Power conversion device, high-voltage DC power transmission system, and power conversion method

Abstract

Provided is a power conversion device disposed at an end of a high-voltage DC power transmission system connected to a renewable energy power source, the power conversion device making it possible to configure a multi-pole high-voltage DC power transmission system by connecting a plurality of power conversion devices in parallel to the same renewable energy power source. The power conversion device includes a power converter and a control device. The power converter converts AC power supplied by the renewable energy power source to DC power and outputs the converted DC power to a DC power transmission line. The control device controls the AC voltage outputted by the power converter to the renewable-energy-power-source side. The control device has a voltage control unit and a current control unit. The voltage control unit generates a voltage command value for controlling the amplitude and the phase of the AC voltage on the basis of at least the power value of active power flowing in from the renewable energy power source. The current control unit corrects the voltage command value via current feedback for reducing at least a DC current component of a lateral-flow current flowing between the current control unit and the power converter of another pole on the renewable-energy-power-source side.

Description

Power conversion device, high-voltage DC transmission system, and power conversion method

[0001] FIELD Embodiments of the present invention relate to a power conversion device, a high-voltage direct current transmission system, and a power conversion method.

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

[0003] In a typical AC system, a synchronous generator establishes a voltage, and a power converter in a high-voltage DC transmission system controls a current based on the established voltage, thereby achieving power interchange between the AC system and the high-voltage DC transmission system. In recent high-voltage DC transmission systems, modular multilevel converters (MMCs) have increasingly been used as power converters using semiconductor power devices.

[0004] On the other hand, when connecting a group of renewable energy power sources, such as offshore wind power generation, to a power grid that does not include large-scale synchronous generators and is primarily composed of renewable energy power sources, a power converter on the transmission side of the HVDC transmission system must establish the voltage of the power grid instead of a synchronous generator. Typically, the generator of the renewable energy power source is connected to the power grid via a power converter for power supply that is separate from the power converter for the HVDC transmission system. The power converter transmits power to the HVDC transmission system by supplying current with control that is dependent on the voltage established by the power converter on the transmission side of the HVDC transmission system.

[0005] However, because a power converter has a limit to the amount of current it can pass, a high-voltage direct current (HVDC) transmission system (so-called a unipolar HVDC transmission system) configured by connecting a single power converter to the grid is limited in the amount of power it can transmit. Therefore, it is conceivable to increase the amount of power that can be transmitted by connecting multiple power converters in parallel to the AC grid on the power transmission side, that is, by increasing the number of poles of the HVDC transmission system. For example, in a bipolar HVDC transmission system in which two power converters are connected in parallel to increase the number of poles to two, it is possible to double the amount of power that can be transmitted.

[0006] However, when the number of power converters connected to a power grid is increased to increase the number of poles in a high-voltage direct current (HDC) transmission system, errors (voltage differences) typically occur in the voltages established by the power converters, even though the power converters at each pole are controlled to maintain the same voltage. This is due to discrepancies in the calculation timing of the control devices that control the power converters and errors in the electrical characteristics of circuit elements. As a result, in a high-voltage direct current transmission system connected to a power grid, unintended currents (so-called cross currents) flow between the power converters in the circulating paths between the power converters. This raises concerns about imbalances in the HDC power transmitted between the poles in a high-voltage direct current transmission system that increases the number of poles by connecting multiple power converters in parallel. Such imbalances can cause a current greater than the maximum current that can flow through a power converter at one pole, potentially resulting in the power converter failing or shutting down. For this reason, in a high-voltage DC transmission system having a configuration with multiple poles, it is necessary to suppress cross currents and eliminate the voltage difference between the voltages output by the power converters of each pole.

[0007] To equalize the voltages output by each power converter, it is possible to implement synchronous control, linking the control devices of each power converter. However, preventing cross currents requires highly accurate and reliable control devices and communication, which is difficult to achieve. Synchronous control increases the cost of the control devices and requires labor to adjust voltage errors. Furthermore, even if adjustment is performed once and the voltages are matched, there is a risk that readjustment will be necessary due to, for example, deterioration of circuit elements over time, which could lead to a decrease in the reliability of the power system.

[0008] Japanese Patent Application Publication No. 2021-044867

[0009] N. Wang, et al.: “Challenges of Connecting Large-scale Renewable Energy Island System for Bipolar VSC-HVDC”, CIGRE B4 International SC Meeting and Colloquium, B4-1108 (2023-09)

[0010] The problem to be solved by the present invention is to provide a power conversion device, a high-voltage DC transmission system, and a power conversion method that can effectively suppress cross currents that flow between power converters of each pole, which can cause unbalance in the power transmitted as high-voltage DC when configuring a multi-pole high-voltage DC transmission system connected to a renewable energy power source.

[0011] A power conversion device according to an embodiment is arranged at an end of a high-voltage DC transmission system interconnected to a renewable energy power source, and can be connected in parallel to the same renewable energy power source to form a multi-pole high-voltage DC transmission system. The power conversion device includes a power converter and a control device. The power converter converts AC power supplied from the renewable energy power source into DC power and outputs the converted DC power to a DC transmission line. The control device controls the AC voltage output by the power converter to the renewable energy power source. The control device includes a voltage control unit and a current control unit. The voltage control unit generates a voltage command value for controlling the amplitude and phase of the AC voltage based on at least the power value of active power flowing from the renewable energy power source. The current control unit corrects the voltage command value by current feedback for reducing at least the DC current component of a cross current flowing between the renewable energy power source and a power converter of the other pole.

[0012] FIG. 1 is a diagram showing an example of the configuration of a high-voltage DC transmission system according to a first embodiment. FIG. 2 is a diagram showing an example of the configuration of a power conversion device provided in the high-voltage DC transmission system according to the first embodiment. FIG. 3 is a diagram showing an example of the configuration of a cell in a leg provided in a power converter in the power conversion device according to the first embodiment. FIG. 4 is a diagram showing an example of the configuration of a converter control unit provided in the power conversion device according to the first embodiment. FIG. 5 is a diagram showing an example of the configuration of a voltage control unit provided in the converter control unit in the power conversion device according to the first embodiment. FIG. 6 is a diagram showing an example of the configuration of a current control unit provided in the converter control unit in the power conversion device according to the first embodiment. FIG. 7 is a diagram showing an example of the operation of a power converter controlled by the converter control unit provided in the power conversion device according to the first embodiment. FIG. 8 is a diagram showing an example of the configuration of a converter control unit provided in the power conversion device according to the second embodiment. FIG. 9 is a diagram showing an example of the configuration of a current control unit provided in the converter control unit in the power conversion device according to the second embodiment.

[0013] Hereinafter, a power conversion device, a high-voltage DC transmission system, and a power conversion method according to embodiments will be described with reference to the drawings.

[0014] First Embodiment [Configuration of High-Voltage DC Transmission System] An example of the configuration of a high-voltage DC transmission system including a power conversion device will now be described. Fig. 1 is a diagram showing an example of the configuration of a high-voltage DC transmission system according to the first embodiment. Fig. 1 shows an example of a high-voltage DC transmission system 1 that transmits power from a renewable energy power source on the power transmitting side to an AC power transmission system on the power receiving side. The high-voltage DC transmission system 1 shown in Fig. 1 is a bipolar (double-pole) high-voltage DC transmission system that converts three-phase AC power supplied from a renewable energy power source RE into DC power, transmits the DC power, and converts it back into three-phase AC power on the power receiving side to supply to an AC power transmission system TS. In FIG. 1 , the renewable energy power source RE includes, for example, a wind turbine generator installed on the sea that generates AC power (alternating current) using wind turbines that rotate in response to wind strength, and a power converter that converts the generated AC power into AC current at a frequency consistent with the AC voltage established by the high-voltage DC transmission system 1 and supplies the AC power to the high-voltage DC transmission system 1. While FIG. 1 illustrates the configuration of the high-voltage DC transmission system 1 connected to one renewable energy power source RE, the number of renewable energy power sources RE connected to the high-voltage DC transmission system 1 is not limited to one, and multiple renewable energy power sources RE may be connected. The AC transmission system TS transmits the AC power generated and transmitted by the renewable energy power source RE to a destination, such as a consumer. The AC transmission system TS may convert the transmitted AC power back into DC power; that is, the AC transmission system TS may operate as a power source on the transmission side of a subsequent high-voltage DC transmission system and transmit the DC power further away.

[0015] The high-voltage DC transmission system 1 includes, for example, four power conversion devices 10 (power conversion device 10-A-1, power conversion device 10-A-2, power conversion device 10-B-1, and power conversion device 10-B-2) and two DC transmission lines LN-DC (DC transmission line LN-DC-1 and DC transmission line LN-DC-2). In the high-voltage DC transmission system 1, the power conversion device 10-A-1 on the power transmission side, the DC transmission line LN-DC-1, and the power conversion device 10-B-1 on the power receiving side form one pole (hereinafter referred to as the "first pole"), and the power conversion device 10-A-2 on the power transmission side, the DC transmission line LN-DC-2, and the power conversion device 10-B-2 on the power receiving side form another pole (hereinafter referred to as the "second pole"), thereby forming a bipolar high-voltage DC transmission system 1.

[0016] In the high-voltage DC transmission system 1, when the active power at each of the first and second poles is balanced, the DC current flowing through the DC transmission line LN-DC-1 returns via the DC transmission line LN-DC-2. On the other hand, when the active power at each of the poles is unbalanced in the high-voltage DC transmission system 1, DC current also flows through the intermediate potential return path. In the high-voltage DC transmission system 1 shown in FIG. 1 , one DC terminal of the power conversion device 10-A-1 and one DC terminal of the power conversion device 10-A-2 are connected and grounded, and one DC terminal of the power conversion device 10-B-1 and one DC terminal of the power conversion device 10-B-2 are connected and grounded, forming an intermediate potential return path LN-GR at the ground level of each connection point (between the ground points of each DC terminal). The return path is not limited to the grounded return path LN-GR shown in FIG. 1 . For example, a dedicated return line may be provided between the connection point of the power conversion device 10-A-1 and the power conversion device 10-A-2 and the connection point of the power conversion device 10-B-1 and the power conversion device 10-B-2. The DC transmission line LN-DC or the dedicated return line may be, for example, a power cable or an overhead line.

[0017] The power conversion device 10 is an AC-DC converter that converts AC power supplied (input) to an AC terminal from a renewable energy power source RE into DC power and outputs it to the DC terminal, or converts DC power input to the DC terminal into AC power and outputs it to the AC terminal. The power conversion device 10 includes a self-excited power converter such as a modular multilevel converter (MMC), and a power converter control unit.

[0018] [Configuration of the power conversion device] Here, an example of the configuration of the power conversion device 10 will be described. Fig. 2 is a diagram showing an example of the configuration of the power conversion device 10 provided in the high-voltage DC transmission system 1 of the first embodiment. The power conversion device 10 includes a power converter 100 and a converter control unit 200.

[0019] The power converter 100 is a double-star-connected modular multilevel converter that converts AC power and DC power mutually in response to control from a converter control unit 200. The power converter 100 includes a plurality of legs LG between a positive terminal P of a DC system and a negative terminal N of the DC system. The number of legs LG included in the power converter 100 corresponds to the number of phases of AC power supplied by the AC system. FIG. 2 illustrates a case in which the AC system supplies three-phase AC power: a first phase (R phase), a second phase (S phase), and a third phase (T phase). Therefore, FIG. 2 illustrates the configuration of the power converter 100 including three legs LG: a leg LG-R, a leg LG-S, and a leg LG-T. The legs LG-R, LG-S, and LG-T each have the same configuration.

[0020] In each leg LG, the AC terminal CA is connected to a terminal of a corresponding phase of the AC system. More specifically, in the leg LG-R corresponding to the R phase, the AC terminal CA-R is connected to the AC terminal R of the R phase of the AC system, in the leg LG-S corresponding to the S phase, the AC terminal CA-S is connected to the AC terminal S of the S phase of the AC system, and in the leg LG-T corresponding to the T phase, the AC terminal CA-T is connected to the AC terminal T of the T phase of the AC system. Fig. 2 shows a case where the AC terminal CA of each leg LG is connected to a terminal of a corresponding phase of the AC system via a transformer TR.

[0021] In each leg LG, a terminal opposite to the AC terminal CA is connected to a respective DC terminal of the DC system. More specifically, in each leg LG, a terminal having the same potential as the positive electrode of the DC voltage Vdc output by the power converter 100 is connected to a positive terminal P, and a terminal having the same potential as the negative electrode of the DC voltage Vdc output by the power converter 100 is connected to a negative terminal N. In the following description, the terminal of the leg LG connected to the positive terminal P is also referred to as a DC terminal CP of the leg LG, and the terminal of the leg LG connected to the negative terminal N is also referred to as a DC terminal CN of the leg LG.

[0022] Each leg LG includes, for example, two arm inductances RT and two arm units AR. Each arm unit AR includes, for example, n cells CL (cell CL-1 to cell CL-n) connected in series (n is a natural number). In FIG. 2 , to distinguish whether each component of the leg LG corresponds to the positive or negative side of the DC system or to which phase of the AC system it corresponds, a hyphen (-) and the letter "P" indicating the positive side or the letter "N" indicating the negative side are added after each reference symbol, followed by a hyphen and the letter "R" indicating the R phase, the letter "S" indicating the S phase, or the letter "T" indicating the T phase. In the following description, if there is no need to distinguish whether each component corresponds to the positive or negative side of the DC system or to which phase of the AC system it corresponds, the hyphen and the identifying letter following the hyphen are omitted from the reference symbol of each component.

[0023] In each leg LG, the positive arm inductance RT-P and the positive arm unit AR-P are connected in series, and the negative arm inductance RT-N and the negative arm unit AR-N are connected in series. In each leg LG, the connection point between the arm inductance RT-P and the arm inductance RT-N is a DC terminal CP. In each leg LG, the terminal of the arm unit AR-P opposite the arm inductance RT-P is the DC terminal CP of the leg LG, and the terminal of the arm unit AR-N opposite the arm inductance RT-N is the DC terminal CN of the leg LG.

[0024] 2 shows a case where the arm inductance RT is arranged on the AC terminal CA side of the arm unit AR in each leg LG, but the arm inductance RT may be arranged on the side opposite the AC terminal CA side of the arm unit AR (i.e., on the DC terminal CP side or the DC terminal CN side), or at any position within the arm unit AR (i.e., a position between any two cells CL connected in series in the arm unit AR). The arm inductance RT may be replaced by a transformer with a special winding structure that has a leakage reactance sufficient to replace the function of the arm inductance. In this case, the arm inductance RT may be integrated with the transformer TR. In this case, a harmonic filter or the like may be provided on the transformer TR side.

[0025] Each arm unit AR generates an AC voltage with a multi-level waveform, which is a stepped sine wave representing the AC waveform to be supplied to the corresponding phase of the AC system, in accordance with control from the converter control unit 200 for each cell CL connected in series.

[0026] [Cell Configuration] Here, an example of the configuration of a cell CL included in the arm unit AR will be described. The cell CL is, for example, a half-bridge circuit. FIG. 3 is a diagram showing an example of the configuration of a cell CL in a leg LG included in the power converter 100 in the power conversion device 10 of the first embodiment. The cell CL includes two switching elements Q (switching element Q1 and switching element Q2) that are semiconductor power devices, two diodes D (diode D1 and diode D2), and a capacitor C. The switching element Q is, for example, a self-extinguishing switching element such as an insulated gate bipolar transistor (IGBT). The switching element Q is not limited to an IGBT. The switching element Q may be any element as long as it is a self-extinguishing semiconductor switching element that can realize a converter or inverter.

[0027] In the cell CL, the switching element Q1 and the switching element Q2 are connected in series. In the cell CL, the series circuit of the switching element Q1 and the switching element Q2 and the capacitor C are connected in parallel. In the cell CL, each switching element Q and its corresponding diode D are connected in parallel. In the cell CL, the connection point between the emitter of the switching element Q1 and the collector of the switching element Q2 is the positive terminal TP(+) connected to the positive terminal P of the leg LG, and the connection point between the emitter of the switching element Q2 and the capacitor C is the negative terminal TN(-) connected to the negative terminal N of the leg LG.

[0028] A control signal from the converter control unit 200 is input to the gates of the switching element Q1 and the switching element Q2 included in the cell CL (a control voltage is applied or a control current is supplied). A gate command gtp is input to the gate of the switching element Q1 as a control signal from the converter control unit 200, and a gate command gtn is input to the gate of the switching element Q2 as a control signal from the converter control unit 200. As a result, each of the switching element Q1 and the switching element Q2 is switched to either an on state or an off state by the converter control unit 200.

[0029] The capacitor C is charged or discharged depending on the state of each switching element Q. In the cell CL, the terminal voltage of the capacitor C (hereinafter referred to as the "capacitor voltage Vc") is generated as the terminal voltage between the positive terminal TP and the negative terminal TN of the cell CL (hereinafter referred to as the "cell voltage Vo") depending on the state of each switching element Q. More specifically, when the switching element Q1 is in the ON state and the switching element Q2 is in the OFF state, the cell voltage Vo is equal to the capacitor voltage Vc, and when the switching element Q1 is in the OFF state and the switching element Q2 is in the ON state, the cell voltage Vo is zero.

[0030] The cell CL is not limited to the configuration shown in FIG. 3, and may have any configuration as long as it achieves the same function as the cell CL.

[0031] By connecting multiple cells CL having such a configuration in series, the arm unit AR outputs a voltage obtained by adding the cell voltages Vo of the respective cells CL. In other words, the arm unit AR outputs a voltage according to the number of cells CL that are controlled by the converter control unit 200 to a state in which the capacitor voltage Vc is generated as the cell voltage Vo. In this way, the arm unit AR generates a multilevel waveform according to the control from the converter control unit 200.

[0032] The power converter 100 is an example of a “power converter.” The power converter 100 included in the power conversion device 10-A-1 is an example of a “first power converter,” and the power converter 100 included in the power conversion device 10-A-2 is an example of a “second power converter of a different polarity from the first power converter.”

[0033] Returning to FIG. 2 , the converter control unit 200 generates a converter AC terminal voltage command value for controlling the voltage output to the AC terminal side and a converter DC terminal voltage command value for controlling the voltage output to the DC terminal side based on the operating state of the power conversion device 10 and detection values ​​(current value, current flow direction (polarity), and voltage value) at each position within the power conversion device 10. Then, based on the generated converter AC terminal voltage command value and converter DC terminal voltage command value, the converter control unit 200 generates a control signal (gate command gt) for controlling (switching control) the switching element Q in the cell CL included in each arm unit AR. The converter control unit 200 then outputs the generated control signal to the corresponding cell CL. For example, consider a case where the converter control unit 200 controls an arm unit AR-P-R, which corresponds to the R phase of the AC system and is connected to the positive side of the DC system. In this case, the converter control unit 200 generates gate commands gtp (gate commands gtp-1-P-R to gate commands gtp-n-P-R) and gate commands gtn (gate commands gtn-1-P-R to gate commands gtn-n-P-R) for controlling the respective switching elements Q in each cell CL (cell CL-1-P-R to cell CL-n-P-R) included in the arm unit AR-P-R, based on the mainly AC voltage component output to the AC terminal CA-R and the mainly DC voltage component output between the DC terminal CP-R and the DC terminal CN-R. The converter control unit 200 outputs the generated gate commands gtp and gtn to the corresponding cells CL. The same applies when the converter control unit 200 controls other arm units AR. The configuration and operation of the converter control unit 200 will be described in detail below.

[0034] The converter control unit 200, for example, implements various functions for controlling the operation of the power converter 100 by causing a hardware processor to execute a program (software) stored in a memory (storage unit) not shown. The memory not shown is implemented by, for example, a semiconductor memory element such as a read-only memory (ROM), a random access memory (RAM), or a flash memory, a hard disk drive (HDD), an optical disk, or the like.

[0035] The hardware processor refers to a circuit such as a central processing unit (CPU), a graphics processing unit (GPU), a large-scale integration (LSI), a system on chip (SOC), an application-specific integrated circuit (ASIC), or a programmable logic device (e.g., a simple programmable logic device (SPLD) or a complex programmable logic device (CPLD), or a field programmable gate array (FPGA)). Instead of storing a program in a memory (not shown), the hardware processor may be configured so that the program is directly embedded in the circuit. In this case, the hardware processor reads and executes the program embedded in the circuit to realize each function for controlling the operation of the power converter 100. The hardware processor is not limited to being configured as a single circuit, but may be configured as a single hardware processor by combining multiple independent circuits to realize each function. Multiple components may be integrated into a single hardware processor to realize each function. Each function may be realized by incorporating multiple components into a single dedicated LSI. Here, the program (software) may be stored in advance in a storage device (storage device having a non-transitory storage medium) constituting a storage device such as a semiconductor memory element such as a ROM, RAM, or flash memory, or a hard disk drive, or may be stored in a removable storage medium (non-transitory storage medium) such as a DVD or CD-ROM, and installed in a storage device (not shown) provided in the power conversion device 10 or the converter control unit 200 by attaching the storage medium to a drive device provided in the power conversion device 10 or the converter control unit 200.The program (software) may be downloaded in advance from another computer device via a network (not shown) and installed in a storage device provided in the power conversion device 10 or the converter control unit 200. The program (software) installed in the storage device provided in the power conversion device 10 or the converter control unit 200 may be transferred to a processing circuit provided in the power conversion device 10 or the converter control unit 200 and executed therein.

[0036] Converter control unit 200 is an example of a "control device."

[0037] In the power conversion device 10, current detectors CT and voltage detectors VT are provided at desired positions to detect current values, current polarity, and voltage values. The example of FIG. 2 shows a case in which each leg LG is provided with a current detector CT-P for detecting a positive arm current Ip flowing from the positive terminal P to the AC terminal CA, and a current detector CT-N for detecting a negative arm current In flowing from the AC terminal CA to the negative terminal N. More specifically, in the leg LG-R, a current detector CT-P-R is provided between the arm unit AR-P-R and the arm inductance RT-P-R for detecting a positive arm current Ipr flowing from the positive terminal P to the AC terminal CA-R, and a current detector CT-N-P is provided between the arm unit AR-N-R and the arm inductance RT-N-R for detecting a negative arm current Inr flowing from the AC terminal CA-R to the negative terminal N. The same applies to the other legs LG. Although not shown in FIG. 3 , each cell CL may also be provided with a voltage detector VT for detecting the capacitor voltage Vc. The R-phase AC side current Isr, the S-phase AC side current Iss, and the T-phase AC side current Ist may each be directly detected by providing a current detector CT at the AC terminals R, S, and T. The R-phase AC side current Isr, the S-phase AC side current Iss, and the T-phase AC side current Ist may each be indirectly detected by calculating the positive arm current Ip and the negative arm current In detected in the respective legs LG. For example, the R-phase AC side current Isr may be indirectly detected by calculating the difference between the positive arm current Ipr and the negative arm current Inr (i.e., Inr−Ipr). The same applies to the AC side currents Is of the other phases. Furthermore, FIG. 2 shows a case where a voltage detector VT is provided for detecting the R-phase grid interconnection point voltage Vsr, the S-phase grid interconnection point voltage Vss, and the T-phase grid interconnection point voltage Vst.

[0038] The current detector CT provided in the power converter 100 is an example of a "current detector." The AC side current Is is an example of a "detected current value."

[0039] Returning to FIG. 1 , power conversion devices 10-B-1 and 10-B-2 connected to an AC transmission system TS typically independently control the AC side currents Isb1 and Isb2 flowing in based on the voltage of the AC transmission system TS. That is, the converter control units 200 included in each of power conversion devices 10-B-1 and 10-B-2 operate using current control or grid following (GFL) operation. In this case, the current Icb flowing in the path circulating between power conversion devices 10-B-1 and 10-B-2 (the so-called cross current) can be calculated using the following equation (1):

[0040] Icb=(Isb1-Isb2) / 2...(1)

[0041] Here, the AC side current Isb1 and the AC side current Isb2 are each controlled to a desired current value by the converter control unit 200 provided therein. Therefore, the cross current Icb is also indirectly controlled, and is unlikely to reach an unintended current value.

[0042] The AC system current Igb on the AC transmission system TS side common to the power conversion device 10-B-1 and the power conversion device 10-B-2 can also be calculated as shown in the following equation (2), and is equivalent to being controlled in the same way.

[0043] Igb=Isb1+Isb2...(2)

[0044] On the other hand, in the power conversion devices 10-A-1 and 10-A-2 that are connected to a power supply system composed of a renewable energy power source RE, it is necessary to establish the voltage to be transmitted by generating and supplying a reference voltage for the connection of the renewable energy power source RE. In other words, the power conversion devices 10-A-1 and 10-A-2 basically operate by voltage control or grid forming (GFM) operation using the converter control unit 200 provided therein, rather than current control or grid following operation like the power conversion devices 10-B-1 and 10-B-2 that are connected to an AC transmission system TS.

[0045] A typical unipolar high-voltage DC transmission system operates using, for example, the constant voltage constant frequency (CVCF) method, which is the simplest voltage control method. However, in a bipolar or higher high-voltage DC transmission system 1 such as that shown in FIG. 1 , if the constant voltage constant frequency method is used to control at least two power conversion devices 10 and there is an error in the voltage (amplitude and phase) output by each power conversion device 10, the error voltage is applied to the impedance between the power conversion devices 10, resulting in an error current (error current) that is not controlled according to Ohm's law. In other words, the cross current Ica flowing in the path circulating between power conversion device 10-A-1 and power conversion device 10-A-2 contains an unintended current component. Like the cross current Icb, this cross current Ica can be expressed as the following equation (3) using the AC side current Isa1 flowing into the power conversion device 10-A-1 and the AC side current Isb2 flowing into the power conversion device 10-A-2.

[0046] Ica=(Isa1-Isa2) / 2...(3)

[0047] Then, using the fact that the power supply system current Iga on the renewable energy power supply RE side common to the power conversion device 10-A-1 and the power conversion device 10-A-2 is expressed by the following equation (4), the AC side current Isa flowing into each of the power conversion device 10-A-1 and the power conversion device 10-A-2 is expressed by the following equation (5).

[0048] Iga=Isa1+Isa2...(4)

[0049] Isa1=Iga / 2+Ica Isa2=Iga / 2-Ica...(5)

[0050] For this reason, if the cross current Ica contains an unintended current component, the currents of the power conversion devices 10-A-1 and 10-A-2 may also have unintended values, which may cause an imbalance in the power transmitted in the high-voltage DC transmission system 1. Therefore, the converter control unit 200 included in the power conversion device 10 controls the power converter 100 to suppress the cross current Ica, which may cause an imbalance in the power transmitted in the high-voltage DC transmission system 1.

[0051] [Configuration of Converter Control Unit] FIG. 4 is a diagram illustrating an example of the configuration of the converter control unit 200 included in the power conversion device 10 of the first embodiment. FIG. 4 illustrates an example of the schematic configuration of the converter control unit 200 included in the power conversion device 10 of one pole (e.g., the power conversion device 10-A-1 of the first pole). FIG. 4 also illustrates an overview of components related to control of the power converter 100 in the converter control unit 200. More specifically, FIG. 4 illustrates the configuration of the power converter 100, including one cell CL in an arm unit AR of one of the legs LG included in the power converter 100, a grid-connection inductance LT (an equivalent inductance formed by an arm inductance RT connected in series to the cell CL and the leakage inductance of the transformer TR), and one current detector CT provided between the cell CL and the arm inductance RT. FIG. 4 also illustrates a state in which a voltage detector VT is provided at a grid-connection point GP where the power converter 100 is connected to a power supply system composed of a renewable energy power source RE.

[0052] The converter control unit 200 acquires the R-phase grid tie point voltage Vsr, the S-phase grid tie point voltage Vss, and the T-phase grid tie point voltage Vst detected by the voltage detector VT, and the R-phase AC side current Isr, the S-phase AC side current Iss, and the T-phase AC side current Ist detected by the current detector CT. The AC side current Is may be detected by the current detector CT located either on the primary side (renewable energy power source RE side) or on the secondary side (leg LG side of the power converter 100) of the transformer TR, as long as appropriate conversion is performed. The converter control unit 200 generates and outputs gate commands gt (gate commands gtp and gtn) for controlling the power converter 100, more specifically, each cell CL included in the power converter 100, based on the acquired grid tie point voltages Vs and AC side currents Is of each phase. The converter control unit 200 includes, for example, a voltage control unit 220, a current control unit 240, an adder 260, and a gate command generation unit 280. The converter control unit 200 has a control unit (not shown) for controlling the capacitor voltage Vc of each cell CL included in the power converter 100 to a predetermined voltage value, and may acquire each capacitor voltage Vc from a voltage detector (not shown).

[0053] The voltage control unit 220 generates a converter AC terminal voltage command value for controlling a reference voltage to be output to the renewable energy power source RE side of the interconnected power system, based on at least the power value of the active power flowing in from the renewable energy power source RE of the power system. More specifically, the voltage control unit 220 generates a converter AC terminal voltage command value Vr for the R phase, based on three-phase grid interconnection point voltages Vsr, Vss, and Vst and three-phase AC side currents Isr, Iss, and Ist. ＊ , S-phase converter AC terminal voltage command value Vs ＊ , T-phase converter AC terminal voltage command value Vt ＊The control method of voltage control in the voltage control unit 220 may be the same as a general control method such as a constant voltage constant frequency method or a droop method. In other words, the control method of voltage control in the voltage control unit 220 may be equivalent to an existing voltage control method in which a converter control unit provided in a power conversion device in a conventional high voltage DC transmission system controls a power converter. The voltage control unit 220 generates the converter AC terminal voltage command values ​​V ＊ is output to the adder 260.

[0054] The voltage control unit 220 is an example of a "voltage control unit." The converter AC terminal voltage command value V ＊ is an example of a "voltage command value."

[0055] The current control unit 240 performs current feedback control to reduce the cross current (at least the DC current) flowing on the renewable energy power source RE side as current control, and therefore, the converter AC terminal voltage command value V ＊ More specifically, the current control unit 240 generates a correction voltage command value for correcting the R-phase converter AC terminal voltage command value Vr based on the three-phase AC side currents Isr, Iss, and Ist. ＊ R-phase correction voltage command value VRr for correcting ＊ , S-phase converter AC terminal voltage command value Vs ＊ The S-phase correction voltage command value VRs for correcting ＊ , T-phase converter AC terminal voltage command value Vt ＊ T-phase correction voltage command value VRt for correcting ＊ The current control in the current control unit 240 is an additional correction control performed on the voltage control in the voltage control unit 220. Therefore, the current control in the current control unit 240 may be added to a general control method such as a constant voltage constant frequency method or a droop method. The current control unit 240 generates the corrected voltage command value VR for each phase that it has generated. ＊ is output to the adder 260.

[0056] The current control unit 240 is an example of a "current control unit." The corrected voltage command value VR generated by the current control unit 240＊ is an example of a "correction value."

[0057] The adder 260 calculates the converter AC terminal voltage command value V ＊ and the corrected voltage command value VR output by the current control unit 240. ＊ and for each phase. As a result, the adder 260 calculates the converter AC terminal voltage command value V ＊ is the corrected voltage command value VR ＊ The corrected converter AC terminal voltage command value V' is ＊ More specifically, the adder 260 obtains the R-phase converter AC terminal voltage command value Vr ＊ and the R-phase corrected voltage command value VRr ＊ By adding these, the converter AC terminal corrected voltage command value Vr' is obtained. ＊ The adder 260 obtains the S-phase converter AC terminal voltage command value Vs ＊ and the S-phase corrected voltage command value VRs ＊ By adding these, the converter AC terminal corrected voltage command value Vs' is obtained. ＊ The adder 260 obtains the T-phase converter AC terminal voltage command value Vt ＊ and the T-phase corrected voltage command value VRt ＊ By adding these, the converter AC terminal corrected voltage command value Vt' is obtained. ＊ The adder 260 obtains the corrected converter AC terminal voltage command value V′ for each phase. ＊ to the gate command generating unit 280.

[0058] The gate command generating unit 280 outputs the converter AC terminal corrected voltage command value V′ to the AC terminal of each phase of the power converter 100. ＊The gate command generating unit 280 generates gate commands gt (gate commands gtp and gtn) for outputting an AC voltage having a voltage value according to the voltage Vcc, i.e., an AC voltage established by the power converter 100. The gate command generating unit 280 performs, for example, phase shift PWM (Pulse Width Modulation) or the like to generate gate commands gt for controlling each switching element Q in each cell CL included in the power converter 100. The gate command generating unit 280 outputs each generated gate command gt to the corresponding cell CL (more specifically, each switching element Q included in the cell CL).

[0059] With this configuration, the converter control section 200 controls the cells CL in each arm unit AR included in the power converter 100, and causes the power converter 100 to output the established AC voltage.

[0060] [Configuration of voltage control unit and current control unit] Next, a more detailed configuration of each of the voltage control unit 220 and the current control unit 240 included in the converter control unit 200 will be described. Fig. 5 is a diagram showing an example of the configuration of the voltage control unit 220 included in the converter control unit 200 in the power conversion device 10 of the first embodiment. Fig. 6 is a diagram showing an example of the configuration of the current control unit 240 included in the converter control unit 200 in the power conversion device 10 of the first embodiment.

[0061] First, a more detailed configuration of the voltage control unit 220 will be described with reference to Fig. 5. The voltage control unit 220 determines the amplitude and phase of the AC voltage to be established in the power converter 100 by amplitude control for controlling the amplitude and phase control for controlling the phase. The voltage control unit 220 calculates the converter AC terminal voltage command value V ＊ (Converter AC terminal voltage command value Vr ＊ , converter AC terminal voltage command value Vs ＊ , and the converter AC terminal voltage command value Vt ＊ ) is generated. ＊The frequency of the AC voltage represented by is, for example, 50 Hz or 60 Hz. The voltage control unit 220 includes, for example, a power calculation unit 2200, an adder 2201, a drooping characteristic calculation unit 2202, an adder 2203, an adder 2204, a proportional-integral control unit 2205, an adder 2210, a drooping characteristic calculation unit 2212, an adder 2214, an oscillator 2216, and a sine wave calculation unit 2220.

[0062] The power calculation unit 2200 calculates the power value of the active power p and the power value of the reactive power q flowing into the power converter 100. More specifically, the power calculation unit 2200 calculates the power value of the active power p and the power value of the reactive power q of each phase based on the R-phase AC side current Isr, the S-phase AC side current Iss, and the T-phase AC side current Ist detected by the current detector CT, the AC system reactive voltage Vsq, and the AC system active voltage Vsd. The AC system reactive voltage Vsq and the AC system active voltage Vsd are voltage values ​​calculated by converting into a rotating coordinate system based on, for example, the R-phase grid tie point voltage Vsr, the S-phase grid tie point voltage Vss, and the T-phase grid tie point voltage Vst detected by the voltage detector VT. There are several known calculation methods for calculating the active power p and the reactive power q, and the method is not limited thereto. The power calculation unit 2200 outputs the calculated power value of the active power p to the adder 2210 , and outputs the calculated power value of the reactive power q to the adder 2201 .

[0063] The adder 2201 calculates the reactive power command value q ＊ and the power value of the reactive power q calculated by the power calculation unit 2200. More specifically, the adder 2201 calculates the deviation between the reactive power command value q ＊ The reactive power deviation value is calculated by subtracting the power value of the reactive power q from the reactive power command value q. ＊ is a command value of a power value representing the reactive power flowing to the power converter 100, and is a command value determined in advance for the power converter 100. The adder 2201 outputs the calculated reactive power deviation value to the drooping characteristic calculation unit 2202. The adder 2201 may be configured as, for example, a subtractor.

[0064] The drooping characteristic calculation unit 2202 calculates an AC system voltage command value VS, which represents the AC voltage to be established in the power converter 100, based on the reactive power deviation value output by the adder 2201. ＊ More specifically, the drooping characteristic calculation unit 2202 generates an established voltage correction value for correcting the reactive power deviation value output by the adder 2201. The drooping characteristic calculation unit 2202 multiplies the reactive power deviation value output by the adder 2201 by a predetermined amplitude control drooping characteristic to generate the established voltage correction value. The amplitude control drooping characteristic is, for example, a gain with a predetermined coefficient. The drooping characteristic calculation unit 2202 outputs the generated established voltage correction value to the adder 2203.

[0065] The adder 2203 calculates the AC system voltage command value VS ＊ The adder 2203 adds the established voltage correction value output by the drooping characteristic calculation unit 2202 to the AC system voltage correction command value, and outputs the AC system voltage correction command value corrected by adding the established voltage correction value to the adder 2204.

[0066] The adder 2204 subtracts the AC system effective voltage Vsd from the AC system voltage corrected command value output by the adder 2203. The adder 2204 outputs the AC system effective voltage corrected command value obtained by subtracting the AC system effective voltage Vsd from the AC system voltage corrected command value to the proportional-integral control unit 2205.

[0067] The proportional-plus-integral control unit 2205 performs proportional-plus-integral control, that is, P (Proportional) control and I (Integral) control, on the AC system effective voltage correction command value output by the adder 2204. As a result, the voltage control unit 220 determines the amplitude of the AC voltage to be established in the power converter 100. The proportional-plus-integral control unit 2205 determines the amplitude of the voltage command value (voltage command value amplitude) VA as a result of performing the proportional-plus-integral control on the AC system effective voltage correction command value. ＊ is output to the sine wave calculation unit 2220.

[0068] The above configuration is the converter AC terminal voltage command value V ＊ The voltage control unit 220 determines the amplitude of the converter AC terminal voltage command value V ＊The configuration related to the determination of the amplitude may be equivalent to a configuration that realizes a general voltage control method such as a constant voltage constant frequency method or a droop method.

[0069] The adder 2210 calculates the active power command value p ＊ and the power value of the active power p calculated by the power calculation unit 2200. More specifically, the adder 2210 calculates the deviation between the active power command value p and the power value of the active power p calculated by the power calculation unit 2200. ＊ The active power deviation value is calculated by subtracting the active power command value p ＊ is a command value of a power value representing the active power flowing to the power converter 100, and is a command value determined in advance for the power converter 100. The adder 2210 calculates the active power command value p ＊ Alternatively, the active power deviation value may be calculated using an active power reference value (the active power reference value may be, for example, an arbitrary constant that is equal for each pole, or a value obtained by equally dividing the predicted power generation value of the renewable energy power source RE by the number of poles). The adder 2210 outputs the calculated active power deviation value to the drooping characteristic calculation unit 2212. The adder 2210 may be configured as, for example, a subtractor.

[0070] The drooping characteristic calculation unit 2212 calculates the frequency deviation Δf of the oscillation frequency of the AC voltage to be established by the power converter 100 based on the active power deviation value output by the adder 2210. ＊ More specifically, the drooping characteristic calculation unit 2212 multiplies the active power deviation value output by the adder 2210 by a predetermined phase control drooping characteristic to generate the frequency deviation Δf ＊ The phase control drooping characteristic is, for example, a gain of a predetermined coefficient. The drooping characteristic calculation unit 2212 calculates the generated frequency deviation Δf ＊ is output to the adder 2214.

[0071] The adder 2214 adds the frequency deviation Δf output from the drooping characteristic calculation unit 2212 to the reference AC system frequency fs0. ＊ The reference AC system frequency fs0 is, for example, 50 [Hz] or 60 [Hz], and is set to the converter AC terminal voltage command value V ＊is a constant representing the rated frequency of the AC voltage. ＊ The oscillation frequency f corrected by adding ＊ is output to oscillator 2216.

[0072] The oscillator 2216 has an oscillation frequency f ＊ The oscillator 2216 oscillates a signal having a frequency according to the voltage command value phase theta ＊ The voltage command value phase theta is output to the sine wave calculation unit 2220. ＊ is, for example, the oscillation frequency f ＊ is 50 [Hz], the phase command value repeats a monotonous increase from 0° to 360° at a cycle of 50 [Hz] and 20 [ms].

[0073] The above configuration is the converter AC terminal voltage command value V ＊ That is, in a general constant voltage constant frequency method, a signal having a frequency oscillated according to the reference AC system frequency fs0 is used as the voltage command value phase theta ＊ In contrast, in the voltage control unit 220, the reference AC system frequency fs0 is calculated by the power feedback control configured by the adder 2210, the drooping characteristic calculation unit 2212, and the adder 2214, and the frequency deviation Δf ＊ The signal of the frequency corrected by the voltage command value phase theta ＊ The converter AC terminal voltage command value V ＊ The configuration related to the determination of the phase may be equivalent to a configuration that realizes a general voltage control method such as a droop method.

[0074] The sine wave calculation unit 2220 calculates the voltage command value amplitude VA output by the proportional-integral control unit 2205. ＊ and the voltage command value phase theta output by the oscillator 2216 ＊ The sine wave calculation unit 2220 performs a sine wave calculation to generate a three-phase sine wave based on the voltage command value amplitude VA ＊ and the voltage command value phase theta ＊The generation of a sine wave based on the above (sine wave calculation) can be performed using existing technology. Once the amplitude and phase are determined, the corresponding sine wave is uniquely determined, and the phases can be shifted at equal intervals between the three phases. The sine wave calculation unit 2220 converts the result of the sine wave calculation into the converter AC terminal voltage command value V ＊ (Converter AC terminal voltage command value Vr ＊ , converter AC terminal voltage command value Vs ＊ , and the converter AC terminal voltage command value Vt ＊ ) and output to the adder 260.

[0075] With this configuration, the voltage control unit 220 determines the amplitude and phase of the AC voltage to be established in the power converter 100, and outputs the converter AC terminal voltage command value V ＊ (Converter AC terminal voltage command value Vr ＊ , converter AC terminal voltage command value Vs ＊ , and the converter AC terminal voltage command value Vt ＊ The voltage control unit 220 generates a voltage command value amplitude VA in a manner equivalent to that of a voltage control unit in a general voltage control method. ＊ Although the voltage command value phase theta is determined, ＊ is the active power command value p ＊ The frequency deviation Δf is calculated based on the deviation between the active power p (which may be an active power reference value) and the active power p calculated by the power calculation unit 2200. ＊ Using the oscillation frequency f ＊ As a result, the voltage control unit 220 determines the converter AC terminal voltage command value V ＊ Even when voltage control for the cells CL in each arm unit AR included in the power converter 100 is performed only by the power converter 10-A-1, it is possible to reduce the unintended cross current Ica that flows between the power converter 10-A-1 of the first pole and the power converter 10-A-2 of the second pole. An example of the operation of the power converter 100 in this case will be described later.

[0076] Next, a more detailed configuration of the current control unit 240 will be described with reference to Fig. 6. The current control unit 240 calculates a converter AC terminal voltage command value V＊ Correction voltage command value VR for correcting ＊ (corrected voltage command value VRr ＊ , corrected voltage command value VRs ＊ , and the correction voltage command value VRt ＊ 6 shows the corrected voltage command value VR generated by the current control unit 240. ＊ The current control unit 240 includes, for example, a three-phase to two-phase conversion unit 2402, a filter calculation unit 2404, a proportional-integral control unit 2406, and a two-phase to three-phase conversion unit 2408.

[0077] The three-phase to two-phase converter 2402 converts the three-phase AC current Is (the R-phase AC current Isr, the S-phase AC current Iss, and the T-phase AC current Ist) detected by the current detector CT into two-phase AC current Is (the AC current Isα and the AC current Isβ). The three-phase to two-phase converter 2402 performs the three-phase to two-phase conversion, for example, using the commonly used αβ conversion. This allows the current control unit 240 to perform subsequent processing for two phases, thereby facilitating processing. This is because, for three-phase AC current Is that does not have a zero-phase current path, once the values ​​of the AC current Is for two phases are known, the value of the AC current Is for the remaining phase can be uniquely determined as the sum of the known values ​​of the two phases. Therefore, for example, when the current control unit 240 performs current control using the values ​​of the AC current Is for three phases, the three-phase to two-phase converter 2402 may be omitted. The three-phase to two-phase conversion unit 2402 outputs the AC side current Is converted into two phases to the filter calculation unit 2404 .

[0078] The AC current Is converted into two phases is an example of a "two-phase variable quantity."

[0079] The filter calculation unit 2404 extracts at least a DC current component contained in the two-phase AC side currents Is (AC side current Isα and AC side current Isβ) converted by the three-phase to two-phase conversion unit 2402. The filter calculation unit 2404 extracts at least a DC current component (a frequency component contained in the AC side current Is) from each AC side current Is using, for example, a low-pass filter (i.e., a low-pass filter: LPF). The cutoff frequency of the low-pass filter in the filter calculation unit 2404 is set lower than the reference AC system frequency fs0. This allows the filter calculation unit 2404 to attenuate AC system frequency components (e.g., 50 Hz, 60 Hz, etc.) contained in each AC side current Is and extract the DC current component. The filter calculation unit 2404 outputs a value including the DC current component extracted from each AC side current Is to the proportional-integral control unit 2406. The reason why the DC current component should be included as a control target will be explained later.

[0080] The value extracted from the AC side current Is by attenuating the frequency component of the AC system is an example of "a value including a DC current component obtained by attenuating the frequency band of the voltage command value."

[0081] The proportional-integral control unit 2406 performs proportional-integral control on the value representing at least the DC current component output by the filter calculation unit 2404. More specifically, the proportional-integral control unit 2406 multiplies the value representing at least the DC current component by a proportional gain and an integral gain. As a result, the proportional-integral control unit 2406 obtains a correction value for correcting the converter AC terminal voltage command value generated by the voltage control unit 220. The proportional-integral control unit 2406 may be configured to perform only either proportional control or integral control by multiplying by only one of the proportional gain and the integral gain. The proportional-integral control unit 2406 outputs the obtained correction value to the two-phase to three-phase conversion unit 2408.

[0082] Proportional gain and integral gain are examples of "gain."

[0083] The two-phase to three-phase converter 2408 converts the correction values ​​output by the proportional-integral controller 2406, i.e., the two-phase correction values, into three-phase correction values. The two-phase to three-phase converter 2408 performs two-phase to three-phase conversion, for example, by using a commonly used inverse αβ conversion. This allows the current controller 240 to obtain correction values ​​for three phases. When the three-phase to two-phase converter 2402 is omitted from the current controller 240, the two-phase to three-phase converter 2408 is also omitted. The two-phase to three-phase converter 2408 converts the correction values ​​converted into three phases into a correction voltage command value VR ＊ (corrected voltage command value VRr ＊ , corrected voltage command value VRs ＊ , and the correction voltage command value VRt ＊ ) to the adder 260.

[0084] With this configuration, the current control unit 240 controls the converter AC terminal voltage command value V ＊ (Converter AC terminal voltage command value Vr ＊ , converter AC terminal voltage command value Vs ＊ , and the converter AC terminal voltage command value Vt ＊ ) to correct the voltage command value VR ＊ (corrected voltage command value VRr ＊ , corrected voltage command value VRs ＊ , and the correction voltage command value VRt ＊ ) is generated in the converter control unit 200. As a result, the adder 260 adds the converter AC terminal voltage command value V ＊ is calculated by the current control unit 240. ＊ The converter AC terminal corrected voltage command value V′ is corrected by ＊ (Converter AC terminal corrected voltage command value Vr' ＊ , converter AC terminal corrected voltage command value Vs' ＊ , and the converter AC terminal corrected voltage command value Vt′ ＊ In the converter control unit 200, the gate command generating unit 280 generates the converter AC terminal corrected voltage command value V' generated by the adder 260. ＊In response to this, the converter control unit 200 generates a gate command gt for controlling each switching element Q in each cell CL of the power converter 100 (see FIG. 4). This allows the converter control unit 200 to reduce unintended cross current Ica flowing between the power conversion devices 10-A-1 of the first pole and the power conversion device 10-A-2 of the second pole.

[0085] The cross current Ica is an example of a "cross current flowing between the power converter of the other pole." The DC current component of the AC side current Is (the frequency component contained in the AC side current Is) is an example of a "DC current component contained in the current detection value."

[0086] [Operation of Power Converter] Next, an example of the operation of the power converter 10 will be described. FIG. 7 is a diagram showing an example of the operation of the power converter 100 controlled by the converter control unit 200 included in the power converter 10 of the first embodiment. FIG. 7 shows an example of the waveform of one power converter 10 (e.g., the power converter 10-A-1 of the first pole) in the high-voltage DC transmission system 1 when the active power transmitted from the renewable energy power source RE is increased to twice the rated capacity of 1.0 [pu] of one power converter 10 in the high-voltage DC transmission system 1. Ideally, the active power is equally distributed between one power converter 10 (e.g., the power converter 10-A-1 of the first pole) and another power converter 10 (e.g., the power converter 10-A-2 of the second pole), and each of them transmits 1.0 [pu] of active power. In the example of the waveform shown in FIG. 7, the converter AC terminal voltage command value V ＊ and the converter AC terminal voltage command value V generated by the converter control unit 200 included in the power conversion device 10-A-2. ＊This is an example of a case where there is an initial phase error of 1% in the phase of the AC voltage established by each power conversion device 10 due to the above. More specifically, the initial phase of the AC voltage established by the power conversion device 10-A-1 is assumed to be delayed by 1% relative to the initial phase of the AC voltage established by the power conversion device 10-A-2. In FIG. 7, an example of the waveform of the active power transmitted from the power conversion device 10-A-1 is shown in the upper row, an example of the waveform of the AC side current (AC side current Is) of each phase (R phase, S phase, and T phase) output (supplied) by the renewable energy power source RE to the power conversion device 10-A-1 according to the AC voltage output (established) by the power converter 100 of the power conversion device 10-A-1 is shown in the middle row, and an example of the waveform of the voltage value (average value for each arm unit AR) of the capacitor C provided in the cell CL in each arm unit AR constituting the power converter 100 of the power conversion device 10-A-1 is shown in the lower row, each corresponding to each other. The horizontal axis in Fig. 7 represents time. Fig. 7(a) is an example of a case where the converter control unit 200 controls the AC voltage to be established using a constant voltage, constant frequency method, which is a common control method, Fig. 7(b) is an example of a case where the converter control unit 200 controls the AC voltage to be established using only voltage control by the voltage control unit 220, and Fig. 7(c) is an example of a case where the converter control unit 200 controls the AC voltage to be established using current control (current feedback control) by the current control unit 240 as well.

[0087] When the AC voltage output by the power converter 100 is established using a typical constant voltage / constant frequency method, if the initial phase of the AC voltage output by the power conversion device 10-A-1 lags behind the phase of the AC voltage output by the power conversion device 10-A-2 by 1% (i.e., the initial phase difference is not corrected). Since each power converter 100 is equipped with a grid-connection inductance LT, active power flows from the phase-leading power conversion device 10-A-2 to the phase-lagging power conversion device 10-A-1. Therefore, as shown in FIG. 7A, in the power conversion device 10-A-1, active power flows in excess of half the power generated by the power system composed of the renewable energy power source RE, exceeding the rated capacity of 1.0 pu. In this case, the excess active power oscillates as shown in FIG. 7A. As a result, the active power flowing from the power supply system, which exceeds the rated capacity of 1.0 pu, may charge the capacitor C in each cell CL with excess active power (i.e., overcharge), causing the voltage value of capacitor C to rise rather than stabilize at 1.0 pu. In this case, the high-voltage DC transmission system 1 may be forced to stop operation to protect the semiconductor power devices (switching element Q, diode D, and capacitor C) that make up the cell CL from overvoltage. Figure 7(a) shows an example of a case where operation is stopped at time t. In this way, when attempting to establish the AC voltage output by the power converter 100 using a general constant voltage constant frequency method, the active power of each power conversion device 10 will not be maintained at the target value (here, rated capacity = 1.0 [pu]), and an imbalance in active power will occur between the power conversion devices 10 of the first pole 10-A-1 and the power conversion device 10-A-2 of the second pole, that is, between the poles of the high-voltage DC transmission system 1, and it may become impossible to continue operating the power conversion devices 10.

[0088] In contrast, when the converter control unit 200 controls the AC voltage established by voltage control based on the amplitude and phase determined by the voltage control unit 220, the converter AC terminal voltage command value V ＊By correcting the phase in the inverter, it is possible to prevent the active power from exceeding the rated capacity of 1.0 [pu]. More specifically, in a general constant voltage constant frequency method, a signal oscillated at a frequency corresponding to the reference AC system frequency fs0 is output as the converter AC terminal voltage command value V ＊ In contrast, the voltage control unit 220 sets the reference AC system frequency fs0 to a frequency deviation Δf ＊ The oscillation frequency f is corrected (power feedback controlled) by adding ＊ The signal oscillated at a frequency corresponding to the converter AC terminal voltage command value V ＊, the initial phase difference is corrected, and as shown in FIG. 7B, the active power can be prevented from exceeding the rated capacity of 1.0 [pu]. FIG. 7B illustrates an example of a state in which the active power is suppressed and its average value converges to 1.0 [pu], which is half the power generated by the power supply system. This indicates that even when the converter control unit 200 only performs voltage control using the voltage control unit 220, it is possible to suppress to some extent the occurrence of active power imbalance between the poles of the HVDC transmission system 1. However, as shown in FIG. 7B, the active power still oscillates after convergence. This is because the AC side current Is contains a DC component in addition to a sinusoidal component of the reference AC system frequency fs0. In the example shown in FIG. 7B, the DC component of one phase (e.g., the R phase) increases, while the DC components of the other two phases (e.g., the S phase and the T phase) decrease. This indicates that DC current is circulating between the phases of the power converter 100. In this state, although the average voltage value of the capacitor C in all arm units AR is maintained near 1.0 [pu], oscillations in active power may cause an imbalance between the arm units AR. In this case, the high-voltage DC transmission system 1 may be forced to stop operation to protect the semiconductor power devices constituting the cells CL of a specific arm unit AR from overvoltage. In other words, if the converter control unit 200 controls the AC voltage established solely by voltage control by the voltage control unit 220, the frequency component of the AC system in the AC side current can be adjusted to the power target value, but the DC component contained in the AC side current is not particularly affected by the control. Therefore, in the high-voltage DC transmission system 1, DC current components may be generated as cross currents circulating between the phases of the power converter 100 and between the poles of the high-voltage DC transmission system 1, which may prevent the power conversion device 10 from continuing operation. FIG. 7B shows an example in which the operation is stopped at time t.The reason why DC current components, in particular, circulate as cross currents between the poles of the power converter 100 is because the impedance between them barely acts to suppress the DC current components. More specifically, there is a grid-connection inductance LT between the first and second poles, but the magnitude of this impedance is proportional to frequency and is nearly zero for DC components at zero frequency. Therefore, even a slight DC voltage error in the converter AC terminal voltage of each power converter 100 easily causes DC current components to flow according to Ohm's law. If only the voltage control unit 220 is provided, and no suppression control means for DC current is provided, the DC voltage error remains, causing DC current components to flow.

[0089] In contrast, when the converter control unit 200 controls the AC voltage to be established, including current control (current feedback control) by the current control unit 240, the DC component included in the AC side current (the DC component of the cross current between the poles of the HVDC transmission system 1) can be suppressed by current control (current feedback control) by the current control unit 240, in addition to adjusting the active power by correcting the reference AC system frequency fs0 (power feedback control) by the voltage control unit 220 as shown in FIG. 7B. FIG. 7C shows an example of a state in which the active power is adjusted and its average value converges to 1.0 pu (the rated capacity), which is half the power generation power of the power supply system, and the oscillation of the active power after convergence is also suppressed, as shown in FIG. 7B. As a result, the average value of the voltage value of the capacitor C in all the arm units AR is maintained near 1.0 pu, and imbalance between the arm units AR is prevented. As a result, there is no need to protect the semiconductor power devices constituting the cells CL of the arm units AR from overvoltage, nor is there any need to stop their operation, in the high-voltage DC power transmission system 1. In other words, when the converter control unit 200 controls the AC voltage established by directly controlling the DC component of the AC side current in current control (current feedback control) by the current control unit 240, in addition to indirectly controlling the frequency component of the AC system in voltage control by the voltage control unit 220 (power feedback control), the frequency component of the AC system in the AC side current is adjusted according to the power target value, and DC current (cross current) circulating between the phases of the power converter 100 and between the poles of the first and second power conversion devices 10 is also suppressed. At this time, in the current control unit 240, the filter calculation unit 2404 extracts the DC current component (the frequency component of the control target) contained in the AC side current, and then the proportional-integral control unit 2406 obtains a correction value for correcting the converter AC terminal voltage command value generated by the voltage control unit 220. Therefore, there is no interference between the indirect control of the frequency component of the AC system in the voltage control unit 220 (power feedback control) and the direct control of the DC component of the AC side current in the current control unit 240 (current feedback control).Therefore, the power conversion device 10 can continue stable operation in the high-voltage DC transmission system 1. Fig. 7(c) shows an example of a case where the operation is not stopped even at time t and the operation continues smoothly.

[0090] When a multi-pole (for example, two-pole (bipolar)) high-voltage DC power transmission system 1 is configured with such a configuration and operation, the converter control unit 200 provided in the power conversion device 10 has a voltage control unit 220 that determines the amplitude and phase of the AC voltage that is established by the power converter 100 and output to the AC terminal side, and outputs a converter AC terminal voltage command value V ＊ In the converter control unit 200, the current control unit 240 generates the converter AC terminal voltage command value V generated by the voltage control unit 220 through current feedback control. ＊ Correction voltage command value VR for correcting ＊ Thereafter, in the converter control unit 200, an adder 260 adds the converter AC terminal voltage command value V generated by the voltage control unit 220 to the ＊ is calculated by the current control unit 240. ＊ The converter AC terminal corrected voltage command value V′ is corrected by ＊ In the converter control unit 200, the gate command generating unit 280 generates the converter AC terminal corrected voltage command value V' generated by the adder 260. ＊ In response to this, the converter control unit 200 generates a gate command gt for controlling each switching element Q in each cell CL included in the power converter 100 and outputs the gate command gt to each switching element Q included in the corresponding cell CL. This allows the converter control unit 200 to reduce (suppress) unintended cross currents that flow between the power conversion devices 10 of each pole.

[0091] As described above, according to the high-voltage DC transmission system 1 of the first embodiment, when a multi-pole high-voltage DC transmission system is configured by a power conversion device 10 including a converter control unit 200 arranged at an end and a plurality of power conversion devices 10 connected in parallel to the same renewable energy power source RE, the converter control unit 200 included in each power conversion device 10 generates a converter AC terminal corrected voltage command value to be established by the power converter 100 included in each power conversion device 10 and output to the AC terminal side. In this case, the converter control unit 200 included in the power conversion device 10 constituting the high-voltage DC transmission system 1 of the first embodiment controls the power converter 100 to suppress cross currents (particularly DC currents) flowing in paths circulating between the power conversion devices 10. This could cause an imbalance in the power transmitted in the high-voltage DC transmission system 1 if there is an error in the AC voltages (amplitude and phase) output by the power conversion devices 10. As a result, in the high-voltage DC transmission system 1 of the first embodiment, even if there is an error in the AC voltage output by each power conversion device 10, the error voltage can suppress unintended cross currents that flow between the power conversion devices 10 of each pole, allowing each power conversion device 10 to continue operating normally. As a result, the high-voltage DC transmission system 1 of the first embodiment can realize a highly reliable multi-pole high-voltage DC transmission system. Because the converter control units 200 provided in each power conversion device 10 operate independently, advanced synchronization and communication are not required, and a low-cost, highly reliable multi-pole high-voltage DC transmission system can be realized.

[0092] Second Embodiment [Configuration of High-Voltage DC Transmission System] An example of the configuration of a high-voltage DC transmission system including a power converter according to a second embodiment will be described below. Like the high-voltage DC transmission system 1 of the first embodiment shown in FIG. 1 , a high-voltage DC transmission system (hereinafter referred to as the “high-voltage DC transmission system 1a”) including a power converter according to the second embodiment (hereinafter referred to as the “power converter 10a”) is a bipolar high-voltage DC transmission system that converts three-phase AC power supplied from a renewable energy power source RE into DC power, transmits the DC power, and then converts the DC power back into three-phase AC power on the power receiving side and supplies it to an AC transmission system TS. The power converter 10a also includes a self-excited power converter, such as a modular multilevel converter, similar to the power converter 100 of the first embodiment shown in FIGS. 2 and 3 . The power converter control unit of the power converter 10a differs from the converter control unit 200 of the first embodiment. Therefore, in the configuration of the power conversion device 10a and the high-voltage DC transmission system 1a including the power conversion device 10a, components that have the same configuration and operation as the power conversion device 10 of the first embodiment and the high-voltage DC transmission system 1 including the power conversion device 10 are assigned the same reference numerals. Further, detailed explanations of these components will be omitted, and only the configuration and operation of the power converter control unit included in the power conversion device 10a, which has a different configuration, will be explained.

[0093] Similarly to the converter control unit 200, the power converter control unit (hereinafter referred to as the "converter control unit 210") included in the power conversion device 10a generates a converter AC terminal voltage command value for controlling the voltage output to the AC terminal side and a converter DC terminal voltage command value for controlling the voltage output to the DC terminal side based on the operating state of the power conversion device 10a and detection values ​​(current value, current flow direction (polarity), and voltage value) at each position within the power conversion device 10a. Similarly to the converter control unit 200, the converter control unit 210 also generates a control signal (gate command gt) for controlling (switching control) the switching element Q in the cell CL included in each arm unit AR based on the generated converter AC terminal voltage command value and converter DC terminal voltage command value, and outputs the generated control signal to the corresponding cell CL.

[0094] [Configuration of converter control unit] Fig. 8 is a diagram showing an example of the configuration of the converter control unit 210 included in the power conversion device 10a of the second embodiment. Like Fig. 4, Fig. 8 also shows an example of the schematic configuration of the converter control unit 210 included in the power conversion device 10a of one pole (for example, the power conversion device 10-A-1 of the first pole shown in Fig. 1). Like Fig. 4, Fig. 8 also shows an overview of the components related to the control of the power converter 100 in the converter control unit 210.

[0095] Similar to the converter control unit 200, the converter control unit 210 also acquires the R-phase grid tie point voltage Vsr, the S-phase grid tie point voltage Vss, and the T-phase grid tie point voltage Vst detected by the voltage detector VT, and the R-phase AC side current Isr, the S-phase AC side current Iss, and the T-phase AC side current Ist detected by the current detector CT. Similar to the converter control unit 200, the converter control unit 210 generates and outputs gate commands gt (gate commands gtp and gtn) for controlling the power converter 100, more specifically, each cell CL included in the power converter 100, based on the acquired grid tie point voltage Vs and AC side current Is of each phase. The converter control unit 210 includes, for example, a voltage control unit 220, a current control unit 242, an adder 260, and a gate command generation unit 280. In the converter control section 210 , the current control section 240 included in the converter control section 200 is replaced with a current control section 242 .

[0096] Similar to the current control unit 240, the current control unit 242 performs current feedback control as current control to reduce the cross current (particularly the DC current) flowing on the renewable energy power source RE side, and therefore, the converter AC terminal voltage command value V ＊However, the current control unit 242 uses not only the AC side current Is of the power converter 100 of its own pole controlled by the converter control unit 210 but also the AC side current Is (hereinafter referred to as "AC side current Is'") of the power converter 100 of the other pole controlled by the converter control unit 210 of the power conversion device 10a of the other pole (for example, the power conversion device 10-A-2 of the second pole shown in FIG. 1) to generate a converter AC terminal voltage command value V generated by the voltage control unit 220 of its own pole. ＊ More specifically, the current control unit 242 generates a correction voltage command value for correcting the R-phase converter AC terminal voltage command value Vr of the own pole based on the three-phase AC side currents Isr, Iss, and Ist of the own pole and the three-phase AC side currents Is'r, Is's, and Is't of the other pole. ＊ R-phase correction voltage command value VRr for correcting ＊ , the S-phase converter AC terminal voltage command value Vs ＊ The S-phase correction voltage command value VRs for correcting ＊ , the converter AC terminal voltage command value Vt of the T-phase of the own pole ＊ T-phase correction voltage command value VRt for correcting ＊ Similar to the current control unit 240, the current control in the current control unit 242 is an additional correction control performed on the voltage control in the voltage control unit 220 of the same pole. Therefore, similar to the current control in the current control unit 240, the current control in the current control unit 242 may be added to a general control method such as a constant voltage constant frequency method or a droop method. The current control unit 242 generates the corrected voltage command value VR for each phase that it has generated. ＊ is output to the adder 260.

[0097] As a result, in the converter control unit 210 as well, similarly to the converter control unit 200, the adder 260 calculates the converter AC terminal voltage command value V ＊ is calculated by the current control unit 242. ＊ The converter AC terminal corrected voltage command value V′ is corrected by ＊ (Converter AC terminal corrected voltage command value Vr' ＊ , converter AC terminal corrected voltage command value Vs' ＊ , and the converter AC terminal corrected voltage command value Vt′＊ In the converter control section 210, similarly to the converter control section 200, the gate command generating section 280 generates the corrected converter AC terminal voltage command value V′ ＊ In response to this, the power converter 100 generates a gate command gt for controlling each switching element Q in each cell CL.

[0098] With this configuration, the converter control unit 210, like the converter control unit 200, controls the cells CL in each arm unit AR included in the power converter 100 to cause the power converter 100 to output an established AC voltage. As a result, like the converter control unit 200, the converter control unit 210 can reduce the unintended cross current Ica that flows between the power conversion devices 10, namely, the power conversion device 10-A-1 of the first pole and the power conversion device 10-A-2 of the second pole.

[0099] The converter control unit 210 is an example of a "control device." The current control unit 242 is an example of a "current control unit." The corrected voltage command value VR generated by the current control unit 242 ＊ is an example of a "correction value." The power converter 100 included in the power conversion device 10-A-1 is an example of a "first power converter," and the power converter 100 included in the power conversion device 10-A-2 is an example of a "second power converter of a different polarity from the first power converter." The current detector CT provided in the power converter 100 included in the power conversion device 10-A-1 is an example of a "first current detector," and the current detector CT provided in the power converter 100 included in the power conversion device 10-A-2 is an example of a "second current detector." The AC side current Is is an example of a "first current detection value," and the AC side current Is' is an example of a "second current detection value."

[0100] [Configuration of power conversion device] Next, a more detailed configuration of the current control unit 242 included in the converter control unit 210 will be described. Fig. 9 is a diagram showing an example of the configuration of the current control unit 242 included in the converter control unit 210 in the power conversion device 10a of the second embodiment.

[0101] Similar to the current control unit 240, the current control unit 242 determines the converter AC terminal voltage command value V based on the amplitude and phase determined by the voltage control unit 220. ＊ Correction voltage command value VR for correcting ＊ (corrected voltage command value VRr ＊ , corrected voltage command value VRs ＊ , and the correction voltage command value VRt ＊ 9 also shows the corrected voltage command value VR generated by the current control unit 242, as in FIG. ＊ The current control unit 242 includes, for example, an adder 2420, a cross current calculation unit 2421, a three-phase to two-phase conversion unit 2402, a filter calculation unit 2425, a proportional-integral control unit 2406, and a two-phase to three-phase conversion unit 2408.

[0102] The adder 2420 calculates the difference between the AC side current Is of the own pole and the AC side current Is' of the other pole for each phase. More specifically, the adder 2420 calculates the AC side current difference value by subtracting the AC side current Is' of the other pole from the AC side current Is of the own pole. Even more specifically, the adder 2420 calculates the R-phase AC side current difference value by subtracting the R-phase AC side current Is'r of the other pole from the R-phase AC side current Isr of the own pole. The adder 2420 calculates the S-phase AC side current difference value by subtracting the S-phase AC side current Is's of the other pole from the S-phase AC side current Iss of the own pole. The adder 2420 calculates the T-phase AC side current difference value by subtracting the T-phase AC side current Is't of the other pole from the T-phase AC side current Ist of the own pole. The adder 2420 outputs the calculated AC current difference value for each phase (three phases) to the cross current calculation unit 2421. The adder 2420 may be configured as, for example, a subtractor.

[0103] The cross current calculation unit 2421 calculates the current value of the cross current Ic flowing between the power conversion device 10a of the own pole and the power conversion device 10a of the other pole. The cross current calculation unit 2421 calculates the cross current Ic of each phase (three phases) flowing between the power conversion device 10a of the own pole and the power conversion device 10a of the other pole by calculating, for each phase, half the value of the AC side current difference value of each phase (three phases) detected by the adder 2420. As a result, in the converter control unit 210, the cross current Ic of each phase (three phases) (R-phase cross current Icr, S-phase cross current Ics, and T-phase cross current Ict) is calculated using the following equation (6) using the configuration of the adder 2420 and the cross current calculation unit 2421.

[0104] Icr=(Isr-Is'r) / 2 Ics=(Iss-Is's) / 2...(6) Ict=(Ist-Is't) / 2

[0105] Here, in the high-voltage DC transmission system 1a, when the power conversion device 10a is configured in three-parallel or three or more poles, the cross current calculation unit 2421 can similarly calculate the AC side current Is' of each phase of the other pole used to calculate the cross current Ic by using the above equation (6) as the sum of the AC side currents Is' of each phase of all the other poles.

[0106] The cross current calculation unit 2421 outputs the calculated cross current Ic to the three-phase to two-phase conversion unit 2402 .

[0107] The cross current Ic calculated by the configuration of the adder 2420 and the cross current calculation unit 2421 is an example of a "cross current flowing between the power converter of the other pole."

[0108] The three-phase to two-phase converter 2402 converts the three-phase cross current Ic (R-phase cross current Icr, S-phase cross current Ics, and T-phase cross current Ict) output by the cross current calculator 2421 into two-phase cross current Ic (cross current Icα and cross current Icβ), instead of the AC side current Is when the current controller 240 is provided with the three-phase to two-phase converter 2402. The three-phase to two-phase converter 2402 may be omitted, for example, when the current controller 242 performs current control using the values ​​of the cross current Ic for three phases, as when the current controller 240 is provided with the three-phase to two-phase converter 2402. The three-phase to two-phase converter 2402 outputs the two-phase converted cross current Ic to the filter calculator 2425.

[0109] The cross current Ic converted into two phases is an example of a "two-phase variable quantity."

[0110] The filter calculation unit 2425 attenuates signals in at least a frequency band near the reference AC system frequency fs0 contained in the two-phase cross currents Ic (cross current Icα and cross current Icβ) converted by the three-phase to two-phase conversion unit 2402. The filter calculation unit 2425 attenuates signals in a frequency band near the reference AC system frequency fs0 from each cross current Ic, for example, using a band-rejection filter (BRF) centered around the reference AC system frequency fs0. As a result, at least the DC current components of the unintended cross currents contained in the cross currents Icα and Icβ remain in the calculation results of the filter calculation unit 2425. In other words, the calculation by the filter calculation unit 2425 extracts frequency components including the DC components contained in the cross currents Ic. To attenuate at least cross current components near the reference AC system frequency fs0, the filter calculation unit 2425 may attenuate signals in a frequency band near the reference AC system frequency fs0 from each cross current Ic using, for example, a low-pass filter (LPF) having a cutoff frequency set lower than the reference AC system frequency fs0. When the three-phase-to-two-phase conversion by the three-phase-to-phase conversion unit 2402 is a dq conversion synchronized with the reference AC system frequency fs0 and the two-phase-to-three-phase conversion by the two-phase-to-three-phase conversion unit 2408 is an inverse dq conversion synchronized with the reference AC system frequency fs0, the filter calculation unit 2425 may, for example, function as a high-pass filter (i.e., a high-pass filter (HPF)) to substantially attenuate signals at the reference AC system frequency fs0. The filter calculation unit 2425 outputs values ​​representing the calculation results for each cross current Ic (at least the DC current components of the remaining cross currents) to the proportional-integral control unit 2406.

[0111] Here, in the high-voltage DC transmission system 1a, when it is assumed that the high-voltage DC transmission system 1a transmits power without matching the active power of the power conversion device 10-A-1 of the first pole and the power conversion device 10-A-2 of the second pole, the filter calculation by the filter calculation unit 2425 to attenuate signals in a frequency band near the reference AC system frequency fs0 is a necessary process. On the other hand, in the high-voltage DC transmission system 1a, when the high-voltage DC transmission system 1a transmits power while always maintaining the active power of the power conversion device 10-A-1 of the first pole and the power conversion device 10-A-2 of the second pole in a balanced state, the filter calculation unit 2425 may be omitted. In other words, the filter calculation unit 2425 does not need to attenuate signals in a frequency band near the reference AC system frequency fs0. In this case, in the converter control unit 210, the DC current component of the reference AC system frequency fs0 contained in the cross current Ic is also the cross current component to be suppressed (the frequency component to be controlled). In this case, in the high-voltage DC transmission system 1a, the amplitudes of the components of the reference AC system frequency fs0 contained in the AC side current Isa (the AC side current Isa1 and the AC side current Isa2) flowing into each power conversion device 10a (the power conversion device 10-A-1 and the power conversion device 10-A-2) are also equal, and the active power at each of the first and second poles is balanced. In this way, in the high-voltage DC transmission system 1a, it is sufficient that at least the cross current component to be controlled (the frequency component to be controlled) and the cross current component that is unintended and undesirable in the operation of the high-voltage DC transmission system 1a are output to the proportional-integral control unit 2406. By adjusting the characteristics of the filter calculation unit 2425, it is possible to selectively suppress cross current components of any frequency.

[0112] The value obtained by attenuating the signal in the frequency band near the reference AC system frequency fs0 contained in the cross current Ic is an example of "a value including a DC current component obtained by attenuating the frequency band of the voltage command value." The calculation result of the filter calculation unit 2425 on the cross current Ic (at least the DC current component of the remaining cross current) is an example of "at least the DC current component of the cross current."

[0113] The proportional-plus-integral control unit 2406 performs proportional-plus-integral control on the value of the cross current Ic output by the filter calculation unit 2425, which includes at least the DC component, instead of the value representing the DC component of the AC side current Is when the current control unit 240 is provided. The proportional-plus-integral control unit 2406 multiplies the value of the cross current Ic, which includes at least the DC component, by a proportional gain and an integral gain that may be different from or the same as the proportional gain and integral gain that the proportional-plus-integral control unit 2406 multiplies on the value representing at least the DC component of the AC side current Is when the current control unit 240 is provided. Thus, the proportional-plus-integral control unit 2406 obtains a correction value for correcting the converter AC terminal voltage command value generated by the voltage control unit 220, based on the cross current Ic calculated by the adder 2420 and the cross current calculation unit 2421 in the converter control unit 210. The proportional-plus-integral control unit 2406 outputs the obtained correction value to the two-phase to three-phase conversion unit 2408.

[0114] The two-phase to three-phase converter 2408 converts the correction value output by the proportional-plus-integral controller 2406, i.e., the two-phase correction value corresponding to the cross current Ic, into a three-phase correction value to obtain correction values ​​for the three phases corresponding to the cross current Ic. As when the two-phase to three-phase converter 2408 is provided in the current controller 240, if the three-phase to two-phase converter 2402 is also omitted, the two-phase to three-phase converter 2408 is also omitted. The two-phase to three-phase converter 2408 converts the correction value corresponding to the cross current Ic converted into three phases into a correction voltage command value VR ＊ (corrected voltage command value VRr ＊ , corrected voltage command value VRs ＊ , and the correction voltage command value VRt ＊ ) to the adder 260.

[0115] With this configuration, the current control unit 242, like the current control unit 240, controls the converter AC terminal voltage command value V ＊ (Converter AC terminal voltage command value Vr ＊ , converter AC terminal voltage command value Vs ＊ , and the converter AC terminal voltage command value Vt ＊ ) to correct the voltage command value VR ＊ (corrected voltage command value VRr＊ , corrected voltage command value VRs ＊ , and the correction voltage command value VRt ＊ At this time, the current control unit 242 uses not only the AC side current Is of the power converter 100 of the own pole controlled by the converter control unit 210 similar to the current control unit 240, but also the AC side current Is (AC side current Is′) of the power converter 100 of the other pole controlled by the converter control unit 210 provided in the power conversion device 10 a of the other pole to generate the converter AC terminal voltage command value V ＊ In other words, the current control unit 242 calculates the cross current Ic itself using the AC side currents Is of all the power converters 100 included in the multi-pole power conversion devices 10a that make up the high-voltage DC transmission system 1a, and generates the correction voltage command value from at least the DC current component contained in the calculated cross current Ic. Therefore, although the converter control unit 210 needs to acquire the AC side currents Is of the power converters 100 between the power conversion devices 10a of each pole, the converter control unit 210 generates the correction voltage command value directly from the calculated cross current Ic itself. Therefore, the converter control unit 210 can generate the correction voltage command value in quicker response to the unintended cross current Ica that flows between the power conversion devices 10a of each pole, and can more effectively reduce (suppress) the cross current Ica.

[0116] [Operation of Power Conversion Device] The operation of the power conversion device 10a of the second embodiment is similar to the operation of the power conversion device 10 of the first embodiment described with reference to Fig. 7. Therefore, detailed description of the operation of the power conversion device 10a will be omitted.

[0117] When a multi-pole (for example, two-pole (bipolar)) high-voltage DC power transmission system 1a is configured with such a configuration and operation, the converter control unit 210 provided in the power conversion device 10a also determines the amplitude and phase of the AC voltage to be established by the power converter 100 and output to the AC terminal side, as in the converter control unit 200, and outputs a converter AC terminal voltage command value V ＊In the converter control unit 210, the current control unit 242 generates the converter AC terminal voltage command value V generated by the voltage control unit 220 through current feedback control that includes the AC side current Is of the power converter 100 of the other pole in addition to the AC side current Is of the power converter 100 of the own pole. ＊ Correction voltage command value VR for correcting ＊ Thereafter, in the converter control section 210, as in the converter control section 200, an adder 260 adds the converter AC terminal voltage command value V ＊ is calculated by the current control unit 242. ＊ The converter AC terminal corrected voltage command value V′ is corrected by ＊ Then, in the converter control section 210, similarly to the converter control section 200, the gate command generating section 280 generates the corrected converter AC terminal voltage command value V′ generated by the adder 260. ＊ In response to this, the converter control unit 210 generates a gate command gt for controlling each switching element Q in each cell CL included in the power converter 100, and outputs the generated gate command to each switching element Q included in the corresponding cell CL. As a result, the converter control unit 210, like the converter control unit 200, can reduce (suppress) unintended cross currents that flow between the power conversion devices 10a of each pole.

[0118] As described above, according to the high-voltage DC transmission system 1a of the second embodiment, when the power conversion device 10a equipped with the converter control unit 210 is placed at an end and a plurality of the power conversion devices 10a are connected in parallel to the same renewable energy power source RE to form a multi-pole high-voltage DC transmission system, the converter control unit 210 equipped in each power conversion device 10a generates a converter AC terminal corrected voltage command value to be established in the power converter 100 equipped in each power conversion device 10a and output to the AC terminal side. In this case, the converter control unit 210 included in the power conversion device 10a constituting the high-voltage DC transmission system 1a of the second embodiment controls the power converter 100 by current feedback control including the AC side current Is of the power converter 100 of the other pole in addition to the AC side current Is of the power converter 100 of its own pole, so as to suppress cross currents (particularly DC currents) flowing in paths circulating between the power conversion devices 10a when there is an error in the AC voltages (amplitude and phase) output by the power conversion devices 10a. This could cause an imbalance in the power transmitted in the high-voltage DC transmission system 1a. As a result, in the high-voltage DC transmission system 1a of the second embodiment, even when there is an error in the AC voltages output by the power conversion devices 10a, unintended cross currents flowing between the power conversion devices 10a of the respective poles due to the error voltages can be more suitably suppressed, thereby allowing the power conversion devices 10a to continue operating normally. As a result, the high voltage DC power transmission system 1a of the second embodiment can realize a more reliable multi-pole high voltage DC power transmission system.

[0119] As described above, in the power conversion device of each embodiment, when the power conversion device is disposed at an end of a high-voltage DC transmission system and multiple power conversion devices are connected in parallel to the same renewable energy power source to form a multi-pole high-voltage DC transmission system, a converter control unit included in each power conversion device generates a converter AC terminal correction voltage command value to be established by the power converter included in each power conversion device and output to the AC terminal side. In this case, when there is an error in the AC voltage (amplitude and phase) output by each power conversion device, the converter control unit included in each power conversion device controls the power converter to suppress cross currents (particularly DC currents) flowing in the paths circulating between each power conversion device, which can cause an imbalance in the power transmitted in the high-voltage DC transmission system. As a result, in a high-voltage DC transmission system including the power conversion device of each embodiment, even if there is an error in the AC voltage output by each power conversion device, the error voltage can suppress unintended cross currents flowing between the power conversion devices of each pole, thereby allowing each power conversion device to continue operating normally. As a result, in a high-voltage DC transmission system including the power conversion device of each embodiment, a highly reliable multi-pole high-voltage DC transmission system can be realized.

[0120] In the high-voltage DC transmission system including the power conversion devices of the above-described embodiments, the converter control unit 200 (or the converter control unit 210) is provided in all of the power conversion devices 10. However, it is also possible to increase the number of poles in an already installed single-pole or multi-pole high-voltage DC transmission system. In this case, if at least one power conversion device 10 on the renewable energy power source RE side constituting the high-voltage DC transmission system is configured to include the converter control unit 200 (or the converter control unit 210), unintended cross-currents flowing between the power conversion devices of each pole can be suppressed, and the power conversion devices 10 can continue to operate smoothly, as in the high-voltage DC transmission system of the above-described embodiments.

[0121] In the power conversion device of each of the above-described embodiments, the power converter 100 is, for example, a double-star-connected MMC that is provided at an interconnection point between an AC system and a DC system and converts AC power supplied from the AC system into DC power supplied from the DC system, in accordance with control from the converter control unit 200 (or the converter control unit 210). However, the power converter 100 may be a power converter with a different configuration. In this case, the configuration, operation, and processing of the power converter 100 and the converter control unit 200 (or the converter control unit 210) may be equivalent to the configuration, operation, and processing of the power conversion device 10 and the converter control unit 200 (or the converter control unit 210) described above.

[0122] In the power conversion device of each of the above-described embodiments, the power conversion device 10 includes the power converter 100 and a converter control unit 200 (or converter control unit 210) that controls the operation of the power converter 100. In other words, the power converter 100 and the converter control unit 200 (or converter control unit 210) are integrated into one unit. However, the power converter 100 and the converter control unit 200 (or converter control unit 210) are not limited to being integrated into one unit. For example, the power converter 100 and the power conversion device including the converter control unit 200 (or converter control unit 210) may be separate units. In this case, the configuration, operation, and processing of the power converter 100 and the power conversion device including the converter control unit 200 (or converter control unit 210) may be equivalent to the configuration, operation, and processing of the power conversion device 10 described above. Therefore, detailed description of the configuration, operation, and processing in the case where the power converter 100 and the power conversion device including the converter control unit 200 (or converter control unit 210) are separate units will be omitted.

[0123] In the high-voltage DC transmission system including the power conversion device of each of the above-described embodiments, the renewable energy power source RE is a wind-powered power generation facility that supplies AC power (alternating current) generated by a wind turbine installed offshore. However, the renewable energy power source RE is not limited to a wind-powered power generation facility. The renewable energy power source RE may be other types of renewable energy power generation facilities, such as solar power generation facilities or geothermal power generation facilities. In this case, the power conversion device and the high-voltage DC transmission system may transmit power using components appropriate for the respective renewable energy power generation facilities. For example, if the power conversion device and the high-voltage DC transmission system are configured to correspond to a solar power generation facility, the solar power generation facility generates DC power. In this case, the solar power generation facility may include a separate power converter for converting DC power to AC power and be connected to the high-voltage DC transmission system. Therefore, the power conversion device and the high-voltage DC transmission system may have a configuration equivalent to the above-described power conversion device and the high-voltage DC transmission system. In this case, the configuration and operation of the power conversion device may be equivalent to the configuration and operation of the power conversion device and high-voltage DC transmission system described above.

[0124] According to at least one embodiment described above, a power conversion device (10) is arranged at an end of a high-voltage DC power transmission system (1) interconnected to a renewable energy power source (RE), and is capable of configuring a multi-pole high-voltage DC power transmission system by being connected in parallel to the same renewable energy power source in multiple units, and includes a power converter (100) that converts AC power supplied by the renewable energy power source into DC power and outputs the converted DC power to a DC transmission line (LN-DC), and a control device (200) that controls the AC voltage output by the power converter to the renewable energy power source side, and the control device controls at least the power value (p ＊ ) to calculate a voltage command value (V ＊and a current control unit (240) that corrects the voltage command value by current feedback for reducing at least the DC current component of the cross current flowing between the power converters of the other pole on the renewable energy power source side. When a multi-pole high-voltage DC power transmission system interconnected to a renewable energy power source is configured, the cross current flowing between the power converters of each pole, which can be a factor in causing an imbalance in the power transmitted as high-voltage DC, can be suitably suppressed.

[0125] 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.

[0126] 1, 1a... High voltage DC transmission system, 10, 10-A-1, 10-A-2, 10-B-1, 10-B-2, 10a... Power conversion device, 100... Power converter, 200, 210... Converter control unit, 220... Voltage control unit, 2200... Power calculation unit, 2201... Adder, 2202... Drooping characteristic calculation unit, 2203... Adder, 2204... Adder, 2205... Proportional and integral control unit, 2210... Adder, 2212... Drooping characteristic calculation unit, 2214... Adder, 2216... Oscillator, 2220... Sine wave calculation unit, 240, 242... Current control unit, 2402... Three-phase to two-phase conversion unit, 2404... Filter calculation unit, 2406... Proportional and integral control unit, 2408... Two-phase to three-phase conversion unit, 2420... Adder, 2421... Cross current calculation unit, 2425... Filter calculation unit, 260... Adder, 280... Gate command generation unit, RE... Renewable energy power source, TS... AC transmission system, LN-DC, LN-DC-1, LN-DC-2... DC transmission line, TR... ...Transformer, R, S, T... AC terminal, P... Positive side terminal, N... Negative side terminal, CA, CA-R, CA-S, CA-T... AC terminal, CP, CP-R, CP-S, CP-T, CN, CN-R, CN-S, CN-T... DC terminal, LG, LG. -R, LG-S, LG-T... Leg, RT, RT-P-R, RT-NR, RT-P-S, RT-NS, RT-PT, RT-NT... Arm inductance, AR, AR-P-R, AR-NR, AR-P-S, AR-NS, AR-P- T, AR-N-T...arm unit, CL, CL-1, CL-n, CL-1-P-R, CL-n-P-R, CL-1-N-R, CL-n-N-R, CL-1-P-S, CL-n-P-S, CL-1-N-S, CL-n-N-S, CL-1-P-T, CL-n-P-T, CL-1-N-T, CL-n-N-T...cell, Q, Q1, Q2...switching elements, D, D1, D2...diodes, C...capacitor, TP...positive terminal, TN...negative terminal, LT...interconnection inductance

Claims

1. A power conversion device that is arranged at an end of a high-voltage DC transmission system that is linked to a renewable energy power source, and that can configure a multi-pole high-voltage DC transmission system by being connected in parallel to the same renewable energy power source in multiple units, the power conversion device comprising: a power converter that converts AC power supplied by the renewable energy power source into DC power and outputs the converted DC power to a DC transmission line; and a control device that controls the AC voltage that the power converter outputs to the renewable energy power source side, wherein the control device comprises: a voltage control unit that generates a voltage command value for controlling the amplitude and phase of the AC voltage based on at least the power value of active power flowing in from the renewable energy power source; and a current control unit that corrects the voltage command value by current feedback to reduce at least the DC current component of a cross current that flows between the renewable energy power source side and a power converter of the other pole.

2. The power conversion device according to claim 1, wherein the current control unit corrects the voltage command value based on a current detection value of an AC current flowing from the renewable energy power source to the power converter, detected by a current detector provided in the power converter.

3. The power conversion device according to claim 2, wherein the current control unit obtains a correction value by multiplying a value representing at least a DC current component included in the current detection value by a gain, and corrects the voltage command value based on the obtained correction value.

4. The power conversion device according to claim 3, wherein the voltage control unit calculates a frequency deviation based on a deviation between an active power command value or an active power reference value and the power value of the active power, and generates the voltage command value in which the phase of the AC voltage is the phase of oscillation at a frequency including the frequency deviation.

5. The power conversion device according to claim 4, wherein the current control unit calculates the correction value by multiplying a value including the DC current component calculated by attenuating the frequency band of the voltage command value included in the current detection value by the gain.

6. The power conversion device according to claim 5, wherein the current control unit obtains a variable amount by performing a three-phase to two-phase conversion of the current detection value, multiplies the obtained two-phase variable amount by the gain to obtain a two-phase correction value, and performs a two-phase to three-phase conversion of the two-phase correction value to obtain the three-phase correction value.

7. The power conversion device according to claim 1, wherein the current control unit determines a current value of at least a DC current component of the cross current flowing between the power converter of the other pole and the renewable energy power source based on a current detection value of the AC side current flowing from the renewable energy power source to the power converter, detected by a current detector provided in the power converter, and corrects the voltage command value based on the determined current value.

8. The power conversion device according to claim 7, wherein the current control unit determines the cross current from the difference between a first current detection value detected by a first current detector provided in a first power converter and a second current detection value detected by a second current detector provided in a second power converter of a different polarity from the first power converter, determines a correction value by multiplying the current value of at least a DC current component of the cross current by a gain, and corrects the voltage command value based on the determined correction value.

9. The power conversion device according to claim 8, wherein the voltage control unit calculates a frequency deviation based on a deviation between an active power command value or an active power reference value and a power value of the active power, and generates the voltage command value in which the phase of the AC voltage is the phase of oscillation at a frequency including the frequency deviation.

10. The power conversion device according to claim 9, wherein the current control unit calculates the correction value by multiplying a value including the DC current component calculated by attenuating the frequency band of the voltage command value included in the cross current by the gain.

11. The power conversion device according to claim 10, wherein the current control unit obtains a variable quantity by performing a three-phase to two-phase conversion of the cross current, multiplies the obtained two-phase variable quantity by the gain to obtain a two-phase correction value, and performs a two-phase to three-phase conversion of the two-phase correction value to obtain the three-phase correction value.

12. A high-voltage DC transmission system that is connected to a renewable energy power source and can form a multi-pole transmission system by connecting multiple power conversion devices arranged at its ends in parallel to the same renewable energy power source, wherein at least one of the power conversion devices is a power conversion device as defined in any one of claims 1 to 11.

13. A power conversion device that is arranged at an end of a high-voltage DC transmission system linked to a renewable energy power source and that can configure a multi-pole high-voltage DC transmission system by being connected in parallel to the same renewable energy power source in multiple units, the power conversion device comprising: a power converter that converts AC power supplied by the renewable energy power source into DC power and outputs the converted DC power to a DC transmission line; and a control device that controls the AC voltage that the power converter outputs to the renewable energy power source side, the power conversion method comprising: a computer of the control device that generates a voltage command value for controlling the amplitude and phase of the AC voltage based on at least the power value of active power flowing in from the renewable energy power source, and corrects the voltage command value by current feedback that reduces at least the DC current component of a cross current that flows between the renewable energy power source side and a power converter of the other pole.

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