Power conversion device and power transmission system

Abstract

To provide a power conversion device capable of performing more appropriate power transmission control at lower cost, and a power transmission system.SOLUTION: A power conversion device according to an embodiment is configured to convert DC power into AC power between a DC system and an AC system and comprises a detection section and a control section. The detection section detects an abnormality of the AC system. In a case where an abnormality of the AC system is detected by the detection section, the control section suppresses a DC current, which flows to the power conversion device, by raising output of a DC voltage of the power conversion device.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] FIELD Embodiments of the present invention relate to a power conversion device and a power transmission system. [Background technology]

[0002] In recent years, DC power transmission systems that transmit DC power between AC systems have been increasingly studied and introduced. Conventional DC power transmission systems cannot output power from the DC system to the AC system when an abnormality, such as a fault, occurs in the AC system on the receiving side. For example, when a DC power transmission system uses a self-commutated power converter for DC interconnection, excess power is generated by the power interchange from the transmitting side until the fault is cleared. This can cause the DC capacitor in the power converter to overcharge due to overvoltage, leading to an abnormality such as an accident and shutdown. In addition, when the AC system on the transmitting side is a power generation facility using offshore wind power, the power converter on the transmitting side is equipped with a DC-ACR (Direct Current Automatic Current Regulator) function, which can be used to suppress the power interchange from the transmitting side. However, this control cannot control the power interchange on the receiving side. Therefore, when an abnormality occurs on the receiving side, the power converter on the receiving side cannot withstand the excess power, causing an abnormality due to overvoltage and resulting in shutdown. Another conventional method to solve the above problem is to install a braking chopper. A braking chopper operates like a step-down chopper, and can prevent overcharging of the DC capacitor by dissipating excess power generated in the DC system using a resistor. However, installing a braking chopper has traditionally had issues, such as increased costs. [Prior art documents] [Non-patent literature]

[0003] [Non-Patent Document 1] Takahiro Kaneko, Kei Sekiguchi, Takuro Arai, Asami Mizutani, Takahiro Ishiguro: "Verification of braking chopper operation during DC faults in multi-terminal DC transmission systems", Proceedings of the 2021 National Convention of the Institute of Electrical Engineers of Japan, 6-232, pp. 402-403 (2021) [Non-patent document 2] V. HUSSENNETHER, J. RITTIGER, A. BARTH, D. WORTHINGTON, G. DELL'ANNA, M. RAPETTI, B. HUHNERBEIN, M. SIEBERT: "Projects BorWin2 and HelWin1 - Large Scale Multilevel Voltage-Sourced Converter Technology for Bundling of Offshore Windpower", CIGRE session, Paris, paper reference, B4-306(2012) Summary of the Invention [Problem to be solved by the invention]

[0004] An object of the present invention is to provide a power conversion device and a power transmission system that can perform more appropriate power transmission control at lower cost. [Means for solving the problem]

[0005] According to an embodiment, a power conversion device converts DC power into AC power between a DC system and an AC system, and includes a detection unit and a control unit. The detection unit detects an abnormality in the AC system. When the detection unit detects an abnormality in the AC system, the control unit increases the DC voltage output of the power conversion device to suppress DC current flowing through the power conversion device. [Brief explanation of the drawings]

[0006] [Figure 1] 1 is a schematic configuration diagram of a power transmission system 1 including a power conversion device according to an embodiment. [Figure 2] FIG. 2 is a diagram showing an example of a specific configuration of the first and second AC / DC converters. [Figure 3] FIG. 2 is a diagram showing an example of the configuration of a cell 230. [Figure 4] 6 is a flowchart showing an example of a power transmission control process in the first embodiment. [Figure 5] 10 is a flowchart showing an example of a power transmission control process in the second embodiment. [Figure 6] 10 is a flowchart showing an example of a power transmission control process in the third embodiment. [Figure 7] 13 is a flowchart showing an example of a power transmission control process in the fourth embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0007] Hereinafter, a power conversion device and a power transmission system according to an embodiment will be described with reference to the drawings.

[0008] (Overall composition) FIG. 1 is a schematic configuration diagram of a power transmission system 1 including a power conversion device according to an embodiment. The power transmission system 1 includes, for example, a power generation facility 100, a first AC / DC converter 200, a second AC / DC converter 300, and an onshore system 400. The power generation facility 100 and the onshore system 400 are each an example of an "AC system." More specifically, the power generation facility 100 is an example of a "transmission-side AC system," and the onshore system 400 is an example of a "receiving-side AC system." The AC system may be an AC power source or an AC load. The first AC / DC converter 200 and the second AC / DC converter 300 are each an example of a "power conversion device." More specifically, the first AC / DC converter 200 is an example of a "transmission-side power conversion device," and the second AC / DC converter 300 is an example of a "receiving-side power conversion device." In the example of FIG. 1, the first AC-DC converter 200 and the second AC-DC converter 300 are provided between the power generation facility 100 and the land system 400. A DC system is formed between the first AC-DC converter 200 and the second AC-DC converter 300, through which DC power (DC current) is transmitted. The DC system may include at least some of the components included in the first AC-DC converter 200 and the second AC-DC converter 300. The DC system may also include a DC power source or a DC load. As shown in FIG. 1, the power transmission system 1 is a DC power transmission system in which the power generation facility 100 and the land system 400 are DC-connected by the first AC-DC converter 200 and the second AC-DC converter 300.

[0009] The power generation facility 100 is, for example, an offshore wind power generation facility (offshore system), but may also be a solar power generation facility or other power generation facility. The first AC / DC converter 200 is connected to the power generation facility 100 via a transformer TR1. The first AC / DC converter 200 converts three-phase AC power from the power generation facility 100 into DC power (AC / DC conversion) and outputs the converted DC power to the DC system (second AC / DC converter 300 side). The second AC / DC converter 300 converts DC power from the DC system (first AC / DC converter 200 side) into three-phase AC power and outputs it to the onshore system 400 via a transformer TR2.

[0010] The transformers TR1 and TR2 may be, for example, three-phase transformers, or may be configured with three single-phase transformers connected, one for each of the three phases. The transformers TR1 and TR2 transform the supplied AC power so that it can be controlled on the output side, and output the transformed AC power to the output side. Note that the power transmission system 1 may be configured without the transformers TR1 and TR2. The land system 400 is an AC system installed on land, but it does not have to be limited to being on land.

[0011] For example, if the power generation facility 100 is an offshore wind power generation facility, the power supplied from the power generation facility 100 to the first AC / DC converter 200 fluctuates mainly due to the influence of the surrounding wind. Furthermore, it is preferable that the power supplied from the DC system (the second AC / DC converter 300 side) to the land system 400 be stable power with little fluctuation. Therefore, the first AC / DC converter 200 may perform, for example, DC-ACR (Direct Current Automatic Current Regulator) control or CVCF (Constant Voltage Constant Frequency) control. Furthermore, the second AC / DC converter 300 may perform, for example, DC-AVR (Direct Current Automatic Voltage Regulator) control.

[0012] In the power transmission system 1 shown in FIG. 1, for example, if an abnormality such as a system fault occurs in the land system 400 on the power receiving side, the second AC / DC converter 300 increases the DC voltage to suppress the DC current flowing through the second AC / DC converter 300 (the DC current supplied from the DC system to the second AC / DC converter 300) so that surplus power is generated due to interchange power from the power transmitting side until the fault is cleared, and the second AC / DC converter 300 does not experience an abnormality due to overvoltage or the like, which would lead to a shutdown. Furthermore, if the first AC / DC converter 200 detects a discrepancy between the power on the power generation equipment 100 side and the power on the DC system side, the first AC / DC converter 200 may perform gate block control (an example of a protective function that disconnects the first AC / DC converter 200 from the system) or control the frequency and voltage value of the input AC voltage to prevent an abnormality due to overvoltage or the like, which would lead to a shutdown. Furthermore, the power transmission system 1 may suppress power output by executing a Fault Ride Through (FRT) operation on the power generation equipment 100 side in accordance with the operation of the first AC / DC converter 200 through the above-described control.

[0013] Next, the specific configurations of the first AC / DC converter 200 and the second AC / DC converter 300 in the above-described power transmission system 1 will be described with reference to the drawings. FIG. 2 is a diagram showing an example of the specific configurations of the first and second AC / DC converters. Note that the example in FIG. 2 also shows a power generation facility 100, a land system 400, and transformers TR1 and TR2 included in the power transmission system 1. The power transmission system 1 shown in FIG. 2 also shows a power generation facility-side control unit 120 that controls the power generation facility 100.

[0014] The first AC / DC converter 200 includes, for example, a first power conversion unit 220, a first detection unit 240, and a first control unit 260. The first detection unit 240 is an example of a "detection unit." The first control unit 260 is an example of a "control unit."

[0015] The first power conversion unit 220 is, for example, a self-commutated power converter (MMC; Modular Multilevel Converter) that can convert AC and DC power even without an AC power system. The first power conversion unit 220 includes, for example, arms AM11 to AM13, each of which has a plurality of cells (sub-modules) 230 connected in series as a unit converter, for each of three phases of AC power (R phase, S phase, and T phase). In the example of FIG. 2, a plurality of cells 230r-1 to 230r-n for a first phase (e.g., R phase) are connected in series to arm AM11. A plurality of cells 230s-1 to 230s-n for a second phase (e.g., S phase) are connected in series to arm AM12. A plurality of cells 230t-1 to 230t-n for a third phase (e.g., T phase) are connected in series to arm AM13. Hereinafter, when the individual cells are collectively described, they may be referred to as "cells 230." Each of the arms AM11 to AM13 may be provided with a reactor (not shown) as an example of an impedance element having an impedance component for suppressing a change in the current flowing through the arm. For example, the first power conversion unit 220 adds up the voltages output by the respective cells 230 under the control of the first control unit 260, thereby coping with high voltage output and suppressing harmonics.

[0016] Each cell 230 shown in FIG. 2 is configured with, for example, a switching element, a diode, a capacitor, etc. FIG. 3 is a diagram illustrating an example of the configuration of the cell 230. The example in FIG. 3A illustrates the configuration of a cell 230α employing a half-bridge. The cell 230α includes, for example, two switch units 232-1 and 232-2 and a capacitor (DC capacitor) 234. Each of the switch units 232-1 and 232-2 may be, for example, a semiconductor switching element, or may be configured such that a semiconductor switching element and a rectifying element such as a diode are connected in parallel. The semiconductor switching element is, for example, a power semiconductor element such as an IGBT (Insulated Gate Bipolar Transistor), but is not limited to an IGBT and may be, for example, a self-extinguishing power semiconductor element capable of realizing a converter or inverter. In the cell 230α, the switch units 232-1 and 232-2 are connected in series, and a capacitor 234 is connected across the series. In cell 230α, the connection point (midpoint of the series circuit) between switch units 232-1 and 232-2 and one end of switch unit 232-2 serve as output terminals (positive terminal (+), negative terminal (-)). Capacitor 234 of cell 230α charges or discharges depending on the state (open / closed state) of each of switch units 232-1 and 232-2 controlled by first control unit 260.

[0017] The example in FIG. 3B shows the configuration of cell 230β to which a full bridge is applied. Cell 230β includes, for example, four switch units 232-1 to 231-4 and a capacitor 234. In cell 230β, a series circuit in which semiconductor switch units 231-1 and 232-2 are connected in series and a series circuit in which switch units 232-4 and 232-4 are connected in series are connected in parallel, and capacitor 234 is connected to both ends of each series circuit. In cell 230β, the midpoint of the series circuit in which switch units 232-1 and 232-2 are connected and the midpoint of the series circuit in which switch units 232-3 and 232-4 are connected serve as output terminals (positive terminal (+), negative terminal (-)). Capacitor 234 of cell 230β is charged or discharged depending on the state (open / closed state) of each of switch units 232-1 to 232-4 controlled by first control unit 260. 3A and 3B, the cells 230α and 230β output the terminal voltage (cell capacitor voltage Vc) of each capacitor 234 as the terminal voltage (cell voltage Vo) between the positive terminal and the negative terminal. Note that the configuration of the cell 230 is not limited to the examples shown in FIGS. 3A and 3B.

[0018] Returning to FIG. 2, the first detection unit 240 detects various states of the first AC / DC converter 200 using a potential transformer (e.g., an ammeter, a voltmeter, a voltage / current transformer, etc.) provided in advance at a predetermined position. For example, the first detection unit 240 detects AC power (e.g., AC system power) from the power generation facility 100 measured by a potential transformer provided between the power generation facility 100 and the transformer TR1. The first detection unit 240 may also detect AC current or AC voltage instead of AC power. The first detection unit 240 may also detect, for example, the current or voltage (e.g., cell voltage Vo) flowing through each of the arms AM11 to AM13 measured by a potential transformer provided in each of the arms AM11 to AM13 of the first power conversion unit 220. Furthermore, the first detection unit 240 may detect the DC voltage Vd1 on the first power conversion unit 220 side (for example, a value based on the sum of the cell voltages Vo), the DC current Idc flowing in the DC system (the second AC / DC converter 300 side), and the power of the DC system using an instrument transformer provided in the first power conversion unit 220 or the DC system.

[0019] The first detection unit 240 may calculate AC power (active power of the AC system) using the effective values ​​and phase difference of the AC current and AC voltage, or may calculate DC power (active power of the DC system) using the DC current Idc and DC voltage Vd1. The first detection unit 240 may compare the active power of the AC system with the active power of the DC system and determine whether there is a discrepancy between the two active powers.

[0020] The first control unit 260 controls the entire first AC / DC converter 200. The first control unit 260 controls the operation of the first AC / DC converter 200 by, for example, having a hardware processor such as a CPU (Central Processing Unit) execute a program (software). The first control unit 260 may be realized by hardware (including circuitry) such as an LSI (Large Scale Integration), an ASIC (Application Specific Integrated Circuit), an FPGA (Field-Programmable Gate Array), or a GPU (Graphics Processing Unit), or may be realized by a combination of software and hardware. The first control unit 260 may be realized by a dedicated LSI. The program may be stored in advance in a storage device (storage device with a non-transitory storage medium) such as an HDD (Hard Disk Drive) or flash memory provided in the first AC / DC converter 200, or may be stored in a removable storage medium (non-transitory storage medium) such as a DVD or CD-ROM, and installed in the HDD or flash memory provided in the first AC / DC converter 200 by attaching the storage medium to a drive device provided in the first AC / DC converter 200.

[0021] For example, the first control unit 260 converts AC power into DC power by controlling the operation of the first power conversion unit 220 (more specifically, the switch state of the switch unit 232 in the cell 230), and outputs the converted DC power (DC current Idc) to the DC system (second AC-DC converter 300). In this case, the first control unit 260 controls the first power conversion unit 220 based on the result of detection by the first detection unit 240 so that a predetermined power (DC current Idc, DC voltage Vd1) is output. Furthermore, the first control unit 260 controls the output of the DC current Idc in accordance with the power (interchange power) from the AC system (power generation equipment side). Furthermore, the first control unit 260 may perform DC-ACR control, CVCF control, and the like. Furthermore, when there is a discrepancy between the active power of the AC system and the active power of the DC system detected by the first detection unit 240, the first control unit 260 may perform control such as gate blocking the first AC / DC converter 200, varying the frequency of the AC voltage, or reducing the AC voltage.

[0022] The second AC / DC converter 300 includes, for example, a second power conversion unit 320, a second detection unit 340, and a second control unit 360. The second detection unit 340 is an example of a "detection unit." The second control unit 360 is an example of a "control unit."

[0023] The second power conversion unit 320 is, for example, a self-commutated power converter (MMC). The second power conversion unit 320 includes arms AM21 to AM23, each of which has a plurality of cells (sub-modules) 330 connected in series as a unit converter, for each phase of AC power of three phases (R phase, S phase, and T phase). In the example of FIG. 2, the arm AM21 has a plurality of cells 330r-1 to 330r-n connected in series for a first phase (for example, R phase). The arm AM22 has a plurality of cells 330s-1 to 330s-n connected in series for a second phase (for example, S phase). The arm AM23 has a plurality of cells 330t-1 to 330t-n connected in series for a third phase (for example, T phase). Hereinafter, the individual cells may be collectively referred to as "cells 330." Each of the arms AM21 to AM23 may be provided with a reactor (not shown). For example, under the control of the second control unit 360, the second power conversion unit 320 converts DC power into AC power using the voltage output by each cell 230. Each cell 330 can have a configuration similar to that of the cell 230 shown in FIG.

[0024] The second detection unit 340 detects various states of the second AC / DC converter 300 using a potential transformer (e.g., an ammeter, a voltmeter, a voltage / current transformer, etc.) provided in advance at a predetermined position. For example, the second detection unit 340 detects the voltage (AC voltage) of the land system 400, which is the power-receiving-side AC system, using a potential transformer provided between the transformer TR2 and the land system 400. The second detection unit 340 may also detect AC current or AC power instead of AC voltage. The second detection unit 340 may also detect the current or voltage (e.g., cell voltage Vo) flowing through each of the arms AM21 to AM23 measured by a potential transformer provided in each of the arms AM21 to AM23 of the second power conversion unit 320. Furthermore, the second detection unit 340 may detect the DC voltage Vd2 on the second power conversion unit 320 side (for example, a value based on the sum of the cell voltages Vo), the DC current Idc supplied from the DC system (first AC / DC converter 200 side), and the power of the DC system using an instrument transformer provided in the DC system or the second power conversion unit 320.

[0025] The second detection unit 340 may calculate AC power (active power of the AC system) using the effective values ​​and phase difference of the AC current and AC voltage, or may calculate DC power (active power of the DC system) using the DC current Idc and DC voltage Vd2. The second detection unit 340 may determine whether or not there is an abnormality on the land system 400 side based on the voltage value of the AC system.

[0026] The second control unit 360 controls the entire second AC / DC converter 300. The second control unit 360 controls the operation of the second AC / DC converter 300 by, for example, having a hardware processor such as a CPU execute a program (software). The second control unit 360 may be realized by hardware (including circuitry) such as an LSI, ASIC, FPGA, or GPU, or may be realized by a combination of software and hardware. The second control unit 360 may be realized by a dedicated LSI. The program may be stored in advance in a storage device (storage device having a non-transitory storage medium) such as a HDD or flash memory provided in the second AC / DC converter 300, or may be stored in a removable storage medium (non-transitory storage medium) such as a DVD or CD-ROM, and installed in the HDD or flash memory provided in the second AC / DC converter 300 by attaching the storage medium to a drive device provided in the second AC / DC converter 300.

[0027] For example, the second control unit 360 converts DC power into AC power by controlling the operation of the second power conversion unit 320 (more specifically, the switch state of the switch unit 232 in the cell 330), and outputs the converted AC power to the land system 400 via the transformer TR2. In this case, the second control unit 360 controls the second power conversion unit 320 based on the result of detection by the second detection unit 340 so that a predetermined power or voltage (for example, AC power or AC voltage based on the DC current Idc and the DC voltage Vd2) is output. The second control unit 360 may also perform DC-AVR control, etc. The first control unit 260 may also control the output of the DC voltage Vd2 of the second AC / DC converter 300 when the second detection unit 340 detects an abnormality in the land system 400.

[0028] Next, several examples of power transmission control in the above-described power transmission system 1 will be specifically described.

[0029] (First Example) Fig. 4 is a flowchart showing an example of a power transmission control process in the first embodiment. The process in the first embodiment is a process that is mainly executed on the side of the second AC / DC converter 300. The process shown in Fig. 4 may be repeatedly executed at a predetermined cycle or at a predetermined timing. The same applies to other processes described in the following embodiments.

[0030] In the example of FIG. 4 , the second detection unit 340 detects the voltage on the land system 400 side, which is the power-receiving-side AC system (step S100). Next, the second detection unit 340 determines whether the detected voltage has dropped below a threshold value (hereinafter referred to as the first threshold value) (step S110). The first threshold value may be a fixed value or may be arbitrarily set by an administrator or the like. If it is determined that the voltage has dropped below the first threshold value, the second detection unit 340 detects an abnormality on the land system 400 side (step S120). Next, the second control unit 360 controls the output of the DC voltage Vd2 of the second power conversion unit 320 of the second AC-DC converter 300. For example, the DC voltage Vd2 of the second power conversion unit 320 is usually set to a value smaller than the DC voltage Vd1 of the first power conversion unit 220 in order to flow the DC current Idc from the first AC-DC converter 200 to the second AC-DC converter 300. Therefore, the second control unit 360 increases the output of the DC voltage Vd2 of the second power conversion unit 320 to suppress the DC current Idc from the first AC / DC converter 200 (DC system) (step S130). This ends the processing of this flowchart. Also, if it is determined in the processing of step S110 that the voltage has not decreased below the first threshold value, the processing of this flowchart ends.

[0031] In the process of step S130, the second control unit 360 increases the DC voltage Vd2 of the second power conversion unit 320 to the target voltage at the time of abnormality detection, for example. The target voltage is, for example, a value that keeps the error from the DC voltage Vd1 within a predetermined range, and may also be set to a voltage value that takes into account the resistance (load) of the DC line. Furthermore, when focusing only on protecting the second AC / DC converter 300, the target voltage may be set based on the upper limit voltage of the first AC / DC converter 200. Furthermore, the target voltage may be set depending on the type, scale, performance, etc. of the power generation facility 100, the first AC / DC converter 200, and the second AC / DC converter 300. Furthermore, when the power generation facility 100 is an offshore wind power generation facility or a solar power generation facility, the power supply varies depending on meteorological conditions such as wind speed and weather. Therefore, the second control unit 360 may externally acquire meteorological information about the vicinity of the power generation facility 100 and set the target voltage based on the acquired meteorological information. Furthermore, the second control unit 360 may set the target voltage based on power information (statistical information or history information) previously supplied by the same or similar power transmission system 1. This makes it possible to more appropriately suppress and control the DC current Idc according to the status of the power transmission system 1. Note that the target voltage may be set to a fixed value or may be arbitrarily set by an administrator or the like.

[0032] Furthermore, when the second control unit 360 detects an abnormality in the onshore system 400 and increases the output of the DC voltage Vd2, the second control unit 360 may control the rate of increase (degree of increase) of the voltage so that the DC voltage Vd2 reaches the target voltage in a shorter time than the time it takes for the second AC / DC converter 300 to shut down due to an abnormality caused by an overvoltage. The time until shutdown may be set, for example, based on the voltage value of the onshore system 400 detected by the second detection unit 340, or may be set based on the scale of the accident that occurred on the onshore system side, the types, scales, performance, etc. of the power generation facility 100, the first AC / DC converter 200, and the second AC / DC converter 300, or may be a fixed time. This makes it possible to more reliably prevent the second AC / DC converter 300 from shutting down due to an overvoltage.

[0033] According to the first embodiment described above, the DC voltage of the second power conversion unit 320 is controlled in accordance with the voltage of the land system 400 to suppress the DC current Idc flowing from the power transmission side, thereby suppressing the supply of interchanged power. Therefore, it is possible to suppress an abnormality, such as an overvoltage of the capacitor of the cell 330 in the second AC / DC converter 300, caused by surplus power generated due to interchanged power from the power transmission side until the fault is cleared, which leads to the second AC / DC converter 300 being shut down. Furthermore, according to the first embodiment, it is possible to suppress interchanged power from the power transmission side without installing a braking chopper. Therefore, for example, when an abnormality occurs in the power receiving side AC system, more appropriate power transmission control can be performed at lower cost.

[0034] (Second Example) FIG. 5 is a flowchart showing an example of a power transmission control process in the second embodiment. The process in the second embodiment is mainly executed on the first AC / DC converter 200 side. In the example of FIG. 5, the first detection unit 240 of the first AC / DC converter 200 detects the active power of the AC system supplied from the power generation facility 100, which is the power transmission-side AC system (step S200). The first detection unit 240 also detects the active power of the DC system (step S210). Next, the first detection unit 240 determines whether or not there is a discrepancy between the active power of the AC system and the active power of the DC system (step S220). For example, the first detection unit 240 determines that there is a discrepancy between the two active powers when the error between the active power of the AC system and the active power of the DC system is equal to or greater than a threshold (hereinafter referred to as the second threshold), and determines that there is no discrepancy when the error is less than the threshold. The second threshold may be a fixed value or may be arbitrarily set by an administrator or the like.

[0035] If it is determined that there is a discrepancy between the two active powers, the first control unit 260 executes control to gate-block the first AC / DC converter 200 (step S230). Specifically, the first control unit 260 executes control (all-cell-off control) to open (set to an open state) all of the switch units 232 of each cell 230 in the first AC / DC converter 200. The first control unit 260 also outputs information indicating that the first AC / DC converter 200 has been gate-blocked to the power generation facility 100 (for example, the power generation facility-side control unit 120 that controls the power generation facility 100) (step S240). This ends the processing of this flowchart. If it is determined in the processing of step S220 that there is no discrepancy, the processing of this flowchart ends.

[0036] By processing step S240, the power generation equipment side control unit 120 can receive information indicating that the first AC / DC converter 200 has been gate-blocked by the first control unit 260, and can further control the power generation equipment 100, such as reducing the power (interchange power) supplied to the first AC / DC converter 200.

[0037] The first control unit 260 performs gate blocking control for a predetermined time (hereinafter referred to as the first predetermined time) from when it is determined that there is a discrepancy between the active power of the AC system (the power generation facility 100 side) and the active power of the DC system. The first predetermined time is a time that is shorter than the time it takes for the first AC / DC converter 200 to shut down due to an abnormality such as overvoltage. The first predetermined time may be set based on the degree of discrepancy between the two active powers being compared, or may be set based on the type, scale, performance, etc. of the power generation facility 100 or the first AC / DC converter 200, or may be a fixed time. This makes it possible to more reliably prevent the first AC / DC converter 200 from being shut down due to an abnormality such as overvoltage.

[0038] According to the second embodiment described above, surplus power in the first AC / DC converter 200 can be detected by comparing the active power on the input and output sides of the first AC / DC converter 200. Furthermore, by controlling the first AC / DC converter 200 to gate-block when there is a discrepancy between the two active powers, it is possible to prevent the first AC / DC converter 200 from shutting down due to an abnormality such as an overvoltage of the capacitor 234 of the cell 230 in the first AC / DC converter 200 caused by surplus power generated due to interchange power until the fault is cleared. Furthermore, according to the second embodiment, by notifying the power generation facility 100 of information indicating that the first AC / DC converter 200 has been gate-blocked, it is possible to suppress the power supply from the power generation facility 100. Furthermore, according to the second embodiment, interchange power can be suppressed without installing a braking chopper. Therefore, more appropriate power transmission control can be performed at lower cost.

[0039] (Third Example) 6 is a flowchart showing an example of a power transmission control process in the third embodiment. The process in the third embodiment is a process that is mainly executed on the side of the first AC / DC converter 200. The third embodiment differs from the second embodiment in that when it is determined that there is a discrepancy between the active power of the AC system and the active power of the DC system, the third embodiment varies the frequency of the AC voltage input to the first AC / DC converter 200 instead of gate-blocking the first AC / DC converter 200. The following description will mainly focus on this difference.

[0040] The example of FIG. 6 differs from the processing of steps S200 to S240 shown in FIG. 5 in that steps S330 to S340 are included instead of steps S230 to S240. If it is determined in the processing of step S220 that there is a discrepancy between the active power of the AC system (the power generation facility 100 side) and the active power of the DC system, the first control unit 260 performs control to lower the frequency of the AC voltage input to the first AC-DC converter 200 (more specifically, the first power conversion unit 220) (step S330). This causes a discrepancy with the frequency of the AC voltage from the power generation facility 100 side, resulting in a phenomenon similar to so-called step-out (loss of synchronization). Therefore, it becomes more difficult for power (voltage) to accumulate in the capacitor 234 of the cell 230 in the first power conversion unit 220 than before the frequency was lowered, and it is possible to prevent the capacitor 234 from becoming overvoltage.

[0041] Next, the first control unit 260 outputs information indicating that the frequency of the AC voltage of the first AC / DC converter 200 has been reduced to the power generation facility 100 side (for example, the power generation facility side control unit 120) (step S340). Note that in the processing of step S340, the first control unit 260 may output to the power generation facility 100 side that a phenomenon similar to a so-called step-out has occurred.

[0042] By processing step S340, the power generation equipment side control unit 120 can receive information indicating that the frequency of the AC voltage input to the first AC / DC converter 200 has been reduced by the first control unit 260, and can further control the power generation equipment 100, such as reducing the power (active power) supplied to the first AC / DC converter 200.

[0043] The first control unit 260 performs the control to reduce the frequency described above until a predetermined time (hereinafter referred to as a second predetermined time) has elapsed since it was determined that there is a discrepancy between the active power of the power generation facility 100 and the active power of the DC system. The second predetermined time may be the same as or different from the first predetermined time described in the second embodiment. In the third embodiment, instead of reducing the frequency of the AC voltage, the first control unit 260 may increase the frequency to cause a discrepancy with the frequency of the AC voltage from the power generation facility 100 side.

[0044] According to the third embodiment described above, when there is a discrepancy between the effective power on the input side and the effective power on the output side of the first AC / DC converter 200, the frequency of the AC voltage of the first AC / DC converter 200 is controlled to vary, thereby achieving the same effect as in the second embodiment described above.

[0045] (Fourth Example) 7 is a flowchart showing an example of power transmission control processing in the fourth embodiment. The processing in the fourth embodiment is mainly processing on the side of the first AC / DC converter 200. The fourth embodiment differs from the second embodiment in that, when it is determined that there is a discrepancy between the active power of the AC system and the active power of the DC system, the first AC / DC converter 200 executes a process to reduce the AC voltage input to the first AC / DC converter 200 (more specifically, the first power conversion unit 220) instead of executing gate block control on the first AC / DC converter 200. The following description will mainly focus on the above difference.

[0046] 7 differs from the processing of steps S200 to S240 shown in FIG. 5 in that the processing of steps S430 to S440 is included instead of steps S230 to S240. If it is determined in the processing of step S220 that there is a discrepancy between the active power of the AC power and the active power of the DC system, the first control unit 260 performs control to reduce the AC voltage of the first AC / DC converter 200 (step S430). For example, the first control unit 260 reduces the AC voltage by reducing the amplitude of the AC voltage. This makes it more difficult for power (voltage) to accumulate in the capacitor 234 of the cell 230 in the first power conversion unit 220 than before the AC voltage was reduced, thereby preventing the capacitor 234 from becoming overvoltage.

[0047] Next, the first control unit 260 outputs information indicating that the AC voltage of the first AC / DC converter 200 has been reduced to the power generation facility 100 side (for example, the power generation facility side control unit 120) (step S440).

[0048] By processing step S440, the power generation equipment side control unit 120 can receive information indicating that the AC voltage input to the first AC / DC converter 200 has been reduced by the first control unit 260, and can further control the power generation equipment 100, such as reducing the power (active power) supplied to the first AC / DC converter 200.

[0049] The first control unit 260 performs the control to reduce the AC voltage described above until a predetermined time (hereinafter referred to as a third predetermined time) has elapsed since it was determined that there is a discrepancy between the active power of the power generation facility 100 and the active power of the DC system. The third predetermined time may be the same as or different from the first and second predetermined times shown in the second and third embodiments.

[0050] According to the fourth embodiment described above, when there is a discrepancy between the effective power on the input side and the effective power on the output side of the first AC / DC converter 200, the AC voltage of the first AC / DC converter 200 is controlled to be reduced, thereby achieving the same effect as the second and third embodiments described above.

[0051] (Fifth Example) The fifth embodiment is a combination of multiple embodiments from the first to fourth embodiments. Specifically, in the fifth embodiment, the second AC / DC converter 300 is protected by controlling the second AC / DC converter 300 to throttle the interchanged power as shown in the first embodiment. Furthermore, in the fifth embodiment, the first AC / DC converter 200 is controlled as shown in the second to fourth embodiments to suppress the power (interchanged power, active power) supplied from the power generation facility 100 to the first AC / DC converter 200, thereby protecting the first AC / DC converter 200. For example, in the power transmission system 1, by controlling the second AC / DC converter 300 as shown in the first embodiment, the DC current Idc flowing in the DC system is reduced, and surplus power accumulates in the first AC / DC converter 200. Therefore, by further controlling the first AC / DC converter 200 as shown in the second to fourth embodiments, the entire power transmission system 1 can be more appropriately protected, and more appropriate power transmission control can be achieved.

[0052] Note that which of the second to fourth embodiments of control is applied to the first AC / DC converter 200 may be set depending on the type, scale, performance, etc. of the first AC / DC converter 200, or the first control unit 260 may switch in response to an instruction from an administrator, etc. In addition, in the fifth embodiment, the first control unit 260 may execute a combination of two or more of the second to fourth embodiments.

[0053] According to the fifth embodiment described above, it is possible to perform more appropriate control over the entire power transmission system 1. Furthermore, according to the fifth embodiment, it is possible to suppress interchange power without installing a braking chopper in the power transmission system 1. Therefore, it is possible to perform more appropriate power transmission control at lower cost.

[0054] (Sixth Example) In a sixth embodiment, when at least one of the control methods shown in the second to fourth embodiments is executed on the first AC / DC converter 200, the first control unit 260 outputs information indicating that the power generated from the first AC / DC converter 200 to the power generation facility 100 side is to be suppressed. For example, when at least one of the control methods shown in the second to fourth embodiments is executed, the first control unit 260 outputs information indicating that the power generated to the power generation facility 100 side is to be suppressed to the power generation facility control unit 120. Furthermore, instead of (or in addition to) the above information, the first control unit 260 may output to the power generation facility control unit 120 execution instruction information for the FRT operation or information indicating the control content executed by the first AC / DC converter 200.

[0055] The power generation facility-side control unit 120 receives the information output from the first control unit 260 and causes the power generation facility 100 to perform the FRT operation. The FRT operation is, for example, an operation that controls voltage drops, frequency fluctuations, etc. so as not to significantly affect the voltage and frequency maintenance of the entire system. In this case, the power generation facility 100 performs the FRT operation so as to satisfy, for example, the requirements for continued operation in the event of an accident (FRT requirements) determined for each type, scale, and performance of the power generation facility.

[0056] The power generation facility-side control unit 120 may, for example, control the power generation facility 100 so that the period from when the power generation facility 100 executes the FRT operation until recovery is within a predetermined time (hereinafter referred to as a fourth predetermined time). The fourth predetermined time may, for example, be a time indicated in the grid interconnection regulations prescribed by the Japan Electrotechnical Standards Committee, a time set by a system administrator, or a fixed time. The fourth predetermined time may also be changed as appropriate due to revisions to regulations or other reasons.

[0057] According to the sixth embodiment described above, even if a grid disturbance such as a voltage drop occurs due to the power transmission control shown in the second to fourth embodiments, the operation of the entire power transmission system 1 can be continued. Furthermore, according to the sixth embodiment, the balance between power supply and demand in the grid is not significantly disrupted even after recovery, so the stability of the grid power can be ensured. Therefore, more appropriate power transmission control can be performed, and the grid interconnection of the power transmission system 1 can be more appropriately protected.

[0058] According to the above-described embodiment, more appropriate power transmission control can be performed at lower cost. For example, in a conventional power transmission system in which the power transmitting side performs DC-AVR control or the like, the land system 400, which is the power receiving side, cannot control interchange power from the power transmitting side, and therefore, a braking chopper or other device is required to absorb surplus power. However, according to the embodiment, more appropriate power control can be achieved without a braking chopper or other device in the power transmission system 1 by controlling power suppression using the AC / DC converters (first AC / DC converter 200, second AC / DC converter 300) provided between the power transmitting side and the land system 400, and absorbing surplus power through FRT operation of the power generation facility 100. Furthermore, according to the embodiment, for example, a highly reliable power conversion device and power transmission system can be provided that suppress converter overvoltages and the like that occur during a power receiving side system fault, thereby improving operational continuity performance.

[0059] Although several embodiments of the present invention have been described, these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their modifications are included within the scope and spirit of the invention, as well as within the scope of the invention described in the claims and their equivalents. [Explanation of symbols]

[0060] REFERENCE SIGNS LIST 1...power transmission system, 100...power generation equipment, 120...power generation equipment side control unit, 200...first AC / DC converter, 220...first power conversion unit, 230, 330...cell, 240...first detection unit, 260...first control unit, 300...second AC / DC converter, 320...second power conversion unit, 340...second detection unit, 360...second control unit, 400...land system, 500...current detection unit, TR1, TR2...transformer

Claims

1. A power conversion device that converts DC power into AC power between a DC system and an AC system, A detection unit that detects an abnormality in the AC system; and a control unit that, when an abnormality in the AC system is detected by the detection unit, increases an output of a DC voltage of the power conversion device to suppress a DC current flowing through the power conversion device. Power conversion equipment.

2. A power conversion device that converts AC power into DC power between a DC system and an AC system, A detection unit that detects the power of the DC system and the power of the AC system; a control unit that controls an output of a direct current in response to power from the AC system; the control unit causes a gate block of the power conversion device when there is a deviation between the active power of the AC system detected by the detection unit and the active power of the DC system. Power conversion equipment.

3. A power conversion device that converts AC power into DC power between a DC system and an AC system, A detection unit that detects the power of the DC system and the power of the AC system; a control unit that controls an output of a direct current in response to power from the AC system; The control unit varies a frequency of the AC voltage from the AC system when a deviation exists between the active power of the AC system detected by the detection unit and the active power of the DC system. Power conversion equipment.

4. A power conversion device that converts AC power into DC power between a DC system and an AC system, A detection unit that detects the power of the DC system and the power of the AC system; a control unit that controls an output of a direct current in response to interchanged power from the AC system; The control unit reduces an AC voltage of the power conversion device when there is a deviation between the active power of the AC system detected by the detection unit and the active power of the DC system. Power conversion equipment.

5. A power converter comprising: the power converter according to claim 1; and the power converter according to any one of claims 2 to 4; and a DC system is provided between the power converters. Power transmission system.

6. A power generation equipment side control unit that controls a power generation equipment that supplies AC power to the power conversion device according to at least one of claims 2 to 4, the power generation facility-side control unit, when control of the power conversion device is executed due to a discrepancy between the active power of the AC system and the active power of the DC system, causes the power generation facility to perform control to suppress power. The power transmission system according to claim 5 .

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