy power source RE-A and renewable energy power source RE-B are examples of "renewable energy power sources." 【0017】 Each of the AC power transmission systems TS is a power transmission facility that transmits AC power. Each of the AC power transmission systems TS transmits the AC power supplied or transmitted to one end (input end) to the other end (output end). AC power transmission system TS-A transmits the AC power supplied by the renewable energy power source RE-A to the AC current interruption device 10-A. As a result, the AC power generated by the renewable energy power source RE-A is supplied to the power converter 20-A via the AC current interruption device 10-A. AC power transmission system TS-B transmits the AC power supplied by the renewable energy power source RE-B to the AC current interruption device 10-B. As a result, the AC power generated by the renewable energy power source RE-B is supplied to the power converter 20-B via the AC current interruption device 10-B. In both AC power transmission systems TS-A and TS-B, the frequency of the AC power is maintained at a constant value by the operation of the power converter 20. AC transmission systems TS-A and TS-B are each located offshore, for example, at an offshore converter station (hereinafter referred to as the "offshore converter station") near the corresponding renewable energy source RE. AC transmission system TS-C is transmitted by DC transmission line LN-AC and transmits AC power supplied by power converter 20-C via AC current interruption device 10-C to the connected consumer side. In this way, AC power is supplied to the consumers of AC transmission system TS-C. 【0018】 Transmission system A, AC transmission system TS-A, transmission system B, and AC transmission system TS-B are examples of "transmission-side AC transmission systems." Receiving system C and AC transmission system TS-C are examples of "receiving-side AC transmission systems including loads connected to the receiving side." 【0019】 Each of the AC current interruption devices 10 is a circuit breaker that interrupts the path where a fault occurs when a power fluctuation (hereinafter simply referred to as "fault") occurs in the path of the connected AC current (AC power) due to an accident or the like. AC current interruption device 10-A is connected to the AC transmission line between the AC transmission system TS-A and the power converter 20-A, and in normal (steady) power transmission in the DC transmission system 1, it is supplied by the renewable energy power source RE-A and transmits the AC power transmitted by the AC transmission system TS-A to the power converter 20-A. AC current interruption device 10-A electrically interrupts (disconnects) the connection point (connection terminal) on the side where the fault occurred when a fault occurs in one or more of the following: the AC transmission system TS-A (which may include the renewable energy power source RE-A), the power converter 20-A and its DC side, and the connected AC transmission line. AC current interruption device 10-B is connected to the AC transmission line between the AC transmission system TS-B and the power converter 20-B. During steady-state power transmission in the DC transmission system 1, it is supplied by the renewable energy power source RE-B and transmits the AC power transmitted by the AC transmission system TS-B to the power converter 20-A. AC current interruption device 10-B electrically disconnects the connection terminal on the side where a fault occurs when one or more of the following occur: the AC transmission system TS-B (which may include the renewable energy power source RE-B), the power converter 20-B and its DC side, or the connected AC transmission line. AC current interruption device 10-C is connected to the AC transmission line between the power converter 20-C and the AC transmission system TS-C. During steady-state power transmission in the DC transmission system 1, it is supplied by the power converter 20-C and transmits the AC power to the AC transmission system TS-C. The AC current interruption device 10-C electrically disconnects the connection terminal on the side where a fault occurs when a fault occurs in one or more of the following: the power converter 20-C and its DC side, the AC power transmission system TS-C, or the connected AC power transmission line. The interruption operation in each AC current interruption device 10 is controlled, for example, from a control device (not shown) located inside or near each AC current interruption device 10. The interruption operation in each AC current interruption device 10 may also be controlled, for example, from a higher-level control device 100.Each of the AC current interrupting devices 10-A and 10-B is, for example, installed at an offshore substation near the corresponding renewable energy power source RE, that is, located offshore. Part or all of the AC current interrupting device 10 may be omitted according to the configuration of the DC power transmission system 1 or the requirements for continuing operation during an accident, etc. 【0020】 Each of the power converters 20 is an AC-DC converter that converts AC power (AC current) input to the AC terminals into DC power (DC current) and outputs it to the DC terminals, or a DC-AC converter that converts DC power input to the DC terminals into AC power and outputs it to the AC terminals. Each of the power converters 20 is, for example, an AC / DC converter or a DC / AC converter. As the power converter 20, for example, a modular multilevel converter (MMC) having arms in which a large number of unit converters equipped with capacitors are connected in series is used. The power converter 20-A converts the AC power supplied by the renewable energy power source RE-A and transmitted through the AC power transmission system TS-A and the AC current interrupting device 10-A into DC power and outputs it to the DC power transmission line LN-AC side. The power converter 20-B converts the AC power supplied by the renewable energy power source RE-B and transmitted through the AC power transmission system TS-B and the AC current interrupting device 10-B into DC power and outputs it to the DC power transmission line LN-AB side. The power converter 20-C converts the DC power transmitted from the DC power transmission line LN-AC side into AC power and outputs it to the AC current interrupting device 10-C. As a result, the AC power converted by the power converter 20-C is transmitted (supplied) to the AC power transmission system TS-C. Each of the power converters 20 is, for example, installed at an offshore substation near the corresponding renewable energy power source RE, that is, located offshore. 【0021】 Each of the power converters 20-A and 20-B is an example of a "power converter on the power transmission side". The power converter 20-C is an example of a "power converter on the power reception side". 【0022】 Each of the DC current interruption devices 30 is a circuit breaker that interrupts the path where a fault occurs when a fault occurs in the path of the connected DC current (DC power). DC current interruption device 30-1 is connected between DC transmission line LN-AB and DC transmission line LN-AC, and in steady-state power transmission in DC power transmission system 1, it transmits the DC power transmitted by DC transmission line LN-AB to DC transmission line LN-AC. DC current interruption device 30-1 electrically disconnects the connection terminal on the side where the fault occurred when a fault occurs in one or more of the DC transmission line LN-AB, power converter 20-A, and DC transmission line LN-AC. The DC current interruption device 30-2 is connected between the power converter 20-B and the DC transmission line LN-AB. During steady-state power transmission in the DC power transmission system 1, it transmits the DC power output from the DC terminal of the power converter 20-B to the DC transmission line LN-AB. When a fault occurs in one or more of the power converter 20-B and / or the DC transmission line LN-AB, the DC current interruption device 30-2 electrically disconnects the connection terminal on the side where the fault occurred. The DC current interruption device 30-3 is connected between the power converter 20-A and the DC current interruption device 30-1 and the DC transmission line LN-AC. In steady-state power transmission in the DC power transmission system 1, it transmits DC power output from the DC terminal of power converter 20-A, and / or DC power output from the DC terminal of power converter 20-B and transmitted via the DC current interruption device 30-2, the DC transmission line LN-AB, and the DC current interruption device 30-1 to the DC transmission line LN-AC side. The DC current interruption device 30-3 electrically disconnects the connection terminal on the side where a fault occurs when a fault occurs in one or more of the power converter 20-A and the DC transmission line LN-AC. The DC current interruption device 30-4 is connected between the DC transmission line LN-AC and the power converter 20-C. During steady-state power transmission in the DC power transmission system 1, it transmits the DC power transmitted by the DC transmission line LN-AC to the DC terminals of the power converter 20-C. When a fault occurs in one or more of the DC transmission line LN-AC and / or power converter 20-C, the DC current interruption device 30-4 electrically disconnects the connection terminal on the side where the fault occurred.The tripping operation of each DC current interruption device 30 is controlled, for example, from a control device (not shown) located inside or near each DC current interruption device 30. The tripping operation of each DC current interruption device 30 may also be controlled, for example, from a higher-level control device 100. Some or all of the DC current interruption devices 30 may be omitted depending on the configuration of the DC power transmission system 1 and the requirements for continuing operation in the event of an accident. 【0023】 Incidentally, each renewable energy power source RE, which is a wind power generation facility, has the inertia of a wind turbine, so even if an accident occurs in any of the paths through which power is transmitted in the DC power transmission system 1, it is not possible to immediately stop the power generation operation in response to the accident. For this reason, if an accident occurs in any of the paths through which power is transmitted in the DC power transmission system 1, the renewable energy power source RE will be disconnected from the transmission system of the DC power transmission system 1 and will become a power source system that operates independently. The wind turbines of the renewable energy power source RE, which have been disconnected from the DC power transmission system 1, will continue to operate until it is detected that they are operating independently, and the power generated during this period of continued operation will become surplus power that has no destination (hereinafter referred to as "surplus power"). This surplus power can cause overvoltage to occur in the AC power transmission system TS of the renewable energy power source RE. Furthermore, if an overvoltage occurs, the power converter 20, which had stopped operating, cannot be restarted, making it impossible to resume power transmission in the DC power transmission system 1. As a result, the DC power transmission system 1 as a whole experiences a prolonged period of power transmission being interrupted. For this reason, in the DC power transmission system 1, when an accident occurs in any of the power transmission paths in the DC power transmission system 1, the power generated by the renewable energy power source RE, which has been disconnected from the DC power transmission system 1, is consumed (absorbed) as surplus power by the AC energy consumption device 40. 【0024】 Each AC energy consumption device 40 is an energy consumption device that consumes (absorbs) surplus AC power (excess power) at the location to which it is connected. In the DC power transmission system 1, the AC energy consumption devices 40 are connected to the transmission side system of each renewable energy power source RE. Each AC energy consumption device 40 is located, for example, at an offshore converter station near the corresponding renewable energy power source RE, i.e., offshore. In other words, the AC energy consumption devices 40 are connected to the transmission side of the AC power supplied by renewable energy power source RE-A (i.e., the transmission path of AC power in transmission side system A) and to the transmission side of the AC power supplied by renewable energy power source RE-B (i.e., the transmission path of AC power in transmission side system B). More specifically, AC energy consumption device 40-A is connected to the location of the AC transmission line between the output terminal of the AC transmission system TS-A and the input side connection terminal of the AC current interruption device 10-A (which may also be the AC terminal of the power converter 20-A). AC energy consumption device 40-A consumes (absorbs) AC power supplied from renewable energy power source RE-A by letting it flow as surplus power when an accident occurs in one or more of the power transmission paths in the DC power transmission system 1. AC energy consumption device 40-B is connected to the AC transmission line between the output terminal of the AC transmission system TS-B and the input connection terminal of the AC current interruption device 10-B (which may also be the AC terminal of the power converter 20-B). AC energy consumption device 40-B consumes (absorbs) AC power supplied from renewable energy power source RE-B by letting it flow as surplus power when an accident occurs in one or more of the power transmission paths in the DC power transmission system 1. As a result, even if an accident occurs in one or more of the power transmission paths in the DC power transmission system 1, the power converter 20, which had stopped operating, can be quickly restarted without generating overvoltage in the AC system of the renewable energy power source RE (transmission side system A and transmission side system B), thereby preventing a power outage on the transmission side. 【0025】 Each AC energy consumption device 40 is a so-called AC chopper circuit, comprising, for example, a semiconductor switch section with two thyristors connected in parallel and in opposite directions, and a resistor. Figure 1 shows the configuration of the semiconductor switch section and resistor as an example of an equivalent circuit of the AC energy consumption device 40. Control of the AC energy consumption device 40 to a state of consuming surplus power is performed when an abnormality in AC power (increase in AC frequency, increase in AC current, increase in AC voltage) is detected in the connected AC transmission line, or when an abnormality in DC power (increase in DC voltage, increase in DC current, or decrease thereof) or a fault (such as a fault in the power converter 20 or AC transmission system TS) is detected in the connected DC transmission line. More specifically, the control of the semiconductor switch section of the AC energy consumption device 40 to either a closed state (on state) or an open state (off state) is performed, for example, based on a detection result obtained by a control device (not shown) of the corresponding power converter 20, which has detected an abnormality in the AC power of the connected AC transmission line or an abnormality in the DC power of the connected DC transmission line. The control of the semiconductor switch section of the AC energy consumption device 40 to either an on state or an off state may also be performed, for example, by control from a higher-level control device 100. The control of the semiconductor switch section of the AC energy consumption device 40 to either an on state or an off state may also be performed, for example, by the higher-level control device 100 operating in response to control from a control device (not shown) of the corresponding power converter 20. In other words, when the higher-level control device 100 receives a signal (information) indicating the detection of an accident transmitted by a control device (not shown) of the power converter 20, the higher-level control device 100 may control the semiconductor switch section to either an on state or an off state. The control of the semiconductor switch section of the AC energy consumption device 40 to either the ON or OFF state may be performed, for example, in accordance with information from a control device (not shown) located inside the AC current interruption device 10 or in the vicinity of the AC current interruption device 10 or the power converter 20.Control of the semiconductor switch section of the AC energy consumption device 40 to either the ON or OFF state may be performed, for example, in accordance with information from a control device (not shown) located inside or near the DC current interruption device 30 (DC current interruption devices 30-1, 30-2, 30-3, etc.) of the DC power transmission system 1. 【0026】 Since the configuration of each component of the AC energy consumption device 40 is the same as that of existing AC chopper circuits, a detailed explanation of the configuration of the AC energy consumption device 40 will be omitted. However, it is also possible to configure the device with a multi-stage semiconductor switch section connected in series, and to control the number of semiconductor switch sections that are turned on according to the amount of surplus power. In this case, the number of stages in which the semiconductor switch section is connected in series in the AC energy consumption device 40, in other words, the amount of energy consumed in the AC energy consumption device 40, the withstand capacity (current withstand capacity) of the thyristors and the withstand capacity of the resistors in the semiconductor switch section, are set to the maximum number of stages and withstand capacity determined in advance. Here, the maximum number of stages and withstand capacity are determined in advance, for example, by analyzing the amount of power generated by the renewable energy power source RE (amount of AC power supplied) and the circuit condition of the AC grid in advance. More specifically, based on the results of prior analysis, the expected overvoltage, that is, the amount of surplus power to be consumed (absorbed), and the time required to operate the AC energy consumption device 40 to consume (absorb) the surplus power (by controlling the semiconductor switch to the ON state) until the surplus power is reduced until no overvoltage occurs (until the overvoltage due to the surplus power falls below a predetermined threshold) are determined in advance, and the maximum value of this (a further margin may be provided) is taken. 【0027】 AC energy consumption device 40 is an example of an "energy absorption device" or "energy consumption device". 【0028】 With this configuration, in the DC power transmission system 1, if a fault occurs in one or more of the power transmission paths, the faulty component is electrically isolated by control from an unillustrated control device located inside or near each component, and the operation of related components connected to the isolated component is stopped. Then, the AC energy consumption device 40 restarts the related components that have stopped operating by consuming (absorbing) the generated power (AC power) from the wind turbine of the renewable energy power source RE, which has been disconnected from the power transmission system of the DC power transmission system 1 and is now operating independently, as surplus power. 【0029】 Control devices (including a higher-level control device 100), which are located inside or near each component, implement control functions that control the operation of each component, for example, by having a hardware processor execute a program (software) stored in memory (not shown). 【0030】 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 programs in memory, the hardware processor may be configured to directly embed programs within its circuits. In this case, the hardware processor reads and executes the programs embedded within the circuits, thereby realizing control functions that manage the operation of each component. A hardware processor is not limited to being configured as a single circuit; it may also be composed of multiple independent circuits combined to form a single hardware processor, realizing control functions that manage the operation of each component. Multiple components may also be integrated into a single hardware processor to realize control functions that manage the operation of each component. Multiple components may be integrated into a single dedicated LSI to implement control functions that manage the operation of each component.Here, the program (software) may be stored in advance in a storage device (a storage device equipped with a non-transient storage medium) such as a ROM (Read Only Memory), RAM (Random Access Memory), flash memory, or hard disk drive (HDD), or it may be stored in a removable storage medium (a non-transient storage medium) such as a DVD or CD-ROM, and the storage medium may be installed in the storage device of the respective component or a nearby control device (including a higher-level control device 100) by being mounted on a drive device of the respective component or a nearby control device (including a higher-level control device 100). If it is a control device (including a higher-level control device 100), the program (software) may be downloaded in advance via a network from, for example, a server device or other computer device including a storage device incorporated into a cloud computing system, and installed in the storage device. The program (software) installed in the storage device of the control device (including a higher-level control device 100) may be transferred to a processing circuit such as a hardware processor of the control device (including a higher-level control device 100) and executed. A control device (including a higher-level control device 100), not shown in the illustration, may be implemented, for example, by a server device or storage device incorporated into a cloud computing system. 【0031】 Each component or a nearby control device (including a higher-level control device 100) may detect whether or not a fault has occurred in each DC transmission line LN or component based on the DC voltage and / or AC voltage supplied to each DC transmission line LN or each component. Each component or a nearby control device (including a higher-level control device 100) may, for example, detect a rise or fall in voltage using a voltage detector (not shown) attached to each DC transmission line LN or each component, and detect whether or not a fault has occurred based on the detected value (voltage value) output by the voltage detector (not shown). Here, the voltage detector detects voltage at the location where it is installed and outputs information representing the detected voltage value (voltage value). The configuration for detecting voltage and outputting information representing the detected value (voltage value) is not limited to a voltage detector and may be any configuration. Furthermore, each component or a control device (including a higher-level control device 100) located nearby (not shown) may detect an increase in current, for example, by using the DC transmission line LN or a current detector (not shown) attached to each component, and detect whether or not an accident has occurred based on the information of the detected value (current value) output by the current detector (not shown). 【0032】 Each component or a nearby control device (including the higher-level control device 100) is an example of a "control device". 【0033】 In the configuration of the DC power transmission system 1 described above, the AC power transmission system TS, AC current interruption device 10, power converter 20, and AC energy consumption device 40 were described as being located at an offshore converter station near the corresponding renewable energy power source RE, i.e., at sea. This is advantageous because having each component located at an offshore converter station near the renewable energy power source RE allows for faster communication between components and faster control of their operation in the event of an accident. However, if it is not possible to place all components at an offshore converter station, some or all components may be located at a land-based converter station. In this case, the maintainability of the components located at the land-based converter station is improved. 【0034】 [Operation of DC power transmission system 1] The following describes an example of the operation of controlling each component in the DC power transmission system 1 when a fault occurs. For the sake of simplicity, the following description assumes that the transmission system B related to the renewable energy power source RE-B in the DC power transmission system 1 is omitted. The following description assumes that a fault occurs in one of the paths related to the renewable energy power source RE-A in the DC power transmission system 1, and describes the control of the power converter 20-A, the AC energy consumption device 40-A, and the renewable energy power source RE-A in this case. In the following description, the AC current interruption device 10 and the DC current interruption device 30 in the DC power transmission system 1 are assumed to suitably interrupt (disconnect) or connect the path where the fault occurred, and a detailed explanation of the control of the AC current interruption device 10 and the DC current interruption device 30 in this case is omitted. 【0035】 Figure 2 is a sequence diagram showing an example of the flow of operations for controlling each component when an accident occurs in the DC power transmission system 1 according to the first embodiment. In the operation of the DC power transmission system 1, the power converter 20 transmits the power generated by the renewable energy power source RE-A via the DC transmission line LN-AC and supplies it to the AC transmission system TS-C. 【0036】 In the steady-state power transmission conditions of DC power transmission system 1, the renewable energy power source RE-A (more specifically, a wind turbine) is in operation, and the power generated from the renewable energy power source RE-A (AC power) is supplied to the power converter 20-A. The power converter 20-A is operating and converting the AC power supplied from the renewable energy power source RE-A into DC power, which is then supplied to the DC transmission line LN-AC. 【0037】 Subsequently, for example, if an accident occurs at time t0, the power converter 20-A will stop operating in accordance with the accident that occurred, that is, due to the effects of the accident, and will enter a so-called gate-block state. The power converter 20-A will then send a command (hereinafter referred to as the "stop command") to the renewable energy power source RE-A to stop operation or to limit (partially stop) operation and suppress (reduce) the power generated. However, because there is a communication delay time (latency) before the stop command reaches the renewable energy power source RE-A, the power converter 20-A will continue to operate and supply generated power (AC power) to the power converter 20-A from the time the stop command is sent by the power converter 20-A until the operation of the renewable energy power source RE-A actually stops or the power generated is suppressed. This generated power becomes surplus power. 【0038】 Subsequently, the AC energy consumption device 40-A, for example, when it detects that the overvoltage due to surplus power from the renewable energy power source RE-A exceeds a predetermined threshold at time t1, that is, when it detects the occurrence of an overvoltage, starts energy consumption operation and begins consuming (absorbing) the surplus power from the renewable energy power source RE-A. The timing of the start of energy consumption in the AC energy consumption device 40-A is not limited to, for example, the result of the determination by the voltage detector (not shown) that has detected a rise in voltage and exceeded a predetermined threshold. For example, the AC energy consumption device 40-A may start energy consumption when it receives a signal (information) transmitted by the power converter 20-A indicating that the power converter 20-A has stopped when an accident occurs, or when it receives an instruction signal to instruct the start of energy consumption. In this case, in order to prioritize the speed of starting energy consumption, the AC energy consumption device 40-A starts energy consumption at the earlier of the determination based on the detection result of the voltage detector (not shown) and the information or instruction from the power converter 20-A. The amount of energy consumed (absorbed) by the AC energy consumption device 40-A at this time is, as described above, the amount of energy consumed (absorbed) corresponding to the amount of surplus power to be consumed (absorbed), which was determined in advance based on the results of prior analysis. As a result, the overvoltage supplied to the power converter 20-A (including the AC transmission system TS-A) due to the continued operation of the renewable energy power source RE-A is suppressed. 【0039】 Subsequently, for example, at time t2, when the renewable energy power source RE-A receives a stop command transmitted by the power converter 20-A, it stops operation or suppresses power generation in accordance with the received stop command. The stopping of operation or suppression of power generation in the renewable energy power source RE-A is not limited to following the stop command transmitted by the power converter 20-A. For example, if the renewable energy power source RE-A receives similar control from the higher-level control device 100 that controls the entire DC transmission system 1, it may stop operation or suppress power generation in accordance with this control. In this case, in order to prioritize the early implementation of stopping operation or suppressing power generation, the renewable energy power source RE-A starts stopping operation or suppressing power generation from the earlier of the stop command from the power converter 20-A and the control from the higher-level control device 100. 【0040】 Subsequently, the AC energy consumption device 40-A terminates its energy consumption operation, for example, at time t3, by consuming (absorbing) surplus power from the renewable energy power source RE-A. The timing of the termination of energy consumption in the AC energy consumption device 40-A, in other words, the time during which the AC energy consumption device 40-A consumes surplus power between time t1 and time t3, is, as described above, the timing of the time required to reduce the surplus power (the time to control the semiconductor switch section to the ON state in order to consume (absorb) the surplus power), which has been determined in advance based on the results of prior analysis. This timing can be determined by, for example, a timer that starts measuring from time t1. 【0041】 Subsequently, for example, at time t4, the power converter 20-A restarts. In other words, the power converter 20-A terminates the state (gate-block state) in which it was stopped in response to (due to the effects of) the accident that occurred. Here, the time during which the power converter 20-A is stopped in operation (gate-block state), in other words, the time from time t0 to time t4, is a predetermined time required for the power converter 20-A, the AC energy consumption device 40-A, and the renewable energy power source RE-A to operate in coordination. More specifically, the time during which the power converter 20-A is in the gate-block state is a predetermined time that takes into account, for example, the communication delay time (delay time) from the time a stop command is sent until the operation of the renewable energy power source RE-A is actually stopped or the generated power is suppressed, and the time required for the AC energy consumption device 40-A to reduce the surplus power from the renewable energy power source RE-A. The power converter 20-A restarts when a predetermined time has elapsed since it stopped in operation (entered the gate-block state). The elapsed time can be determined by a timer, for example, which starts timing at time t0. The time during which the power converter 20-A is in a gate block state is not limited to a predetermined time. For example, the power converter 20-A may be restarted when it receives a consumption termination signal from the AC energy consumption device 40-A indicating the end of energy consumption, and an operating status signal from the renewable energy power supply RE-A indicating that operation has stopped or power generation has been suppressed. 【0042】 Subsequently, in the DC power transmission system 1, for example, at time t5, after a (steady-state) power transmission state is established in the transmission system A, the renewable energy power source RE-A resumes steady-state operation. That is, if the renewable energy power source RE-A was stopped in response to a stop command, it returns to an operating state, and if it was suppressing power generation, it releases the suppression, thereby resuming steady-state operation. The resumption of steady-state operation in the renewable energy power source RE-A is performed, for example, by a voltage detector (not shown) detecting a rise in voltage and determining that a power transmission state has been established. The determination of resuming steady-state operation in the renewable energy power source RE-A is not limited to the detection result by the voltage detector (not shown). For example, the power converter 20-A may send a command to the renewable energy power source RE-A to resume operation (hereinafter referred to as the "resumption command"), and the renewable energy power source RE-A may resume steady-state operation when it receives the resumption command. 【0043】 Thus, in the operation of the DC power transmission system 1, if an accident occurs, the surplus power generated by the renewable energy power source RE-A, which becomes surplus power due to the power converter 20-A entering a gate block state, is consumed (absorbed) by the AC energy consumption device 40-A. In the operation of the DC power transmission system 1, the consumption (absorption) of surplus power by the AC energy consumption device 40-A continues until the operation of the renewable energy power source RE-A is actually stopped or the reduction of power generation is completed, and before the power converter 20-A restarts. In other words, in the operation of the DC power transmission system 1, the power converter 20-A restarts after the operation of the renewable energy power source RE-A has actually stopped or the reduction of power generation is completed, and the consumption (absorption) of surplus power by the AC energy consumption device 40-A has ended. Subsequently, in the operation of the DC power transmission system 1, after a (steady-state) power transmission state is established, the renewable energy power source RE-A resumes steady-state operation. 【0044】 As a result, in the operation of the DC power transmission system 1, even if a fault occurs in one or more of the power transmission paths, the power converter 20-A, which had stopped operating (gate-blocked state) in response to the fault, can be suitably restarted without generating an overvoltage in the transmission-side system A of the renewable energy power source RE-A. In other words, in the operation of the DC power transmission system 1, the power converter 20-A, the AC energy consumption device 40-A, and the renewable energy power source RE-A can be operated in coordination to suitably resume steady-state operation. 【0045】 [Variations in the operation of DC power transmission system 1] The operation of the DC power transmission system 1 described above explained how the AC energy consumption device 40-A consumes (absorbs) the surplus power generated by the renewable energy power source RE-A. In other words, the operation of the DC power transmission system 1 described an example of how the AC energy consumption device 40 operates in the event of a fault. However, the AC energy consumption device 40 can also be operated in a steady-state power transmission condition when no fault has occurred. 【0046】 Here, we will describe the operation of a modified version of DC power transmission system 1. In the following description, as with the operation of DC power transmission system 1, for the sake of simplicity, we will assume that the transmission system B related to the renewable energy power source RE-B in DC power transmission system 1 is omitted. 【0047】 Figure 3 is a sequence diagram showing an example of another operation flow for controlling each component in the DC power transmission system 1 according to the first embodiment. The operation shown in the sequence diagram in Figure 3 (modified operation) is an example of operating the AC energy consumption device 40-A in a steady-state power transmission condition in which no fault has occurred in the DC power transmission system 1. In the modified operation of the DC power transmission system 1, it is assumed that the power converter 20 transmits the power generated by the renewable energy power source RE-A via the DC transmission line LN-AC and supplies it to the AC transmission system TS-C. 【0048】 Even in the steady-state power transmission conditions of the modified DC power transmission system 1, the renewable energy power source RE-A (more specifically, the wind turbine) is operating, and the power generated from the renewable energy power source RE-A (AC power) is supplied to the power converter 20-A, which is operating and converting the AC power supplied from the renewable energy power source RE-A into DC power and supplying it to the DC transmission line LN-AC. At this time, the AC energy consumption device 40-A is performing an adjustment operation. The adjustment operation is the operation of a device that adjusts the frequency of the receiving system C and the power generation at the renewable energy power source RE-A. In other words, the adjustment operation is the operation of a power control buffer that responds to the frequency of the receiving system C. As a result, the DC power transmission system 1 can suppress fluctuations in the frequency of the receiving system C caused by the AC power generated by the renewable energy power source RE-A and contribute to maintaining the frequency of the receiving system C at a constant value. 【0049】 Subsequently, for example, if a fault occurs at time t0, the power converter 20-A will stop operating (enter a gate-block state) in accordance with the fault (due to the effects of the fault), similar to the operation shown in the sequence diagram in Figure 2. The subsequent operation of the DC power transmission system 1 is the same as the operation shown in the sequence diagram in Figure 2. Therefore, a further detailed explanation of the control and operation of each component when a fault occurs in the DC power transmission system 1 is omitted. 【0050】 Thus, in the modified operation of DC power transmission system 1, the frequency of AC power is maintained at a constant value by operating the AC energy consumption device 40-A in a steady power transmission state where no fault has occurred. Furthermore, even in the modified operation of DC power transmission system 1, if a fault occurs, the AC energy consumption device 40-A consumes (absorbs) the surplus power generated by the renewable energy power source RE-A, which has become excess power due to the power converter 20-A entering a gate block state. Furthermore, in the modified operation of DC power transmission system 1, the consumption (absorption) of surplus power by the AC energy consumption device 40-A continues until the operation of the renewable energy power source RE-A is actually stopped or the reduction of generated power is completed, and terminates before the power converter 20-A restarts. In other words, even in the modified operation of DC power transmission system 1, the power converter 20-A restarts after the operation of the renewable energy power source RE-A has actually stopped or the reduction of generated power is completed, and the consumption (absorption) of surplus power by the AC energy consumption device 40-A has ended. Subsequently, in the modified operation of DC power transmission system 1, after the (steady-state) power transmission state is established, the renewable energy power source RE-A resumes steady-state operation. At this time, the AC energy consumption device 40-A may perform adjustment operation again and resume operation to maintain the frequency of AC power at a constant value. 【0051】 As a result, even in the modified operation of the DC power transmission system 1, if an accident occurs in one or more of the power transmission paths, the power converter 20-A, which had stopped operating (gate-blocked state) in response to the accident, can be suitably restarted without generating an overvoltage in the transmission-side system A of the renewable energy power source RE-A. In other words, even in the modified operation of the DC power transmission system 1, the power converter 20-A, the AC energy consumption device 40-A, and the renewable energy power source RE-A can be operated in coordination, similar to the operation of the sequence diagram of the DC power transmission system 1 shown in Figure 2, to suitably resume steady-state operation. 【0052】 As described above, in the DC power transmission system 1 of the first embodiment, an AC energy consumption device 40 is connected to the offshore converter station located between the corresponding renewable energy power source RE and the power converter 20 in the transmission-side system of each renewable energy power source RE. In the DC power transmission system 1 of the first embodiment, the AC energy consumption device 40 consumes (absorbs) the surplus power generated by the renewable energy power source RE when an accident occurs and the power converter 20 stops operating. As a result, in the DC power transmission system 1 of the first embodiment, even if an accident occurs in one or more of the power transmission paths, the power converter 20, which had stopped operating in response to the accident, can be suitably restarted without generating an overvoltage in the transmission-side system of the renewable energy power source RE. In this way, in the DC power transmission system 1 of the first embodiment, the power converter 20, the AC energy consumption device 40, and the renewable energy power source RE can be operated in coordination to suitably return to a steady-state power transmission, enabling the operation of a more reliable DC power transmission system. 【0053】 (Second embodiment) [Configuration of a DC power transmission system] The following describes an example of the configuration of the DC power transmission system according to the second embodiment. Figure 4 is a diagram showing an example of the configuration of the DC power transmission system according to the second embodiment. Similar to the DC power transmission system 1 of the first embodiment shown in Figure 1, Figure 4 also shows an example of a multi-terminal (two-terminal-one-terminal) DC power transmission system (high-voltage direct current (HVDC) system) that converts three-phase AC power supplied from each of two renewable energy sources (renewable energy sources RE-A and RE-B) into DC power for transmission, converts it back into three-phase AC power, and supplies it to a single AC power transmission system (AC power transmission system TS-C). In Figure 4, renewable energy sources RE-A and RE-B are, for example, renewable energy sources equipped with wind turbines installed offshore that supply generated power (AC power). 【0054】 The DC power transmission system 2 includes, for example, three AC current interruption devices 10 (AC current interruption devices 10-A, 10-B, and 10-C), three power converters 20 (power converters 20-A, 20-B, and 20-C), two DC power transmission lines LN (DC power transmission lines LN-AB and LN-AC), four DC current interruption devices 30 (DC current interruption devices 30-1 to 30-4), and two AC energy storage devices 50 (AC energy storage devices 50-A and 50-B). The DC power transmission system 2 shown in Figure 4, similar to the DC power transmission system 1, shows an example configuration in which the AC power transmission system TS-A is located between the renewable energy power source RE-A and the AC current interruption device 10-A, and the AC power transmission system TS-B is located between the renewable energy power source RE-B and the AC current interruption device 10-B. Figure 4, like Figure 1, also shows a higher-level control device 100 as an example of a control device that controls each component of the DC power transmission system 2. 【0055】 In DC power transmission system 2, the AC energy consumption device 40 in DC power transmission system 1 is replaced by an AC energy storage device 50. The other components of DC power transmission system 2 are the same as those of DC power transmission system 1 shown in Figure 1. In Figure 4, components of DC power transmission system 2 that have the same function as those of DC power transmission system 1 are denoted by the same reference numerals. Therefore, a further detailed explanation of components of DC power transmission system 2 that have the same configuration and operation as those of DC power transmission system 1 will be omitted, and only the different configurations and operations will be explained. 【0056】 In DC power transmission system 1, when an accident occurred, the AC energy consumption device 40 consumed (absorbed) the power generated by the renewable energy power source RE by releasing it as surplus power. In contrast, in DC power transmission system 2, when an accident occurred, the AC energy storage device 50 absorbed the surplus power by storing (accumulating) the power generated by the renewable energy power source RE. Therefore, even when it is necessary to absorb surplus power for a long period of time, the AC energy storage device 50 can absorb surplus power more effectively than the AC energy consumption device 40, which is an AC chopper circuit composed of a thyristor (semiconductor switch) and a resistor. 【0057】 Each AC energy storage device 50 is an energy storage device that stores AC power at the location to which it is connected. Each AC energy storage device 50 stores or discharges the generated power (AC power) generated by the corresponding renewable energy power source RE during steady-state power transmission in the DC power transmission system 2, and also absorbs excess AC power (surplus power) generated by the corresponding renewable energy power source RE in the event of an accident by storing it. In the DC power transmission system 2, each AC energy storage device 50 is also connected to the transmission side system of each renewable energy power source RE. Each AC energy storage device 50 is also located, for example, at an offshore converter station near the corresponding renewable energy power source RE, i.e., offshore. In other words, the AC energy storage device 50 is connected to both the transmission side of the AC power supplied by the renewable energy power source RE-A (i.e., the AC power transmission path in transmission system A) and the transmission side of the AC power supplied by the renewable energy power source RE-B (i.e., the AC power transmission path in transmission system B). More specifically, the AC energy storage device 50-A is connected to the AC transmission line between the output terminal of the AC transmission system TS-A and the input connection terminal of the AC current interruption device 10-A (which may also be the AC terminal of the power converter 20-A). In the steady-state transmission conditions of the DC transmission system 2, the AC energy storage device 50-A stores or discharges the AC power supplied from the renewable energy power source RE-A, and when an accident occurs in one or more of the power transmission paths in the DC transmission system 2, it stores (absorbs) the AC power supplied from the renewable energy power source RE-A as surplus power. The AC energy storage device 50-B is connected to the AC transmission line between the output terminal of the AC transmission system TS-B and the input terminal of the AC current interruption device 10-B (which may also be the AC terminal of the power converter 20-B).The AC energy storage device 50-B stores or discharges AC power supplied from the renewable energy power source RE-B during steady-state power transmission in the DC power transmission system 2. In the event of a fault in one or more of the power transmission paths in the DC power transmission system 2, it stores (absorbs) the AC power supplied from the renewable energy power source RE-B as surplus power. This allows the power converter 20, which had stopped operating, to be quickly restarted in the event of a fault in one or more of the power transmission paths in the DC power transmission system 2 without generating overvoltage in the AC systems of the renewable energy power source RE (transmission side system A and transmission side system B). 【0058】 Each AC energy storage device 50 is, for example, an energy storage device equipped with a converter and a power storage unit capable of grid forming (GFM) operation. The converter in the AC energy storage device 50 is, for example, a self-commutated converter. The power storage unit in the AC energy storage device 50 is, for example, a battery. Figure 4 shows an example of an equivalent circuit of the AC energy storage device 50, showing the configuration of the converter and the power storage unit represented by the symbol of a capacitor. Control of the operation of the AC energy storage device 50, that is, control to one of the states of power storage state, discharge state, or stopped state, is performed not only when an abnormality in AC power (increase in AC frequency or increase in AC current) is detected in the connected AC transmission line, or when an abnormality in DC power (increase in DC voltage or increase in DC current, or a decrease thereof) or a fault (such as a fault in the power converter 20 or AC transmission system TS) is detected in the connected DC transmission line, in order to control the state to store (absorb) surplus power, but also in the steady-state power transmission state. This control is performed, for example, by a control device (not shown) provided in the corresponding power converter 20. Control of the AC energy storage device 50 to any of the states of stored, discharged, or stopped may be performed, for example, by control from a higher-level control device 100. Control of the AC energy storage device 50 to any of the states of stored, discharged, or stopped may be performed, for example, by the higher-level control device 100 operating in response to control from a control device (not shown) provided in the corresponding power converter 20. In other words, when the higher-level control device 100 receives a control signal transmitted by a control device (not shown) provided in the power converter 20, the higher-level control device 100 may control the AC energy storage device 50 to any of the states of stored, discharged, or stopped. Control of the AC energy storage device 50 to any of the states of stored or discharged may be performed, for example, in response to information from a control device (not shown) provided inside the AC current interruption device 10 or in a component located near the AC current interruption device 10 or the power converter 20.Control of the AC energy storage device 50 to either a stored state or a discharge state may be performed, for example, in response to information from a control device (not shown) located inside or near a DC current interruption device 30 (DC current interruption devices 30-1, 30-2, 30-3, etc.) of the DC power transmission system 2. 【0059】 The AC power capacity (storage capacity) that the AC energy storage device 50 can store is at least the capacity to store (absorb) surplus power. The storage capacity corresponding to surplus power in the AC energy storage device 50 is determined in advance, for example, by analyzing the amount of power generated by the renewable energy power source RE (amount of AC power supplied) and the circuit condition of the AC grid in advance. More specifically, based on the results of the prior analysis, the expected overvoltage, that is, the amount of surplus power to be stored (absorbed), and the time required to reduce the surplus power until no overvoltage occurs (until the overvoltage due to the surplus power falls below a predetermined threshold) by putting the AC energy storage device 50 into a storage state to store (absorb) the surplus power, are determined in advance, and the capacity is set to the maximum value of this (a margin may also be provided). The total storage capacity of the AC energy storage device 50 is set to include the storage capacity corresponding to the surplus power mentioned above, as well as the capacity required to store the AC power generated by the corresponding renewable energy power source RE under steady-state power transmission conditions. 【0060】 AC energy storage device 50 is an example of an "energy absorption device" or "energy storage device". 【0061】 With this configuration, in the DC power transmission system 2, if an accident occurs in one or more of the power transmission paths, the faulty component is electrically isolated by control from an unillustrated control device located inside or near each component, and the operation of related components connected to the isolated component is stopped. Then, the AC energy storage device 50 stores (absorbs) the generated power (AC power) from the wind turbine of the renewable energy power source RE, which has been disconnected from the power transmission system of the DC power transmission system 2 and is now operating independently, as surplus power, thereby restarting the related components that have stopped operating. 【0062】 In the configuration of the DC power transmission system 2 described above, similar to the DC power transmission system 1, the AC power transmission system TS, the AC current interruption device 10, the power converter 20, and the AC energy storage device 50 were described as being located at an offshore converter station near the corresponding renewable energy source RE, i.e., located offshore. This is for the same reasons as in the DC power transmission system 1. However, in the DC power transmission system 2, similar to the DC power transmission system 1, some or all of the components may be located at an onshore converter station. In this case, similar to the DC power transmission system 1, the maintainability of the components located at the onshore converter station is improved in the DC power transmission system 2. 【0063】 [Operation of DC power transmission system 2] The following describes an example of the operation of controlling each component in the DC power transmission system 2 when a fault occurs. In the following description, as with the operation of DC power transmission system 1, for the sake of simplicity, the transmission system B related to the renewable energy power source RE-B in DC power transmission system 2 will be omitted. Then, in DC power transmission system 2, for example, a fault will occur in one of the paths related to the renewable energy power source RE-A, and the control of the power converter 20-A, AC energy storage device 50-A, and renewable energy power source RE-A in this case will be described. In the following description, as with the operation of DC power transmission system 1, in DC power transmission system 2, the AC current interruption device 10 and the DC current interruption device 30 will suitably interrupt (disconnect) or connect the path where the fault occurred, and a detailed explanation of the control of the AC current interruption device 10 and the DC current interruption device 30 in this case will be omitted. 【0064】 Figure 5 is a sequence diagram showing an example of the flow of operations for controlling each component when an accident occurs in the DC power transmission system 2 according to the second embodiment. In the operation of the DC power transmission system 2, it is assumed that the power converter 20 transmits the power generated by the renewable energy power source RE-A via the DC transmission line LN-AC and supplies it to the AC power transmission system TS-C. 【0065】 In the steady-state power transmission of DC power transmission system 2, the renewable energy power source RE-A (more specifically, a wind turbine) is in operation, and the generated power (AC power) from the renewable energy power source RE-A is supplied to the power converter 20-A, which is operating and converting the AC power supplied from the renewable energy power source RE-A into DC power and supplying it to the DC transmission line LN-AC. At this time, the AC energy storage device 50-A repeatedly switches between a storage state and a discharge state by synchronous control (current control). Synchronous control is, for example, grid following (GFL) control. The storage capacity used for storage and discharge in the AC energy storage device 50-A at this time is the capacity provided in addition to the storage capacity corresponding to the surplus power. The AC energy storage device 50-A operates by synchronous control as a control device between the frequency of the receiving system C and the power generation in the renewable energy power source RE-A, similar to the adjustment operation of the AC energy consumption device 40-A in the modified version of the DC power transmission system 1. In other words, the AC energy storage device 50-A operates by synchronous control as a power control buffer corresponding to the frequency of the receiving system C, etc. This allows the DC power transmission system 2 to contribute to maintaining the frequency of the receiving system C at a constant value, similar to the operation in the modified version of the DC power transmission system 1. 【0066】 Subsequently, for example, if an accident occurs at time t0, the power converter 20-A, like the DC power transmission system 1, will stop operating (enter a gate-block state) in response to the accident (due to the effects of the accident) and transmit a stop command to the renewable energy power source RE-A. Then, in the DC power transmission system 2, just like the DC power transmission system 1, the renewable energy power source RE-A will continue to operate and supply surplus power to the power converter 20-A. 【0067】 Subsequently, the AC energy storage device 50-A switches its current operating control when, for example, at time t1, the overvoltage caused by surplus power from the renewable energy power source RE-A exceeds a predetermined threshold (detecting the occurrence of overvoltage). More specifically, the AC energy storage device 50-A switches from synchronous control, which repeatedly stores and discharges energy, to constant voltage constant frequency (CVCF) control, which fixes the amplitude and phase of the AC voltage, and begins storing (absorbing) surplus power from the renewable energy power source RE-A. The timing of the switch to CVCF control in the AC energy storage device 50-A, that is, the timing of the start of storing (absorbing) surplus power, is not limited to, for example, the result of overvoltage detection by a voltage detector (not shown), but may also be when, similar to the start of energy consumption in the AC energy consumption device 40-A of the DC power transmission system 1, the device receives a signal (information) indicating that the power converter 20-A has stopped, which was transmitted by the power converter 20-A when an accident occurred, or when it receives an instruction signal to instruct the switch to CVCF control. In the DC power transmission system 2, in order to prioritize the speed of switching to CVCF control, the AC energy storage device 50-A switches to CVCF control at the earlier of the timing of the judgment based on the detection result of the voltage detector (not shown) and the information or instructions from the power converter 20-A. At this time, the storage capacity stored in the AC energy storage device 50-A is, as described above, the storage capacity corresponding to the amount of surplus power to be stored (absorbed), which has been determined in advance based on the results of prior analysis. As a result, the overvoltage supplied to the power converter 20-A (including the AC power transmission system TS-A) due to the continued operation of the renewable energy power source RE-A is suppressed. 【0068】 Subsequently, for example, at time t2, when the renewable energy power source RE-A receives a stop command transmitted by the power converter 20-A, it stops operation or suppresses power generation in accordance with the received stop command, similar to the DC power transmission system 1. The stopping of operation or suppression of power generation in the renewable energy power source RE-A is not limited to following the stop command transmitted by the power converter 20-A, but may also be done in accordance with control from the higher-level control device 100, similar to the DC power transmission system 1. In the DC power transmission system 2 as well, in order to prioritize the early stopping of operation or suppression of power generation in the renewable energy power source RE-A, the stopping of operation or suppression of power generation is started at the earlier timing of the stop command from the power converter 20-A or the control from the higher-level control device 100. 【0069】 Subsequently, the AC energy storage device 50-A terminates the storage (absorption) of surplus power from the renewable energy power source RE-A, for example, at time t3. At this time, the AC energy storage device 50-A switches its current operating control. More specifically, the AC energy storage device 50-A switches from CVCF control, which stores (absorbs) surplus power, to an operating control that repeatedly stores and discharges AC power from the renewable energy power source RE-A. This is because if the power converter 20-A is restarted while the AC energy storage device 50-A is under CVCF control, the CVCF control in the AC energy storage device 50-A and the control in the power converter 20-A that converts AC power to DC power may interfere with each other, making it impossible to control them appropriately (coordinate control). However, the operating control switched here is the same as the steady-state power transmission state in the DC power transmission system 2, and is different from synchronous control such as grid following control. The operating control switched to here is, for example, voltage-controlled droop control. In this case, the droop control in the AC energy storage device 50-A and the conversion control in the power converter 20-A do not interfere with each other, and both can be controlled in a coordinated manner. The timing of switching to droop control in the AC energy storage device 50-A, that is, the timing of ending the storage (absorption) of surplus power, in other words, the consumption time of surplus power in the AC energy storage device 50-A between time t1 and time t3, is, as described above, the timing of the time required to reduce the surplus power, which has been determined in advance based on the results of prior analysis. This timing can be determined by, for example, a timer that starts measuring from time t1. 【0070】 Subsequently, for example, at time t4, the power converter 20-A, similar to the DC power transmission system 1, terminates the state in which it was stopped (gate-blocked state) in response to the accident (due to the effects of the accident) and restarts. Here, the time during which the power converter 20-A is stopped (gate-blocked), in other words, the time from time t0 to time t4, is a predetermined time required for the power converter 20-A, the AC energy storage device 50-A, and the renewable energy power source RE-A to operate in coordination, similar to the DC power transmission system 1. In other words, in the DC power transmission system 2 as well, the time during which the power converter 20-A is in the gate-blocked state is a predetermined time, taking into account, for example, the communication delay time (delay time) from the time a stop command is sent until the operation of the renewable energy power source RE-A is actually stopped or the generated power is suppressed, and the time required for the AC energy storage device 50-A to reduce the surplus power from the renewable energy power source RE-A. The power converter 20-A restarts after a predetermined time has elapsed since it stopped operating (since entering the gate block state). This predetermined time can be determined by a timer, for example, which starts counting from time t0, similar to the DC power transmission system 1. The time during which the power converter 20-A is in the gate block state is not limited to a predetermined time. Similar to the DC power transmission system 1, the power converter 20-A may also restart when it receives, for example, a storage termination signal transmitted by the AC energy storage device 50-A indicating the end of storage (absorption) of surplus power from the renewable energy power source RE-A, and an operating status signal transmitted by the renewable energy power source RE-A. 【0071】 Subsequently, the AC energy storage device 50-A stops operating, for example, at time t5. The timing for stopping the operation of the AC energy storage device 50-A is a predetermined timing (time) based on, for example, the elapsed time since the timing when the operation control of the AC energy storage device 50-A was switched to CVCF control (time t1), or the timing when the operation control of the AC energy storage device 50-A was switched to droop control (time t3), or the time until a (steady) power transmission state is established in the power transmission system A, as assumed from the prior analysis results. The AC energy storage device 50-A may, for example, not stop operating at time t5, but continue with the current operation control (droop control). 【0072】 Subsequently, in the DC power transmission system 2, for example, at time t6, after a (steady-state) power transmission condition is established in the transmission system A, the renewable energy power source RE-A resumes steady-state operation, similar to the DC power transmission system 1. When the steady-state power transmission condition is resumed in the DC power transmission system 2, the AC energy storage device 50-A resumes operation control (for example, synchronous control such as grid following control). The AC energy storage device 50-A may remain in a stopped state even after the steady-state power transmission condition is resumed in the DC power transmission system 2. 【0073】 Thus, in the operation of the DC power transmission system 2, by synchronously controlling the AC energy storage device 50-A (for example, grid-following control), it contributes to maintaining the frequency of the receiving system C at a constant value in a steady-state power transmission state without faults, similar to the operation of the modified DC power transmission system 1. Furthermore, in the operation of the DC power transmission system 2, if a fault occurs, the operation control of the AC energy storage device 50-A is switched from synchronous control to CVCF control, and the AC energy storage device 50-A absorbs the surplus power generated by the renewable energy power source RE-A, which has become excess power due to the power converter 20-A entering a gate-block state, by storing (accumulating) it. Subsequently, in the operation of the DC power transmission system 2, when the operation of the renewable energy power source RE-A is actually stopped or the reduction of generated power is completed, in other words, before the power converter 20-A is restarted, the operation control of the AC energy storage device 50-A is switched to voltage-controlled droop control so that the AC energy storage device 50-A and the power converter 20-A operate in coordination. Then, in the operation of the DC power transmission system 2, the power converter 20-A is restarted. Subsequently, in the operation of the DC power transmission system 2, the operation of the AC energy storage device 50-A may be stopped. After that, in the operation of the DC power transmission system 2, once a (steady-state) power transmission state is established, the renewable energy power source RE-A resumes steady-state operation. At this time, in the operation of the DC power transmission system 2, the operation control of the AC energy storage device 50-A (for example, synchronous control such as grid following control) is restarted, or the stopped state of operation is continued. 【0074】 As a result, even when the DC power transmission system 2 is operating, if an accident occurs in one or more of the power transmission paths, the power converter 20-A, which had stopped operating (gate-blocked state) in response to the accident, can be suitably restarted without generating an overvoltage in the transmission-side system A of the renewable energy power source RE-A. In other words, even when the DC power transmission system 2 is operating, just like with the DC power transmission system 1, the power converter 20-A, the AC energy consumption device 40-A, and the renewable energy power source RE-A can be operated in coordination to suitably resume steady-state operation. 【0075】 As described above, in the DC power transmission system 2 of the second embodiment, an AC energy storage device 50 is connected to the offshore converter station located between the corresponding renewable energy power source RE and the power converter 20 in the transmission-side system of each renewable energy power source RE. In the DC power transmission system 2 of the second embodiment, the AC energy storage device 50 absorbs the surplus power generated by the renewable energy power source RE when an accident occurs and the power converter 20 stops operating. As a result, in the DC power transmission system 2 of the second embodiment, as in the DC power transmission system 1 of the first embodiment, even if an accident occurs in one or more of the power transmission paths, the power converter 20, which had stopped operating in response to the accident, can be suitably restarted without generating an overvoltage in the transmission-side system of the renewable energy power source RE. As a result, in the DC power transmission system 2 of the second embodiment, similar to the DC power transmission system 1 of the first embodiment, the power converter 20, the AC energy consumption device 40, and the renewable energy power source RE can be operated in coordination to suitably return to a steady power transmission state, enabling the operation of a more reliable DC power transmission system. 【0076】 As described above, the DC power transmission system of the embodiment includes energy absorption devices (AC energy consumption device 40 and AC energy storage device 50) for absorbing surplus power. In the DC power transmission system of the embodiment, control devices (not shown) located inside or near each component, and / or a higher-level control device 100, detect an accident occurring in one or more paths transmitting power in the DC power transmission system. In the DC power transmission system of the embodiment, the control devices (not shown) located inside or near each component, and / or a higher-level control device 100, detect an accident and stop the operation of the power converter 20 (power converter 20-A in the embodiment), causing the renewable energy power source RE, which has become a power source system that is disconnected from the power transmission system of the DC power transmission system and operates independently, to absorb the surplus power (AC power) generated by the renewable energy power source RE, which has no destination for transmission, using the corresponding energy absorption device. In the DC power transmission system of this embodiment, control devices (not shown) located inside or near each component, and / or a higher-level control device 100, stop the operation of the renewable energy power source RE (or suppress the generated power), then terminate the absorption of surplus power by the energy absorption device, and then restart the power converter. In other words, in the DC power transmission system of this embodiment, the power converter 20, the AC energy consumption device 40, and the renewable energy power source RE are operated in coordination. As a result, in the DC power transmission system of this embodiment, surplus power can be suitably absorbed by the energy absorption device, and steady-state operation can be suitably resumed (returned to a steady-state power transmission). This enables the DC power transmission system of this embodiment to be operated with greater reliability. 【0077】 The DC power transmission systems described above (DC power transmission system 1 and DC power transmission system 2) describe a case where the renewable energy power source RE is a wind power generation facility that supplies generated electricity (AC power) from a wind turbine installed offshore. However, the renewable energy power source RE is not limited to wind power generation facilities. The renewable energy power source RE may be other types of renewable energy power generation facilities, such as solar power generation facilities or geothermal power generation facilities. In this case, the DC power transmission system can transmit electricity using components corresponding to each renewable energy power generation facility. For example, if the DC power transmission system is configured to correspond to a solar power generation facility, the solar power generation facility generates DC power. In this case, the solar power generation facility can be equipped with another power converter that converts DC power to AC power and can be connected to the DC power transmission system. Therefore, the DC power transmission system can be configured to be equivalent to the DC power transmission system described above. 【0078】 In the DC power transmission systems of the embodiments described above (DC power transmission system 1 and DC power transmission system 2), a two-terminal-one-terminal DC power transmission system is shown and explained as an example of a multi-terminal DC power transmission system (high-voltage direct current (HVDC) system), in which two renewable energy power sources RE (renewable energy power source RE-A and renewable energy power source RE-B) are connected and one AC power transmission system TS (AC power transmission system TS-C) is connected. However, the number of renewable energy power sources RE and AC power transmission systems TS connected to the DC power transmission system, that is, the number of terminals in the DC power transmission system, is not limited to the number of terminals in the embodiments described above. For example, the number of terminals on the transmitting side of the DC power transmission system may be one terminal or three or more terminals, and the number of terminals on the receiving side may be two or more terminals. In these cases, the control of the DC power transmission system 1, that is, the operation of control devices (not shown) located inside or near each component, and the operation of the higher-level control device 100, should be equivalent to the control of the DC power transmission system (DC power transmission system 1 or DC power transmission system 2) described above. 【0079】 According to at least one embodiment described above, the system comprises at least one DC transmission line (LN) for transmitting DC power, a transmission-side power converter (20-A, 20-B) that converts AC power supplied by renewable energy sources (RE-A, RE-B) connected to the transmission-side AC transmission system (transmission-side system A, transmission-side system B) into DC power and outputs it to the DC transmission line, a receiving-side power converter (20-C) that converts DC power transmitted by the DC transmission line into AC power and supplies it to the receiving-side AC transmission system (receiving-side system C) including a load connected to the receiving side, and an energy absorption device (40, 50) connected between the transmission-side AC transmission system and the transmission-side power converter, which absorbs AC power supplied by the renewable energy source, wherein the energy absorption device comprises at least Furthermore, if the AC power supplied by a renewable energy source becomes surplus power due to the transmission-side power converter ceasing to operate as a result of an accident occurring in any of the components that transmit power, the energy absorption device will absorb the surplus power. The energy absorption device will begin absorbing the surplus power when it detects either or both of the overvoltage generated in the transmission-side AC transmission system due to the surplus power and / or the ceasing to operate the transmission-side power converter. The absorption of the surplus power will end after the operation of the renewable energy source has been stopped or the supply of AC power has been reduced, and before the transmission-side power converter restarts. In this way, the energy absorption device can suitably absorb surplus power from a renewable energy source generated as a result of an accident in a multi-terminal DC transmission system (1, 2). 【0080】 While several embodiments of the present invention have been described, these embodiments are presented as examples only and are not intended to limit the scope of the invention. These embodiments can be carried out in a variety of other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their variations are included in the scope and spirit of the invention, as well as in the claims and their equivalents. [Explanation of Symbols] 【0081】 1,2...DC power transmission system, 10,10-A,10-B,10-C...AC current interruption device, 20,20-A,20-B,20-C...Power converter, 30,30-1,30-2,30-3,30-4...DC current interruption device, 40,40-A,40-B...AC energy consumption device, 50,50-A,50-B...AC energy storage device, 100...Higher-level control device, RE,RE-A,RE-B...Renewable energy power source, TS,TS-A,TS-B,TS-C...AC power transmission system, LN,LN-AB,LN-AC...DC power transmission line

Claims

[Claim 1] A DC power transmission line for transmitting DC power, A power converter on the transmission side converts AC power supplied by a renewable energy source connected to the AC power transmission system on the transmission side into DC power and outputs it to the DC transmission line, A receiving-side power converter that converts the DC power transmitted by the DC transmission line into AC power and supplies it to the AC transmission system on the receiving side, including the load connected to the receiving side. An energy absorption device is connected to a position between the AC power transmission system on the transmission side and the power converter on the transmission side, and absorbs the AC power supplied by the renewable energy power source, Equipped with, The energy absorption device absorbs the excess power when the AC power supplied by the renewable energy source becomes excess power due to the transmission-side power converter ceasing to operate as a result of an accident occurring in at least one of the components that transmits power, The aforementioned energy absorption device is When either or both of the following are detected—an overvoltage generated in the AC power transmission system on the transmission side due to the surplus power, or the cessation of operation of the power converter on the transmission side—the absorption of the surplus power is initiated. After the operation of the renewable energy power source has been stopped or the reduction of the AC power supplied has been completed, and before the power converter on the transmission side restarts, the absorption of the surplus power will be terminated. DC power transmission system. [Claim 2] The energy absorption device is an energy consumption device that absorbs the surplus power by consuming the surplus power, The amount of surplus electricity consumed in the energy consumption device is: The amount of AC power supplied from the renewable energy source and the amount of surplus power, which is assumed from the results of a prior analysis of the circuit conditions of the AC power transmission system on the transmission side, are predetermined. The consumption time of the surplus power in the energy consumption device is: Based on the time required to reduce the surplus power, which is assumed from the above analysis results, this is predetermined. The DC power transmission system according to claim 1. [Claim 3] The aforementioned power converter on the power transmission side is The system restarts when either a predetermined time has elapsed, taking into account the delay time until the operation of the renewable energy power source is stopped or the supply of AC power is reduced in response to the stop command transmitted when the system stopped, and the time the energy consumption device consumes the surplus power, or when an operating status signal indicating that the operation of the renewable energy power source has stopped or the supply of AC power has been reduced is received, or both of the above. The DC power transmission system according to claim 2. [Claim 4] The aforementioned renewable energy source is After the power converter on the transmission side has restarted and the power transmission state of the AC power transmission system on the transmission side has been established, operation will be resumed. The DC power transmission system according to claim 3. [Claim 5] The energy consumption device absorbs the AC power supplied by the renewable energy source during a steady-state power transmission of the AC transmission system on the transmitting side where no accident has occurred, and adjusts the frequency of the AC transmission system on the receiving side with the supply of AC power from the renewable energy source. A DC power transmission system according to any one of claims 2 to 4. [Claim 6] The energy absorption device comprises a converter and a power storage unit capable of grid forming operation, and is an energy storage device that absorbs the surplus power by storing the surplus power. The aforementioned energy storage device is In a steady-state power transmission state of the AC power transmission system on the transmitting side where the aforementioned accident has not occurred, the AC power supplied by the renewable energy power source is repeatedly stored and discharged by synchronous control operation, thereby adjusting the frequency of the AC power transmission system on the receiving side and the supply of AC power by the renewable energy power source. In the event of the aforementioned accident, the operation control is switched from the synchronous control to constant voltage constant frequency control to begin absorbing the surplus power. The DC power transmission system according to claim 1. [Claim 7] The energy storage device, after the operation of the renewable energy power source has been stopped or the reduction of the AC power supplied has been completed, and before the power transmission power converter is restarted, switches the operation control from constant voltage constant frequency control to droop control to end the absorption of the surplus power. The DC power transmission system according to claim 6. [Claim 8] The energy storage capacity corresponding to the surplus power in the aforementioned energy storage device is: The amount of AC power supplied from the renewable energy source and the amount of surplus power, which is assumed from the results of a prior analysis of the circuit conditions of the AC power transmission system on the transmission side, are predetermined. The consumption time of the surplus power in the energy storage device is: Based on the time required to reduce the surplus power, which is assumed from the above analysis results, this is predetermined. The DC power transmission system according to claim 7. [Claim 9] The aforementioned power converter on the power transmission side is The system restarts when either a predetermined time has elapsed, taking into account the delay time until the operation of the renewable energy power source is stopped or the reduction of the AC power supplied is completed in response to the stop command transmitted when the system stopped operating, and the consumption time of the surplus power in the energy storage device, or when an operating status signal indicating that the operation of the renewable energy power source has stopped or the reduction of the AC power supplied is completed is received, or when both of these occur. The DC power transmission system according to claim 8. [Claim 10] The aforementioned renewable energy source is After the power converter on the transmission side has restarted and the power transmission state of the AC power transmission system on the transmission side has been established, operation will be resumed. The DC power transmission system according to claim 9. [Claim 11] A DC power transmission line for transmitting DC power, A power converter on the transmission side converts AC power supplied by a renewable energy source connected to the AC power transmission system on the transmission side into DC power and outputs it to the DC transmission line, A receiving-side power converter that converts the DC power transmitted by the DC transmission line into AC power and supplies it to the AC transmission system on the receiving side, including the load connected to the receiving side. An energy absorption device is connected to a position between the AC power transmission system on the transmission side and the power converter on the transmission side, and absorbs the AC power supplied by the renewable energy power source, Equipped with, The energy absorption device is a DC power transmission method in a DC power transmission system that absorbs surplus power when the AC power supplied by the renewable energy source becomes surplus power due to the power transmission side power converter stopping due to an accident occurring in at least one of the components that transmits power, Computers The energy absorption device, When either or both of the following are detected—an overvoltage generated in the AC power transmission system on the transmission side due to the surplus power, or the cessation of operation of the power converter on the transmission side—the absorption of the surplus power is initiated. After the shutdown of the renewable energy power source or the reduction of the AC power supplied has been completed, and before the power converter on the transmission side is restarted, the absorption of the surplus power is terminated. A method of transmitting power using direct current.

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