JP7688482B2 - Power conversion device, control method, and program - Google Patents

Description

Embodiments of the present invention relate to a power conversion device, a control method, and a program.

In recent years, the use of renewable energy has been promoted, and technology to utilize electricity generated by offshore wind power has been actively developed. In such technology, electricity generated at offshore wind power plants is transmitted to an onshore AC system. When the electricity needs to be transmitted over a long distance to the AC system, high voltage direct current (HVDC) transmission is applied, which allows the electricity to be transmitted efficiently. When HVDC transmission is performed, the AC power generated at the wind power plant is converted to DC power by an AC/DC converter (hereinafter sometimes simply called a converter), and the converted DC power is connected to the DC power system to transmit it to another converter connected to the onshore AC system.

In the following, the converter used for HVDC transmission and the equipment including peripheral devices (current meters, reactors, DC circuit breakers, etc.) installed on the DC transmission line connected to the converter are referred to as "terminals." In addition, when there is no distinction between the converter and the terminal, the terminal may be referred to as the "converter."

Conventional HVDC transmission involves two-terminal DC transmission, where two distant converters are connected by a set of DC transmission lines, transmitting and receiving power one-to-one. However, recent technological improvements have led to the development of multi-terminal HVDC systems, where multiple converters share power with each other. In a multi-terminal HVDC system, one converter is connected to multiple other converters via a DC system. This makes it possible to secure multiple transmission routes from an offshore wind farm to the AC system, diversifying the power transmission routes.

International Publication No. 2018/154783

Makoto Hagiwara and Yasufumi Akagi, "PWM Control Method and Operation Verification of Modular Multilevel Converter (MMC)", Transactions of the Institute of Electrical Engineers of Japan, Industrial Applications Division, Vol. 128, No. 7, pp. 957-965 (July 2008)

In a multi-terminal HVDC system, an accident may occur on the DC cable section. In this case, the amount of power that can be transmitted may decrease, or the receiving converter may be stopped due to an accident or malfunction. In this case, the amount of power that can be received by the entire system may decrease. When such an unsteady state occurs, if the DC transmission system continues to operate at rated power, an imbalance in power transfer may occur. For example, the total amount of transmitted power output by the transmitting converter (hereinafter referred to as the transmitting end) may exceed the total amount of power that can be received by the receiving converter (hereinafter referred to as the receiving end) (hereinafter referred to as an excess amount of transmitted power). As a result of the imbalance in power transfer, one of the converters may stop for a long time due to overvoltage.

If the converter is stopped for a long period of time, it will result in a loss of power generation opportunities. To prevent an excessive amount of power being transmitted, it is necessary to take measures to prevent overvoltage and loss of power generation opportunities by safely stopping some transmitting ends and quickly reducing the amount of power being transmitted.

The problem that this invention aims to solve is to provide a power conversion device, a control method, and a program that can determine whether or not to stop the converter at its own terminal when an accident occurs at another terminal.

The power conversion device of the embodiment constitutes a multi-terminal DC transmission system in which three or more power conversion devices are connected by DC transmission lines, and each of the power conversion devices is installed at the boundary between the AC system and the DC system. The power conversion device has an AC/DC converter and a control device. The AC/DC converter converts AC power into DC power and vice versa. The control device controls the AC/DC converter. When the control device detects that an accident has occurred on the DC side of another power conversion device other than the own device (own power conversion device), it detects the fault section from the current value flowing in the DC transmission line near the own device (own power conversion device) and the impedance of the DC transmission line, and determines whether the other power conversion device has been paralleled off from the DC system based on the time-series change in the current value. When it is determined that the other power conversion device has been paralleled off, it determines whether to stop the AC/DC converter based on the transmitted and received power information of the multi-terminal DC transmission system and the calculation result of the fault section.

1 is a block diagram showing an example of the configuration of a multi-terminal HVDC system 1 according to an embodiment. 2 is a diagram for explaining a state in which an accident occurs on the DC bus 20 of the multi-terminal HVDC system 1 of the embodiment. FIG. 4 is a flowchart showing a flow of processing performed by a control device 60 of the embodiment. FIG. 4 is a diagram for explaining the process of step S110 in the flowchart of FIG. 3 . FIG. 4 is a diagram for explaining the process of step S120 in the flowchart of FIG. 3 . 2 is a diagram for explaining a state in which an accident occurs on the DC bus 20 of the multi-terminal HVDC system 1 of the embodiment. FIG. FIG. 4 is a diagram for explaining the process of step S140 in the flowchart of FIG. 3 . FIG. 4 is a diagram for explaining the process of step S160 in the flowchart of FIG. 3 . FIG. 4 is a diagram for explaining the process of step S170 in the flowchart of FIG. 3 . 2 is a diagram showing an example of a time series change in current flowing through a terminal 100 in the multi-terminal HVDC system 1 of the embodiment. 2 is a diagram showing an example of a time series change in current flowing through a terminal 100 in the multi-terminal HVDC system 1 of the embodiment. FIG. 1 is a block diagram showing an example of the configuration of a multi-terminal HVDC system 1A according to a modified example of an embodiment. 10A to 10C are diagrams illustrating processes performed by a control device 60 according to a modified example of the embodiment.

The power conversion device, control method, and program of the embodiment will be described below with reference to the drawings. In the following, an example will be described in which the multi-terminal HVDC system 1 of the embodiment has four terminals (terminals 100-1 to 100-4 described later), and each of the terminals is connected to a DC system to form a loop-shaped DC system. Each terminal is an example of a "power conversion device." The multi-terminal HVDC system 1 is also an example of a "multi-terminal DC transmission system." In addition, when applying the embodiment, the multi-terminal HVDC system 1 is not limited to four terminals, and may have at least three or more terminals. The DC system of the multi-terminal HVDC system 1 may also be configured in a radial or mesh shape.

Figure 1 is a block diagram showing an example of the configuration of a multi-terminal HVDC system 1 to which a terminal 100 of an embodiment is applied. The multi-terminal HVDC system 1 includes, for example, an AC side substation 2 (AC side substations 2-1 to 2-4), a terminal 100 (terminals 100-1 to 100-4), and a DC system 200.

The AC side substation equipment 2 constitutes part of an AC system that includes an AC power source and an AC transmission line. Each of the AC side substation equipment 2 is connected to each other via one of the terminals 100 and the DC system 200. For example, the AC side substation equipment 2-1 is connected to the AC side substation equipment 2-2 via terminal 100-1, DC transmission lines 203 and 204 in the DC system 200, and terminal 100-2. The AC side substation equipment 2-1 is connected to the AC side substation equipment 2-3 via terminal 100-1, DC transmission lines 201 and 202 in the DC system 200, and terminal 100-3. The AC side substation 2-1 is connected to the AC side substation 2-4 via DC transmission lines 203, 204 and terminal 100-2 in the DC system 200, and DC transmission lines 205, 207 in the DC system 200, and terminal 100-4.

The terminals 100 are installed at the boundary between the DC system and the AC system in the multi-terminal HVDC system 1. In the multi-terminal HVDC system 1, each of the terminals 100 has the function of supplying power to the other terminals 100 and the function of receiving power from the other terminals 100. However, the flow of power is often roughly constant, with some terminals 100 being exclusively connected to power generation equipment and other terminals 100 being exclusively connected to consumers.

In the following, a terminal 100 that converts AC power received from the AC side into DC power and supplies the power to another terminal 100 via the DC system 200 may be referred to as a "power transmitting end", and a terminal 100 that receives power from another terminal 100 via the DC system 200, converts it into AC power, and supplies it to the AC side may be referred to as a "power receiving end". In this embodiment, terminals 100-1 and 100-2 are the "power transmitting end", and terminals 100-3 and 100-4 are the "power receiving end". The power transmitting end is an example of a "power transmitting device". The power receiving end is an example of a "power receiving device". Furthermore, the functions described in this specification relate exclusively to the power transmitting device.

The DC system 200 includes, for example, DC transmission lines 201 to 208. The DC transmission lines 201 to 208 transmit DC power. DC power transmission can suppress power loss compared to AC power transmission, and is therefore particularly applicable to large-capacity, long-distance power transmission. The DC transmission lines 201 to 208 may be cables, overhead transmission lines, or any other type. In this embodiment, the DC transmission line refers to the section between the DC bus bar 20 of a certain terminal 100 and the DC bus bar 20 of another terminal 100. For example, the DC transmission line 201 refers to the section between the DC bus bar 20-1 and the DC bus bar 20-3. The DC transmission line 202 refers to the section between the DC bus bar 21-1 and the DC bus bar 21-3. The DC transmission line 203 refers to the area between the DC bus bar 20-1 and the DC bus bar 20-2. The DC transmission line 204 refers to the area between the DC bus bar 21-1 and the DC bus bar 21-2. The DC transmission line 205 refers to the area between the DC bus bar 20-2 and the DC bus bar 20-4. The DC transmission line 206 refers to the area between the DC bus bar 21-2 and the DC bus bar 21-4. The DC transmission line 207 refers to the area between the DC bus bar 20-4 and the DC bus bar 20-3. The DC transmission line 208 refers to the area between the DC bus bar 21-4 and the DC bus bar 21-3. The DC transmission lines 201 to 208 are examples of "electrical equipment".

The terminal 100 includes, for example, an AC/DC converter 10, DC busbars 20 and 21, current detectors 30-33, reactors 40-43, DC circuit breakers 50-53, and a control device 60. For example, the terminal 100-1 includes an AC/DC converter 10-1, DC busbars 20-1 and 21-1, current detectors 30-1, 31-1, 32-1, and 33-1, reactors 40-1, 41-2, 42-1, and 43-1, DC circuit breakers 50-1, 51-1, 52-1, and 53-1, and a control device 60-1. The current detectors 30-33, reactors 40-43, and DC circuit breakers 50-53 are examples of "electrical equipment." The reactors 40-43 are examples of "current limiters."

In this embodiment, the configuration connected to the DC bus 20 corresponds to the positive electrode, and the configuration connected to the DC bus 21 corresponds to the negative electrode. In the following description, the symbols following the hyphens will be omitted as appropriate.

The AC/DC converter 10 is a converter that converts AC power to DC power and vice versa, and is, for example, a self-excited power converter.

The DC busbar 20 is connected to the DC side of the AC/DC converter 10. The DC busbar 20 is connected to one side of the AC/DC converter 10, and a positive DC voltage is applied to it. The DC busbar 21 is connected to the other side of the AC/DC converter 10, and a negative DC voltage is applied to it.

The DC bus 20 is connected to the DC system 200 via current detectors 30, 32, reactors 40, 42, and DC circuit breakers 50, 52. The DC bus 21 is connected to the DC system 200 via current detectors 31, 33, reactors 41, 43, and DC circuit breakers 51, 53. For example, the DC bus 20-1 is connected to the DC transmission line 201 via the current detector 30-1, reactor 40-1, and DC circuit breaker 50-1. The DC bus 20-1 is connected to the DC transmission line 203 via the current detector 32-1, reactor 42-1, and DC circuit breaker 52-1. The DC bus 21-1 is connected to the DC transmission line 202 via the current detector 31-1, reactor 41-1, and DC circuit breaker 51-1. The DC bus 21-1 is connected to the DC transmission line 204 via a current detector 33-1, a reactor 43-1, and a DC breaker 53-1.

The current detectors 30 and 32 are connected between the DC bus 20 and the DC system 200. The current detectors 30 and 32 detect the value of a current flowing between the DC bus 20 and the DC system 200. For example, the current detector 30-1 detects the value of a current flowing in the DC transmission line 201. The current detector 32-1 detects the value of a current flowing in the DC transmission line 203. The current detectors 30 and 32 output the detected current value to the control device 60. The current detectors 31 and 33 are connected between the DC bus 21 and the DC system 200. The current detectors 31 and 33 detect the value of a current flowing between the DC bus 21 and the DC system 200. For example, the current detector 31-1 detects the value of a current flowing in the DC transmission line 202. The current detector 33-1 detects the value of a current flowing in the DC transmission line 204. The current detectors 31 and 33 output the detected current values to the control device 60.

The reactors 40 and 42 are connected between the DC bus 20 and the DC system 200. The reactors 40 and 42 suppress abrupt changes in the current flowing between the DC bus 20 and the DC system 200. The reactors 41 and 43 are connected between the DC bus 21 and the DC system 200. The reactors 41 and 43 suppress abrupt changes in the current flowing between the DC bus 21 and the DC system 200. The reactors 40 to 43 are used as current limiters that smooth (limit) a rapidly changing current that occurs, for example, during an accident (for example, a surge current that occurs during a lightning strike). The reactors 40 to 43 may be any type of reactor, such as a series reactor.

The DC circuit breakers 50 and 52 are connected between the DC bus 20 and the DC system 200. The DC circuit breakers 50 and 52 electrically connect or disconnect between the DC bus 20 and the DC system 200 in response to control by the control device 60. The DC circuit breakers 51 and 53 are connected between the DC bus 21 and the DC system 200. The DC circuit breakers 51 and 53 electrically connect or disconnect between the DC bus 21 and the DC system 200 in response to control by the control device 60.

The control device 60 is realized, for example, by a hardware processor such as a CPU (Central Processing Unit) executing a program (software). Some or all of these components may be realized by hardware (including circuitry) such as an LSI (Large Scale Integration), ASIC (Application Specific Integrated Circuit), FPGA (Field-Programmable Gate Array), or GPU (Graphics Processing Unit), or may be realized by a combination of software and hardware. The program may be stored in advance in a storage device (a storage device with a non-transient storage medium) such as an HDD (Hard Disk Drive) or flash memory, or may be stored in a removable storage medium (non-transient storage medium) such as a DVD or CD-ROM, and installed by inserting the storage medium into the drive device.

The control device 60 acquires the current values detected by the current detectors 30 to 33. For example, the control device 60-1 acquires the current values detected by the current detectors 30-1, 31-1, 32-1, and 33-1.

The control device 60 also controls the DC circuit breakers 50 to 53. For example, the control device 60-1 controls the DC circuit breakers 50-1, 51-1, 52-1, and 53-1, and shuts off the DC circuit breakers as necessary.

The control device 60 also controls the AC/DC converter 10. For example, the control device 60-1 controls the AC/DC converter 10-1 and stops the operation of the AC/DC converter 10-1 as necessary.

The control device 60 detects the occurrence of an accident at its own terminal based on the current values acquired from the current detectors 30 to 33. The own terminal is the terminal 100 at which the control device 60 is provided. For example, the own terminal of the control device 60-1 is the terminal 100-1. The own terminal of the control device 60-2 is the terminal 100-2. An accident at the own terminal is an accident that occurs in the DC bus bars 20, 21 to which the own terminal is connected.

For example, the control device 60 detects the occurrence of an accident at its own terminal based on the change over time in the current value acquired from the current detectors 30-33, and identifies the location of the detected accident (fault section). The control device 60 opens the DC circuit breakers 50-53 at its own terminal according to the location of the accident. This removes the location of the accident from the DC system 200. The method by which the control device 60 detects an accident at its own terminal and the method by which the control device 60 opens the DC circuit breakers 50-53 as necessary may be any method similar to the conventional method. For this reason, a detailed description thereof will be omitted.

In this embodiment, when an accident occurs at the other terminal, the control device 60 determines whether or not to stop the AC/DC converter 10 at its own terminal. The other terminal is a terminal 100 different from the terminal 100 at which the control device 60 is provided. For example, the other terminals of the control device 60-1 are terminals 100-2, 100-3, and 100-4. The other terminals of the control device 60-2 are terminals 100-1, 100-3, and 100-4. An accident that occurs at the other terminal refers to an accident that occurs at the DC busbars 20 and 21 to which the other terminal is connected.

In the event of such an accident occurring at the other terminal, the DC circuit breakers 50-53 at the other terminal are opened, and the location of the accident is removed from the DC system 200. In other words, the other terminal is disconnected. In this case, if there is no effect from the removal of the location of the accident, the own terminal may continue to exchange power, but if an imbalance in power exchange occurs in the system as a whole, it is necessary to stop the AC/DC converter 10 of the own terminal and stop the exchange of power through the own terminal.

As described above, in the multi-terminal HVDC system 1, various power transmission routes are constructed. Therefore, when an accident occurs at another terminal, whether or not it is necessary to stop the AC/DC converter 10 at the own terminal must be determined by comprehensively taking into consideration the location of the accident, the connection state of the DC transmission line, the state of the current flowing through the DC transmission line, and other factors. On the other hand, if the unbalanced state of power transfer continues, any of the AC/DC converters 10 included in the multi-terminal HVDC system 1 may reach an overvoltage state, which may result in the AC/DC converter 10 being forced to stop for a long period of time. For this reason, it is necessary to determine whether or not it is necessary to stop the AC/DC converter 10 at the own terminal as quickly as possible.

As a countermeasure, in the case of an accident at another terminal, the control device 60 of this embodiment maintains or stops the operation of the AC/DC converter 10 based on the DC current flowing through its own terminal, i.e., the change over time in the current value acquired from the current detectors 30 to 33. This makes it possible to quickly determine whether or not it is necessary to stop the AC/DC converter 10 at its own terminal, while minimizing the communication time with the higher-level device and delays due to signal processing related to communication.

The higher-level device here is a device that aggregates information about the power status of multiple terminals 100 in the multi-terminal HVDC system 1, such as a central control device. The higher-level device has the function of using the aggregated information to detect the presence or absence of an accident and the location of the accident, and protecting the DC system 200 by outputting commands to open the DC circuit breakers 50-53 and to stop the AC-DC converter 10 as necessary.

As an example, consider the process performed by the control device 60-1 when an accident occurs on the DC bus 20-3 of terminal 100-3 (see FIG. 2). Also assume that before the accident, 1 pu of power was transmitted from each of terminals 100-1 and 100-2, and 1 pu of power was received from each of terminals 100-3 and 100-4. Also assume that the rated power capacity of each of the terminals 100 is 1 pu. Below, we will explain the process from when a DC accident occurs until the AC/DC converter 10 is restarted after an emergency shutdown, with examples of the processes performed at terminals 100-1 and 100-2.

In the following, the current value of the DC current detected by current detectors 30 to 33 is expressed as current value Idcx, with the direction from the own terminal to the other terminal being positive. x indicates the numerical value following the hyphen of the connection source and destination terminals 100. For example, the current value detected by current detector 30-1 is expressed as current value Idc13. The current value detected by current detector 32-1 is expressed as current value Idc12. The current value detected by current detector 30-2 is expressed as current value Idc21. The current value detected by current detector 32-2 is expressed as current value Idc24. The current value detected by current detector 30-3 is expressed as current value Idc31. The current value detected by current detector 32-3 is expressed as current value Idc34. The current value detected by current detector 30-4 is expressed as current value Idc43. The current value detected by current detector 32-4 is represented as current value Idc42 (see Figure 2).

The processing performed by the control device 60 will now be described with reference to Figures 2 to 9. Figure 2 is a diagram illustrating the state immediately after an accident occurs on the DC busbar 20 of the multi-terminal HVDC system 1 of the embodiment. Figure 3 is a flowchart illustrating the overall flow of the processing performed by the control device 60 of the embodiment. Figures 4 and 5 are diagrams illustrating the processing of steps S110 and S120 in the flowchart of Figure 3. Figure 6 is a diagram illustrating the state in which the terminal 100-3 is disconnected after an accident occurs on the DC busbar 20 of the multi-terminal HVDC system 1 of the embodiment. Figures 7 to 9 are diagrams illustrating the processing of steps S140, S160, and S170 in the flowchart of Figure 3.

In step S100 of FIG. 3, the control device 60 calculates the time rate of change ΔIdcx of the DC current Idcx. For example, the control device 60 periodically acquires the current values detected by the current detectors 30-33 (e.g., every 10 μs to 1 ms), and calculates the time rate of change ΔIdcx using the current value acquired this time and the current value acquired last time. For example, the control device 60-1 calculates the time rate of change ΔIdc12 of the DC current Idc12 and the time rate of change ΔIdc13 of the DC current Idc13 as the flow of DC current on the positive electrode side.

In step S110, the control device 60 uses the calculated time rate of change ΔIdcx to determine whether an accident has occurred. The method by which the control device 60 uses the calculated time rate of change ΔIdcx to determine whether an accident has occurred is described in detail in FIG. 4.

In step S120, if the control device 60 determines that an accident has occurred in step S110, it determines which terminal 100 may be disconnected. The terminal 100 that may be disconnected here is a terminal that may open any of the DC circuit breakers 50 to 53 provided at the terminal 100 due to the occurrence of an accident. The method by which the control device 60 determines which terminal 100 may be disconnected is described in detail in FIG. 5.

In step S130, the control device 60 determines whether the terminal 100 that may be disconnected is another terminal based on the determination result of step S120. If the terminal 100 that may be disconnected is another terminal, the control device 60 proceeds to step S140. If the terminal 100 that may be disconnected is not another terminal, the control device 60 ends the process. Note that if the terminal 100 that may be disconnected is the own terminal, the control device 60 uses a conventional protective relay function to open the DC circuit breakers 50-53 of the own terminal as necessary.

In step S140, the control device 60 determines whether the other terminal has been disconnected. If an accident is detected in step S110, the control device 60 determines whether the other terminal has been disconnected using the time rate of change ΔIdcx until a predetermined time has elapsed. The method by which the control device 60 determines whether the other terminal has been disconnected is described in detail in FIG. 7.

In step S150, if the control device 60 determines in step S140 that the other terminal has been disconnected, it determines whether the disconnected terminal 100 is a receiving end. Whether each of the multiple terminals 100 included in the multi-terminal HVDC system 1 is a sending end or a receiving end is determined in advance, for example, when the multi-terminal HVDC system 1 is constructed. The control device 60 stores in advance, for example, a correspondence table indicating whether each of the terminals 100 included in the multi-terminal HVDC system 1 is a sending end or a receiving end. The control device 60 determines whether the disconnected terminal 100 is a receiving end by referring to the correspondence table using information indicating the other terminal determined to be disconnected.

In step S160, if there is a transmission end that may be disconnected between the own terminal and the disconnected other terminal, the control device 60 calculates the power state of that transmission end. The method by which the control device 60 calculates the power state of the transmission end is described in detail in FIG. 8.

In step S170, the control device 60 determines whether to stop the operation of the AC/DC converter 10 at its own terminal based on the power state at the sending end calculated in step S160. The method by which the control device 60 determines whether to stop the operation of the AC/DC converter 10 at its own terminal is described in detail in FIG. 9.

In step S180, if the control device 60 determines that the AC/DC converter 10 of its own terminal should be stopped, it stops (gate blocks) the AC/DC converter 10 of its own terminal. In step S190, the control device 60 determines whether or not a restart command has been received. The restart command is notified to the control device 60 by a higher-level device such as a central processing unit. In step S200, if the control device 60 receives a restart command in step S190, it performs a process related to restart (restart process). The restart process is, for example, a process of resetting an integrator used for various controls and setting a command value for restart (power command value). In step S210, the control device 60 performs deblocking (release of gate block) by issuing a command to operate the AC/DC converter 10 of its own terminal. Then, the control device 60 operates the AC/DC converter 10 after restart based on the power command value.

By performing processing in this manner, the control device 60 can independently and safely perform gate block operation without waiting for commands from a higher-level device or the like. Therefore, it can be restarted more quickly than when the AC/DC converter 10 is stopped due to a major fault. A major fault here refers to a fault that causes a long-term stoppage, such as when any of the AC/DC converters 10 in the multi-terminal HVDC system 1 stops as a result of a sustained overvoltage state.

Figure 2 shows the path of the fault current that flows immediately after an accident occurs on the DC bus 20-3 of terminal 100-3. When an accident occurs on the DC bus 20-3 of terminal 100-3, the voltage at the accident point drops instantaneously. As a result, currents flow steeply from each part of the DC system 200 toward the accident point. In this embodiment, the occurrence of an accident is detected by capturing the phenomenon of currents flowing steeply toward the accident point.

Figure 4 shows a detailed flow of the process shown in step S110 in Figure 3. In step S111, the control device 60 calculates the absolute value of the time change rate ΔIdcx calculated in step S100. In step S112, the control device 60 compares the absolute value calculated in step S111 with a preset current threshold ΔIlim. The current threshold ΔIlim is set to a value sufficiently larger than the time change rate of the DC current during normal operation when no accident has occurred. For example, the current threshold ΔIlim is set to 0.5 [A/μs]. If the absolute value |ΔIdcx| is larger than the current threshold ΔIlim, the control device 60 determines that a DC accident has occurred at some point in the DC system 200 and proceeds to step S120. On the other hand, if the absolute value |ΔIdcx| is smaller than the current threshold value ΔIlim, the control device 60 determines that no DC fault has occurred anywhere in the DC system 200 and ends the process.

Figure 5 shows details of the process shown in step S120 in Figure 3. Figure 5 shows a judgment table in which the upper row shows the judgment formula and the lower row shows the judgment result. The judgment formula is a mathematical expression of the condition for determining which terminals may be disconnected. Here, the judgment formula is a mathematical expression of the condition when making a judgment based on the magnitude of the time rate of change ΔIdcx. The judgment result shows which terminals may be disconnected when the judgment condition is met.

The time rate of change ΔIdcx after an accident occurs is expressed by the following (Equation 1). In (Equation 1), Idcx is the DC current value acquired this time. Idcx_p is the DC current value acquired last time. Δt is the time from the last time to this time. ΔVI indicates the voltage applied to inductance Ldc during time Δt. Ldc is the total value of the DC side inductance from the own terminal to the other terminal.

ΔIdcx={(Idcx)-(Idcx_p)}/Δt
=ΔVI/Ldc...(Formula 1)

The inductance Ldc in (Equation 1) mainly includes the impedance of reactors 40-43 and the DC transmission line (any of DC transmission lines 201-208). Therefore, when the accident point is far away, the inductance Ldc is larger than when the accident point is nearby. As a result, when the accident point is far away, the time rate of change ΔIdcx is smaller.

Using this property, the control device 60 determines which terminals may be disconnected, i.e., which terminals may have experienced an accident, based on the magnitude relationship of ΔIdcx, as shown in step S120. In this embodiment, this determination is made for each terminal. That is, each terminal makes the determination using the current value detected at its own terminal, without using the current values detected at other terminals. For example, the control device 60-1 makes the determination using the time rate of change ΔIdc13 of the DC current Idc13 detected by the current detector 30-1.

The example in FIG. 5 shows a judgment table for determining which terminals may be disconnected based on the time rate of change ΔIdc13 of the DC current Idc13 flowing from terminal 100-1 to terminal 100-3. The time rate of change ΔIdc13 is in [A/us].

In this example, it is shown that if the time rate of change ΔIdc13 is smaller than -5.0 [A/us], the terminal that may disconnect is terminal 100-1 (own terminal). This indicates that if the DC current Idc13 increases suddenly in the direction flowing from terminal 100-3 to terminal 100-1, the accident point is at terminal 100-1. In other words, it indicates that if the current flows sharply toward the accident point at terminal 100-1, the time rate of change ΔIdc13 becomes smaller than -5.0 [A/us].

When the time rate of change ΔIdc13 is -2.0 [A/us] or more and less than -0.5 [A/us], it is shown that the terminal that may disconnect is terminal 100-2. This indicates that when the DC current Idc13 increases somewhat suddenly in the direction flowing from terminal 100-3 to terminal 100-1, the accident point is at terminal 100-2. In other words, when the current flows sharply toward the accident point at terminal 100-2, the time rate of change ΔIdc13 is in the range of -2.0 to -0.5 [A/us].

When the time rate of change ΔIdc13 is -0.2 [A/us] or more and less than -0.05 [A/us], it is shown that the terminal that may disconnect is terminal 100-4. This indicates that when the DC current Idc13 increases slightly in the direction flowing from terminal 100-3 to terminal 100-1, the accident point is at terminal 100-4. In other words, when the current flows steeply toward the accident point at terminal 100-4, the time rate of change ΔIdc13 is in the range of -0.2 to -0.05 [A/us].

When the time rate of change ΔIdc13 is equal to or greater than 0.05 [A/us] and smaller than 0.2 [A/us], it is shown that the terminal that may disconnect is terminal 100-4. This indicates that when the DC current Idc13 increases slightly in the direction flowing from terminal 100-1 to terminal 100-3, the accident point is at terminal 100-4. In other words, it indicates that when the current flows steeply toward the accident point at terminal 100-4, the time rate of change ΔIdc13 is in the range of 0.05 to 0.2 [A/us].

When the time rate of change ΔIdc13 is equal to or greater than 0.5 [A/us] and smaller than 2.0 [A/us], it is shown that the terminal that may disconnect is terminal 100-3. This indicates that when the DC current Idc13 increases somewhat suddenly in the direction flowing from terminal 100-1 to terminal 100-3, the accident point is at terminal 100-3. In other words, it indicates that when the current flows steeply toward the accident point at terminal 100-3, the time rate of change ΔIdc13 is in the range of 0.5 to 2.0 [A/us].

Figure 6 shows the path of the fault current that flows after DC circuit breakers 50-3, 51-3, 52-3, and 53-3 of terminal 100-3 are opened due to an accident that occurs on DC bus 20-3 of terminal 100-3. In this case, terminal 100-3 may be disconnected due to DC circuit breaker 50-1 or DC circuit breaker 50-4 opening.

In the state shown in FIG. 6, if operation continues at terminals (terminals 100-1, 100-2, 100-4) other than the disconnected terminal (terminal 100-3), terminals 100-1 and 100-2, which are the sending ends, will each continue to transmit 1 pu of power. On the other hand, AC/DC converter 10-4, which is the receiving end, will receive 2 pu of power despite only having a receiving capacity of 1 pu, exceeding its rated capacity and potentially resulting in an overvoltage state.

On the other hand, when the terminal 100-3 having the fault point is disconnected, the sudden inflow to the fault point is eliminated, and the current flow in the direction in which it had been flowing is suddenly reduced. Taking advantage of this property, in this embodiment, the sudden change in the current flow is detected to determine whether the other terminals are disconnected.

Figure 7 shows a detailed flow of the process shown in step S140 in Figure 3. In step S141, the control device 60 determines whether the polarity of the time change rate ΔIdcx and the time change rate ΔIdcx_p has reversed. The time change rate ΔIdcx is a time change rate calculated using the current DC current value Idcx. The time change rate ΔIdcx_p is a time change rate calculated using the previous DC current value Idcx_p. When the time change rate ΔIdcx_p is a positive value, the control device 60 determines that the polarity has reversed when the time change rate ΔIdcx becomes a negative value. When the time change rate ΔIdcx_p is a negative value, the control device 60 determines that the polarity has reversed when the time change rate ΔIdcx becomes a positive value. When the control device 60 determines that the polarity has reversed, the control device 60 proceeds to step S150.

In step S142, if the control device 60 determines in step S141 that the polarity has not been reversed, it stores the value (current value) of the time change rate ΔIdcx as the time change rate ΔIdcx_p. In step S143, the control device 60 counts up the variable T (timer T). In step S144, the control device 60 determines whether the variable T is greater than the timer threshold Tlim. The control device 60 ends the process if the variable T is greater than the timer threshold Tlim. If the variable T is less than the timer threshold Tlim, the control device 60 returns to step S141 and continues to determine whether the polarity has been reversed. In this way, after determining in step S130 that the terminal 100 that may be disconnected is another terminal, the control device 60 detects whether or not there is a terminal 100 that has been disconnected until the predetermined timer threshold Tlim has elapsed.

Figure 8 shows a detailed flow of the process shown in step S160 in Figure 3. In step S161, the control device 60 determines whether there is another transmission end other than the own terminal in the direction in which the fault current flows as viewed from the own terminal. The direction in which the fault current flows as viewed from the own terminal is the direction of the time rate of change ΔIdcx immediately after the fault is detected in step S110.

For example, as described above, if an accident occurs at terminal 100-3, current will flow steeply toward the accident point. In this case, it is considered that the current will increase in the direction from terminal 100-1 toward terminal 100-3. At this time, the time rate of change ΔIdc13 will be positive. Also, the time rate of change ΔIdc12 will be negative. In this case, the control device 60-1 will determine that the direction in which the accident current flows as viewed from its own terminal is toward terminal 100-3, and that there is "no" other power transmission end other than its own terminal in that direction.

On the other hand, if an accident occurs at terminal 100-3, it is considered that the current increases in the direction from terminal 100-2 to terminal 100-3. At this time, for example, the time rate of change ΔIdc21 becomes positive. In this case, the control device 60-2 determines that the direction in which the accident current flows as viewed from its own terminal is the direction toward terminal 100-3, that there is another power transmission end other than its own terminal in that direction, and determines that the other power transmission end is terminal 100-1.

In this embodiment, a position table indicating the relationship between the positions of the multiple terminals 100 included in the multi-terminal HVDC system 1 is stored in advance in the control device 60 at each terminal. Based on the direction in which the fault current determined in step S161 flows, the control device 60 refers to the position table to determine whether there is another transmission end other than the own terminal in that direction.

In step S162, if the control device 60 determines in step S161 that there is another power transmission end other than the own terminal in the direction in which the fault current flowed, it calculates the transmission power value Psd [pu] of the corresponding power transmission end. The transmission power value Psd is an example of "power transmission/reception information." Here, it is assumed that the transmission power value at the power transmission end does not change before and after the accident occurs, and that each of terminals 100-1 and 100-2 transmits 1 [pu] of DC power. In this case, the control device 60-2 determines that there is another power transmission end other than the own terminal in the direction in which the fault current flowed as viewed from the own terminal, and that the power transmission end is terminal 100-1, and therefore determines that the transmission power value Psd is 1 [pu].

In step S163, if the control device 60 determines in step S161 that there is "no" other power transmission end other than the own terminal in the direction in which the fault current flows, the control device 60 sets the transmission power value Psd to 0 (zero) [pu]. For example, the control device 60-1 sets the transmission power value Psd to 0 (zero) [pu] because there is "no" other power transmission end other than the own terminal in the direction in which the fault current flows as viewed from the own terminal.

Figure 8 shows a detailed flow of the process shown in step S170 in Figure 3. In step S171, the control device 60 calculates the total value Psc [pu] of the transmitted power at all transmitting ends that continue to operate. The total value Psc is an example of "transmitted and received power information." Here, the transmitting ends that continue to operate are all transmitting ends in the multi-terminal HVDC system 1, excluding the transmitting ends that may be disconnected. The transmitting ends that may be disconnected are the transmitting ends that are determined in step S161 to be between the own terminal and the terminal that has been disconnected due to the accident.

For example, the control device 60 uses the following (Equation 2) to calculate the total value Psc [pu] of the transmitted power at all transmitting ends that continue to operate. In (Equation 2), Psc is the total value of the transmitted power at all transmitting ends that continue to operate after the accident occurs. Pss is the total value of the transmitted power (command value) at all transmitting ends in the multi-terminal HVDC system 1. The command value Pss is an example of "transmitted and received power information." Psd is the total value of the transmitted power at other transmitting ends other than the own terminal that are in the direction in which the fault current flows.

Psc=Pss-Psd...(Formula 2)

In step S172, the control device 60 calculates the total value Prc [pu] of the received power at all receiving ends that continue to operate. The total value Prc is an example of "transmitted and received power information." Here, the receiving end that continues to operate refers to all receiving ends in the multi-terminal HVDC system 1 excluding the disconnected receiving end. The disconnected receiving end refers to the terminal that was determined to be the disconnected receiving end in step S150.

For example, the control device 60 uses the following (Equation 3) to calculate the total value Prc [pu] of the received power at all receiving ends that continue to operate. In (Equation 3), Prc is the total value of the received power at all receiving ends that continue to operate after the accident occurs. Prs is the total value of the received power (command value) at all receiving ends in the multi-terminal HVDC system 1. The command value Prs is an example of "transmitted and received power information." Prd is the total value of the received power at receiving ends that are disconnected due to the fault current. The received power value Prd is an example of "transmitted and received power information."

Prc=Prs-Prd...(Formula 3)

In step S173, the control device 60 compares the total value Psc of the transmitted power at the transmitting end that continues to operate with the total value Prc of the received power at the receiving end. If the total value Psc of the transmitted power is greater than the total value Prc of the received power, the control device 60 determines to stop the AC/DC converter 10 at its own terminal.

For example, when the multi-terminal HVDC system 1 is in the situation shown in FIG. 6, at terminal 100-1, the total transmitted power Psc is 2 [pu], and the total received power Prc is 1 [pu]. In other words, the total transmitted power Psc exceeds the total received power Prc. For this reason, the control device 60-1 determines to stop the AC/DC converter 10-1 of its own terminal (terminal 100-1).

In the situation shown in FIG. 6, the total transmitted power Psc is 1 [pu] and the total received power Prc is 1 [pu] at terminal 100-2. This is because in step S162, the total transmitted power Psd of the other transmitting ends other than the own terminal in the direction in which the fault current flows is 1 [pu]. In other words, at terminal 100-2, the total transmitted power Psc and the total received power Prc have the same capacity. Therefore, the control device 60-2 determines to continue operation of the AC/DC converter 10-2 at its own terminal (terminal 100-2) without stopping it.

In this way, even if an accident causes a receiving end to disconnect and the amount of power transmitted becomes excessive in the multi-terminal HVDC system 1, the AC/DC converters 10 at the transmitting ends can be stopped in order, starting from the transmitting end with the shortest DC transmission path to the disconnected receiving end, until the excessive power transmission state is resolved. This prevents an unbalanced state of power transmission and reception, for example, a state in which the amount of transmitted power exceeds the amount of received power, from continuing, and avoids a situation in which a healthy terminal is stopped due to a major fault. Here, a state in which a receiving end is disconnected due to an accident and the AC/DC converters 10 at the transmitting ends are stopped in order, starting from the transmitting end with the shortest DC transmission path to the disconnected receiving end, is an example of a "state after disconnection".

Figures 10 and 11 are diagrams showing examples of time series changes in currents flowing through each of the terminals 100 in the multi-terminal HVDC system 1 of the embodiment. In Figures 10 and 11, the first diagram from the top shows an example of time series changes in DC currents Idc13 and Idc12 at terminal 100-1. The second diagram from the top shows an example of time series changes in DC currents Idc21 and Idc24 at terminal 100-2. The third diagram from the top shows an example of time series changes in DC currents Idc31 and Idc34 at terminal 100-3. The fourth diagram from the top shows an example of time series changes in DC currents Idc43 and Idc42 at terminal 100-4.

Figure 10 shows a schematic diagram of the time series changes in the current values flowing through each terminal, from normal operation when no accidents have occurred, through the occurrence of an accident at terminal 100-3 and the disconnection of terminal 100-3, until the AC/DC converter 10 at terminal 100-1 is spontaneously shut down (not due to a major fault).

In this example, terminals 100-1 and 100-2 function as transmitting ends and each transmits 1 pu of power under normal circumstances. Terminals 100-3 and 100-4 function as receiving ends and each receives 1 pu of power.

At time T1, an accident occurs on the DC bus 20 at terminal 100-3. As current flows steeply toward the accident point, DC current Idc13 increases significantly and Idc12 decreases significantly. DC current Idc21 increases and Idc24 decreases. DC currents Idc31 and Idc34 decrease significantly. DC current Idc43 increases significantly and Idc42 decreases significantly.

At time T2, terminal 100-3 is disconnected. As a result, the phenomenon of current flowing sharply toward the fault point is eliminated. Therefore, a large increase in DC current Idc13 changes to a decrease, and a large decrease in Idc12 changes to an increase. Furthermore, an increase in DC current Idc21 changes to a decrease, and a decrease in Idc24 changes to an increase. Furthermore, DC currents Idc31 and Idc34 become 0 (zero). Furthermore, a large increase in DC current Idc43 changes to a decrease, and a large decrease in Idc42 changes to an increase.

At time T3, the AC/DC converter 10-1 at terminal 100-1 is stopped. As a result, the DC currents Idc13 and Idc12 approach 0 (zero). The amount of power stabilizes after the DC current Idc21 decreases to near 0 (zero) [pu], and Idc24 increases to near 1 [pu] and then stabilizes. The DC current Idc43 decreases to near 0 [pu] and then stabilizes, and Idc42 increases to near -1 [pu] and then stabilizes. In this way, the amount of power transmission is reduced by quickly stopping the AC/DC converter 10-1 at terminal 100-1 before the amount of power transmission becomes excessive, and as a result, the power transmission of 1 [pu] is continued between terminals 100-2 and 100-4.

Figure 11 shows a schematic diagram of the time series changes in the current values flowing through each terminal, from normal operation when no accidents have occurred, through the occurrence of an accident at terminal 100-3 and the disconnection of terminal 100-3, to the point where the AC/DC converter 10 at terminal 100-1 is shut down voluntarily (not due to a major fault), but the imbalance in power transfer is not resolved, and the AC/DC converter 10 at terminal 100-2 is also shut down voluntarily (not due to a major fault).

In this example, it is shown that under normal circumstances, terminals 100-1 and 100-2 function as transmitting ends, with terminal 100-1 transmitting 0.5 [pu] and terminal 100-2 transmitting 1 [pu]. It is also shown that terminals 100-3 and 100-4 function as receiving ends, with terminal 100-3 receiving 1 [pu] and terminal 100-4 receiving 0.5 [pu].

Of the 0.5 [pu] of transmission power transmitted from terminal 100-2 to terminal 100-3, 0.25 [pu] passes through DC transmission line 203 provided between terminals 100-1 and 100-2, and the remaining 0.25 [pu] passes through DC transmission lines 205 and 207 provided between terminals 100-2, 100-4, and 100-3.

At time T4, an accident occurs on the DC bus 20 at terminal 100-3. As current flows steeply toward the accident point, DC current Idc13 increases significantly and Idc12 decreases significantly. DC current Idc21 increases and Idc24 decreases. DC currents Idc31 and Idc34 decrease significantly. DC current Idc43 increases significantly and Idc42 decreases significantly.

At time T5, terminal 100-3 is disconnected. As a result, the phenomenon of current flowing sharply toward the fault point is eliminated. Therefore, the large increase in DC current Idc13 changes to a decrease, and the large decrease in Idc12 changes to an increase. Furthermore, the increase in DC current Idc21 changes to a decrease, and the decrease in Idc24 changes to an increase. Furthermore, DC currents Idc31 and Idc34 become 0 (zero). Furthermore, the large increase in DC current Idc43 changes to a decrease, and the large decrease in Idc42 changes to an increase. At terminal 100-1, the total transmitted power Psc is 1.5 [pu] and the total received power Prc is 0.5 [pu], so it is determined that AC/DC converter 10-1 should be stopped, and gate blocking is executed. At terminal 100-2, the total transmitted power Psc is 1.0 [pu] and the total received power Prc is 0.5 [pu], so it is determined that AC/DC converter 10-2 should be stopped and gate blocking is executed.

At time T6, the AC/DC converter 10-1 at terminal 100-1 is stopped. At time T7, the AC/DC converter 10-2 at terminal 100-2 is stopped. As a result, the DC currents Idc13, Idc12, Idc21, and Idc24 asymptotically approach 0 (zero). In addition, the DC currents Idc43 and Idc42 also asymptotically approach 0 (zero).

Note that at time T7, terminal 100-4, which is in good condition and has no accidents, has a power receiving capacity of 0.5 [pu], and terminal 100-1 has a power transmission capacity of 0.5 [pu], and is in a state where it is stopped in a good condition and not in a major fault state. Therefore, it is possible to resume the transmission of 0.5 [pu] by restarting terminal 100-1 based on a command from a higher-level device, etc.

As described above, the terminal 100 of the embodiment includes an AC/DC converter 10 and a control device 60. The AC/DC converter 10 converts AC power into DC power and vice versa. The control device 60 controls the AC/DC converter 10. When the control device 60 detects an accident at another terminal (another power conversion device), it detects the fault section from the current value flowing in the DC transmission line (any of the DC transmission lines 201 to 208) near the own terminal (own device) and the impedance of the nearby DC transmission line. The control device 60 determines whether the other terminal has been disconnected from the DC system 200 based on the time-series change in the current value flowing in the nearby DC transmission line. When it is determined that the other terminal has been disconnected, the control device 60 determines whether to stop the AC/DC converter 10 based on the transmission and reception power information of the multi-terminal HVDC system 1 and the calculation result of the fault section.

In this way, when an accident occurs at another terminal, the terminal 100 of the embodiment can identify the accident section and determine whether the other terminal where the accident occurred has been disconnected. Therefore, when the terminal 100 at the accident point is disconnected, it can be determined whether an imbalance in power transmission and reception will occur even if the operation of the own terminal is continued based on the relationship between the transmitted and received power of the multi-terminal HVDC system 1 and the accident section. Therefore, it can be determined whether to stop the converter of the own terminal.

In addition, in the terminal 100 of the embodiment, the control device 60 identifies the fault section based on the magnitude relationship of the rate of change (ΔIdcx) of the current value per unit time. As a result, the control device 60 can detect whether an accident has occurred (without obtaining the status of other terminals from a higher-level device) through a process closed within its own terminal, that is, by monitoring the rate of change (ΔIdcx) of the current in the nearby DC transmission line, and in the event of an accident, it is possible to identify how far away from its own terminal the accident occurred at the other terminal.

In addition, in the terminal 100 of the embodiment, when the control device 60 determines that the other terminal where the accident occurred has been disconnected, it uses the command value (Pss) of the transmission power in the multi-terminal HVDC system 1 and the command value (Prs) of the receiving power in the multi-terminal HVDC system 1 to calculate the total transmission power value (Pss-Psd) and the receiving power value (Prs-Prd) of the multi-terminal HVDC system 1 in the state after parallel-off (for example, a state in which the other terminal is parallel-off and the AC-DC converter 10 at the sending end connected between the own terminal and the other terminal is stopped). If the calculated transmission power value (Pss-Psd) is greater than the receiving power value (Prs-Prd) and the own terminal is a terminal on the power transmitting side, the control device 60 decides to stop the AC-DC converter 10 (at the own terminal). As a result, in the terminal 100 of the embodiment, depending on the state after parallel-off, for example, when the other terminal where the accident occurred is parallel-off and all power transmission at the power transmission ends connected between the terminal and the other terminal is stopped, but it is expected that the imbalance in power transfer in the entire system will not be resolved, it is possible to determine to stop the AC/DC converter 10 of the own terminal. Therefore, it is possible to resolve the imbalance in power transfer caused by the parallel-off of the other terminal where the accident occurred.

In the above-described embodiment, an example has been described in which the command values (Pss and Prs) are used to determine the total value of the transmitted power at the transmitting end included in the multi-terminal HVDC system 1 and the total value of the received power at the receiving end included in the multi-terminal HVDC system 1. However, this is not limited to this. Instead of using the command values, periodically measured power values may be used. Here, the periodically measured power values are an example of "transmitted and received power information".

In this case, for example, the control device 60 of each terminal 100 periodically measures the power value transmitted (or received) by the AC/DC converter 10 of its own terminal, and notifies, for example, a higher-level device of the measured power value. The higher-level device stores the power value received from each control device 60 in a storage area that can be referenced by the other control devices 60. Alternatively, the higher-level device periodically notifies the other control devices 60 of the power value received from each control device 60.

When the terminal 100 determines that the other terminal where the accident occurred has been disconnected, it uses the measured value of the transmitted power (referred to as Pss_m) at the transmitting end included in the multi-terminal HVDC system 1 and the measured value of the received power (referred to as Prs_m) at the receiving end included in the multi-terminal HVDC system 1 to calculate the total transmitted power value (Pss_m-Psd) and received power value (Prs_m-Prd) of the multi-terminal HVDC system 1 in the state after disconnection (for example, a state in which the other terminal is disconnected and the AC-DC converter 10 at the transmitting end connected between the own terminal and the other terminal is stopped). If the calculated transmitted power value (Pss_m-Psd) is greater than the received power value (Prs_m-Prd) and the own terminal is a terminal on the transmitting side, the control device 60 decides to stop the AC-DC converter 10 (at the own terminal). This provides an effect equivalent to that described above. In addition, because periodically measured values are used instead of command values, it is possible to accurately calculate whether the imbalance in power transfer is resolved.

In this case, the control device 60 may use a command value or a measurement value to calculate the total value of the transmission power Psd of the power transmission end connected between the own terminal and the other terminal, and the power receiving value (Prd) of the other terminal where the accident occurred. Since the current value is measured periodically, the control device 60 can calculate the total value of the transmission power (Psd) and the power receiving value (Prd) of the other terminal where the accident occurred, using the measurement value measured immediately before the accident, when an accident occurs. Since there is no need to communicate with a higher-level device after an accident occurs, there is no need to communicate with a higher-level device to detect the accident and to decide whether or not to stop the AC-DC converter 10 in response to the occurrence of the accident. Therefore, there is no delay in the decision due to communication delays, etc. Therefore, it is possible to prevent the adverse effects of the accident from spreading to healthy electrical equipment.

In addition, in the terminal 100 of the embodiment, when the other terminal where an accident has occurred is disconnected and the AC/DC converter 10 (at its own terminal) is stopped, the control device 60 sets a power command value for the transmission power by the AC/DC converter within a range that does not exceed the amount of power that can be received by the multi-terminal HVDC system 1 in the state after disconnection (for example, the value (Prd-{Psc+Psc_j}) obtained by subtracting the power value of the transmission power (Psc+Psc_j) from the received power value (Prd) at the disconnected other terminal).

Psc is the total value of the transmission power of the power transmission ends connected between the own terminal and the other terminal. Psc_j is the transmission power value of the own terminal. The control device 60 restarts the AC/DC converter 10 (of the own terminal) and operates the restarted AC/DC converter 10 based on the determined power command value.

In this way, in the terminal 100 of the embodiment, the imbalance in power transfer that results in excessive transmitted power is eliminated, but if the received power is excessive, the AC/DC converter 10 can be restarted to resume power transmission. In this case, the power command value is set to a range in which the transmitted power will not become excessive after restart. Therefore, restarting the AC/DC converter 10 does not put the entire system into a state in which the transmitted power is excessive. This makes it possible to prevent a situation in which the receiving end is shut down due to a major failure.

The terminal 100 of the embodiment also includes reactors 40-43 that limit the current flowing through the DC transmission line (any of the DC transmission lines 201-208). The reactors 40-43 are an example of a "current limiter." The control device 60 calculates the fault section from the value of the current flowing through the DC transmission line near the device and the impedance of the DC transmission line including the reactors 40-43 of the device. This allows the control device 60 to achieve the same effects as those described above.

In addition, in the terminal 100 of the embodiment, the AC system (connected to the AC side) includes a wind power plant or an AC side substation 2. As a result, in the terminal 100 of the embodiment, it is possible to convert AC power generated in a wind power plant or AC power supplied from the AC side substation 2 into DC power and supply it to the DC system 200.

(Modification of the embodiment)
Here, a modified example of the embodiment will be described. This modified example differs from the above-described embodiment in that the terminals 100-3 and 100-4 are not directly connected. FIG. 12 is a block diagram showing an example of the configuration of a multi-terminal HVDC system 1A of this modified example. The configuration of each terminal 100 is equivalent to that of the above-described embodiment. Therefore, the description thereof will be omitted. In this modified example, the terminal 100-1 transmits DC power to the terminal 100-3, and also transmits DC power to the terminal 100-4 via the terminal 100-2. The terminal 100-2 transmits DC power to the terminal 100-4, and also transmits DC power to the terminal 100-3 via the terminal 100-1.

Figure 13 is a diagram explaining the details of the process corresponding to step S120 in Figure 3 of the above-mentioned embodiment. Figure 13 corresponds to the judgment table of Figure 5, with the judgment formula shown in the upper row and the judgment result shown in the lower row. The judgment formula and judgment result are similar to the judgment formula and judgment result in Figure 5.

In this modified example, the relationship between the own terminal (terminal 100-1) and terminals 100-2 and 100-3 is the same as in the embodiment described above. Therefore, in FIG. 13, the relationship between the judgment formula and the judgment result when the terminals that may be disconnected are terminals 100-1, 100-2, and 100-3 is the same as in FIG. 5. On the other hand, the relationship between the own terminal (terminal 100-1) and terminal 100-4 is different from the embodiment described above. Therefore, in FIG. 13, the relationship between the judgment formula and the judgment result when the terminal that may be disconnected is terminal 100-4 is different from that in FIG. 5.

Specifically, when the time rate of change ΔIdc13 is -0.5 [A/us] or more and less than -0.05 [A/us], it is shown that the terminal that may disconnect is terminal 100-4. This indicates that when the DC current Idc13 decreases slightly in the direction flowing from terminal 100-1 to terminal 100-3, the accident point is at terminal 100-4 (since it is considered that the current flowing to terminal 100-4 is increasing). In other words, when current flows toward the accident point at terminal 100-4, the time rate of change ΔIdc13 is in the range of -0.5 to -0.05 [A/us].

According to at least one embodiment described above, in the terminal 100, when the control device 60 detects that an accident has occurred in the other terminal, it detects the fault section from the current value flowing in the DC transmission line (any of the DC transmission lines 201 to 208) near the own terminal and the impedance of the nearby DC transmission line. The control device 60 determines whether the other terminal has been disconnected from the DC system 200 based on the time-series change in the current value flowing in the nearby DC transmission line. When it is determined that the other terminal has been disconnected, the control device 60 determines whether to stop the converter of the own terminal based on the transmitted and received power information of the multi-terminal HVDC system 1 and the calculation result of the fault section. This makes it possible to avoid a situation in which the converter is stopped for a long time due to a major fault and prevent loss of power generation opportunities.

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 gist of the invention. These embodiments and their modifications are within the scope of the invention and its equivalents as set forth in the claims, as well as the scope and gist of the invention.

1...Multi-terminal HVDC system, 2...AC side substation equipment, 10...AC/DC converter, 20, 21...DC busbar, 30-33...Current detector, 40-43...Reactor, 50-53...DC circuit breaker, 60...Control device, 100...Terminal

Claims (8)

A multi-terminal DC transmission system is configured in which three or more power conversion devices are connected by a DC transmission line, and each of the power conversion devices is installed at a boundary between an AC system and a DC system including the DC transmission line,
an AC/DC converter that converts AC power into DC power and vice versa;
A control device for controlling the AC/DC converter;
Equipped with
The control device includes:
determining whether or not an accident has occurred in the multi-terminal DC transmission system based on the magnitude of time-series change in the current value flowing through the DC transmission line connected to the power conversion device in which the control device is provided, and if it is determined that an accident has occurred, determining whether or not there is any power conversion device that may be disconnected from the DC system due to the accident, and if there is any power conversion device that may be disconnected, determining whether or not the power conversion device that may be disconnected is another power conversion device different from the power conversion device in which the control device is provided;
When there is a possibility that the other power electronics device will be disconnected, determining whether or not the other power electronics device has been disconnected from the DC system based on a change in a direction of a current flowing through the DC transmission line connected to the power electronics device itself;
when it is determined that the other power conversion device has been disconnected from the DC system, the power conversion device determines whether or not to stop the AC/DC converter based on whether the disconnected other power conversion device is a power receiving device that receives power via the DC system and based on the amount of power transmitted by a power transmitting side power conversion device that supplies power via the DC system and is located between the power conversion device and the disconnected other power conversion device;
Power conversion equipment. the control device identifies the power electronics device that may be disconnected based on a determination table in which the rate of change per unit time of the current value is associated with the power electronics device that may be disconnected.
The power conversion device according to claim 1 . when the control device determines that the other power electronics device is disconnected and that the disconnected other power electronics device is the power receiving side device, the control device calculates a first transmission power value and a first received power value of the multi-terminal DC transmission system using a command value for transmission power in the multi-terminal DC transmission system and a command value for received power in the multi-terminal DC transmission system;
the first transmission power value is a value obtained by subtracting a sum of command values of the amount of power to be supplied to the DC system that was instructed to a target device before the occurrence of an accident from a total amount of transmission power that is a sum of command values of the amount of power to be supplied to the DC system that was instructed to the power conversion device on the power transmitting side before the occurrence of an accident among all the power conversion devices in the multi-terminal DC transmission system,
the target device is a power electronics device on the power transmission side located between the target device and the other power electronics device that has been disconnected,
the first received power value is a value obtained by subtracting the sum of command values of the amount of power that was instructed to be supplied via the DC system to the other power conversion devices that were disconnected due to the accident before the accident from a total received power amount that is the sum of command values of the amount of power that was instructed to be supplied via the DC system to the power conversion device on the power receiving side before the accident occurred among all the power conversion devices in the multi-terminal DC transmission system,
determining to stop the AC/DC converter when the first transmission power value is greater than the first reception power value and the device is a power electronics device on the power transmitting side;
The power conversion device according to claim 1 or 2. when the control device determines that the other power electronics device is disconnected and that the disconnected other power electronics device is the power receiving device, calculates a first transmission power value and a first received power value of the multi-terminal DC transmission system using a measurement value of transmitted power measured before a fault occurs in the multi-terminal DC transmission system and a measurement value of received power measured before a fault occurs in the multi-terminal DC transmission system;
the first transmission power value is a value obtained by subtracting a sum of measured values of amounts of power supplied to the DC system by a target device before the occurrence of an accident from a total transmission power amount which is a sum of measured values of amounts of power supplied to the DC system by the power conversion devices on the power transmission side before the occurrence of an accident among all the power conversion devices in the multi-terminal DC transmission system,
the target device is a power electronics device on the power transmission side located between the target device and the other power electronics device that has been disconnected,
the first received power value is a value obtained by subtracting a sum of measured values of the amount of power supplied from the DC system to the other power conversion devices that have been disconnected due to the accident from a total received power amount that is a sum of measured values of the amount of power supplied from the DC system to the power conversion device on the power receiving side before the accident occurred among all the power conversion devices in the multi-terminal DC transmission system,
determining to stop the AC/DC converter when the first transmission power value is greater than the first reception power value and the device is a power electronics device on the power transmitting side;
The power conversion device according to claim 1 or 2. The power transmission system further includes a current limiter that limits a current flowing through the DC transmission line,
the control device identifies the power conversion device that may be disconnected based on a rate of change per unit time of the current value corresponding to a relationship between an amount of change in a voltage value applied to the DC transmission line connected to the control device and an impedance of the DC transmission line connected to the control device including the current limiter.
The power conversion device according to claim 2 . It is installed at the boundary between a wind power plant or an AC system including an AC side substation and the DC system.
The power conversion device according to any one of claims 1 to 5 . A method for controlling each of the power conversion devices, which constitutes a multi-terminal DC transmission system in which three or more power conversion devices are connected by a DC transmission line, and which includes an AC/DC converter that is installed at a boundary between an AC system and a DC system including the DC transmission line and converts AC power into DC power and vice versa, and a control device that controls the AC/DC converter, comprising:
determining whether or not an accident has occurred in the multi-terminal DC transmission system based on the magnitude of time-series change in the current value flowing through the DC transmission line connected to the power conversion device in which the control device is provided, and if it is determined that an accident has occurred, determining whether or not there is any power conversion device that may be disconnected from the DC system due to the accident, and if there is any power conversion device that may be disconnected, determining whether or not the power conversion device that may be disconnected is another power conversion device different from the power conversion device in which the control device is provided;
When there is a possibility that the other power electronics device will be disconnected, determining whether or not the other power electronics device has been disconnected from the DC system based on a change in a direction of a current flowing through the DC transmission line connected to the power electronics device itself;
when it is determined that the other power conversion device has been disconnected from the DC system, the power conversion device determines whether or not to stop the AC/DC converter based on whether the disconnected other power conversion device is a power receiving device that receives power via the DC system and based on the amount of power transmitted by a power transmitting side power conversion device that supplies power via the DC system and is located between the power conversion device and the disconnected other power conversion device;
Control methods. A multi-terminal DC transmission system is configured in which three or more power conversion devices are connected by a DC transmission line, and the power conversion devices are provided with an AC/DC converter that converts AC power into DC power and vice versa, the AC/DC converter being installed at a boundary between an AC system and a DC system including the DC transmission line, and a control device that controls the AC/DC converter.
determine whether or not an accident has occurred in the multi-terminal DC transmission system based on the magnitude of time-series change in the current value flowing through the DC transmission line connected to the power conversion device in which the control device is provided, and if it is determined that an accident has occurred, determine whether or not there is any power conversion device that may be disconnected from the DC system due to the accident, and if there is any power conversion device that may be disconnected, determine whether or not the power conversion device that may be disconnected is another power conversion device different from the control device;
when there is a possibility that the other power electronics device will be disconnected, determining whether or not the other power electronics device has been disconnected from the DC system based on a change in a direction of a current flowing through the DC transmission line connected to the own device;
when it is determined that the other power conversion device has been disconnected from the DC system, a determination is made as to whether or not to stop the AC/DC converter based on whether or not the disconnected other power conversion device is a power receiving side device that receives power via the DC system, and based on an amount of power transmitted by a power transmitting side power conversion device that supplies power via the DC system and is located between the device itself and the disconnected other power conversion device;
program.

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