Power conversion device, power conversion method, and program

Abstract

A power conversion device according to an embodiment has a power converter and a control device. The power converter converts AC power supplied by a renewable energy power generation system connected to an AC system on a power transmission side into DC power, and outputs the DC power to a DC power transmission line. The control device controls the operation of the power converter. The control device has an overcurrent suppression control unit, a during-fault command value calculation unit, and a command value output unit. If an overcurrent flowing in the power converter in response to a fault that had occurred in the AC system is detected, the overcurrent suppression control unit switches an operation control scheme of the power converter from a normal operation voltage control scheme to an overcurrent suppression control scheme before the fault is cleared by a cutoff device disposed on the AC power transmission line of the AC system, and causes the power converter to be activated when the fault is cleared. The during-fault command value calculation unit generates a during-fault command value corresponding to the overcurrent suppression control. The command value output unit selects either a normal-time command value of the voltage control or the during-fault command value to output the selected command value to the power converter.

Description

Power conversion device, power conversion method, and program

[0001] FIELD Embodiments of the present invention relate to a power conversion device, a power conversion method, and a program.

[0002] In recent years, active consideration and introduction of renewable energy power generation systems, such as offshore wind power generation systems that generate electricity using wind turbines installed on the sea, has been progressing. Renewable energy power generation systems are installed in locations suitable for generating electricity and are far from areas with large power demand. For this reason, the development of high-voltage direct current (HVDC) transmission systems for efficiently transmitting power from renewable energy power generation systems over long distances has become important.

[0003] In conventional high-voltage direct current (HVDC) transmission systems, a generator included in the AC system on the transmission side supplies voltage to establish the voltage to be transmitted, and a renewable energy power generation system included in the AC system on the transmission side supplies current according to the established voltage, thereby transmitting HVDC. In recent HVDC transmission systems, power converters using semiconductor power devices have been increasingly introduced in place of conventional generators included in the AC system on the transmission side.

[0004] In conventional high-voltage direct current (HDC) transmission systems, the size of the generators included in the AC grid on the transmission side is significantly larger than the size of the facilities of the interconnected renewable energy power generation system. Therefore, in conventional HDC transmission systems, even if a fault occurs in any path of the AC grid on the transmission side to which the renewable energy power generation system is connected, the generator absorbs the excess current (overcurrent) corresponding to the power fluctuations caused by the fault, allowing the renewable energy power generation system to continue operating. However, power converters cannot continue to pass overcurrents that exceed the short-circuit withstand capability of semiconductor power devices. Therefore, in HDC transmission systems incorporating power converters, if a fault occurs in any path of the AC grid on the transmission side, the operation of the power converter operating in grid forming (GFM) must be stopped (including temporarily) to prevent the semiconductor power devices from failing or being destroyed. In this case, the HDC transmission system incorporating the power converter cannot transmit power (current) from the renewable energy power generation system while the power converter operation is stopped. Furthermore, in a high-voltage DC transmission system incorporating a power converter, the power converter must be activated (resuming operation) when the fault is resolved (recovered from the fault). Therefore, in a high-voltage DC transmission system incorporating a power converter, if the fault is resolved while the power converter is stopped, the renewable energy power generation system will also stop operating due to an overvoltage in the AC system. For this reason, in a high-voltage DC transmission system incorporating a power converter, if a fault occurs on any path of the AC system, there is a high possibility that situations will arise in which opportunities for power generation by the renewable energy power generation system and opportunities for high-voltage DC transmission will be lost.

[0005] Japanese Patent Application Publication No. 2021-141704

[0006] The problem to be solved by the present invention is to provide a power conversion device, a power conversion method, and a program that can effectively suppress overcurrents caused by power fluctuations due to an accident in an AC system, thereby starting up a power converter and enabling the operation of a renewable energy power generation system to continue until recovery from the accident.

[0007] A power conversion device according to an embodiment includes a power converter and a control device. The power converter converts AC power supplied from a renewable energy power generation system connected to a power transmission-side AC system that transmits DC power into DC power and outputs the converted DC power to a DC transmission line. The control device controls operation of the power converter. The control device includes an overcurrent suppression control unit, a fault command value calculation unit, and a command value output unit. When the overcurrent suppression control unit detects an overcurrent flowing through the power converter due to a fault occurring in the AC system, the overcurrent suppression control unit switches the control method for the operation of the power converter from a voltage control method for normal operation of the power converter to an overcurrent suppression control method before the fault is cleared by a circuit breaker arranged on an AC transmission line of the AC system, and causes the power converter to enter an activated state when the fault is cleared. The fault command value calculation unit generates a fault command value corresponding to the overcurrent suppression control. The command value output unit selects either a normal command value, which is a predetermined voltage command value in the voltage control, or the fault command value, and outputs the selected value to the power converter.

[0008] 1 is a diagram showing an example of the configuration of a high-voltage DC transmission system including a power conversion apparatus according to a first embodiment. FIG. 2 is a diagram schematically showing an example of control of a power converter by a control device included in the power conversion apparatus according to the first embodiment when a fault occurs. FIG. 3 is a diagram schematically showing an example of a time flow of control of a power converter by a control device included in the power conversion apparatus according to the first embodiment when a fault occurs. FIG. 4 is a diagram showing an example of specifications for a voltage dip tolerance of a renewable energy power generation system constituting the high-voltage DC transmission system including the power conversion apparatus according to the first embodiment. A flowchart showing an example of a process by which a control device included in the power conversion apparatus according to the first embodiment controls a power converter. FIG. 5 is a diagram showing an example of the configuration of a high-voltage DC transmission system including a power conversion apparatus according to a second embodiment. A flowchart showing an example of a process by which a control device included in the power conversion apparatus according to the second embodiment controls a power converter. FIG. 6 is a diagram schematically showing an example of a time flow of control of a power converter by a control device included in a power conversion apparatus according to a third embodiment when a fault occurs. A flowchart showing an example of a process by which a control device included in a power conversion apparatus according to the third embodiment controls a power converter. A flowchart showing an example of a process by which a control device included in a power conversion apparatus according to a fourth embodiment controls a power converter. FIG. 10 is a diagram showing an example of the configuration of a high-voltage DC power transmission system including a power conversion device according to a fifth embodiment.

[0009] Hereinafter, a power conversion device, a power conversion method, and a program according to an embodiment will be described with reference to the drawings.

[0010] (First embodiment) [Configuration of high-voltage DC transmission system] An example of the configuration of a high-voltage DC transmission system including a power conversion device will be described below. Fig. 1 is a diagram showing an example of the configuration of a high-voltage DC transmission system including a power conversion device according to a first embodiment. Fig. 1 shows an example of the configuration of the power transmission side of a high-voltage DC transmission system 1. The high-voltage DC transmission system 1 includes, for example, a renewable energy power generation system 10 and a power conversion device 40 on the power transmission side.

[0011] In the high-voltage DC transmission system 1, AC power generated by a renewable energy power generation system 10, which is connected as an AC system on the power transmission side, is converted into DC power by a power conversion device 40, and the DC power is transmitted to the power receiving side via two DC transmission lines LN-DC (DC transmission line LN-DC-A and DC transmission line LN-DC-B). In the high-voltage DC transmission system 1, the power conversion device 40 performs grid forming (GFM) operation to operate the power converter 100 as a voltage source, i.e., controls the voltage of the power converter 100 to establish an AC system voltage (high voltage) for DC transmission. In the high-voltage DC transmission system 1, the renewable energy power generation system 10 generates a current (AC current) according to the established AC system voltage (hereinafter referred to as "system voltage") and supplies the generated current to the power conversion device 40. As a result, in the high-voltage DC transmission system 1, the power conversion device 40 (more specifically, the power converter 100) converts the AC power of the AC current supplied by the renewable energy power generation system 10 into DC power at the system voltage established by the power converter 100, and transmits the high-voltage DC power.

[0012] The high-voltage DC transmission system 1 converts the transmitted DC power back into AC power at the power receiving side (not shown) and supplies it to the AC system of the power receiving side. The high-voltage DC transmission system 1 may convert the AC power back into DC power in the AC system of the power receiving side (not shown), that is, the AC system of the power receiving side (not shown) may operate as the AC system of the power transmitting side in the next-stage high-voltage DC transmission system, and transmit the DC power further away.

[0013] The renewable energy power generation system 10 includes, for example, a wind power generator installed on the sea, which generates AC power (alternating current) using wind turbines that rotate in response to wind strength. While FIG. 1 shows the configuration of a high-voltage direct current (HVDC) transmission system 1 interconnected with one renewable energy power generation system 10, the number of renewable energy power generation systems 10 interconnected to the HVDC transmission system 1 is not limited to one, and multiple renewable energy power generation systems 10 may also be interconnected. In the HVDC transmission system 1 shown in FIG. 1 , AC current generated by the renewable energy power generation system 10 is supplied (transmitted) to the AC side of a power conversion device 40 via two AC transmission lines LN-AC (AC transmission line LN-AC-A and AC transmission line LN-AC-B). A circuit breaker 20 is connected (disposed) to each of the AC transmission lines LN-AC. More specifically, a circuit breaker 20-A-1 is connected to the end of the AC transmission line LN-AC-A on the renewable energy power generation system 10 side, and a circuit breaker 20-A-2 is connected to the end of the AC side of the power conversion device 40. A circuit breaker 20-B-1 is connected to the end of the AC transmission line LN-AC-B on the renewable energy power generation system 10 side, and a circuit breaker 20-B-2 is connected to the end of the AC side of the power conversion device 40.

[0014] Each of the circuit breakers 20 is an AC current circuit breaker. When a power fluctuation (hereinafter simply referred to as an "accident") occurs in a connected AC power (AC current) path (AC transmission line LN-AC) due to an accident or the like, each circuit breaker interrupts the path where the accident occurred. When an accident occurs in one or more of the renewable energy power generation system 10, the AC side of the power conversion device 40, and the connected AC transmission line LN-AC, each of the circuit breakers 20 electrically interrupts (disconnects) the connection point (connection terminal) on the side where the accident occurred. During normal (steady) power transmission in the high-voltage DC power transmission system 1, each of the circuit breakers 20 transmits AC current supplied by the renewable energy power generation system 10 to the AC side of the power conversion device 40. The circuit breaker operation of each of the circuit breakers 20 may be performed, for example, under the control of a higher-level control device (not shown) or under the control of a control device (not shown) located inside or near each of the circuit breakers 20. Each of the circuit breakers 20 outputs a signal (hereinafter referred to as a "relay signal R") indicating the circuit breaker operating state to the power conversion device 40. Only the relay signal R from the circuit breaker 20-B-2 is shown in Fig. 1. The relay signal R may be output to the power conversion device 40 by a higher-level control device (not shown) or a control device (not shown) located inside or near each of the circuit breakers 20.

[0015] In the high-voltage DC transmission system 1, a current detector 31 and a voltage detector 32 are further connected to the AC transmission line LN-AC between the connection point of the circuit breaker 20-A-2 and the circuit breaker 20-B-2 and the power conversion device 40. The current detector 31 and the voltage detector 32 are connected to the AC transmission line LN-AC at positions close to the AC terminals of the power converter 100 included in the power conversion device 40.

[0016] The shutoff device 20 is an example of a "shutoff device."

[0017] The current detector 31 detects the current flowing through the AC transmission line LN-AC. The current detector 31 outputs information (a signal) representing the detected current value (hereinafter referred to as the “current detection value I”) to the power conversion device 40.

[0018] The voltage detector 32 detects the voltage of the AC transmission line LN-AC. The voltage detector 32 outputs information (a signal) representing the detected voltage value (hereinafter referred to as the “detected voltage value V”) to the power conversion device 40.

[0019] [Configuration of Power Conversion Device] The power conversion device 40 includes, for example, a power converter 100 and a control device 200. The power conversion device 40 converts AC power, which is an AC current supplied by the renewable energy power generation system 10, into DC power at a grid voltage established by the power converter 100 in grid-forming operation. At this time, in the power conversion device 40, the control device 200 controls the operation of the power converter 100 based on the interruption operation state indicated by the relay signal R output by each interruption device 20, the detected current value I output by the current detector 31, and the detected voltage value V output by the voltage detector 32, thereby converting the AC power into DC power. The power conversion device 40 transmits (supplies) the DC power converted by the power converter 100 to the power receiving side via the DC transmission line LN-DC.

[0020] The power converter 100 is a high-voltage direct current (HVDC) AC-DC converter that converts AC power (AC current) supplied to its AC terminals by the renewable energy power generation system 10 into DC power (DC current) and outputs the DC power to a DC terminal connected to a DC transmission line LN-DC. In other words, the power converter 100 converts the AC power generated by the renewable energy power generation system 10 and transmitted via the AC transmission line LN-AC and a corresponding circuit breaker 20 into DC power and outputs the DC power to the DC transmission line LN-DC side. FIG. 1 shows the configuration of the power converter 100, which includes two semiconductor switches each composed of a semiconductor power device including a semiconductor switching element and a diode connected in parallel with each other, and one coil. The configuration of the power converter 100 shown in FIG. 1 is merely an example, and the power converter 100 may have any configuration. The power converter 100 may be, for example, a modular multilevel converter (MMC) having arms in which multiple unit converters each equipped with a capacitor are connected in series. The power converter 100 may be an AC / DC converter that has a function of converting DC power input to a DC terminal into AC power and outputting it to an AC terminal.

[0021] The power converter 100 is an example of a "power converter."

[0022] The control device 200 controls the operation of the power converter 100 based on the relay signal R output by each circuit breaker 20, the current detection value I output by the current detector 31, and the voltage detection value V output by the voltage detector 32.

[0023] More specifically, in the case of normal (steady) power transmission in the high-voltage DC transmission system 1, the control device 200 controls the voltage of the power converter 100 to output the established grid voltage from the AC terminal. As a result, AC current according to the grid voltage established by the voltage control is supplied from the renewable energy power generation system 10 to the AC terminal of the power converter 100. Then, the power converter 100 converts the AC power of the AC current supplied by the renewable energy power generation system 10 into DC power at the established grid voltage and outputs it from the DC terminal.

[0024] On the other hand, if a fault occurs in the AC system of the high-voltage DC transmission system 1, the control device 200 controls the operation of the power converter 100 to suppress an excess current (overcurrent) corresponding to a power fluctuation caused by the fault. In the following description, the control by the control device 200 on the power converter 100 to suppress the overcurrent is referred to as "overcurrent suppression control." After that, when the fault is resolved (the AC system recovers from the fault), the control device 200 returns the operation of the power converter 100 to voltage control and resumes normal (steady) power transmission.

[0025] 2A and 2B are diagrams schematically illustrating an example of control of the power converter 100 by the control device 200 included in the power conversion device 40 of the first embodiment when a fault occurs. Fig. 2A illustrates an example of a transition of control methods when a fault occurs during normal (steady) power transmission in the high-voltage DC power transmission system 1, and the control device 200 controls the operation of the power converter 100 in response to the fault.

[0026] In the voltage control during normal (steady) power transmission by the control device 200 (see (a) of FIG. 2), the established grid voltage is output from the power converter 100 as described above. In (b) of FIG. 2, the grid voltage V Ｓ is output, and this system voltage V Ｓ AC current I according to ＡＣ is supplied by the renewable energy power generation system 10.

[0027] If an accident occurs thereafter, the power converter 100 reduces the system voltage V ＳIn FIG. 2(c), an overcurrent corresponding to the short circuit (ground fault) fault X occurs in the AC transmission line LN-AC-B, and the system voltage V output from the voltage-controlled power converter 100 decreases. Ｓ Overcurrent I according to Ｘ Even in this case, the renewable energy power generation system 10 continues to operate for a predetermined period of time, with the AC current I ＡＣ However, as shown in (c) of FIG. 2, the AC current I supplied by the renewable energy power generation system 10 ＡＣ flows toward the ground (earth) due to the fault X that has occurred. Therefore, the control device 200 switches the control method of the power converter 100 from voltage control to overcurrent suppression control (see (a) of FIG. 2). (c) of FIG. 2 shows that the control device 200 has switched the control method of the power converter 100 to overcurrent suppression control, and as a result, the system voltage V Ｓ voltage at which the overcurrent suppression voltage V Ｃ "), and as a result, the overcurrent I Ｘ current (hereinafter referred to as the suppression current I Ｘ２ In other words, in (c) of FIG. 2, the overcurrent I Ｘ indicates a suppressed state.

[0028] Thereafter, when the fault is resolved (the fault is removed) (see (a) of FIG. 2), the control device 200 switches the control method of the power converter 100 from overcurrent suppression control to voltage control (returns to voltage control). As a result, normal (steady-state) power transmission is resumed in the high-voltage DC power transmission system 1. In (d) of FIG. 2, the fault X that occurred on the AC power transmission line LN-AC-B causes the circuit breakers 20-B-1 and 20-B-2 to break the paths, electrically disconnecting the AC power transmission line LN-AC-B (removing the fault), and the control device 200 controls the voltage of the power converter 100, thereby re-establishing the system voltage V from the power converter 100 to the AC terminals. Ｓ 2(d) shows the state in which the system voltage V is re-established. Ｓ AC current I according toＡＣ is supplied by the renewable energy power generation system 10.

[0029] In this way, the control device 200 controls the operation of the power converter 100 so as to suppress overcurrent corresponding to power fluctuations caused by an accident. At this time, the control device 200 controls the operation of the power converter 100 so that a series of controls is completed within the operation duration while the renewable energy power generation system 10 continues to generate AC current. In other words, the control device 200 controls the operation of the power converter 100 so that a series of controls is completed before the renewable energy power generation system 10, which continues to supply AC current even in a state where an accident has occurred, stops operating. Here, the operation duration is a time determined by the configuration and specifications of the renewable energy power generation system 10. The operation duration is determined, for example, based on the time during which operation can be continued even when the voltage is 0 [V], in the specifications of the voltage drop tolerance of the renewable energy power generation system 10.

[0030] The control device 200 is an example of a "control device."

[0031] [Configuration of Control Device] Returning to Fig. 1 , the control device 200 includes, for example, a comparator 212, an overcurrent detection unit 214, a comparator 222, a low-voltage detection unit 224, a fault removal detection unit 230, a gate block control unit 240, an overcurrent suppression control unit 250, a normal-state voltage command value calculation unit 260, a control selection unit 270, and a power converter control unit 280. The gate block control unit 240 includes, for example, a gate block determination unit 242 and a gate block signal generation unit 244. The overcurrent suppression control unit 250 includes, for example, a fault-state operation control unit 251, a control switching unit 252, an adder 253, a fault-state voltage command value calculation unit 254, and a fault-state voltage command value control unit 255. The control selection unit 270 includes, for example, a voltage command value selection unit 272 and a converter 274.

[0032] Some or all of the components of the control device 200 are realized by, for example, a hardware processor executing a program (software) stored in a memory (storage unit) (not shown), thereby realizing the functions of the respective components. The memory (not shown) is realized by, for example, a semiconductor memory element such as a read-only memory (ROM), a random access memory (RAM), or a flash memory, a hard disk drive (HDD), an optical disk, or the like.

[0033] The term "hardware processor" refers to a circuit such as a central processing unit (CPU), a graphics processing unit (GPU), a large-scale integration (LSI), a system on chip (SOC), an application-specific integrated circuit (ASIC), or a programmable logic device (e.g., a simple programmable logic device (SPLD) or a complex programmable logic device (CPLD), or a field programmable gate array (FPGA)). Instead of storing a program in a memory (not shown), the hardware processor may be configured so that the program is directly embedded in the circuit. In this case, the hardware processor realizes each function by reading and executing the program embedded in the circuit. The hardware processor is not limited to being configured as a single circuit, but may be configured as a single hardware processor by combining multiple independent circuits to realize each function. Multiple components may be integrated into a single hardware processor to realize each function. Each function may be realized by incorporating multiple components into a single dedicated LSI. Here, the program (software) may be stored in advance in a storage device (a storage device having a non-transitory storage medium) constituting a storage device such as a semiconductor memory element such as a ROM, RAM, or flash memory, or a hard disk drive, or may be stored in a removable storage medium (a non-transitory storage medium) such as a DVD or CD-ROM, and may be installed in a storage device (not shown) provided in the power conversion device 40 or the control device 200 by attaching the storage medium to a drive device provided in the power conversion device 40 or the control device 200.The program (software) may be downloaded in advance from another computer device via a network (not shown) and installed in a storage device provided in the power conversion device 40 or the control device 200. The program (software) installed in the storage device provided in the power conversion device 40 or the control device 200 may be transferred to a processing circuit provided in the power conversion device 40 or the control device 200 and executed therein.

[0034] The comparator 212 compares the current detection value I output by the current detector 31 with the overcurrent threshold I ＯＣ The magnitude of the overcurrent threshold I is compared with ＯＣ is a threshold current value for determining whether the current flowing through the AC transmission line LN-AC is an overcurrent. The comparator 212 compares the current detection value I with the overcurrent threshold I ＯＣ The comparison result between the overcurrent detection unit 214 and the low voltage detection unit 224 (hereinafter referred to as the “current comparison result”) is output to the overcurrent detection unit 214 and the low voltage detection unit 224, respectively.

[0035] The overcurrent detection unit 214 detects whether or not an overcurrent is flowing in the AC transmission line LN-AC based on the current comparison result output by the comparator 212. More specifically, the overcurrent detection unit 214 detects whether or not the current comparison result output by the comparator 212 is greater than or equal to the overcurrent threshold I ＯＣ If the current comparison result output by the comparator 212 indicates that the current detection value I is greater than the overcurrent threshold value I, it is determined that an overcurrent is flowing in the AC transmission line LN-AC. ＯＣ If the difference (a) indicates that the difference (b) is smaller than the value (b), it is determined that no overcurrent is flowing in the AC transmission line LN-AC. The overcurrent detection unit 214 outputs the determination result (hereinafter referred to as the "overcurrent determination result") of detecting (determining) whether or not an overcurrent is flowing in the AC transmission line LN-AC to the gate block control unit 240 (more specifically, the gate block determination unit 242).

[0036] The comparator 222 compares the voltage detection value V output by the voltage detector 32 with the low voltage threshold V ＬＶ Compare the magnitude with the low voltage threshold V ＬＶ is a threshold voltage value for determining whether the voltage of the AC transmission line LN-AC is a low voltage. The comparator 222 compares the voltage detection value V with the low voltage threshold VＬＶ The comparison result (hereinafter referred to as the “voltage comparison result”) is output to the low voltage detection unit 224 and the fault removal detection unit 230, respectively.

[0037] The low voltage detection unit 224 detects whether the voltage of the AC transmission line LN-AC is low based on the current comparison result output by the comparator 212 and the voltage comparison result output by the comparator 222. More specifically, the low voltage detection unit 224 detects whether the current comparison result output by the comparator 212 indicates that the current detection value I is equal to or lower than the overcurrent threshold I ＯＣ , that is, when an overcurrent is flowing, the voltage comparison result output by the comparator 222 indicates that the voltage detection value V is greater than the low voltage threshold V ＬＶ , the voltage of the AC transmission line LN-AC is determined to be a low voltage, and the voltage comparison result output by the comparator 222 indicates that the voltage detection value V is lower than the low voltage threshold V ＬＶ If the current comparison result output by the comparator 212 indicates that the current detection value I is higher than the overcurrent threshold I, the low voltage detection unit 224 determines that the voltage of the AC transmission line LN-AC is not low. ＯＣ , that is, when the voltage comparison result output by the comparator 222 indicates that the voltage detection value V is smaller than the low voltage threshold V ＬＶ , the voltage of the AC transmission line LN-AC is determined to be a low voltage, and the voltage comparison result output by the comparator 222 indicates that the voltage detection value V is lower than the low voltage threshold V ＬＶ If the voltage of the AC transmission line LN-AC is higher than 0, the low voltage detection unit 224 determines that the voltage of the AC transmission line LN-AC is not a low voltage. The low voltage detection unit 224 outputs a determination result (hereinafter referred to as a "low voltage determination result") obtained by detecting (determining) whether the voltage of the AC transmission line LN-AC is a low voltage to each of the gate block control unit 240 (more specifically, the gate block determination unit 242) and the overcurrent suppression control unit 250 (more specifically, the accident operation control unit 251).

[0038] The fault removal detection unit 230 detects whether or not the fault that occurred in the AC system has been removed (the fault has been resolved) based on the voltage comparison result output by the comparator 222 and the relay signal R output by each breaker device 20. More specifically, the fault removal detection unit 230 detects whether or not the voltage comparison result output by the comparator 222 indicates that the voltage detection value V is equal to or lower than the low voltage threshold V ＬＶ , that is, when the relay signal R output by any one of the circuit breakers 20 indicates that the circuit breaker operation state is in the "blocked" state, it is determined that the fault that occurred in the AC system has been removed, and when the relay signals R output by all of the circuit breakers 20 indicate that the circuit breaker operation state is not in the "blocked" state, it is determined that the fault that occurred in the AC system has not been removed. ＬＶ In the case where the relay signal R output from any one of the circuit breakers 20 indicates that the fault occurring in the AC system has been removed, that is, when the relay signal R output from any one of the circuit breakers 20 indicates that the circuit breaker operation state is in the "blocked" state, it is determined that the fault occurring in the AC system has not been removed, and when the relay signals R output from all of the circuit breakers 20 indicate that the circuit breaker operation state is not in the "blocked" state, it is determined that the fault occurring in the AC system has not been removed. The fault removal detection unit 230 outputs a determination result (hereinafter referred to as "fault removal determination result") of detecting (determining) whether or not the fault occurring in the AC system has been removed to the overcurrent suppression control unit 250 (more specifically, the fault operation control unit 251).

[0039] When an accident occurs in the AC system, the gate block control unit 240 performs control to electrically isolate the power converter 100 from the AC system in which the accident occurred, that is, to perform so-called gate block. This control is performed to protect the semiconductor power devices constituting the power converter 100 from an overcurrent that flows through the power converter 100 in response to the accident, so that the semiconductor power devices do not break down or are destroyed. In the following description, the control over the power converter 100 to isolate it from the AC system in which the accident occurred is referred to as "gate block control."

[0040] The gate block determination unit 242 determines whether to gate block the power converter 100 based on the overcurrent determination result output by the overcurrent detection unit 214 and the undervoltage determination result output by the undervoltage detection unit 224. More specifically, the gate block determination unit 242 determines to gate block the power converter 100 when the overcurrent determination result output by the overcurrent detection unit 214 indicates that an overcurrent is flowing in the AC transmission line LN-AC or when the undervoltage determination result output by the undervoltage detection unit 224 indicates that the voltage of the AC transmission line LN-AC is an undervoltage. On the other hand, the gate block determination unit 242 determines not to gate block the power converter 100 when the overcurrent determination result output by the overcurrent detection unit 214 indicates that an overcurrent is not flowing in the AC transmission line LN-AC and the undervoltage determination result output by the undervoltage detection unit 224 indicates that the voltage of the AC transmission line LN-AC is not an undervoltage. The gate block determining unit 242 outputs the determination result of whether or not to gate block the power converter 100 (hereinafter referred to as the “gate block determination result”) to the gate block signal generating unit 244 .

[0041] The gate block signal generation unit 244 generates a gate block signal for controlling gate blocking of the power converter 100 based on the gate block determination result output by the gate block determination unit 242. More specifically, when a gate block determination result indicating that the power converter 100 is to be gate blocked is input from the gate block determination unit 242, the gate block signal generation unit 244 sets the gate block signal to a state in which the power converter 100 is gate blocked and starts measuring time. Then, when the measured time has elapsed a predetermined gate block time, the gate block signal generation unit 244 sets the gate block signal to a state in which the gate block of the power converter 100 is released. In other words, the gate block signal generation unit 244 generates a gate block signal that gate blocks the power converter 100 only for the gate block time and then resets (restarts) the power converter 100 to return to a normal operating state. Here, the gate block time is a time shorter than the interruption possible time, which is determined based on the specifications of the interruption possible time required for each of the circuit breakers 20 to interrupt the path (disconnect the power converter 100 from the AC system where the accident occurred) after the occurrence of the accident. The gate block time may be determined, for example, by multiplying the interruption possible time by a predetermined time coefficient. The gate block signal generation unit 244 outputs the gate block signal thus generated to each of the power converter control unit 280 and the overcurrent suppression control unit 250 (more specifically, the accident operation control unit 251).

[0042] The gate block control unit 240 (including the gate block determination unit 242 and the gate block signal generation unit 244) is an example of a "gate block control unit."

[0043] The overcurrent suppression control unit 250 performs overcurrent suppression control when a fault occurs in the AC system. In order to directly suppress an overcurrent flowing through the power converter 100 in response to the fault, the overcurrent suppression control unit 250 performs overcurrent suppression control by switching the control of the power converter 100 from voltage control to current control (hereinafter referred to as "overcurrent suppression current control").

[0044] The accident operation control unit 251 controls the overall sequence (time flow) of the operation of overcurrent suppression current control performed on the power converter 100. To this end, the accident operation control unit 251 determines whether or not an accident has occurred in the AC system based on the low voltage determination result output by the low voltage detection unit 224. Furthermore, the accident operation control unit 251 checks the state of the gate block of the power converter 100 based on the gate block signal output by the gate block control unit 240 (more specifically, the gate block signal generation unit 244). Furthermore, the accident operation control unit 251 determines whether or not the accident that occurred in the AC system has been removed based on the fault removal determination result output by the fault removal detection unit 230. The accident operation control unit 251 generates an accident operation control signal for performing overcurrent suppression control (overcurrent suppression current control) on the power converter 100 based on the determination result of the occurrence of an accident, the confirmation result of the gate block state, and the determination result of the fault removal. More specifically, after determining that an accident has occurred and confirming that the gate block of the power converter 100 has been released, the accident operation control unit 251 sets the accident operation control signal to a state in which the power converter 100 starts overcurrent suppression current control. This restarts the power converter 100 through overcurrent suppression current control. After determining that the accident has been resolved, the accident operation control unit 251 starts counting time. When the counting time reaches a predetermined termination process start time, the accident operation control signal is set to a state in which termination process is started to terminate the overcurrent suppression current control. The termination process is, for example, a control that gradually restores the system voltage output by the power converter 100 to the normal (steady) system voltage over a predetermined termination process time to prevent the system voltage output by the power converter 100 from suddenly becoming the normal (steady) system voltage when the overcurrent suppression current control is subsequently terminated and control of the power converter 100 is returned to voltage control. In other words, the termination process is a control equivalent to a so-called soft start function.As a result, in the high-voltage DC transmission system 1, when the control of the power converter 100 is switched from overcurrent suppression current control to voltage control, it is possible to prevent a large current (i.e., an overcurrent different from the overcurrent caused by the power fluctuation due to the accident) from flowing if the system voltage output by the power converter 100 changes suddenly. Then, when the elapsed time since the start of the termination process reaches the termination process time, the accident-time operation control unit 251 sets the accident-time operation control signal to a state that terminates the overcurrent suppression current control for the power converter 100, that is, a state that returns the control of the power converter 100 to voltage control. The accident-time operation control unit 251 outputs the accident-time operation control signal generated in this manner to each of the control switching unit 252 and the accident-time voltage command value control unit 255.

[0045] 3 is a diagram schematically illustrating an example of a time flow of control of the power converter 100 by the control device 200 included in the power conversion device 40 of the first embodiment when a fault occurs. In FIG. 3, the total control time from the occurrence of the fault to the end of the overcurrent suppression current control is represented as a control time T ＦＲＴ The time required for the breaker 22 to clear the fault after the fault occurs is defined as the breaking time T ＣＢｏｐｅｎ 3 shows an example of the relationship between the respective times when the control device 200 performs overcurrent suppression current control. Furthermore, FIG. 3 shows the relationship between the system voltage V Ｓ Or the overcurrent suppression voltage V Ｃ An example of the above is shown schematically.

[0046] When the control device 200 is performing voltage control during normal (steady) power transmission in the high-voltage DC power transmission system 1, an accident occurs in the AC system, and if the accident is detected, the gate block control unit 240 in the control device 200 outputs a gate block signal for gate-blocking the power converter 100. Thereafter, the gate block control unit 240 outputs a gate block signal for a predetermined gate block time T ＧＢWhen the time T has elapsed, the gate block signal being output is set to a state that releases the gate block of the power converter 100 (restarts the power converter 100). As a result, the fault operation control unit 251 sets the fault operation control signal to a state that starts overcurrent suppression current control for the power converter 100. Thereafter, the fault operation control unit 251 starts the termination process after a predetermined fixed termination process start time T ＢＳＳ When the termination processing time T has elapsed, the accident operation control signal is set to a state in which the termination processing is started. ＳＳ When the time has elapsed, the fault operation control signal is set to a state in which the overcurrent suppression current control for the power converter 100 is ended.

[0047] Here, the relationship between the operation duration of the renewable energy power generation system 10 and the control of the operation of the power converter 100 by the control device 200 will be described. Fig. 4 is a diagram showing an example of the specification of the voltage drop tolerance of the renewable energy power generation system 10 constituting the high-voltage DC power transmission system 1 including the power conversion device 40 of the first embodiment. In the example shown in Fig. 4, even if the voltage established by the power converter 100 drops to 0 [V] due to an accident occurring in the high-voltage DC power transmission system 1, the renewable energy power generation system 10 can continue to operate for the operation duration T ＳＯＡ = 150 [ms]. Here, the interruption time required for each of the interrupting devices 20 to clear the fault that has occurred is defined as the interruption time T Ｃｙｃｌｅ The interruption time T Ｃｙｃｌｅ In such a case, the fault operation control unit 251 determines each of the times shown in Fig. 3 using the following relational expressions, and controls the operation of the power converter 100 at each of the determined times.

[0048] T ＣＢｏｐｅｎ = T Ｃｙｃｌｅ ...(1)

[0049] T ＧＢ < K.T. ＣＢｏｐｅｎ ... (2)

[0050] In the above equation (2), K is a coefficient determined by the configuration and specifications of the power converter 100. The coefficient K is, for example, a value between 0 and 1 (0<K<1). The coefficient K is, for example, 0.5.

[0051] T ＦＲＴ = T ＣＢｏｐｅｎ + T ＢＳＳ + T ＳＳ < T ＳＯＡ ...(3)

[0052] In this way, in the high-voltage DC transmission system 1, the control device 200 controls the operation of the power converter 100 so that, in the event of a fault, the control device 200 starts overcurrent suppression current control before the fault has been cleared by the circuit breaker 20, and returns control to voltage control after the fault has been cleared by the circuit breaker 20. At this time, as described above, the control device 200 performs a series of controls on the power converter 100 during the operation duration during which the renewable energy power generation system 10 continues to generate AC current. More specifically, the fault operation control unit 251 calculates the total control time T from the occurrence of the fault to the end of the overcurrent suppression current control as shown in the above formula (3). ＦＲＴ is the operation duration T ＳＯＡ , and completes a series of controls on the power converter 100 so that the overcurrent generated by the fault does not exceed 1. As a result, in the high-voltage DC transmission system 1, the semiconductor power devices constituting the power converter 100 are protected by suppressing the overcurrent caused by the fault, and when the fault is cleared, the power converter 100 is in a started-up state (resuming operation), thereby preventing the renewable energy power generation system 10 from shutting down.

[0053] Returning to FIG. 1 , the control switching unit 252 switches the selection of the control method of the power converter 100 by the control selection unit 270 in accordance with the accident-time operation control signal output by the accident-time operation control unit 251. More specifically, when the accident-time operation control signal output by the accident-time operation control unit 251 indicates that overcurrent suppression current control is to be performed on the power converter 100, that is, during the period from when the accident-time operation control signal starts overcurrent suppression current control to when it ends overcurrent suppression current control, the control switching unit 252 outputs a switching signal to the control selection unit 270 to switch the control method of the power converter 100 to overcurrent suppression current control. On the other hand, when the accident-time operation control signal output by the accident-time operation control unit 251 indicates that control other than overcurrent suppression current control is to be performed on the power converter 100, that is, voltage control, the control switching unit 252 outputs a switching signal to the control selection unit 270 to switch the control method of the power converter 100 to voltage control.

[0054] The adder 253 calculates the difference between the current command value and the current detection value I detected by the current detector 31. More specifically, the adder 253 calculates the difference between the current command value I ＊ The current difference value is calculated by subtracting the current detection value I from the current command value I. ＊ is a command value of the current value for suppressing the overcurrent flowing to the power converter 100 in response to the fault that has occurred, and the current value of the overcurrent flowing to the power converter 100 in response to the fault that has occurred is equal to or greater than the overcurrent threshold I ＯＣ The adder 253 outputs the calculated current difference value to the fault-time voltage command value calculation unit 254. The adder 253 may be configured as, for example, a subtractor.

[0055] The fault-time voltage command value calculation unit 254 generates a voltage command value to be output to the power converter 100 in overcurrent suppression current control based on the current difference value output by the adder 253. More specifically, the voltage command value is generated by multiplying the current difference value output by the adder 253 by a predetermined current control coefficient Ki. The current control coefficient Ki is, for example, a control gain equivalent to a resistance value, and is a value used to obtain the voltage command value by multiplying the current difference value. The fault-time voltage command value calculation unit 254 outputs the generated voltage command value to the fault-time voltage command value control unit 255.

[0056] The fault-time voltage command value control unit 255 controls the voltage command value output by the fault-time voltage command value calculation unit 254 in accordance with the fault-time operation control signal output by the fault-time operation control unit 251. The fault-time voltage command value control unit 255 outputs the controlled voltage command value (hereinafter referred to as the "fault-time voltage command value") to the control selection unit 270. More specifically, when the fault-time operation control signal output by the fault-time operation control unit 251 becomes a state in which overcurrent suppression current control is started for the power converter 100, the fault-time voltage command value control unit 255 outputs the voltage command value output by the fault-time voltage command value calculation unit 254 to the control selection unit 270 as the fault-time voltage command value. As a result, in the overcurrent suppression current control, the system voltage output (established) by the power converter 100 is suppressed to be lower than the system voltage in normal (steady) conditions. As a result, the overcurrent generated in the power converter 100 according to the system voltage is suppressed to be lower than the overcurrent according to the system voltage when an accident occurs. Thereafter, when the accident-time operation control signal output by the accident-time operation control unit 251 reaches a state in which termination processing for terminating the overcurrent suppression current control is to be started, the accident-time voltage command value control unit 255 gradually increases the voltage command value output by the accident-time voltage command value calculation unit 254, and controls (changes) it to the voltage command value of the system voltage in normal (steady) conditions. At this time, the accident-time voltage command value control unit 255 controls (changes) it to the voltage command value of the system voltage in normal (steady) conditions for a predetermined certain termination processing time (the termination processing time T ＳＳ) to gradually increase the voltage value represented by the voltage command value to the voltage value represented by the voltage command value in normal (steady) state. As a method for gradually increasing the voltage value represented by the voltage command value to the voltage value represented by the voltage command value in normal (steady) state, the fault-time voltage command value control unit 255 may perform filtering using a low-pass filter (LPF) to suppress voltage values ​​of frequencies higher than a predetermined frequency (cutoff frequency). The fault-time voltage command value control unit 255 outputs the voltage command value obtained by controlling (changing) the voltage value to the control selection unit 270 as the fault-time voltage command value. As a result, in the overcurrent suppression current control, the grid voltage output (established) by the power converter 100 does not suddenly become the grid voltage in normal (steady) state but gradually becomes the grid voltage in normal (steady) state over a time period for the termination process, thereby making it possible to suppress an overcurrent from flowing in the power converter 100 when the overcurrent suppression current control is terminated.

[0057] The overcurrent suppression control unit 250 is an example of an "overcurrent suppression control unit." The configuration of the adder 253 and the fault-time voltage command value calculation unit 254 is an example of an "fault-time command value calculation unit" and a "first fault-time command value calculation unit." The voltage command value output by the fault-time voltage command value calculation unit 254 is an example of an "fault-time command value" and a "first fault-time command value." The fault-time voltage command value control unit 255 is an example of an "fault-time command value control unit."

[0058] The normal-time voltage command value calculation unit 260 generates a voltage command value to be output to the power converter 100 during normal (steady) power transmission, based on the voltage value of the grid voltage established during normal (steady) times in the high-voltage DC power transmission system 1. More specifically, the normal-time voltage command value calculation unit 260 calculates a voltage command value to be output to the power converter 100 during normal (steady) power transmission, based on the voltage value of the grid voltage V Ｓ The voltage value of the power converter 100 is multiplied by a predetermined voltage control coefficient Kv to generate a voltage command value. The voltage control coefficient Kv is a control gain that represents the voltage value. For example, when Kv=1, that is, when the system voltage established by the power converter 100 is equal to or greater than the system voltage V Ｓ The normal-time voltage command value calculation unit 260 outputs the generated voltage command value (hereinafter referred to as the “normal-time voltage command value”) to the control selection unit 270.

[0059] In response to a switching signal output by overcurrent suppression control unit 250 (more specifically, control switching unit 252), control selection unit 270 selects either the fault-time voltage command value output by overcurrent suppression control unit 250 (more specifically, fault-time voltage command value control unit 255) or the normal-time voltage command value output by normal-time voltage command value calculation unit 260. Control selection unit 270 outputs the selected voltage command value to power converter control unit 280 as the voltage command value to be output to power converter 100.

[0060] The voltage command value selection unit 272 selects either the fault-time voltage command value output by the fault-time voltage command value control unit 255 or the normal-time voltage command value output by the normal-time voltage command value calculation unit 260 in accordance with the switching signal output by the control switching unit 252. The voltage command value selection unit 272 sets the selected voltage command value as the voltage command value V ＊ (command value having amplitude information) to the converter 274.

[0061] The converter 274 converts the voltage command value V having the amplitude information output by the voltage command value selector 272 into ＊ The converter 274 outputs the voltage command value having the amplitude information and phase information to the power converter control unit 280 as the voltage command value to be output to the power converter 100.

[0062] The power converter control unit 280 generates a pulse signal representing the phase angle output by the control selection unit 270 (more specifically, the converter 274). That is, the power converter control unit 280 generates a pulse signal representing a voltage command value in voltage control or overcurrent suppression current control. More specifically, in order to perform switching control of semiconductor switches configured by semiconductor power devices included in the power converter 100, the power converter control unit 280 performs, for example, phase-shift PWM (Pulse Width Modulation) to generate a multi-step, stepped control waveform in which the phases of the control signals input to each semiconductor switch are uniformly shifted. The power converter control unit 280 outputs the generated control waveform to the power converter 100, causing the power converter 100 to output a grid voltage.

[0063] The configuration of control selection section 270 (including voltage command value selection section 272 and converter 274) and power converter control section 280 is an example of a "command value output section".

[0064] [Processing of Control Device] Next, an example of processing when the control device 200 in the power conversion device 40 controls the power converter 100 will be described. FIG. 5 is a flowchart showing an example of processing when the control device 200 included in the power conversion device 40 of the first embodiment controls the power converter 100. FIG. 5 shows an example of a processing flow for controlling the operation of the power converter 100, which is performed by the gate block control unit 240 and the fault operation control unit 251 in the overcurrent suppression control unit 250 included in the control device 200. More specifically, FIG. 5 shows an example of a processing flow for controlling the operation of the power converter 100, which is performed by the gate block control unit 240 and the fault operation control unit 251 in the overcurrent suppression control unit 250 included in the control device 200. In a state where a grid voltage is established in the power converter 100 for normal (steady-state) power transmission in the high-voltage DC transmission system 1, when an fault occurs in the AC grid, the series of controls is completed while the renewable energy power generation system 10 continues to supply AC current while suppressing overcurrent due to the fault, and the power converter 100 is returned to a normal (steady-state) operating state.

[0065] The processing of this flowchart may be repeatedly executed while power is being transmitted in the high-voltage DC power transmission system 1. In the following description, for ease of explanation, it is assumed that the control device 200 controls the operation of the power converter 100.

[0066] In the following description, in the power conversion device 40, the overcurrent threshold I ＯＣ , low voltage threshold V ＬＶ , current command value I ＊ , and the normal (steady) system voltage V Ｓ It is assumed that the values ​​of these are determined in advance and input (set) to the control device 200. Furthermore, in the following description, it is assumed that the power conversion device 40 has an operation duration T ＳＯＡ , and the interruption time T Ｃｙｃｌｅ Control time T based on ＦＲＴ , interruption time T ＣＢｏｐｅｎ , gate block time T ＧＢ (Gate block time T ＧＢ), the termination process start time T ＢＳＳ , and the final processing time T ＳＳ It is assumed that the respective times are predetermined and input (set) or determined in the control device 200. Furthermore, in the following description, it is assumed that the respective coefficients of the current control coefficient Ki and the voltage control coefficient Kv in the power conversion device 40 are predetermined and input (set) in the control device 200.

[0067] The control device 200 performs voltage control on the power converter 100, causing the power converter 100 to output a system voltage established for normal (steady) power transmission (step S100). The control device 200 then determines whether or not a fault has occurred in the AC system (step S110). If it is determined in step S110 that no fault has occurred, the control device 200 returns to step S100 and repeats voltage control on the power converter 100.

[0068] On the other hand, if it is determined in step S110 that an accident has occurred, the control device 200 gate-blocks the power converter 100 (step S200). Then, the control device 200 starts measuring the time in the gate-block control. Thereafter, the control device 200 counts the gate-block time T ＧＢ In step S202, it is determined whether the gate block time T ＧＢ If it is determined that the time has not elapsed, the control device 200 returns the process to step S200 and repeats the counting of time.

[0069] On the other hand, in step S202, the gate block time T ＧＢ If it is determined that the predetermined time has elapsed, the control device 200 releases the gate block of the power converter 100 (step S204).

[0070] Then, the control device 200 starts the overcurrent suppression current control for the power converter 100 (step S300). At this time, the control device 200 restarts the power converter 100 and continues the overcurrent suppression current control for the power converter 100 (step S302). Then, the control device 200 starts measuring time.

[0071] Thereafter, the control device 200 determines whether or not the interrupter 20 corresponding to the fault has detected the completion of fault clearance (step S304). If it is determined in step S304 that the completion of fault clearance has not been detected, the control device 200 returns the process to step S302 and continues the overcurrent suppression current control for the power converter 100.

[0072] On the other hand, if it is determined in step S304 that the completion of the fault removal has been detected, the control device 200 starts measuring the time in the overcurrent suppression current control. ＢＳＳ In step S306, it is determined whether the end process start time T ＢＳＳ If it is determined that the time has not elapsed, the control device 200 repeats the counting of time.

[0073] On the other hand, in step S306, the termination process start time T ＢＳＳ If it is determined that the termination time T has elapsed, the control device 200 starts the termination process of the overcurrent suppression current control for the power converter 100 (step S308). ＳＳ In step S310, it is determined whether the end processing time T ＳＳ If it is determined that the time has not elapsed, the control device 200 repeats counting the time and continues the process of ending the overcurrent suppression current control for the power converter 100 .

[0074] On the other hand, in step S310, the end processing time T ＳＳIf it is determined that the time has elapsed, the control device 200 terminates the overcurrent suppression current control and the termination process for the power converter 100. Then, the control device 200 returns the control for the power converter 100 to voltage control (step S320). Then, the control device 200 returns the process to step S100 and causes the power converter 100 to output the established grid voltage for normal (steady) power transmission.

[0075] With this configuration and processing, if a fault occurs in the AC system during normal (steady) power transmission in the high-voltage DC power transmission system 1, the control device 200 gate-blocks the power converter 100 to suppress overcurrent caused by the fault. Thereafter, the control device 200 releases the gate block and restarts the power converter 100, and also changes the control of the operation of the power converter 100 from voltage control to current command value I ＊ and after detecting the completion of fault clearance by the circuit breaker 20, performs processing to terminate the overcurrent suppression current control and then switches back to voltage control. The control device 200 completes this series of control operations for the power converter 100 while the renewable energy power generation system 10 continues to supply AC current. As a result, in the high-voltage DC transmission system 1, overcurrent due to a fault is suppressed to protect the semiconductor power devices that make up the power converter 100, and when the fault is cleared, the power converter 100 is restarted, thereby preventing the renewable energy power generation system 10 from shutting down.

[0076] As described above, in the high-voltage DC transmission system 1 including the power conversion device 40 of the first embodiment, even when the power converter 100 using semiconductor power devices is introduced instead of the generator provided in the AC system on the power transmission side in a conventional high-voltage DC transmission system, the high-voltage DC transmission system 1 includes the power conversion device 40 of the first embodiment. This prevents the renewable energy power generation system 10 from stopping operation due to an accident, and when the accident is resolved, the high-voltage DC transmission system 1 includes the power conversion device 40 of the first embodiment. Therefore, the high-voltage DC transmission system 1 including the power conversion device 40 of the first embodiment can prevent the renewable energy power generation system 10 from stopping operation due to an accident. Therefore, the high-voltage DC transmission system 1 includes the power conversion device 40 of the first embodiment. Therefore, when an accident occurs in a conventional high-voltage DC transmission system including a power converter using semiconductor power devices, power transmission to the power receiving side is stopped for several minutes until the renewable energy power generation system is disconnected (de-paralleled off) from the AC system and stops. In contrast, in the high-voltage DC transmission system 1 equipped with the power conversion device 40 of the first embodiment, the time during which the renewable energy power generation system 10 stops operating due to an accident can be set to 0 seconds, and the time during which power transmission to the power receiving side stops can be shortened to several tens of milliseconds, which corresponds to the period during which the power converter 100 is gate-blocked.

[0077] Second Embodiment [Configuration of High-Voltage DC Transmission System] An example of the configuration of a high-voltage DC transmission system including a power conversion device according to a second embodiment will now be described. Fig. 6 is a diagram showing an example of the configuration of a high-voltage DC transmission system including a power conversion device according to the second embodiment. Similar to the high-voltage DC transmission system 1 including the power conversion device 40 of the first embodiment shown in Fig. 1, the high-voltage DC transmission system 2 including the power conversion device of the second embodiment shown in Fig. 6 also shows an example of the configuration of the power transmission side. The high-voltage DC transmission system 2 includes, for example, a renewable energy power generation system 10 and a power conversion device 50 on the power transmission side.

[0078] The high-voltage DC transmission system 2 including the power conversion device 50 also includes the same components as those in the high-voltage DC transmission system 1. In Fig. 6, the same reference numerals are used to denote components of the high-voltage DC transmission system 2 and the power conversion device 50 that have the same functions as those of the components of the high-voltage DC transmission system 1 and the power conversion device 40. Therefore, detailed descriptions of components of the high-voltage DC transmission system 2 and the power conversion device 50 that have the same configurations and operations as those of the components of the high-voltage DC transmission system 1 and the power conversion device 40 will be omitted, and only different configurations and operations will be described.

[0079] Similar to the HVDC transmission system 1, the HVDC transmission system 2 also converts AC power generated by a renewable energy power generation system 10, which is connected as an AC system on the power transmission side, into DC power using a power conversion device 50, and transmits the DC power to the power receiving side via two DC transmission lines LN-DC (DC transmission line LN-DC-A and DC transmission line LN-DC-B). Similar to the HVDC transmission system 1, the power conversion device 50 also performs grid forming (GFM) operation (voltage control) to operate the power converter 100 as a voltage source, thereby establishing an AC system voltage (high voltage) for DC transmission. Similarly to the HVDC transmission system 1, the HVDC transmission system 2 also generates a current (AC current) according to the established system voltage of the AC system and supplies it to the power conversion device 50. As a result, in the high-voltage DC transmission system 2, as in the high-voltage DC transmission system 1, the power conversion device 50 (more specifically, the power converter 100) converts the AC power of the AC current supplied by the renewable energy power generation system 10 into DC power at the system voltage established by the power converter 100, and transmits the high-voltage DC power.

[0080] In the high-voltage DC transmission system 2, similarly to the high-voltage DC transmission system 1, the transmitted DC power is converted back into AC power at the power receiving side (not shown) and supplied to the AC system at the power receiving side. In the high-voltage DC transmission system 2, similarly to the high-voltage DC transmission system 1, the AC power may be converted back into DC power at the AC system at the power receiving side (not shown) (the AC system at the power receiving side (not shown) may operate as the AC system at the power transmitting side in the high-voltage DC transmission system at the next stage), and the DC power may be transmitted further away.

[0081] [Configuration of Power Conversion Apparatus] The power conversion apparatus 50 includes, for example, a power converter 100 and a control device 300. In the power conversion apparatus 50, the control device 200 included in the power conversion apparatus 40 is replaced with the control device 300. Like the power conversion apparatus 40, the power conversion apparatus 50 also converts AC power, which is an AC current supplied by the renewable energy power generation system 10, into DC power at a grid voltage established by the power converter 100 in grid-forming operation. At this time, in the power conversion apparatus 50, like the control device 200 included in the power conversion apparatus 40, the control device 300 converts AC power into DC power by controlling the operation of the power converter 100 based on the interruption operation state indicated by the relay signal R output by each interruption device 20, the current detection value I output by the current detector 31, and the voltage detection value V output by the voltage detector 32. Like the power conversion device 40, the power conversion device 50 also transmits (supplies) DC power converted by the power converter 100 to the power receiving side via the DC transmission line LN-DC.

[0082] Like the control device 200, the control device 300 also controls the operation of the power converter 100 based on the relay signal R output by each circuit breaker 20, the detected current value I output by the current detector 31, and the detected voltage value V output by the voltage detector 32. That is, during normal (steady) power transmission in the high-voltage DC transmission system 2, the control device 300 also performs voltage control on the power converter 100 to output the established system voltage from the AC terminals, and performs overcurrent suppression control on the power converter 100 when a fault occurs in the AC system of the high-voltage DC transmission system 2. In this case, the control of the power converter 100 by the control device 300 is similar to the control of the power converter 100 by the control device 200 (see FIG. 2 ). However, the method of overcurrent suppression control in the control device 300 is different from that in the control device 200.

[0083] The control device 300 is an example of a "control device."

[0084] [Configuration of Control Device] The control device 300 includes, for example, a comparator 212, an overcurrent detection unit 214, a comparator 222, a low-voltage detection unit 224, a fault removal detection unit 230, a gate block control unit 240, an overcurrent suppression control unit 350, a normal-state voltage command value calculation unit 260, a control selection unit 270, and a power converter control unit 280. The gate block control unit 240 includes, for example, a gate block determination unit 242 and a gate block signal generation unit 244. The overcurrent suppression control unit 350 includes, for example, a fault-time operation control unit 251, a control switching unit 252, a fault-time voltage command value calculation unit 356, and a fault-time voltage command value control unit 357. The control selection unit 270 includes, for example, a voltage command value selection unit 272 and a converter 274. In the control device 300, the overcurrent suppression control unit 250 provided in the control device 200 is replaced with an overcurrent suppression control unit 350. In the overcurrent suppression control unit 350, the adder 253, the fault-time voltage command value calculation unit 254, and the fault-time voltage command value control unit 255 provided in the overcurrent suppression control unit 250 are replaced with a fault-time voltage command value calculation unit 356 and a fault-time voltage command value control unit 357.

[0085] In the control device 300, as in the control device 200, some or all of the components of the control device 300 realize the functions of each component, for example, by a hardware processor executing a program (software) stored in a memory (storage unit) not shown.

[0086] Like the overcurrent suppression control unit 250, the overcurrent suppression control unit 350 also performs overcurrent suppression control when a fault occurs in the AC system. In order to suppress an overcurrent flowing through the power converter 100 in response to the fault, the overcurrent suppression control unit 350 performs overcurrent suppression control while maintaining voltage control of the power converter 100. In other words, unlike the overcurrent suppression control unit 250, the overcurrent suppression control unit 350 does not switch the control of the power converter 100 from voltage control to overcurrent suppression current control, but performs overcurrent suppression control while maintaining voltage control. In the following description, in order to distinguish between voltage control during normal (steady-state) power transmission in the high-voltage DC transmission system 2 and voltage control when overcurrent suppression control is performed, the voltage control for overcurrent suppression control will be referred to as "overcurrent suppression voltage control."

[0087] In the overcurrent suppression control unit 350, the accident-time operation control signal generated by the accident-time operation control unit 251 controls the overall sequence (time flow) of the operation of the overcurrent suppression voltage control performed on the power converter 100. The accident-time operation control unit 251 outputs the generated accident-time operation control signal to the control switching unit 252 and the accident-time voltage command value control unit 357. The time flow of control of the power converter 100 by the control device 300 provided in the power conversion device 50 when an accident occurs is equivalent to the example of the time flow of control of the power converter 100 by the control device 200 when an accident occurs shown in FIG. 3, with overcurrent suppression current control replaced by overcurrent suppression voltage control. Furthermore, the relationship between the operation duration of the renewable energy power generation system 10 and the control of the operation of the power converter 100 by the control device 300 is equivalent to the relationship between the operation duration of the renewable energy power generation system 10 and the control of the operation of the power converter 100 by the control device 200 described using FIG. 4. Therefore, we will not provide a detailed explanation of the relationship between the time flow of control of the power converter 100 by the control device 300 when an accident occurs, the continuous operation time of the renewable energy power generation system 10, and the control of the operation of the power converter 100 by the control device 300.

[0088] The fault-time voltage command value calculation unit 356 generates a voltage command value to be output to the power converter 100 in the overcurrent suppression voltage control based on the voltage value of the grid voltage established in normal (steady) conditions, which is the same as that of the normal-time voltage command value calculation unit 260. More specifically, the fault-time voltage command value calculation unit 356 calculates the voltage command value to be output to the power converter 100 in the overcurrent suppression voltage control based on the voltage value of the grid voltage established in normal (steady) conditions. Ｓ The voltage command value is generated by multiplying the voltage value of the power converter 100 by a predetermined overcurrent suppression voltage control coefficient Kvs. The overcurrent suppression voltage control coefficient Kvs is a control gain that represents the voltage value, and is a value that is (much) smaller than the voltage control coefficient Kv used by the normal-state voltage command value calculation unit 260. For example, when the voltage control coefficient Kv is set to a value that is smaller than the system voltage V ＳWhereas the overcurrent suppression voltage control coefficient Kvs is a value (Kv=1) that is equal to the voltage command value itself, the overcurrent suppression voltage control coefficient Kvs is a (very) small value, such that Kvs<<1. For example, the overcurrent suppression voltage control coefficient Kvs is 0.1. As a result, in the overcurrent suppression voltage control, the system voltage output (established) by the power converter 100 is suppressed to be lower than the system voltage in normal (steady) conditions, and as a result, the overcurrent generated in the power converter 100 according to the system voltage is suppressed to be smaller than the overcurrent according to the system voltage when a fault occurs. The fault-time voltage command value calculation unit 356 outputs the generated voltage command value to the fault-time voltage command value control unit 357.

[0089] Similar to the fault-time voltage command value control unit 255 included in the overcurrent suppression control unit 250, the fault-time voltage command value control unit 357 controls the voltage command value output by the fault-time voltage command value calculation unit 356 in accordance with the fault-time operation control signal output by the fault-time operation control unit 251. The fault-time voltage command value control unit 357 outputs the controlled voltage command value (hereinafter referred to as the "fault-time voltage command value") to the control selection unit 270. More specifically, when the fault-time operation control signal output by the fault-time operation control unit 251 reaches a state in which overcurrent suppression voltage control is started for the power converter 100, the fault-time voltage command value control unit 357 outputs the voltage command value output by the fault-time voltage command value calculation unit 356 to the control selection unit 270 as the fault-time voltage command value. Thereafter, when the accident-time operation control signal output by the accident-time operation control unit 251 reaches a state in which termination processing for terminating the overcurrent suppression voltage control is to be started, the accident-time voltage command value control unit 357 gradually increases the voltage command value output by the accident-time voltage command value calculation unit 356, and controls (changes) it to the voltage command value of the system voltage in normal (steady) conditions. At this time, the accident-time voltage command value control unit 357 also controls (changes) the voltage command value output by the accident-time voltage command value calculation unit 356 to the voltage command value of the system voltage in normal (steady) conditions, similar to the accident-time voltage command value control unit 255, for a predetermined certain termination processing time (the termination processing time T ＳＳ(see section 270) to gradually increase the voltage value represented by the voltage command value to the voltage value represented by the voltage command value in normal (steady) conditions. The method by which the fault-time voltage command value control unit 357 gradually increases the voltage value represented by the voltage command value to the voltage value represented by the voltage command value in normal (steady) conditions is known as soft start. As a method by which the fault-time voltage command value control unit 357 gradually increases the voltage value represented by the voltage command value to the voltage value represented by the voltage command value in normal (steady) conditions, for example, the fault-time voltage command value control unit 357 may perform filtering using a low-pass filter (LPF) to suppress voltage values ​​of frequencies higher than a predetermined frequency (cutoff frequency). The fault-time voltage command value control unit 357 outputs the voltage command value obtained by controlling (changing) the voltage value to the control selection unit 270 as the fault-time voltage command value. As a result, in the overcurrent suppression voltage control, as in the overcurrent suppression current control, the system voltage output (established) by the power converter 100 does not suddenly become the normal (steady) system voltage, but gradually becomes the normal (steady) system voltage over the time required for the termination processing, thereby preventing overcurrent from flowing in the power converter 100 when the overcurrent suppression voltage control is terminated.

[0090] The overcurrent suppression control unit 350 is an example of an "overcurrent suppression control unit." The accident-time voltage command value calculation unit 356 is an example of an "accident-time command value calculation unit" and a "second accident-time command value calculation unit." The voltage command value output by the accident-time voltage command value calculation unit 356 is an example of an "accident-time command value" and a "second accident-time command value." The accident-time voltage command value control unit 357 is an example of an "accident-time command value control unit."

[0091] In response to a switching signal output by the overcurrent suppression control unit 350 (more specifically, the control switching unit 252), the control selection unit 270 selects either the fault-time voltage command value output by the overcurrent suppression control unit 350 (more specifically, the fault-time voltage command value control unit 357) or the normal-time voltage command value output by the normal-time voltage command value calculation unit 260. The control selection unit 270 outputs the selected voltage command value to the power converter control unit 280 as the voltage command value to be output to the power converter 100.

[0092] The voltage command value selection unit 272 selects either the fault-time voltage command value output by the fault-time voltage command value control unit 357 or the normal-time voltage command value output by the normal-time voltage command value calculation unit 260 in accordance with the switching signal output by the control switching unit 252. The voltage command value selection unit 272 sets the selected voltage command value as the voltage command value V ＊ (command value having amplitude information) to the converter 274. As a result, the converter 274 converts the voltage command value V ＊ and outputs a voltage command value having amplitude information and phase information to the power converter control unit 280 as a voltage command value to be output to the power converter 100 .

[0093] [Processing of Control Device] Next, an example of processing when the control device 300 in the power conversion device 50 controls the power converter 100 will be described. Fig. 7 is a flowchart showing an example of processing when the control device 300 provided in the power conversion device 50 of the second embodiment controls the power converter 100. Like the flowchart shown in Fig. 5, Fig. 7 shows an example of the flow of processing for controlling the operation of the power converter 100 performed by the gate block control unit 240 and the accident operation control unit 251 in the overcurrent suppression control unit 350 provided in the control device 300. However, in the flowchart shown in Fig. 7, the control device 300 performs overcurrent suppression voltage control instead of the overcurrent suppression current control in the flowchart shown in Fig. 5.

[0094] The processing of this flowchart may also be repeatedly executed while power is being transmitted in the high-voltage DC power transmission system 2. For ease of explanation, the following description will also be given assuming that the control device 300 controls the operation of the power converter 100. However, in the flowchart shown in Figure 7, the same steps as those in the flowchart shown in Figure 5 are assigned the same step numbers, and detailed explanations thereof will not be repeated.

[0095] In the following description, the overcurrent threshold I ＯＣ , low voltage threshold V ＬＶ , and the normal (steady) system voltage V ＳIt is assumed that the values ​​of the above are determined in advance and input (set) to the control device 300. Furthermore, in the following description, it is assumed that the values ​​of the above are determined in advance and input (set) to the control device 300. ＳＯＡ , and the interruption time T Ｃｙｃｌｅ Control time T based on ＦＲＴ , interruption time T ＣＢｏｐｅｎ , gate block time T ＧＢ (Gate block time T ＧＢ ), the termination process start time T ＢＳＳ , and the final processing time T ＳＳ It is assumed that the respective times are predetermined and input (set) or determined in the control device 300. Furthermore, in the following description, it is assumed that the respective coefficients of the voltage control coefficient Kv and the overcurrent suppression voltage control coefficient Kvs are predetermined in the power conversion device 50 and input (set) in the control device 300.

[0096] The control device 300 also performs voltage control on the power converter 100 in the processes of steps S100 to S110, causing the power converter 100 to output the system voltage established for normal (steady) power transmission, and while it determines in step S110 that an accident has not been detected, it repeats the voltage control on the power converter 100 by the process of step S100. Then, when it determines in step S110 that an accident has been detected, the control device 300 also gate-blocks the power converter 100 in the processes of steps S200 to S204, and in step S202, it sets the gate block time T ＧＢ While it is determined that the gate block time T has not elapsed, the control device 300 repeats counting of time in the gate block control and maintains the gate-blocked state of the power converter 100. Then, in step S202, the control device 300 also determines that the gate block time T ＧＢ If it is determined that the predetermined time has elapsed, the gate block of the power converter 100 is released in the process of step S204.

[0097] When the gate block of the power converter 100 is released in the processing of step S204, the control device 300 starts overcurrent suppression voltage control for the power converter 100 (step S400). At this time, the control device 300 restarts the power converter 100 and continues overcurrent suppression voltage control for the power converter 100 (step S402). Then, the control device 300 starts measuring time.

[0098] Thereafter, the control device 300 determines whether or not the interrupter 20 corresponding to the fault has detected the completion of fault clearance (step S404). If it is determined in step S404 that the completion of fault clearance has not been detected, the control device 300 returns the process to step S402 and continues the overcurrent suppression voltage control for the power converter 100.

[0099] On the other hand, if it is determined in step S404 that the completion of fault removal has been detected, the control device 300 starts measuring the time in the overcurrent suppression voltage control. ＢＳＳ In step S406, it is determined whether the end process start time T ＢＳＳ If it is determined that the time has not elapsed, the control device 300 repeats the counting of time.

[0100] On the other hand, in step S406, the termination process start time T ＢＳＳ If it is determined that the termination time T has elapsed, the control device 300 starts the termination process of the overcurrent suppression voltage control for the power converter 100 (step S408). ＳＳ In step S410, it is determined whether the end processing time T ＳＳ If it is determined that the time has not elapsed, the control device 300 repeats counting the time and continues the process of ending the overcurrent suppression voltage control for the power converter 100 .

[0101] On the other hand, in step S410, the end processing time T ＳＳIf it is determined that the time has elapsed, the control device 300 terminates the overcurrent suppression voltage control and the termination process for the power converter 100. Then, the control device 300 returns the process to step S100 and causes the power converter 100 to output the established grid voltage for normal (steady) power transmission.

[0102] With this configuration and processing, if an accident occurs in the AC system during normal (steady) power transmission in the high-voltage DC power transmission system 2, the control device 300 gate-blocks the power converter 100 to suppress overcurrent caused by the accident. Thereafter, the control device 300 releases the gate block and restarts the power converter 100, and also changes the control of the operation of the power converter 100 from voltage control to the system voltage V established during normal (steady) power transmission. Ｓ , and then switches to overcurrent suppression voltage control, which is a similar voltage control, based on the voltage value of the fault detected by the circuit breaker 20, and performs a process to terminate the overcurrent suppression voltage control after detecting the completion of fault clearance by the circuit breaker 20. That is, the control device 300 keeps controlling the operation of the power converter 100 under voltage control, and causes the power converter 100 to output a grid voltage having a voltage value different from the grid voltage value established for normal (steady) power transmission. Like the control device 200, the control device 300 also completes this series of control operations for the power converter 100 while the renewable energy power generation system 10 continues to supply AC current. As a result, like the high-voltage direct current transmission system 1, the high-voltage direct current transmission system 2 also suppresses overcurrent due to a fault to protect the semiconductor power devices constituting the power converter 100, and, when the fault is cleared, the power converter 100 is restarted, thereby preventing the renewable energy power generation system 10 from shutting down.

[0103] As described above, the high-voltage DC transmission system 2 including the power conversion device 50 of the second embodiment, like the high-voltage DC transmission system 1 including the power conversion device 40 of the first embodiment, suppresses overcurrent due to an accident to protect the semiconductor power devices, and completes a series of controls on the power converter 100 before the renewable energy power generation system 10 stops operating. As a result, like the high-voltage DC transmission system 1 including the power conversion device 40 of the first embodiment, the high-voltage DC transmission system 2 including the power conversion device 50 of the second embodiment also prevents the renewable energy power generation system 10 from stopping operating due to an accident, and when the accident is cleared, it is possible to quickly resume power transmission and continue power transmission.

[0104] Third Embodiment The overcurrent suppression control unit 250 included in the power conversion device 40 of the first embodiment gate-blocks the power converter 100 to suppress overcurrent caused by an accident in the AC system. However, for example, if the overcurrent suppression control unit 250 can quickly detect the accident or quickly switch the voltage control of the power converter 100 to overcurrent suppression current control (which may include a case where the short-circuit tolerance of the semiconductor power devices constituting the power converter 100 has increased), the overcurrent suppression control unit 250 may switch the operation control of the power converter 100 before gate-blocking the power converter 100. In other words, the overcurrent suppression control unit 250 does not necessarily gate-block the power converter 100 after detecting the occurrence of an accident.

[0105] Hereinafter, as a third embodiment, an example of a case where overcurrent suppression control is performed by switching the control of the power converter 100 from voltage control to overcurrent suppression current control without gate-blocking the power converter 100 will be described. In the following description, a power conversion device that performs overcurrent suppression current control without gate-blocking the power converter 100 will be referred to as a "power conversion device 60," and a high-voltage DC transmission system including the power conversion device 60 will be referred to as a "high-voltage DC transmission system 3."

[0106] [Configuration of High-Voltage DC Transmission System and Configuration of Power Conversion Device] The configuration of the high-voltage DC transmission system 3 and the power conversion device 60 is similar to that of the high-voltage DC transmission system 1 including the power conversion device 40 of the first embodiment shown in FIG. 1 , except that components (which may include the comparator 212 and the overcurrent detection unit 214) related to the gate block control unit 240 for gate-blocking the power converter 100 are omitted. The configuration of the high-voltage DC transmission system 3 and the power conversion device 60 can be easily imagined based on the configuration of the high-voltage DC transmission system 1 and the power conversion device 40 shown in FIG. 1 . Therefore, a detailed description of the configuration of the high-voltage DC transmission system 3 and the power conversion device 60 will not be repeated. In the following description, the operations and processes of the components included in the high-voltage DC transmission system 3 and the power conversion device 60 will be described as being performed by the components included in the high-voltage DC transmission system 1 and the power conversion device 40 shown in FIG. 1 . However, when distinguishing between each component of the first embodiment and each component of the third embodiment with the same reference numeral, the reference numeral of each component of the third embodiment will be followed by "a."

[0107] 8 is a diagram schematically illustrating an example of a time flow of control of the power converter 100 by the control device 200a included in the power conversion device 60 of the third embodiment when a fault occurs. In FIG. 8, similar to the example of a time flow of control of the power converter 100 by the control device 200 included in the power conversion device 40 of the first embodiment shown in FIG. 3 when a fault occurs, the total control time from the occurrence of the fault to the end of the overcurrent suppression current control is defined as a control time T ＦＲＴ The time required for the breaker 22 to clear the fault after the fault occurs is defined as the breaking time T ＣＢｏｐｅｎ 8 shows an example of the relationship between the respective times when the control device 200a performs overcurrent suppression current control. Furthermore, similar to FIG. 3, FIG. 8 also shows the system voltage V Ｓ Or the overcurrent suppression voltage V Ｃ An example of the above is shown schematically.

[0108] When the control device 200a is performing voltage control during normal (steady) power transmission in the high-voltage DC power transmission system 3, if a fault occurs in the AC system and the control device 200a detects the fault, the control device 200a causes the power converter 100 to continue operation. Then, the fault operation control unit 251a sets the fault operation control signal to a state in which overcurrent suppression current control for the power converter 100 is started. After that, the fault operation control unit 251a starts the termination process for a predetermined fixed termination process start time T ＢＳＳ When the termination processing time T has elapsed, the accident operation control signal is set to a state in which termination processing is started. ＳＳ When the time has elapsed, the fault operation control signal is set to a state in which the overcurrent suppression current control for the power converter 100 is ended.

[0109] Here, also in the high-voltage DC transmission system 3, the relationship between the operation duration of the renewable energy power generation system 10 and the control of the operation of the power converter 100 by the control device 200a is equivalent to the relationship between the operation duration of the renewable energy power generation system 10 and the control of the operation of the power converter 100 by the control device 200, which was explained using Fig. 4. Therefore, a detailed explanation of the relationship between the operation duration of the renewable energy power generation system 10 and the control of the operation of the power converter 100 by the control device 200a will not be given again.

[0110] [Processing of Control Device] Next, an example of processing when the control device 200a in the power conversion device 60 controls the power converter 100 will be described. FIG. 9 is a flowchart showing an example of processing when the control device 200a included in the power conversion device 60 controls the power converter 100 according to the third embodiment. FIG. 9 shows an example of a processing flow for controlling the operation of the power converter 100, which is performed by an accident operation control unit 251a in an overcurrent suppression control unit 250a included in the control device 200a. More specifically, FIG. 9 shows an example of a processing flow for returning the power converter 100 to a normal (normal) operating state by completing a series of controls while the renewable energy power generation system 10 continues to supply AC current while suppressing an overcurrent due to the accident, when an accident occurs in the AC system while the power converter 100 is establishing a system voltage for normal (steady) power transmission in the high-voltage DC transmission system 3.

[0111] The processing of this flowchart may also be repeatedly executed while power is being transmitted in the high-voltage DC power transmission system 3. For ease of explanation, in the following explanation, it is assumed that the control device 200a controls the operation of the power converter 100. However, in the flowchart shown in Figure 9, the same steps as those in the flowchart shown in Figure 5 are assigned the same step numbers, and detailed explanations thereof will not be repeated.

[0112] In the following description, the overcurrent threshold I ＯＣ , low voltage threshold V ＬＶ , current command value I ＊ , and the normal (steady) system voltage V Ｓ It is assumed that the values ​​of the above are determined in advance and input (set) to the control device 200a. Furthermore, in the following description, it is assumed that the values ​​of the above are determined in advance and input (set) to the control device 200a. ＳＯＡ , and the interruption time T Ｃｙｃｌｅ Control time T based on ＦＲＴ , interruption time T ＣＢｏｐｅｎ , termination processing start time T ＢＳＳ , and the final processing time T ＳＳIt is assumed that the respective times are predetermined and input (set) or determined in the control device 200a. Furthermore, in the following description, it is assumed that the respective coefficients of the current control coefficient Ki and the voltage control coefficient Kv in the power conversion device 60 are predetermined and input (set) in the control device 200a.

[0113] The control device 200a also performs voltage control on the power converter 100 in the processing of steps S100 to S110, causing the power converter 100 to output the established system voltage for normal (steady-state) power transmission, and repeats voltage control on the power converter 100 by the processing of step S100 while it determines in step S110 that no accident has been detected.

[0114] On the other hand, if it is determined in step S110 that an accident has been detected, the control device 200a starts overcurrent suppression current control for the power converter 100 (step S300). At this time, the control device 200a starts overcurrent suppression current control for the power converter 100 without gate blocking the power converter 100, while allowing the power converter 100 to continue operating.

[0115] Thereafter, the control device 200a performs overcurrent suppression current control on the power converter 100 in the processes of steps S302 to S310. Thereafter, in step S310, the control device 200a performs the overcurrent suppression current control on the power converter 100 in the process of steps S302 to S310. ＳＳ If it is determined that the time has elapsed, the control device 200a terminates the overcurrent suppression current control and termination process for the power converter 100. Then, in the process of step S320, the control device 200a returns the control for the power converter 100 to voltage control, returns the process to step S100, and causes the power converter 100 to output the established grid voltage for normal (steady) power transmission.

[0116] With this configuration and processing, when an accident occurs in the AC system during normal (steady) power transmission in the high-voltage DC power transmission system 3, the control device 200a changes the control of the operation of the power converter 100 from voltage control to current command value I ＊and after detecting the completion of fault clearance by the circuit breaker 20, performs processing to terminate the overcurrent suppression current control and then switches back to voltage control. That is, the control device 200a switches the operation control of the power converter 100 from voltage control to overcurrent suppression current control and from overcurrent suppression current control to voltage control without gate-blocking the power converter 100. Like the control device 200, the control device 200a also completes this series of control operations for the power converter 100 while the renewable energy power generation system 10 continues to supply AC current. As a result, like the high-voltage direct current transmission system 1, the high-voltage direct current transmission system 3 can also protect the semiconductor power devices constituting the power converter 100 by suppressing overcurrent due to a fault, and can keep the power converter 100 activated even when the fault is cleared, thereby preventing the renewable energy power generation system 10 from shutting down.

[0117] As described above, the high-voltage DC transmission system 3 including the power conversion device 60 of the third embodiment, like the high-voltage DC transmission system 1 including the power conversion device 40 of the first embodiment, suppresses overcurrent due to an accident to protect the semiconductor power devices, and completes a series of controls on the power converter 100 before the renewable energy power generation system 10 stops operating. As a result, like the high-voltage DC transmission system 1 including the power conversion device 40 of the first embodiment, the high-voltage DC transmission system 3 including the power conversion device 60 of the third embodiment also prevents the renewable energy power generation system 10 from stopping operating due to an accident, and when the accident is cleared, it is possible to quickly resume power transmission and continue power transmission.

[0118] Fourth Embodiment In the third embodiment, the overcurrent suppression control unit 250a, which performs operations and processes equivalent to those of the overcurrent suppression control unit 250 included in the power conversion device 40 of the first embodiment, switches the control of the power converter 100 from voltage control to overcurrent suppression current control without gate-blocking the power converter 100, thereby performing overcurrent suppression control. However, the overcurrent suppression control unit 350 included in the power conversion device 50 of the second embodiment may also switch the operation control of the power converter 100 before gate-blocking the power converter 100, for example, in cases where an accident can be detected early or where the voltage control of the power converter 100 can be switched to overcurrent suppression voltage control early (which may include cases where the short-circuit tolerance of the semiconductor power devices constituting the power converter 100 has increased). In other words, the overcurrent suppression control unit 350 of the second embodiment does not necessarily have to gate-block the power converter 100 after detecting the occurrence of an accident.

[0119] Hereinafter, as a fourth embodiment, an example of a case where overcurrent suppression control is performed by switching the control of the power converter 100 from voltage control to overcurrent suppression voltage control without gate-blocking the power converter 100 will be described. In the following description, a power conversion device that performs overcurrent suppression voltage control without gate-blocking the power converter 100 will be referred to as a "power conversion device 70," and a high-voltage DC transmission system including the power conversion device 70 will be referred to as a "high-voltage DC transmission system 4."

[0120] [Configuration of High-Voltage DC Transmission System and Configuration of Power Converter] The configuration of the high-voltage DC transmission system 4 and the power converter 70 is similar to that of the high-voltage DC transmission system 3 including the power converter 60 of the third embodiment, except that the components (which may include the comparator 212 and the overcurrent detection unit 214) related to the gate block control unit 240 for gate-blocking the power converter 100 are omitted from the configuration of the high-voltage DC transmission system 2 and the power converter 50 shown in FIG. 6 . The configurations of the high-voltage DC transmission system 4 and the power converter 70 can be easily imagined based on the configurations of the high-voltage DC transmission system 2 and the power converter 50 shown in FIG. 6 . Therefore, a detailed description of the configurations of the high-voltage DC transmission system 4 and the power converter 70 will not be repeated. In the following description, the operations and processes of the components included in the high-voltage DC transmission system 4 and the power converter 70 will be described as being performed by the components included in the high-voltage DC transmission system 2 and the power converter 50 shown in FIG. 6 . However, when distinguishing between each component of the second embodiment and each component of the fourth embodiment with the same reference numeral, the reference numeral of each component of the fourth embodiment will be described with an "a" added after it.

[0121] The time flow of control of the power converter 100 by the control device 300a included in the power conversion device 70 when an accident occurs is equivalent to the example of the time flow of control of the power converter 100 by the control device 200a when an accident occurs shown in Figure 8, with overcurrent suppression current control replaced with overcurrent suppression voltage control. Furthermore, in the high-voltage DC transmission system 4, the relationship between the operation duration of the renewable energy power generation system 10 and the control of the operation of the power converter 100 by the control device 300a is also equivalent to the relationship between the operation duration of the renewable energy power generation system 10 and the control of the operation of the power converter 100 by the control device 200 described using Figure 4. Therefore, detailed explanations of the time flow of control of the power converter 100 by the control device 300a when an accident occurs and the relationship between the operation duration of the renewable energy power generation system 10 and the control of the operation of the power converter 100 by the control device 300a will not be repeated.

[0122] [Processing of Control Device] Next, an example of processing when the control device 300a in the power conversion device 70 controls the power converter 100 will be described. Fig. 10 is a flowchart showing an example of processing when the control device 300a provided in the power conversion device 70 of the fourth embodiment controls the power converter 100. Like the flowchart shown in Fig. 9, Fig. 10 shows an example of the processing flow for controlling the operation of the power converter 100 performed by the accident operation control unit 251a in the overcurrent suppression control unit 350a provided in the control device 300a. However, in the flowchart shown in Fig. 10, the control device 300a performs overcurrent suppression voltage control instead of the overcurrent suppression current control in the flowchart shown in Fig. 9.

[0123] The processing of this flowchart may also be repeatedly executed while power transmission is being performed in the high-voltage DC transmission system 4. For ease of explanation, in the following description, it is assumed that the control device 300a controls the operation of the power converter 100. However, in the flowchart shown in Fig. 10 , the same steps as those in the flowchart for the high-voltage DC transmission system 2 including the power conversion device 50 of the second embodiment shown in Fig. 7 are assigned the same step numbers, and detailed explanations thereof will not be repeated.

[0124] In the following description, the overcurrent threshold I ＯＣ , low voltage threshold V ＬＶ , and the normal (steady) system voltage V Ｓ It is assumed that the values ​​of the above are determined in advance and input (set) to the control device 300a. Furthermore, in the following description, it is assumed that the values ​​of the above are determined in advance and input (set) to the control device 300a. ＳＯＡ , and the interruption time T Ｃｙｃｌｅ Control time T based on ＦＲＴ , interruption time T ＣＢｏｐｅｎ , termination processing start time T ＢＳＳ , and the final processing time T ＳＳIt is assumed that the respective times are predetermined and input (set) or determined in the control device 300a. Furthermore, in the following description, it is assumed that the respective coefficients of the voltage control coefficient Kv and the overcurrent suppression voltage control coefficient Kvs in the power conversion device 70 are predetermined and input (set) in the control device 300a.

[0125] The control device 300a also performs voltage control on the power converter 100 in the processing of steps S100 to S110, causing the power converter 100 to output the established system voltage for normal (steady-state) power transmission, and repeats voltage control on the power converter 100 by the processing of step S100 while it determines in step S110 that no accident has been detected.

[0126] On the other hand, if it is determined in step S110 that an accident has been detected, the control device 300a starts overcurrent suppression voltage control for the power converter 100 (step S400). At this time, the control device 300a starts overcurrent suppression voltage control for the power converter 100 without gate-blocking the power converter 100, while allowing the power converter 100 to continue operating.

[0127] Thereafter, the control device 300a performs overcurrent suppression voltage control on the power converter 100 in the processes of steps S402 to S410. Thereafter, in step S410, the control device 300a performs the overcurrent suppression voltage control on the power converter 100 in the process of steps S402 to S410. ＳＳ If it is determined that the time has elapsed, the control device 300a terminates the overcurrent suppression voltage control and the termination process for the power converter 100. Then, the control device 300a returns the process to step S100 and causes the power converter 100 to output the established grid voltage for normal (steady) power transmission.

[0128] With this configuration and processing, when an accident occurs in the AC system during normal (steady) power transmission in the high-voltage DC power transmission system 4, the control device 300a changes the control of the operation of the power converter 100 from voltage control to the system voltage V established during normal (steady) power transmission, while allowing the power converter 100 to continue operating. Ｓ, and then switches to overcurrent suppression voltage control, which is a similar voltage control, based on the voltage value of the power converter 100. After detecting the completion of fault clearance by the circuit breaker 20, the control device 300a performs a process of terminating the overcurrent suppression voltage control. That is, the control device 300a switches the operation control of the power converter 100 from voltage control to overcurrent suppression voltage control and from overcurrent suppression voltage control to voltage control without gate-blocking the power converter 100. Like the control device 300, the control device 300a also completes this series of control operations for the power converter 100 while the renewable energy power generation system 10 continues to supply AC current. As a result, in the high-voltage DC transmission system 4, as in the high-voltage DC transmission system 2, overcurrent due to a fault is suppressed to protect the semiconductor power devices constituting the power converter 100, and the power converter 100 remains activated even when the fault is cleared, thereby preventing the renewable energy power generation system 10 from shutting down.

[0129] As described above, the high-voltage DC transmission system 4 including the power conversion device 70 of the fourth embodiment, like the high-voltage DC transmission system 2 including the power conversion device 50 of the second embodiment, suppresses overcurrent due to an accident to protect the semiconductor power devices, and completes a series of controls on the power converter 100 before the renewable energy power generation system 10 stops operating. As a result, like the high-voltage DC transmission system 2 including the power conversion device 50 of the second embodiment, the high-voltage DC transmission system 4 including the power conversion device 70 of the fourth embodiment also prevents the renewable energy power generation system 10 from stopping operating due to an accident, and when the accident is cleared, it is possible to quickly resume power transmission and continue power transmission.

[0130] Fifth Embodiment Incidentally, in the high-voltage DC transmission system including the power conversion devices (power conversion device 40, power conversion device 50, power conversion device 60, power conversion device 70) of the first to fourth embodiments, the overcurrent suppression control units (overcurrent suppression control unit 250, overcurrent suppression control unit 350, overcurrent suppression control unit 250a, overcurrent suppression control unit 350a) have been described as performing either overcurrent suppression control, i.e., overcurrent suppression current control or overcurrent suppression voltage control. However, the overcurrent suppression control units are not limited to performing either overcurrent suppression control, i.e., overcurrent suppression current control or overcurrent suppression voltage control. For example, the overcurrent suppression control unit may be configured to perform both overcurrent suppression control, i.e., overcurrent suppression current control and overcurrent suppression voltage control, and may be configured to determine (select) which overcurrent suppression control to perform, i.e., overcurrent suppression current control or overcurrent suppression voltage control, when a fault occurs in the AC grid.

[0131] Hereinafter, an example in which the overcurrent suppression control unit is configured to be able to perform both overcurrent suppression control, ie, overcurrent suppression current control and overcurrent suppression voltage control, will be described as a fifth embodiment.

[0132] [Configuration of High-Voltage DC Transmission System] Fig. 11 is a diagram showing an example of the configuration of a high-voltage DC transmission system including a power conversion device according to the fifth embodiment. The high-voltage DC transmission system 5 including the power conversion device of the fifth embodiment shown in Fig. 11 also shows an example of the configuration of the power transmission side, similar to the high-voltage DC transmission system 1 including the power conversion device 40 of the first embodiment shown in Fig. 1 and the high-voltage DC transmission system 2 including the power conversion device 50 of the second embodiment shown in Fig. 6. The high-voltage DC transmission system 5 includes, for example, a renewable energy power generation system 10 and a power conversion device 80 on the power transmission side.

[0133] The high-voltage DC transmission system 5 including the power conversion device 80 includes the same components as the high-voltage DC transmission system 1 or the high-voltage DC transmission system 2. In Fig. 11 , the same reference numerals are used to denote components of the high-voltage DC transmission system 5 or the power conversion device 80 that have the same functions as components of the high-voltage DC transmission system 1 or the power conversion device 40, or the high-voltage DC transmission system 2 or the power conversion device 50. Therefore, detailed descriptions of components of the high-voltage DC transmission system 5 or the power conversion device 80 that have the same configurations or operations as the components of the high-voltage DC transmission system 1 or the power conversion device 40, or the high-voltage DC transmission system 2 or the power conversion device 50 will be omitted, and only different configurations and operations will be described.

[0134] Like the high-voltage DC transmission systems 1 and 2, the high-voltage DC transmission system 5 also converts AC power generated by a renewable energy power generation system 10, which is connected as an AC system on the power transmission side, into DC power using a power conversion device 80, and transmits the DC power to the power receiving side via two DC transmission lines LN-DC (DC transmission line LN-DC-A and DC transmission line LN-DC-B). In the high-voltage DC transmission system 5, like the high-voltage DC transmission systems 1 and 2, the power conversion device 80 performs grid forming (GFM) operation (voltage control) to operate the power converter 100 as a voltage source, thereby establishing the AC system voltage (high voltage) for DC transmission. Also in the high-voltage DC transmission system 5, similarly to the high-voltage DC transmission systems 1 and 2, the renewable energy power generation system 10 generates a current (AC current) according to the system voltage of the established AC system and supplies the generated current to the power conversion device 80. As a result, also in the high-voltage DC transmission system 5, similarly to the high-voltage DC transmission systems 1 and 2, the power conversion device 80 (more specifically, the power converter 100) converts the AC power of the AC current supplied by the renewable energy power generation system 10 into DC power at the system voltage established by the power converter 100, and transmits the high-voltage DC power.

[0135] In the high-voltage DC transmission system 5, similarly to the high-voltage DC transmission systems 1 and 2, the transmitted DC power is converted back into AC power at the power receiving side (not shown) and supplied to the AC system at the power receiving side. In the high-voltage DC transmission system 5, similarly to the high-voltage DC transmission systems 1 and 2, the AC power may be converted back into DC power at the AC system at the power receiving side (not shown) (the AC system at the power receiving side (not shown) may operate as the AC system at the power transmitting side in the high-voltage DC transmission system at the next stage), and the DC power may be transmitted further away.

[0136] [Configuration of Power Conversion Apparatus] The power conversion apparatus 80 includes, for example, a power converter 100 and a control device 400. In the power conversion apparatus 80, the control device 200 included in the power conversion apparatus 40 or the control device 300 included in the power conversion apparatus 50 is replaced with the control device 400. Like the power conversion apparatus 40 and the power conversion apparatus 50, the power conversion apparatus 80 also converts AC power, which is an AC current supplied by the renewable energy power generation system 10, into DC power at a grid voltage established by the power converter 100 in grid-forming operation. At this time, in the power conversion apparatus 80, like the control device 200 included in the power conversion apparatus 40 and the control device 300 included in the power conversion apparatus 50, the control device 400 converts AC power into DC power by controlling the operation of the power converter 100 based on the interruption operation state indicated by the relay signal R output by each interruption device 20, the current detection value I output by the current detector 31, and the voltage detection value V output by the voltage detector 32. Like power conversion devices 40 and 50, power conversion device 80 also transmits (supplies) DC power converted by power converter 100 to the power receiving side via DC transmission line LN-DC.

[0137] Like the control devices 200 and 300, the control device 400 also controls the operation of the power converter 100 based on the relay signal R output by each circuit breaker 20, the detected current value I output by the current detector 31, and the detected voltage value V output by the voltage detector 32. That is, during normal (steady) power transmission in the high-voltage DC transmission system 5, the control device 400 also performs voltage control on the power converter 100 to output the established system voltage from the AC terminals, and performs overcurrent suppression control on the power converter 100 when a fault occurs in the AC system of the high-voltage DC transmission system 5. In this case, the control of the power converter 100 by the control device 400 is similar to the control of the power converter 100 by the control device 200 or the control device 300 (see FIG. 2 ). However, the control device 400 can determine (select) the method of overcurrent suppression control.

[0138] The control device 400 is an example of a "control device."

[0139] [Configuration of Control Device] The control device 400 includes, for example, a comparator 212, an overcurrent detection unit 214, a comparator 222, a low-voltage detection unit 224, a fault removal detection unit 230, a gate block control unit 240, an overcurrent suppression control unit 450, a normal-state voltage command value calculation unit 260, a control selection unit 270, and a power converter control unit 280. The gate block control unit 240 includes, for example, a gate block determination unit 242 and a gate block signal generation unit 244. The overcurrent suppression control unit 450 includes, for example, a fault operation control unit 251, a control switching unit 252, an adder 253, a fault-state voltage command value calculation unit 254, a fault-state voltage command value control unit 255, a fault-state voltage command value calculation unit 356, a fault-state voltage command value control unit 357, an overcurrent suppression control method determination unit 458, and an overcurrent suppression control method selection unit 459. The control selection unit 270 includes, for example, a voltage command value selection unit 272 and a converter 274. In the control device 400, the overcurrent suppression control unit 250 included in the control device 200 is replaced with an overcurrent suppression control unit 450, or the overcurrent suppression control unit 350 included in the control device 300 is replaced with an overcurrent suppression control unit 450. In the overcurrent suppression control unit 450, the fault-time voltage command value calculation unit 356 and the fault-time voltage command value control unit 357 included in the overcurrent suppression control unit 350 are added to the overcurrent suppression control unit 250, or the adder 253, the fault-time voltage command value calculation unit 254, and the fault-time voltage command value control unit 255 included in the overcurrent suppression control unit 250 are added to the overcurrent suppression control unit 350. In other words, the overcurrent suppression control unit 450 is a combination of the overcurrent suppression control unit 250 and the overcurrent suppression control unit 350. Furthermore, the overcurrent suppression control unit 450 is additionally provided with an overcurrent suppression control method determination unit 458 and an overcurrent suppression control method selection unit 459 .

[0140] In the control device 400, as in the control device 200 and the control device 300, some or all of the components of the control device 400 realize the functions of each component, for example, by a hardware processor executing a program (software) stored in a memory (storage unit) not shown.

[0141] Like the overcurrent suppression control units 250 and 350, the overcurrent suppression control unit 450 also performs overcurrent suppression control when a fault occurs in the AC system. The overcurrent suppression control unit 450 controls the operation of the power converter 100 using a preset or determined (selected) overcurrent suppression control method to suppress overcurrent flowing through the power converter 100 in response to the fault that has occurred. That is, like the overcurrent suppression control unit 250, the overcurrent suppression control unit 450 switches the control of the power converter 100 from voltage control to overcurrent suppression current control, or like the overcurrent suppression control unit 350, switches the control of the power converter 100 from voltage control to overcurrent suppression voltage control while maintaining voltage control.

[0142] In the overcurrent suppression control unit 450, the accident operation control signal generated by the accident operation control unit 251 controls the overall sequence (time flow) of the operation of overcurrent suppression current control or overcurrent suppression voltage control performed on the power converter 100. The accident operation control unit 251 outputs the generated accident operation control signal to the control switching unit 252, the accident voltage command value control unit 255, and the accident voltage command value control unit 357. The time flow of control over the power converter 100 by the control device 400 included in the power conversion device 80 when an accident occurs is equivalent to a flow in which the overcurrent suppression current control is replaced with the overcurrent suppression current control or the overcurrent suppression voltage control in the example of the time flow of control over the power converter 100 by the control device 200 when an accident occurs shown in FIG. Furthermore, the relationship between the operation duration of the renewable energy power generation system 10 and the control of the operation of the power converter 100 by the control device 400 is equivalent to the relationship between the operation duration of the renewable energy power generation system 10 and the control of the operation of the power converter 100 by the control device 200, which was explained using Fig. 4. Therefore, detailed explanations of the time flow of control of the power converter 100 by the control device 400 when an accident occurs, and the relationship between the operation duration of the renewable energy power generation system 10 and the control of the operation of the power converter 100 by the control device 400 will not be repeated.

[0143] In the overcurrent suppression control unit 450, the fault-time voltage command value control unit 255 and the fault-time voltage command value control unit 357 each output a fault-time voltage command value to the overcurrent suppression control method selection unit 459. In the following description, the fault-time voltage command value for overcurrent suppression current control output by the fault-time voltage command value control unit 255 is referred to as the "fault-time current control voltage command value," and the fault-time voltage command value for overcurrent suppression voltage control output by the fault-time voltage command value control unit 357 is referred to as the "fault-time voltage control voltage command value," to distinguish between the respective fault-time voltage command values.

[0144] The overcurrent suppression control method determination unit 458 determines the method of overcurrent suppression control to be performed by the overcurrent suppression control unit 450. More specifically, the overcurrent suppression control method determination unit 458 determines the control method of overcurrent suppression control to be performed on the power converter 100 when a fault occurs in the AC system to be a preset or selected overcurrent suppression control method. The control method of overcurrent suppression control for the power converter 100 may be preset by, for example, a higher-level control device (not shown), or may be selected in advance based on the configuration of each component included in the HVDC transmission system 5 and the configuration of the power conversion device 80 (the configuration of the power converter 100 and the configuration of the control device 400) when constructing the HVDC transmission system 5 or designing and installing the power conversion device 80. The control method of overcurrent suppression control for the power converter 100 may be determined depending on the type of fault (such as a short-circuit fault, a ground fault, or a break in the AC transmission line LN-AC) that has occurred in the AC system, determined based on the low-voltage determination result output by the low-voltage detection unit 224. In this case, the overcurrent suppression control method determination unit 458 determines whether an accident has occurred in the AC system based on the low voltage determination result output by the low voltage detection unit 224. Then, the overcurrent suppression control method determination unit 458 outputs a determination signal to the overcurrent suppression control method selection unit 459, indicating the control method that has been determined (selected) for overcurrent suppression control, that is, indicating whether overcurrent suppression current control or overcurrent suppression voltage control is to be performed.

[0145] The overcurrent suppression control method selection unit 459 selects either the voltage command value for fault-time current control output by the fault-time voltage command value control unit 255 or the voltage command value for fault-time voltage control output by the fault-time voltage command value control unit 357, in accordance with the determination signal output by the overcurrent suppression control method determination unit 458. The overcurrent suppression control method selection unit 459 outputs the selected fault-time voltage command value to the control selection unit 270 (more specifically, the voltage command value selection unit 272).

[0146] The overcurrent suppression control unit 450 is an example of an "overcurrent suppression control unit." The configuration of the adder 253 and the fault-time voltage command value calculation unit 254 included in the overcurrent suppression control unit 450 is an example of a "first fault-time command value calculation unit." The voltage command value output by the fault-time voltage command value calculation unit 254 included in the overcurrent suppression control unit 450 is an example of a "first fault-time command value." The fault-time voltage command value calculation unit 356 included in the overcurrent suppression control unit 450 is an example of a "second fault-time command value calculation unit." The voltage command value output by the fault-time voltage command value calculation unit 356 included in the overcurrent suppression control unit 450 is an example of a "second fault-time command value." The fault-time voltage command value control unit 255 and the fault-time voltage command value control unit 357 are each an example of an "fault-time command value control unit." The configuration of the overcurrent suppression control method determination unit 458 and the overcurrent suppression control method selection unit 459 is an example of an "overcurrent suppression control method selection unit."

[0147] In response to a switching signal output by overcurrent suppression control unit 450 (more specifically, control switching unit 252), control selection unit 270 selects either the fault-time voltage command value output by overcurrent suppression control unit 450 (more specifically, overcurrent suppression control method selection unit 459) or the normal-time voltage command value output by normal-time voltage command value calculation unit 260. Control selection unit 270 outputs the selected voltage command value to power converter control unit 280 as the voltage command value to be output to power converter 100.

[0148] The voltage command value selection unit 272 selects either the fault-time voltage command value output by the overcurrent suppression control method selection unit 459 or the normal-time voltage command value output by the normal-time voltage command value calculation unit 260 in accordance with the switching signal output by the control switching unit 252. The voltage command value selection unit 272 sets the selected voltage command value as the voltage command value V ＊ (command value having amplitude information) to the converter 274. As a result, the converter 274 converts the voltage command value V ＊ and outputs a voltage command value having amplitude information and phase information to the power converter control unit 280 as a voltage command value to be output to the power converter 100 .

[0149] [Processing of Control Device] The processing performed by the control device 400 in the power conversion device 80 when controlling the power converter 100 is equivalent to a combination of the flowchart of the processing performed by the control device 200 of the power conversion device 40 of the first embodiment shown in FIG. 5 to control the power converter 100 and the flowchart of the processing performed by the control device 300 of the power conversion device 50 of the second embodiment shown in FIG. 7 to control the power converter 100. Furthermore, if the control device 400 is configured to perform overcurrent suppression control by switching control of the power converter 100 without gate-blocking the power converter 100, the processing may be equivalent to a combination of the flowchart of the processing performed by the control device 200a of the power conversion device 60 of the third embodiment shown in FIG. 9 to control the power converter 100 and the flowchart of the processing performed by the control device 300a of the power conversion device 70 of the fourth embodiment shown in FIG. 10 to control the power converter 100. In these cases, when it is determined that a fault has occurred, the control device 400 executes the processing of one of the flowcharts corresponding to the overcurrent suppression control method determined by the overcurrent suppression control method determination unit 458.

[0150] More specifically, in steps S100 to S110 common to the flowcharts shown in FIGS. 5 , 7 , 9 , and 10 , the control device 400 performs voltage control on the power converter 100, causing the power converter 100 to output a grid voltage established for normal (steady) power transmission. While determining in step S110 that no fault has been detected, the control device 400 repeats the voltage control on the power converter 100 through the process of step S100. When determining in step S110 that a fault has been detected, the control device 400 selects and executes the process of one of the corresponding flowcharts based on the overcurrent suppression control method determined by the overcurrent suppression control method determination unit 458 and whether or not to gate-block the power converter 100. Such flowcharts of the process performed by the control device 400 to control the power converter 100 can be easily conceived based on the flowcharts shown in FIGS. 5 , 7 , 9 , and 10 . Therefore, a detailed description of the flowcharts of the process performed by the control device 400 to control the power converter 100 will be omitted.

[0151] With this configuration and processing, when a fault occurs in the AC system during normal (steady) power transmission in the high-voltage DC transmission system 5, the control device 400 controls the operation of the power converter 100 using the overcurrent suppression control method determined by the overcurrent suppression control method determination unit 458, and performs processing to terminate the overcurrent suppression voltage control after detecting completion of fault clearance by the circuit breaker 20. The control device 400 also completes a series of control operations for the power converter 100 while the renewable energy power generation system 10 continues to supply AC current. As a result, in the high-voltage DC transmission system 5, overcurrent due to a fault is suppressed to protect the semiconductor power devices constituting the power converter 100, and when the fault is cleared, the power converter 100 is kept activated, thereby preventing the renewable energy power generation system 10 from shutting down.

[0152] Furthermore, the control device 400 has the components and functions of all of the control devices, namely the control device 200 included in the power conversion device 40 of the first embodiment, the control device 300 included in the power conversion device 50 of the second embodiment, the control device 200a included in the power conversion device 60 of the third embodiment, and the control device 300a included in the power conversion device 70 of the fourth embodiment. Therefore, by preparing the control device 400 as a basic control device for a power conversion device, and then, for example, when constructing a high-voltage DC transmission system or designing or installing a power conversion device, it is possible to select components and functions to be used from the components and functions included in the control device 400 and realize a new power conversion device. In other words, by omitting unused components or stopping the operation of functions among the components and functions included in the control device 400, it is possible to realize a new power conversion device having necessary components (components to be used) and functions. For example, by fixing the determination signal output by the overcurrent suppression control method determination unit 458 to a state in which the fault-time current control voltage command value output by the fault-time voltage command value control unit 255 is selected, the function of the control device 400 can be made equivalent to that of the control device 200, and a power conversion device equivalent to the power conversion device 40 of the first embodiment can be realized. In this case, unused components such as the fault-time voltage command value calculation unit 356 and the fault-time voltage command value control unit 357 included in the overcurrent suppression control unit 450 may be omitted. Power conversion devices equivalent to the power conversion devices of the other embodiments can also be realized in a similar manner.

[0153] As described above, the high-voltage DC transmission system 5 including the power conversion device 80 of the fifth embodiment, like any of the high-voltage DC transmission system 1 including the power conversion device 40 of the first embodiment, the high-voltage DC transmission system 2 including the power conversion device 50 of the second embodiment, the high-voltage DC transmission system 3 including the power conversion device 60 of the third embodiment, and the high-voltage DC transmission system 4 including the power conversion device 70 of the fourth embodiment, suppresses overcurrent due to a fault to protect the semiconductor power devices, and completes a series of controls on the power converter 100 before the renewable energy power generation system 10 stops operating. Thus, like any of the high-voltage DC transmission systems 1 including the power conversion device 40 of the first embodiment to the high-voltage DC transmission system 4 including the power conversion device 70 of the fourth embodiment, the high-voltage DC transmission system 5 including the power conversion device 80 of the fifth embodiment also prevents the renewable energy power generation system 10 from stopping operating due to a fault, and when the fault is cleared, power transmission can be quickly resumed and continued.

[0154] Moreover, the power conversion device 80 of the fifth embodiment has the components and functions of all of the power conversion devices, namely the power conversion device 40 of the first embodiment, the power conversion device 50 of the second embodiment, the power conversion device 60 of the third embodiment, and the power conversion device 70 of the fourth embodiment. Therefore, by preparing the power conversion device 80 of the fifth embodiment as the basic configuration of a power conversion device and omitting components that are not used or stopping the operation of functions as necessary, it is possible to easily realize a power conversion device equivalent to any of the power conversion devices 40 of the first embodiment to the power conversion device 70 of the fourth embodiment, or a new power conversion device.

[0155] As described above, the power conversion devices of the embodiments (including power conversion device 40, power conversion device 50, power conversion device 60, power conversion device 70, and power conversion device 80) include a control device (including control device 200, control device 200a, control device 300, control device 300a, and control device 400) that controls the operation of the power converter 100. When a fault occurs in an AC system of a high-voltage DC transmission system (including high-voltage DC transmission system 1, high-voltage DC transmission system 2, high-voltage DC transmission system 3, high-voltage DC transmission system 4, and high-voltage DC transmission system 5) that includes the power conversion device, the power conversion device controls the operation of the power converter 100 so as to suppress excess current (overcurrent) corresponding to power fluctuations caused by the fault. More specifically, the overcurrent suppression control units (including the overcurrent suppression control unit 250, the overcurrent suppression control unit 250a, the overcurrent suppression control unit 350, the overcurrent suppression control unit 350a, and the overcurrent suppression control unit 450) included in the power conversion device switch the control method for controlling the operation of the power converter 100 in order to suppress an overcurrent flowing through the power converter 100 in response to the fault that has occurred. At this time, the power conversion device of the embodiment completes a series of control operations for the power converter 100 while the renewable energy power generation system 10 constituting the high-voltage direct current transmission system continues to supply AC current. In this way, the power conversion device of the embodiment suppresses an overcurrent caused by the fault to protect the semiconductor power devices constituting the power converter 100, and when the fault is removed, the power converter 100 is kept in an activated state, thereby preventing the renewable energy power generation system 10 from shutting down. As a result, the power conversion device of the embodiment prevents the renewable energy power generation system 10 from stopping operation due to an accident, and when the accident is cleared by the circuit breaker 20 connected to the AC transmission line LN-AC that constitutes the high-voltage DC power transmission system, power transmission can be quickly resumed and continued. As a result, in the high-voltage DC power transmission system including the power conversion device of the embodiment, it is possible to prevent a power outage caused by the operation of the renewable energy power generation system on the power transmission side stopping, and it is possible to operate the high-voltage DC power transmission system with higher reliability.

[0156] In the power conversion device of the above-described embodiment, the power conversion device includes the power converter 100 and a control device that controls the operation of the power converter 100. In other words, the power converter 100 and the control device are integrated into one unit. However, the power converter 100 and the control device are not limited to being integrated into one unit. For example, the power converter 100 and the power conversion device including the control device may be separate units. In this case, the configurations, operations, and processes of the power converter 100 and the power conversion device including the control device may be equivalent to the configurations, operations, and processes of the power conversion device described above. Therefore, detailed descriptions of the configurations, operations, and processes of a configuration in which the power converter 100 and the power conversion device including the control device are separate units will be omitted.

[0157] In the above-described embodiment of the high-voltage DC transmission system including the power conversion device, the renewable energy power generation system 10 is a wind-powered power generation facility that supplies AC power (alternating current) generated by a wind turbine installed offshore. However, the renewable energy power generation system 10 is not limited to a wind-powered power generation facility. The renewable energy power generation system 10 may be other types of renewable energy power generation facilities, such as thermal power generation facilities, solar power generation facilities, or geothermal power generation facilities. In this case, the power conversion device and the high-voltage DC transmission system may transmit power using components appropriate for the respective renewable energy power generation facilities. For example, if the power conversion device and the high-voltage DC transmission system are 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 may include a separate power converter that converts DC power to AC power and be connected to the high-voltage DC transmission system. Therefore, the power conversion device and the high-voltage DC transmission system may have a configuration equivalent to the above-described power conversion device and the high-voltage DC transmission system. In this case, the configuration and operation of the power conversion device may be equivalent to the configuration and operation of the power conversion device and high-voltage DC transmission system described above.

[0158] According to at least one embodiment described above, a power converter (100) is provided that converts AC power supplied by a renewable energy power generation system (10) interconnected to an AC system on the power transmission side that transmits DC power into DC power and outputs the converted DC power to a DC transmission line (LN-DC), and a control device (200) that controls the operation of the power converter, and when the control device detects an overcurrent flowing in the power converter in response to a fault that has occurred in the AC system, the control device switches the control method for the operation of the power converter from a voltage control method for normal operation of the power converter to an overcurrent suppression control method before the fault is cleared by a breaker (20) arranged on the AC transmission line of the AC system. By providing an overcurrent suppression control unit (250) that switches the power converter to an activated state when the fault is removed, an accident command value calculation unit (253, 254) that generates an accident command value corresponding to the overcurrent suppression control, and a command value output unit (270, 272, 274, 280) that selects either a normal command value (normal voltage command value) that is a predetermined voltage command value in voltage control or an accident command value (accident voltage command value) and outputs the selected value to the power converter, it is possible to suitably suppress overcurrent corresponding to power fluctuations due to an accident in the AC system, start up the power converter until recovery from the accident, and continue operation of the renewable energy power generation system.

[0159] Although several embodiments of the present invention have been described, these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their modifications are included within the scope and spirit of the invention, as well as within the scope of the invention described in the claims and their equivalents.

[0160] 1, 2, 3, 4, 5... High voltage DC transmission system, 10... Renewable energy power generation system, 20, 20-A-1, 20-A-2, 20-B-1, 20-B-2... Circuit breaker, 31... Current detector, 32... Voltage detector, 40, 50, 60, 70, 80... Power conversion device, LN-DC, LN-DC-A, LN-DC-B... DC transmission line, LN-AC, LN-AC-A, LN-AC-B... AC transmission line, 100... Power converter, 200, 200a, 300, 300a, 400... Control device, 212... Comparator, 214... Overcurrent detection unit, 222... Comparator, 224... Low voltage detection unit, 230... Fault removal detection unit, 240... Gate Block control unit, 242... gate block determination unit, 244... gate block signal generation unit, 250, 250a, 350, 350a, 450... overcurrent suppression control unit, 251, 251a... fault operation control unit, 252... control switching unit, 253... adder, 254... fault voltage command value calculation unit, 255... fault voltage command value control unit, 260... normal time voltage command value calculation unit, 270... control selection unit, 272... voltage command value selection unit, 274... converter, 280... power converter control unit, 356... fault voltage command value calculation unit, 357... fault voltage command value control unit, 458... overcurrent suppression control method determination unit, 459... overcurrent suppression control method selection unit

Claims

1. A power conversion device comprising: a power converter that converts AC power supplied by a renewable energy power generation system connected to an AC system on the transmission side that transmits DC power into DC power and outputs the converted DC power to a DC transmission line; and a control device that controls the operation of the power converter, wherein the control device comprises: an overcurrent suppression control unit that, when detecting an overcurrent flowing in the power converter in response to a fault that has occurred in the AC system, switches a control method for the operation of the power converter from a voltage control method for normal operation of the power converter to an overcurrent suppression control method before the fault is cleared by a circuit breaker arranged on the AC transmission line of the AC system, and brings the power converter into an activated state when the fault is cleared; a fault command value calculation unit that generates a fault command value corresponding to the overcurrent suppression control; and a command value output unit that selects either a normal command value that is a predetermined voltage command value for the voltage control or the fault command value and outputs the selected value to the power converter.

2. The power conversion device according to claim 1, wherein the control device further comprises a fault command value control unit that gradually changes the fault command value so that the fault command value becomes the normal command value over a predetermined certain termination processing time, and the overcurrent suppression control unit starts changing the fault command value when a predetermined certain termination processing start time has elapsed after the fault is removed.

3. The power conversion device according to claim 2, wherein the fault command value calculation unit generates the fault command value based on a current command value that is predetermined so that the current value of the overcurrent before the fault is removed is smaller than an overcurrent threshold value used to detect the overcurrent.

4. The power conversion device according to claim 3, wherein the fault command value calculation unit generates the fault command value by multiplying a difference between the current value of the overcurrent before the fault is removed and the current command value by a predetermined current control coefficient.

5. The power conversion device according to claim 2, wherein the fault command value calculation unit generates the fault command value based on the normal command value so that the overcurrent before the fault is removed is smaller than an overcurrent threshold value used to detect the overcurrent.

6. The power conversion device according to claim 5, wherein the fault command value calculation unit generates the fault command value by multiplying the normal command value by a predetermined overcurrent suppression voltage control coefficient.

7. The power conversion device according to claim 2, wherein the fault command value calculation unit includes: a first fault command value calculation unit that generates a first fault command value based on a predetermined current command value so that the current value of the overcurrent before the fault is removed is smaller than an overcurrent threshold value used to detect the overcurrent; and a second fault command value calculation unit that generates a second fault command value based on the normal command value so that the overcurrent before the fault is removed is smaller than the overcurrent threshold value used to detect the overcurrent; and the control device further includes: an overcurrent suppression control method selection unit that determines the overcurrent suppression control method to be either a method using the first fault command value or a method using the second fault command value, selects either the first fault command value or the second fault command value based on the determined method, and outputs the selected value as the fault command value.

8. The power conversion device according to claim 7, wherein the first fault command value calculation unit multiplies a difference between the current command value and the current value of the overcurrent before the fault is removed by a predetermined current control coefficient to generate the first fault command value, and the second fault command value calculation unit multiplies the normal command value by a predetermined overcurrent suppression voltage control coefficient to generate the second fault command value.

9. The power conversion device according to any one of claims 2 to 8, wherein the control device further comprises a gate block control unit that, when a fault occurring in the AC system is detected, disconnects the power converter from the AC system for a predetermined gate block time and restarts the power converter when the gate block time has elapsed, and the overcurrent suppression control unit switches the control method for operating the power converter to the overcurrent suppression control method when the gate block time has elapsed before the fault is removed.

10. The power conversion device according to claim 9, wherein the overcurrent suppression control unit completes a series of controls, including switching the control method for operation of the power converter from the voltage control method to the overcurrent suppression control method after the accident occurs, and switching the control method for operation of the power converter from the overcurrent suppression control method to the voltage control method after the accident is removed, within the elapse of an operation duration during which the renewable energy power generation system can continue to supply the AC power even in the event of the accident.

11. A power conversion method in a power conversion device comprising: a power converter that converts AC power supplied by a renewable energy power generation system connected to an AC system on the transmission side that transmits DC power into DC power and outputs the converted DC power to a DC transmission line; and a control device that controls the operation of the power converter, wherein when a computer of the control device detects an overcurrent flowing in the power converter in response to a fault that has occurred in the AC system, the computer switches the control method for the operation of the power converter from a voltage control method for normal operation of the power converter to an overcurrent suppression control method before the fault is cleared by a circuit breaker arranged on the AC transmission line of the AC system, and when the power converter is brought into an activated state when the fault is cleared, the computer generates a fault command value corresponding to the overcurrent suppression control, and selects either a normal command value that is a predetermined voltage command value in the voltage control or the fault command value and outputs it to the power converter.

12. A program for controlling a power conversion device comprising: a power converter that converts AC power supplied by a renewable energy power generation system connected to an AC system on the transmission side that transmits DC power into DC power and outputs the converted DC power to a DC transmission line; and a control device that controls the operation of the power converter, wherein the program causes a computer of the control device to, when detecting an overcurrent flowing in the power converter in response to a fault that has occurred in the AC system, switch the control method for the operation of the power converter from a voltage control method for normal operation of the power converter to an overcurrent suppression control method before the fault is cleared by a circuit breaker arranged on the AC transmission line of the AC system, and to generate a fault command value corresponding to the overcurrent suppression control when the power converter is in a started state when the fault is cleared, and select either a normal command value that is a predetermined voltage command value in the voltage control or the fault command value and output it to the power converter.

Images