Power conversion device, control method, and computer program

Abstract

To provide a power conversion device which reduces excessive power output to a power system at the time of instantaneous voltage drop and improves stability of the power system.SOLUTION: The power conversion device includes a bidirectional converter having a first end connected to a distributed power supply and a second end connected to a DC bus, a bidirectional inverter having a first end connected to a power system and a second end connected to the DC bus, and a control device. The control device includes a detection unit configured to detect an instantaneous voltage drop of the system voltage, a first determination unit configured to determine, when the detection unit detects the instantaneous voltage drop, a second current target value of an output current of the bidirectional inverter after the occurrence of the instantaneous voltage drop based on a ratio between a first system voltage before the occurrence of the instantaneous voltage drop and a second system voltage after the occurrence of the instantaneous voltage drop and a first current target value of the output current of the bidirectional inverter before the occurrence of the instantaneous voltage drop, and a first inverter control unit configured to control the bidirectional inverter based on the second current target value determined by the first determination unit during the instantaneous voltage drop.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a power conversion device, a control method, and a computer program. [Background technology]

[0002] In recent years, it has become common for distributed power sources (DGs) based on renewable energy to be connected to the power grid, and it is expected that many DGs will be connected to the power grid. In such a situation, if a power grid fault occurs, causing a momentary voltage drop, the power converters of the DGs connected to the power grid may disconnect all of them simultaneously due to the islanding prevention function, which may significantly affect the voltage and frequency of the power grid. Therefore, to ensure the quality of the power grid in the event of a fault, power converters connected to the power grid are required to meet the Fault Ride Through (FRT) requirement, which allows them to continue operating in the event of a fault (see the Grid Interconnection Regulation JEAC9701-2012). The islanding prevention function is a function that disconnects individual DGs from the power grid in the event of a power outage or other problem.

[0003] The power conversion device described in Patent Document 1 continues to output a current up to its upper limit in order to achieve continuous operation performance (FRT requirement) during an instantaneous voltage drop. In this grid-connected power conversion device, the upper limit of the output current is set to a value between the rated current and the overcurrent level. However, when a large number of distributed power sources are connected to the same power system, if a large number of power conversion devices output currents up to their upper limit during an instantaneous voltage drop, this may destabilize the power system.

[0004] Therefore, in order to reduce the risk of destabilizing the power system, the power conversion device described in Patent Document 2 outputs an adjustment coefficient FTratio proportional to the magnitude of the amplitude value of the system voltage to a limiter circuit that limits the output current of the inverter when an instantaneous voltage drop occurs, thereby lowering the output current limiter value and reducing the output current value from the inverter. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-223038 [Patent Document 2] Japanese Patent Application Publication No. 2018-82569 Summary of the Invention [Problem to be solved by the invention]

[0006] However, the power conversion device described in Patent Document 2 reduces the output current value from the inverter during an instantaneous voltage drop based on an adjustment coefficient FRTratio that is proportional to the magnitude of the amplitude value of the system voltage. Therefore, FRTratio does not take into account the operating state of the inverter immediately before the instantaneous voltage drop, and the power conversion device may output excessive power to the power system, which may destabilize the power system.

[0007] In view of the above problems, the present disclosure aims to provide a power conversion device that reduces the excessive power output from the power conversion device to a power grid during an instantaneous voltage drop, thereby improving the stability of the power grid. [Means for solving the problem]

[0008] a first current target value that is a target value of an output current of the bidirectional inverter after the instantaneous voltage sag occurs, based on a ratio between a first system voltage before the instantaneous voltage sag and a second system voltage after the instantaneous voltage sag, and based on a first current target value that is a target value of an output current of the bidirectional inverter before the instantaneous voltage sag occurs; and a first inverter control unit that controls the bidirectional inverter while the instantaneous voltage sag occurs, based on the second current target value determined by the first determination unit. [Effects of the Invention]

[0009] According to the present disclosure, it is possible to provide a power conversion device that reduces excessive power output to a power grid during an instantaneous voltage drop and improves the stability of the power grid. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a schematic diagram showing the overall configuration including the power conversion device according to the first embodiment. [Figure 2] FIG. 2 is a functional block diagram showing functions of the control device in the first embodiment. [Figure 3] FIG. 3 is a waveform diagram showing an example of voltage and current waveforms at various points during an instantaneous voltage drop. [Figure 4] FIG. 4 is a flowchart illustrating an example of control of the power conversion device according to the first embodiment. [Figure 5] FIG. 5 is an explanatory diagram illustrating the changes in voltage and current at each point before and after an instantaneous voltage drop. [Figure 6]FIG. 6 is a waveform diagram of an example of the system voltage and the inverter output current before and after an instantaneous voltage drop. [Figure 7] FIG. 7 is an explanatory diagram illustrating changes in voltage and current at various points before and after an instantaneous voltage drop in the second and third embodiments. [Figure 8] FIG. 8 is a functional block diagram showing functions of the control device in the third embodiment. [Figure 9] FIG. 9 is a flowchart showing an example of control of the power conversion device according to the third embodiment. [Figure 10] FIG. 10 is a functional block diagram showing functions of the control device in the fourth embodiment. [Figure 11] FIG. 11 is a flowchart showing an example of control of the power conversion device according to the fourth embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0011] <Summary of Embodiments of the Present Disclosure> The following provides an outline of embodiments of the present disclosure. (1) A power conversion device of this embodiment is a power conversion device for connecting a distributed power source to a power grid, and includes: a bidirectional converter having a first end connected to the distributed power source and a second end connected to a DC bus; a bidirectional inverter having a first end connected to the power grid and a second end connected to the DC bus; and a control device that controls the bidirectional converter and the bidirectional inverter. The control device includes: a detection unit that detects an instantaneous voltage sag in a system voltage that is a voltage of the power grid; a first determination unit that, when the detection unit detects the instantaneous voltage sag, determines a second current target value that is a target value for the output current of the bidirectional inverter after the instantaneous voltage sag, based on a ratio between a first system voltage before the instantaneous voltage sag and a second system voltage after the instantaneous voltage sag and a first current target value that is a target value for the output current of the bidirectional inverter before the instantaneous voltage sag; and a first inverter control unit that controls the bidirectional inverter while the instantaneous voltage sag is occurring, based on the second current target value determined by the first determination unit. According to the power conversion device of this embodiment, the inverter is controlled based on the inverter current target value before the instantaneous voltage drop occurs and the system voltage before and after the instantaneous voltage drop occurs, so it is possible to provide a power conversion device that reduces excessive power output to the power system during an instantaneous voltage drop and improves the stability of the power system.

[0012] (2) In the power conversion device described in (1) above, after the detection unit detects the instantaneous voltage drop, the first inverter control unit may control the bidirectional inverter based on a third current target value that is smaller than the first current target value, and after control based on the third current target value, may control the bidirectional inverter based on the second current target value. Immediately after an instantaneous voltage drop, the waveform of the system voltage is likely to become distorted, but the power conversion device can improve the stability of the power system because the inverter is controlled using a current target value that is smaller than the current target value before the instantaneous voltage drop.

[0013] (3) In the power conversion device described in (1) above, the control device may further include a determination unit that determines whether the grid voltage is stable, and the first inverter control unit may control the bidirectional inverter based on a third current target value that is smaller than the first current target value after the detection unit detects the instantaneous voltage drop, and may control the bidirectional inverter based on the second current target value after the determination unit determines that the grid voltage is stable. Immediately after an instantaneous voltage drop, the waveform of the system voltage is likely to become distorted. However, until the system voltage stabilizes, the inverter is controlled using a current target value that is smaller than the current target value before the instantaneous voltage drop, thereby enabling the power conversion device to improve the stability of the power system.

[0014] (4) In the power conversion device described in (2) or (3) above, the third current target value may be smaller than the second current target value. Since the inverter is controlled using a current target value that is smaller than the current target value after an instantaneous voltage drop, the power conversion device can further improve the stability of the power system.

[0015] (5) In the power conversion device according to any one of (1) to (4) above, the control device may further include a first converter control unit that controls the bidirectional converter so as to cause the voltage of the DC bus to converge to a target value when the detection unit detects the instantaneous voltage drop. During an instantaneous voltage drop, the bidirectional inverter does not control the DC bus voltage, but the first converter control unit controls the bidirectional converter so that the DC bus voltage converges to the target value, thereby keeping the DC bus voltage constant.

[0016] (6) In the power conversion device according to any one of (1) to (5) above, the control device may further include a second inverter control unit that controls the bidirectional inverter to cause the voltage of the DC bus to converge to a target value when the detection unit does not detect the instantaneous voltage drop. Before the instantaneous voltage drop, the bidirectional converter does not control the DC bus voltage, but the second inverter control unit controls the bidirectional inverter to converge the DC bus voltage to the target value, so the DC bus voltage can be kept constant.

[0017] (7) In the power conversion device described in (5) above, the control device may further include a second converter control unit that controls the bidirectional converter so that, when the detection unit does not detect the instantaneous voltage drop, the bidirectional converter outputs, to the DC bus, power corresponding to power consumption of a load connected to the first end of the bidirectional inverter. As a result, the bidirectional converter outputs power commensurate with the power consumption of the load before the instantaneous voltage drop occurs, thereby preventing reverse power flow from the power conversion device to the power grid.

[0018] (8) In the power conversion device according to any one of (1) to (7) above, the control device may further include a second determination unit that, when the detection unit detects the instantaneous voltage drop, determines whether the first inverter control unit controls the bidirectional inverter based on the second current target value or executes a stop control process to stop the bidirectional inverter. This makes it possible to switch between controlling the bidirectional inverter based on the second current target value and controlling the bidirectional inverter to stop it when an instantaneous voltage drop occurs.

[0019] (9) A control method for a power conversion device of this embodiment is a control method for a power conversion device for connecting a distributed power source to a power grid, the power conversion device including: a bidirectional converter having a first terminal connected to the distributed power source and a second terminal connected to a DC bus; a bidirectional inverter having a first terminal connected to a power grid and a second terminal connected to the DC bus; and a control device that controls the bidirectional converter and the bidirectional inverter. The control method includes the steps of: detecting an instantaneous voltage sag of a system voltage that is a voltage of the power grid; determining, when the instantaneous voltage sag is detected, a second current target value that is a target value of the output current of the bidirectional inverter after the instantaneous voltage sag, based on a ratio between a first system voltage before the instantaneous voltage sag and a second system voltage after the instantaneous voltage sag and a first current target value that is a target value of the output current of the bidirectional inverter before the instantaneous voltage sag; and controlling the bidirectional inverter while the instantaneous voltage sag is occurring, based on the determined second current target value. According to the control method for the power conversion device of this embodiment, the inverter is controlled taking into consideration the inverter current target value before the instantaneous voltage drop occurs and the system voltage before and after the instantaneous voltage drop occurs, so it is possible to control the power conversion device so as to reduce excessive power output to the power system and improve the stability of the power system.

[0020] (10) A computer program of this embodiment is a computer program for operating a control device that controls a power conversion device for connecting a distributed power source to a power grid, the power conversion device including: a bidirectional converter having a first terminal connected to the distributed power source and a second terminal connected to a DC bus; a bidirectional inverter having a first terminal connected to a power grid and a second terminal connected to the DC bus; and a control device that controls the bidirectional converter and the bidirectional inverter. The computer program includes the steps of: detecting an instantaneous voltage sag of a system voltage that is a voltage of the power grid; determining, when the instantaneous voltage sag is detected, a second current target value that is a target value of the output current of the bidirectional inverter after the instantaneous voltage sag, based on a ratio between a first system voltage before the instantaneous voltage sag and a second system voltage after the instantaneous voltage sag and a first current target value that is a target value of the output current of the bidirectional inverter before the instantaneous voltage sag; and controlling the bidirectional inverter while the instantaneous voltage sag is occurring, based on the determined second current target value. The computer program of this embodiment controls the control device to control the inverter in consideration of the inverter current target value before the instantaneous voltage sag and the grid voltage before and after the instantaneous voltage sag, thereby reducing excess power output to the grid and improving the stability of the grid.

[0021] <Details of the embodiment of the present disclosure> 1. Embodiment 1 Hereinafter, with reference to the drawings, a power conversion device according to a first embodiment, which is an example of the present disclosure, will be described in detail.

[0022] [1-1. Overall Structure] FIG. 1 is a schematic diagram showing an overall configuration including a power conversion device according to an embodiment of the present disclosure. First, the overall configuration will be described based on FIG. 1. The overall configuration includes a power conversion device 1, a power system 2, and a distributed power source 3. A current sensor CT is provided on a power line PL1a connecting the power system 2 and a consumer's receiving end RP. The power line PL1a branches off from the receiving end RP, with a first power line PL1b connected to the power conversion device 1 (described later) and a second power line PL1c connected to a consumer's load 4. The consumer's load 4 is, for example, an electrical appliance such as an LCD television or a refrigerator.

[0023] <Power conversion device> The power conversion device 1 is a power conversion device having, for example, a first function and a second function. The first function is to receive power from a power grid 2 (described later), convert it to a predetermined voltage and current, and output it to a power storage device mounted on a distributed power source 3 (described later). The second function is to receive power stored in the power storage device, convert it to a predetermined voltage and current, and output it to the power grid 2 side.

[0024] <Power system> The power grid 2 is a system that integrates power generation, transformation, transmission, and distribution to supply power to the power receiving equipment of consumers. The power exchanged between the power grid 2 and the consumers is AC power exchanged at an AC voltage of, for example, 101V±6V.

[0025] <Distributed power source> The distributed power source 3 is, for example, a power storage device mounted on an electric vehicle (hereinafter referred to as "EV (Electric Vehicle) 3"). The power storage device is, for example, a secondary battery such as a lithium-ion battery. The power exchanged with the power storage device mounted on the EV 3 is DC power, and its voltage and current comply with, for example, CHAdeMO (registered trademark), which is one standard for rapid charging methods. The power storage device of the EV 3 stores DC power from the power conversion device 1 and outputs the stored power to the power conversion device 1.

[0026] Hereinafter, the distributed power source will be described using a power storage device as an example, but is not limited to this. For example, the distributed power source may be a solar power generation device or a wind power generation device that uses renewable energy. In the case of a solar power generation device or a wind power generation device, the distributed power source 3 may not have the function of storing DC power from the power conversion device 1.

[0027] <Current sensor> A current sensor CT provided on a power line PL1a connecting the power system 2 and a receiving end RP of a consumer detects the current flowing from the power system 2 to the consumer. The current sensor CT is, for example, a current transformer type sensor. A signal indicating the detected current is sent via a communication line to an AD converter (hereinafter referred to as "ADC") 14d, which will be described later and is included in the control device 14 of the power conversion device 1.

[0028] 1-2. Configuration of the power conversion device according to the present disclosure The power conversion device 1 includes a bidirectional inverter 11, a bidirectional converter 12, a capacitor 13, and a control device 14. The power conversion device 1 may further include a relay 15. The relay 15 is inserted between the power line PL1b and the power line PL2, and switches the connection relationship between the power conversion device 1 and the electric power grid 2 between connected and disconnected under the control of the control device 14.

[0029] <Bidirectional inverter> The first terminal 11a of the bidirectional inverter 11 is connected to the power line PL2. The first terminal 11a is a terminal from which the bidirectional inverter 11 outputs or receives AC power. The second terminal 11b of the bidirectional inverter 11 is connected to the DC bus PL3. The second terminal 11b is a terminal from which the bidirectional inverter 11 outputs or receives DC power. The bidirectional inverter 11 is an AC / DC inverter having a first function and a second function. The first function is to receive AC power from the first terminal 11a, convert it into DC power of a predetermined voltage and current, and output the DC power from the second terminal 11b to the DC bus PL3. The second function is to receive DC power from the DC bus PL3 connected to the second terminal 11b, convert it into AC power of a predetermined voltage and current, and output it from the first terminal 11a to the power line PL2.

[0030] The bidirectional inverter 11 includes a switching element, a coil, a capacitor, an MCU (microcontroller unit), etc. The bidirectional inverter 11 realizes the first and second functions by the cooperative operation of the switching element, coil, capacitor, MCU, etc. The bidirectional inverter 11 receives control information from the control device 14 and operates in accordance with the received control information. The control information includes, for example, information indicating whether the bidirectional inverter 11 operates using the first or second function, a current target value indicating a target value of the output current, a current value to be received, etc.

[0031] <Bidirectional converter> The first end 12a of the bidirectional converter 12 is connected to the power line PL4. The first end 12a is a terminal from which the bidirectional converter 12 outputs DC power to or receives DC power from the power line PL4. The power line PL4 connects the first end 12a of the bidirectional converter to the power storage device of the EV3. The second end 12b of the bidirectional converter 12 is connected to the DC bus PL3. The second end 12b is a terminal from which the bidirectional converter 12 outputs DC power to or receives DC power from the DC bus PL3. The bidirectional converter 12 is a DC / DC converter having the following first and second functions. The first function is to receive DC power from the power line PL4 connected to the first end 12a, convert it into DC power of a predetermined voltage and current, and output it from the second end 12b to the DC bus PL3. The second function is to receive DC power from the second terminal 12b, convert it into DC power of a predetermined voltage and current, and output the DC power from the first terminal 12a to the power line PL4.

[0032] The bidirectional converter 12 includes a switching element, a coil, a capacitor, an MCU, etc. The bidirectional converter 12 realizes the first and second functions by the cooperative operation of the switching element, coil, capacitor, MCU, etc. The bidirectional converter 12 receives control information from the control device 14 and operates in accordance with the received control information. The control information includes, for example, information indicating whether the bidirectional converter 12 operates as the first or second function, and information indicating the output voltage value, current value, and received current value.

[0033] <Capacitor> The capacitor 13 is connected to the DC bus PL3. The capacitor 13 can stabilize the voltage of the DC bus PL3 by discharging the stored charge to the DC bus PL3 in response to a sudden current request from the bidirectional inverter 11 or the bidirectional converter 12. For example, since DC power of 300 V is applied to the DC bus PL3, the capacitor 13 has a withstand voltage that can withstand a DC voltage of 300 V.

[0034] <Control device> The control device 14 includes a processor 14a, a memory 14b, an input / output interface (I / F) 14c, and an ADC 14d. The control device 14 controls the bidirectional inverter 11, the bidirectional converter 12, and the relay 15. The control device 14 also detects the voltage Vps and current of the power line PL1a of the power system, the voltage Vbus of the DC bus PL3 (hereinafter referred to as the "DC bus voltage Vbus"), and the voltage of the power line PL4.

[0035] <Processor> The processor 14a is, for example, a CPU (Central Processing Unit). However, the processor 14a is not limited to a CPU. The processor 14a may be a GPU (Graphics Processing Unit). The processor 14a is, for example, a multi-core processor. The processor 14a may be a single-core processor. The processor 14a may be, for example, an ASIC (Application Specific Integrated Circuit) or a programmable logic device such as a gate array or FPGA (Field Programmable Gate Array).

[0036] <Memory> The memory 14b includes volatile memory and nonvolatile memory. The volatile memory is, for example, a semiconductor memory such as a static random access memory (SRAM) or a dynamic random access memory (DRAM). The nonvolatile memory is, for example, a flash memory, a hard disk, or a read-only memory (ROM). The nonvolatile memory stores a control program for controlling the rail system, which is a computer program, and data used to execute the control program. The functions of the control device 14 are realized when the processor 14a executes the control program. The control program can be stored in a recording medium such as a flash memory, a ROM, or a CD-ROM. The processor 14a controls the bidirectional inverter 11, the bidirectional converter 12, and the relay 15 using the control program.

[0037] <Input / output interface> The input / output I / F 14c is connected to the bidirectional inverter 11, the bidirectional converter 12, and the relay 15. The input / output I / F 14c transmits commands to control the devices connected to each of them, and receives data from the devices.

[0038] <adc> The ADC 14d is an AD converter that converts analog signals into digital data. The ADC 14d has the following first and second functions. The first function is to convert an analog signal from a current sensor that detects the current flowing through the power line PL1a connected to the power system 2 into digital data and pass it to the processor 14a. The second function is to receive the voltages of the voltage detection signal lines 16 connected to the power line PL1a connected to the power system 2, the DC bus PL3, and the power line PL4 connected to the EV3, convert them into digital data, and pass them to the processor 14a. The voltage detection signal line from which the voltage is received may be switched by a relay, or multiple ADCs 14d may be provided. Furthermore, a voltage divider may be inserted in the voltage detection signal line 16 to reduce the voltage to a level that can be input to the ADC 14d.

[0039] [1-3. Functions of the control device] FIG. 2 is a functional block diagram showing the functions of the control device 14. As shown in FIG. FIG. 3 is a waveform diagram showing an example of voltage and current waveforms at various points during an instantaneous voltage drop. The functions of the power conversion device 1 will be described below with reference to the drawings.

[0040] The control device 14 of the power conversion device 1 includes a storage unit 20, a detection unit 21, a first determination unit 22, a first inverter control unit 23, a second inverter control unit 24, a first converter control unit 25, and a second converter control unit 26. Note that the control mode in which normal operation is performed before an instantaneous voltage drop occurs in the power grid 2 is sometimes referred to as a normal mode, and the control mode after it is determined that an instantaneous voltage drop occurs in the power grid 2 is sometimes referred to as an FRT mode.

[0041] <Storage section> The storage unit 20 stores the voltage Vps (hereinafter referred to as "system voltage Vps") of the power system 2 (power line PL1a) in the memory 14b. In addition, the storage unit 20 stores a first current target value Itg1, which is the target value of the current output by the bidirectional inverter 11 in normal mode, in the memory 14b. The current is, for example, an active current. The first current target value Itg1 is determined by a second inverter control unit 24, which will be described later. This is because, when an instantaneous voltage sag is determined, a first determination unit 22, which will be described later, determines a second current target value Itg2, which is the current target value of the bidirectional inverter 11 after the instantaneous voltage sag, using the system voltage Vps before and after the instantaneous voltage sag and the current target value Itg1 before the instantaneous voltage sag.

[0042] Specifically, to acquire the system voltage Vps, the storage unit 20 controls the ADC 14d to detect the voltage of the voltage detection signal line 16 connected to the power line PL1a and acquire digital data corresponding to the detected voltage. When acquiring the digital data, the storage unit 20 acquires, for example, 128 points of data for one cycle of the AC voltage on the power line PL1a.

[0043] When storing the system voltage Vps, for example, the effective value Vrms of the system voltage Vps is stored. To calculate the effective value Vrms of the system voltage Vps, the control device 14 calculates the effective value based on Equation 1. Specifically, first, the acquired data for each point is squared. Next, the squared data for 128 points is added together. The sum is then divided by the period T (the time for one cycle) to calculate the square root.

[0044]

number

[0045] In the above description, when storing the system voltage Vps, for example, the effective value of the system voltage Vps is stored, but this is not limiting and the average value of the system voltage Vps may also be stored. To calculate the average value Vav of the system voltage Vps, the control device 14 calculates the average value based on Equation 2. Specifically, first, the absolute value of the acquired data at each point is calculated. Next, the absolute value data for 128 points is added together. Then, the sum is divided by the period T (the time for one cycle).

number

[0046] <Detection unit> The detection unit 21 detects an instantaneous voltage drop in the grid voltage, which is the voltage of the power grid. Specifically, the detection unit 21 detects an instantaneous voltage drop based on whether the effective value Vrms of the grid voltage Vps is equal to or greater than a threshold. If the effective value Vrms of the grid voltage Vps is equal to or greater than the threshold, it is determined that no instantaneous voltage drop has occurred. If the effective value Vrms of the grid voltage Vps is lower than the threshold, it is determined that an instantaneous voltage drop has occurred. The threshold is, for example, 80% of the nominal voltage Vn (e.g., 100 V) of the power grid 2. The effective value Vrms of the grid voltage Vps, which is the basis for the determination, can be acquired from the memory 14b in which the effective value Vrms is stored.

[0047] Since the detection unit 21 determines the occurrence of an instantaneous voltage drop based on data for one or several cycles of the system voltage Vps, a period (period 2 in Figure 3) is required from the time the instantaneous voltage drop occurs until the detection unit 21 determines that an instantaneous voltage drop has occurred in the power system 2.

[0048] The above method for determining an instantaneous voltage drop is merely an example and is not intended to be limiting. For example, an instantaneous voltage drop may be determined to have occurred when the grid voltage Vps crosses 0 V, i.e., zero-crosses, within a certain period of time, e.g., within 1 mS.

[0049] <First Decision Section> When the detection unit 21 detects an instantaneous voltage sag, the first determination unit 22 determines a second current target value, which is a target value of the output current of the bidirectional inverter 11 after the instantaneous voltage sag, based on the ratio between the first system voltage before the instantaneous voltage sag and the second system voltage after the instantaneous voltage sag, and the first current target value, which is a target value of the output current of the bidirectional inverter before the instantaneous voltage sag. That is, the first determination unit 22 determines a second current target value Itg2 of the bidirectional inverter 11 in FRT mode. The second current target value Itg2 is the target value of the output current of the bidirectional inverter 11 in FRT mode.

[0050] First, the first determination unit 22 obtains data indicating the first system voltage Vps1 before the instantaneous voltage sag, the second system voltage Vps2 after the instantaneous voltage sag, and the first current target value Itg1 in normal mode from the memory 14b.

[0051] Next, the first determination unit 22 determines the second current target value Itg2 in the FRT mode using Equation 3 based on the acquired data.

number

[0052] <First inverter control unit> The first inverter control unit 23 controls the bidirectional inverter 11 while the instantaneous voltage sag is occurring, based on the second current target value Itg2 determined by the first determination unit 22. Specifically, the first inverter control unit 23 sends data indicating the determined second current target value Itg2 to the bidirectional inverter 11 via the input / output I / F 14c. The bidirectional inverter 11 receives the data indicating the second current target value Itg2 and controls the output current based on the second current target value Itg2 included in the received data.

[0053] <Second inverter control unit> The second inverter control unit 24 controls the bidirectional inverter 11 to converge the DC bus voltage Vbus to a target value (target voltage Vtg) when the detection unit 21 does not detect an instantaneous voltage drop. The target voltage Vtg is a target voltage, such as 300 V, for controlling the DC bus voltage Vbus to a constant voltage. The second inverter control unit 24 also controls the bidirectional inverter 11 based on the first current target value Itg1.

[0054] Specifically, first, the second inverter control unit 24 acquires the voltage value of the DC bus PL3 in order to converge the DC bus voltage Vbus to the target voltage Vtg. Specifically, the voltage value of the voltage appearing on the voltage detection signal line 16 connected to the DC bus PL3 is converted into digital data by the ADC 14d, and the second inverter control unit 24 acquires the digital data of the voltage value of the DC bus PL3.

[0055] The second inverter control unit 24 then controls the bidirectional inverter 11 based on the difference between the acquired digital data and the digital data indicating the target voltage Vtg. When the bidirectional inverter 11 converts power from the second terminal 11b to the first terminal 11a, the second inverter control unit 24 controls the current drawn by the bidirectional inverter 11 from the second terminal 11b. When the bidirectional inverter 11 converts power from the first terminal 11a to the second terminal 11b, the second inverter control unit 24 controls the current output from the second terminal 11b.

[0056] More specifically, the second inverter control unit 24 sends data indicating the value of the current to be output or drawn to the bidirectional inverter 11 via the input / output I / F 14c. The bidirectional inverter 11 receives the data and controls the current to be output or drawn based on the current value indicated by the received data. As a result, the second inverter control unit 24 controls the DC bus voltage Vbus connected to the DC bus PL3 to the target voltage Vtg.

[0057] Furthermore, in order to control the output current of the bidirectional inverter 11 to the first current target value Itg1, the second inverter control unit 24 determines the first current target value Itg1 in normal mode using Equation 4.

number

[0058] The second inverter control unit 24 controls the bidirectional inverter 11 based on the determined first current target value Itg1. The second inverter control unit 24 sends data indicating the determined first current target value Itg1 to the bidirectional inverter 11 via the input / output I / F 14c. The bidirectional inverter 11 receives the data indicating the first current target value Itg1 and controls the output current based on the first current target value Itg1 indicated by the received data.

[0059] The first current target value Itg1 is determined by Equation 4, but can be modified based on multiple conditions. One condition for modifying the first current target value Itg1 is, for example, a condition that takes into account reverse power flow to the power grid 2. The power conversion device 1 may be operated in a mode that does not allow reverse power flow from the power conversion device 1 to the power grid 2. In this case, the second inverter control unit 24 modifies the first current target value Itg1 based on the current value of the power line PL1a detected by the current sensor CT, and controls the bidirectional inverter 11 so that reverse power flow does not occur.

[0060] Another condition is, for example, a condition that takes into consideration the amount of electricity stored in the electricity storage device 3. When the amount of electricity stored in the electricity storage device 3 decreases, the first current target value Itg1 is corrected, and the second inverter control unit 24 controls the bidirectional inverter 11 based on the corrected first current target value Itg1.

[0061] <First converter control unit> When the detector 21 detects an instantaneous voltage drop, the first converter controller 25 controls the bidirectional converter 12 to cause the DC bus voltage Vbus to converge to a target value (target voltage Vtg). Specifically, the first converter controller 25 sends data indicating the target value to the bidirectional converter 12. Upon receiving the data, the bidirectional converter 12 operates to cause the DC bus voltage Vbus to converge to the target value.

[0062] In the normal mode, the second inverter control unit 24 controls the bidirectional inverter 11 so that the DC bus voltage Vbus converges to the target voltage Vtg current. Therefore, in the normal mode and the FRT mode, if the target voltage Vtg is the same, the DC bus voltage Vbus is always maintained at the same voltage.

[0063] <Second converter control unit> When the detector 21 does not detect an instantaneous voltage drop, the second converter control unit 26 controls the bidirectional converter 12 so that the bidirectional converter 12 outputs, to the DC bus PL3, power corresponding to the power consumption of the load 4 connected to the first end 11a of the bidirectional inverter 11. Specifically, the second converter control unit 26 first acquires the power consumption of the load 4. For example, a current sensor is attached to the power line PL1c, and a detection signal from the current sensor is received by the ADC 14d. The second converter control unit 26 then acquires the data converted into digital data by the ADC 14d, thereby acquiring the current consumption of the load 4. In parallel with this, the system voltage Vps is acquired by the ADC 4d. The second converter control unit 26 then multiplies the acquired current consumption by the system voltage Vps to acquire the power consumption of the load 4.

[0064] The second converter control unit 26 then controls the bidirectional converter 12 to output power corresponding to the acquired power consumption of the load 4. Specifically, for example, in normal mode, the second inverter control unit 24 controls the voltage of the DC bus PL3 to converge to the target voltage Vtg, so the second converter control unit 26 acquires data indicating this target voltage Vtg. The second converter control unit 26 then determines a current value to be output by the bidirectional converter 12 based on the acquired power consumption of the load 4 and the target voltage Vtg, and sends data indicating the determined current value to the bidirectional converter 12. Upon receiving the sent data, the bidirectional converter 12 outputs power to the DC bus PL3 based on this data. Note that in normal mode and FRT mode, if the target voltage Vtg is the same, the DC bus voltage Vbus is always maintained at the same voltage.

[0065] [1-4. Control of power conversion equipment] FIG. 4 is a flowchart showing an example of control of the power conversion device 1. Next, the control of the power conversion device 1 will be described with reference to FIG. It is assumed that the power conversion device 1, the power system 2, the EV 3, and the load 4 are in an operating state.

[0066] <Step S01> First, the storage unit 20 acquires the effective value Vrms of the system voltage Vps and stores it in the memory 14b. The storage unit 20 also stores the first current target value Itg1 of the bidirectional inverter 11 in the memory 14b (step S01).

[0067] Specifically, to acquire the system voltage Vps, the storage unit 20 controls the ADC 14d to acquire digital data corresponding to the voltage of the voltage detection signal line 16 connected to the power line PL1a. Then, to obtain the effective value Vrms of the system voltage Vps, the storage unit 20 obtains the effective value based on Equation 1 and stores the obtained effective value Vrms in the memory 14b.

[0068] Furthermore, in the normal mode, the storage unit 20 acquires the first current target value Itg1 determined by the second inverter control unit 24. After the storage unit 20 stores Vrms and Itg1 in the memory 14b, the control device 14 proceeds to step S02.

[0069] <Step S02> Next, the detection unit 21 detects an instantaneous voltage drop by determining whether the effective value Vrms of the grid voltage Vps is equal to or greater than a threshold value (step S02). If the effective value Vrms of the grid voltage Vps is equal to or greater than the threshold value, it is determined that an instantaneous voltage drop has not occurred (YES in step S02), and the process proceeds to step S03. On the other hand, if the effective value Vrms of the grid voltage Vps is lower than the threshold value, it is determined that an instantaneous voltage drop has occurred (NO in step S02), and the process proceeds to step S05. The threshold value is, for example, 80% of the nominal voltage Vn (e.g., 100 V) of the power grid 2. Since the occurrence of an instantaneous voltage drop is determined based on data for one or several cycles of the grid voltage Vps, it may take several cycles for the detection unit 21 to determine the occurrence of the instantaneous voltage drop after the instantaneous voltage drop occurs.

[0070] <Step S03> If the answer is YES in step S02, that is, in the normal mode, step S03 is executed. In step S03, the second inverter control unit 24 causes the DC bus voltage Vbus to converge to the target voltage Vtg.

[0071] Furthermore, in order to control the output current of the bidirectional inverter 11 to the first current target value Itg1, the second inverter control unit 24 determines the first current target value Itg1 in normal mode using Equation 4. The second inverter control unit 24 controls the bidirectional inverter 11 based on the determined first current target value Itg1 in normal mode. Then, the control device 14 proceeds to step S04.

[0072] <Step S04> Next, the second converter control unit 26 performs load tracking control on the bidirectional converter 12 to make the output power follow the power consumption of the load 4 (step S04). The second converter control unit 26 acquires the power consumption of the load 4 and controls the bidirectional converter 12 to output power according to the acquired power consumption of the load 4.

[0073] Then, the control device 14 returns to step S01 and executes step S01 again. In the normal mode, there is no instantaneous voltage drop in the power grid 2. Therefore, the determination in step S02 is YES, and the control device 14 sequentially repeats steps S01, S02, S03, and S04.

[0074] <Step S05> If the result of step S02 is NO, i.e., if the FRT mode is selected, step S05 is executed. In step S05, the first determination unit 22 determines the second current target value Itg2. After the second current target value Itg2 is determined, the control device 14 proceeds to step S06.

[0075] <Step S06> Next, the first inverter control unit 23 controls the bidirectional inverter 11 based on the second current target value Itg2 determined by the first determination unit 22. After the bidirectional inverter 11 is controlled based on the second current target value Itg2, the control device 14 proceeds to step S07.

[0076] <Step S07> Next, the first converter control unit 25 controls the bidirectional converter 12 so that the DC bus voltage Vbus converges to the target voltage Vtg (step S07). The first converter control unit 25 sends data indicating the target voltage Vtg to the bidirectional converter 12. Upon receiving the data, the bidirectional converter 12 operates so that the DC bus voltage Vbus converges to the target voltage Vtg.

[0077] Then, the control device 14 returns to step S01 and executes step S01 again. In the FRT mode, an instantaneous voltage drop occurs in the power grid 2. For this reason, the determination in step S02 is NO, and the control device 14 sequentially repeats steps S01, S02, S05, S06, and S07.

[0078] In the above description, the control device 14 sequentially repeats steps S01, S02, S03, and S04, or sequentially repeats steps S01, S02, S05, S06, and S07, but this is not limiting. The detection unit 21 may be configured to constantly monitor the grid voltage Vps, and execute steps S05, S06, and S07 when the determination of the detection unit 21 switches from not having an instantaneous voltage drop to having an instantaneous voltage drop, and execute steps S03 and S04 when the determination switches from having an instantaneous voltage drop to not having an instantaneous voltage drop.

[0079] 1-5. Operation of the power conversion device Next, the operation of the power conversion device 1 will be described with reference to FIG. FIG. 5 is an explanatory diagram illustrating the changes in voltage and current at each point before and after the occurrence of an instantaneous voltage drop.

[0080] <Period 1> Period 1 is the state before the power grid 2 experiences an instantaneous voltage drop. Because the detection unit 21 determines that the power grid 2 does not experience an instantaneous voltage drop, the control device 14 repeatedly performs the control of steps S01, S02, S03, and S04. Therefore, the second inverter control unit 24 controls the bidirectional inverter 11 so that the DC bus voltage Vbus converges to the target voltage Vtg. As a result, the DC bus voltage is maintained at the target voltage Vtg. Furthermore, the second inverter control unit 24 controls the bidirectional inverter 11 with the first current target value Itg1. Furthermore, the second converter control unit 26 performs load following control on the bidirectional converter 12 to cause the output power to follow the power consumption of the load 4.

[0081] <Period 2> Period 2 is a period during which an instantaneous voltage drop occurs in the power grid 2 but is not yet detected by the detector 21. Although the grid voltage Vps drops, the detector 21 has not yet detected the instantaneous voltage drop. Therefore, the control device 14 executes steps S03 and S04. Therefore, the bidirectional inverter 11 is controlled by the first current target value Itg1. However, because the grid voltage Vps drops, the power output by the bidirectional inverter 11 also drops. Meanwhile, the bidirectional converter 12 is controlled by the second converter control unit 26 of the control device 14 to follow the power consumed by the load 4. Therefore, the power output by the bidirectional converter 12 becomes greater than the power output by the bidirectional inverter 11. As a result, the DC bus voltage Vbus gradually rises from the target voltage Vtg.

[0082] <Period 3> Period 3 is the period after the detection unit 21 detects an instantaneous voltage drop. Therefore, the control device 14 repeatedly performs the control of steps S01, S02, S05, S06, and S07. Therefore, the first inverter control unit 23 controls the bidirectional inverter 11 based on the second current target value Itg2. As a result, the current target value of the bidirectional inverter 11 is a value that takes into account Itg1, the current target value in normal mode, thereby suppressing the occurrence of reverse power flow to the power grid 2 and improving the stability of the power grid 2. Meanwhile, the first converter control unit 25 controls the bidirectional converter 12 so that the DC bus voltage Vbus converges to the target voltage Vtg. As a result, the DC bus voltage Vbus, which was higher than the target voltage Vtg during Period 2, drops to the target voltage Vtg and is maintained at the target voltage Vtg.

[0083] <Period 4> Period 4 is a period in which the instantaneous voltage sag in the power grid 2 has been resolved, but the detection unit 21 has not yet detected the resolution of the instantaneous voltage sag. That is, although the grid voltage Vps has recovered, the detection unit 21 still determines that an instantaneous voltage sag has occurred, and therefore the control device 14 continues to execute steps S05, S06, and S07. Therefore, the bidirectional converter 12 is controlled by the first converter control unit 25 so that the DC bus voltage Vbus converges to the target voltage Vtg, and therefore the DC bus voltage Vbus is maintained at the target voltage Vtg.

[0084] <Period 5> Period 5 is the period after the detection unit 21 detects the elimination of the instantaneous voltage sag. Therefore, the control device 14 repeatedly performs the control of steps S01, S02, S03, and S04. Therefore, the second inverter control unit 24 controls the bidirectional inverter 11 so that the DC bus voltage Vbus converges to the target voltage Vtg. Furthermore, since the second inverter control unit 24 controls the bidirectional inverter 11 with the first current target value Itg1, the current target value returns to the first current target value Itg1. Meanwhile, the second converter control unit 26 performs load following control on the bidirectional converter 12 to cause the output power to follow the power consumption of the load 4.

[0085] During period 5, after the detection unit 21 determines that the voltage dip is not an instantaneous voltage drop, the second inverter control unit 24 may increase the second current target value from Itg2 to the first current target value Itg1 over a certain period of time rather than immediately restoring the bidirectional inverter 11 from the second current target value Itg2 to the first current target value Itg1. In this case, the current target value increases slowly, further improving the stability of the power grid 2. However, because the bidirectional converter 12 is already controlled to track the load power and the current target value of the bidirectional inverter 11 increases slowly, the power supplied by the bidirectional converter 12 may temporarily exceed the power output by the bidirectional inverter 11, causing the DC bus voltage Vbus to temporarily exceed the target voltage Vtg.

[0086] [1-5. Summary] When an instantaneous voltage sag occurs in the power grid 2, the power conversion device 1 of the present application operates the bidirectional inverter 11, which outputs power to or receives power from the power grid 2, at a second current target value Itg2 determined by Equation 3 based on the first system voltage Vps1 before the instantaneous voltage sag, the second system voltage Vps2 after the instantaneous voltage sag, and the first current target value Itg1 before the instantaneous voltage sag. In other words, the power conversion device 1 determines the operating state after the instantaneous voltage sag based on the operating state of the bidirectional inverter 11 before the instantaneous voltage sag. As a result, it is possible to provide a power conversion device that reduces excessive power output to the power grid and improves the stability of the power grid.

[0087] Furthermore, the power conversion device 1 of the present application controls the bidirectional inverter 11 in normal mode so that the DC bus voltage Vbus converges to the target voltage Vtg, and controls the bidirectional converter 12 in FRT mode so that the DC bus voltage Vbus converges to the target voltage Vtg. When the target voltage Vtg is the same in normal mode and FRT mode, the DC bus voltage Vbus is maintained at the same voltage in both normal mode and FRT mode. As a result, a constant voltage is always applied to the capacitor 13, preventing excessive voltage application to the capacitor 13 and improving reliability. Furthermore, since the need for an excessively high withstand voltage is reduced, the use of capacitors with excessively high withstand voltages is avoided, resulting in reduced costs and installation space. Furthermore, because fluctuations in the DC bus voltage Vbus are limited to the vicinity of the target voltage Vtg, high conversion efficiency can be maintained by setting the target voltage Vtg to a voltage that maximizes the conversion efficiency of the bidirectional inverter 11 and the bidirectional converter 12.

[0088] 2. Embodiment 2 Hereinafter, a power conversion device according to a second embodiment of the present disclosure will be described with reference to the drawings. The second embodiment differs from the first embodiment in that some of the functions of the first inverter control unit 23 are different, but the rest are the same. The same components as those in the first embodiment are denoted by the same reference numerals, and descriptions of the same components, functions, and operations will be omitted.

[0089] 2-1. Problems to be Solved by the Present Embodiment In the example of the instantaneous voltage drop shown in FIG. 3 of the first embodiment, no sudden change is observed in the system voltage Vps during period 2, when the system transitions from period 1 (normal mode) to period 3 (FRT mode). However, changes in the system voltage during an instantaneous voltage drop are diverse. An example is shown in FIG. 6. FIG. 6 is a waveform diagram of an example of the system voltage before and after the occurrence of an instantaneous voltage drop and the output current of a conventional bidirectional inverter.

[0090] In the example shown in FIG. 6, the grid voltage Vps drops suddenly after the momentary voltage drop occurs, dropping below 0 V. Accordingly, the output current of the bidirectional inverter, which attempts to output a current that follows the grid voltage Vps to the power grid, exhibits unstable behavior. This is because the bidirectional inverter attempts to output a current that follows the grid voltage, but is unable to adapt to the rapidly changing grid voltage, resulting in unstable behavior. In the second embodiment, an attempt is made to provide a power conversion device that operates stably even with the grid voltage Vps fluctuating in various ways immediately after the occurrence of an instantaneous voltage drop.

[0091] [2-2. Functions of the control device] <Functions of the first inverter control unit> As described above, the control device 14 for the power conversion device of the second embodiment differs from that of the first embodiment in the function of the first inverter control unit 23, and therefore the function of the first inverter control unit 23 will be described in detail.

[0092] Although the system voltage changes variously during an instantaneous voltage sag, the system voltage Vps stabilizes after a certain period of time has passed. Therefore, after detecting an instantaneous voltage sag, the first inverter control unit 23 of the second embodiment controls the bidirectional inverter 11 based on a third current target value Itg3 that is smaller than the first current target value Itg1, and after control based on the third current target value Itg3, controls the bidirectional inverter 11 based on the second current target value Itg2.

[0093] First, the first inverter control unit 23 acquires data indicating a first current target value Itg1, which is the current target value in normal mode, from the memory 14b. Then, the first inverter control unit 23 controls the bidirectional inverter 11 using a third current target value Itg3, which is smaller than the acquired first current target value Itg1. The third current target value is, for example, a value obtained by multiplying the acquired first current target value Itg1 by a coefficient. The coefficient is, for example, 0.1. For example, if the first current target value Itg1 in normal mode is 20 A, the third current target value Itg3 is 2 A.

[0094] Specifically, the first inverter control unit 23 sends data indicating the third current target value Itg3 to the bidirectional inverter 11 via the input / output I / F 14c. The bidirectional inverter 11 receives the data indicating the third current target value Itg3 and controls the output current based on the third current target value Itg3 included in the received data.

[0095] The first inverter control unit 23 then controls the bidirectional inverter 11 based on the third current target value Itg3, and then controls the bidirectional inverter 11 based on the second current target value Itg2. The period from when the instantaneous voltage drop is detected to when the bidirectional inverter is controlled based on the second current target value Itg2 is several cycles, for example, two cycles. When the frequency of the power grid 2 is 50 Hz, two cycles is 40 ms.

[0096] [2-3. Control of power conversion equipment] The control flow of the power conversion device 1 in the second embodiment is the same as that in Fig. 4 of the first embodiment, except that in step S06, after detecting an instantaneous voltage drop, the first inverter control unit 23 controls the bidirectional inverter 11 based on a third current target value Itg3 that is smaller than the first current target value Itg1, and after the control based on the third current target value Itg3, controls the bidirectional inverter 11 based on the second current target value Itg2.

[0097] 2-4. Operation of the power conversion device FIG. 7 is an explanatory diagram illustrating the changes in voltage and current at each part before and after the occurrence of an instantaneous voltage drop. The operation of the power conversion device 1 of the second embodiment will be described with reference to FIG.

[0098] <Period 1 and Period 2> Periods 1 and 2 in FIG. 7 are the same as periods 1 and 2 in the first embodiment, respectively.

[0099] <Period 3-1> After the detection unit 21 determines that an instantaneous voltage drop has occurred, the period 3-1 begins, and the control device 14 switches control to execute steps S05 and S06. In step S05, the first inverter control unit 23 of the second embodiment first acquires data indicating the first current target value Itg1 from the memory 14b and controls the bidirectional inverter 11 based on the third current target value Itg3 obtained by multiplying the acquired first current target value Itg1 by, for example, a coefficient of 0.1. Meanwhile, the first converter control unit 25 controls the bidirectional converter 12 so that the DC bus voltage Vbus converges to the target voltage Vtg (step S06). As a result, the DC bus voltage Vbus of the DC bus PL3 decreases toward the target voltage Vtg and is maintained at the target voltage Vtg.

[0100] <Period 3-2> Next, the first inverter control unit 23 controls the bidirectional inverter 11 based on the third current target value Itg3, and then enters period 3-2, where it controls the bidirectional inverter 11 based on the second current target value Itg2. In the case of Figure 6, the period from detecting the instantaneous voltage drop to controlling the bidirectional inverter based on the second current target value Itg2 is two cycles.

[0101] As described above, after detecting the occurrence of an instantaneous voltage drop, the power conversion device 1 controls the bidirectional inverter 11 with the third current target value Itg3, which is smaller than the first current target value Itg1. As a result, if the first current target value Itg1 in normal mode is 20 A, for example, after detecting an instantaneous voltage drop, the current target value becomes 2 A, and the power conversion device 1 operates with a current target value that is extremely small relative to the power grid 2. As a result, the power conversion device 1 can improve the stability of the power grid 2. The third current target value Itg3 may be smaller than the second current target value Itg2. Furthermore, the power conversion device 1 can improve the stability of the power system 2.

[0102] 3. Embodiment 3 Hereinafter, a power conversion device 1 according to a third embodiment of the present disclosure will be described with reference to the drawings. The third embodiment differs from the second embodiment in that some of the functions of the first inverter control unit 23 are different and that a determination unit that determines whether the grid voltage is stable is further included, but the rest is the same. The same components as those in the second embodiment are denoted by the same reference numerals, and descriptions of the same components, functions, and operations will be omitted.

[0103] 3-1. Problems to be Solved by the Present Embodiment The problem to be solved in the third embodiment is the same as that in the second embodiment. That is, the object is to provide a power conversion device that operates stably even when the system voltage Vps fluctuates in various ways immediately after the occurrence of an instantaneous voltage drop.

[0104] [3-2. Functions of the control device] 8 is a functional block diagram showing functions of a control device in embodiment 3. The functions of the control device 14 of the power conversion device 1 in embodiment 3 are different from those in embodiment 1 in that it further includes a determination unit 81 and the functions of the first inverter control unit 23 are partially different. As described above, although the changes in the grid voltage during an instantaneous voltage sag vary widely, the grid voltage Vps eventually stabilizes. Therefore, the control device 14 of the third embodiment controls the bidirectional inverter 11 based on the third current target value Itg3, which is smaller than the first current target value Itg1, during the period from when the instantaneous voltage sag is detected until the grid voltage Vps stabilizes.

[0105] <Judgment part> The determination unit 81 determines whether the system voltage is stable. Specifically, it determines whether the second system voltage Vps2 after the instantaneous voltage drop is stable. Whether the second system voltage Vps2 is stable or not is determined, for example, by acquiring the waveform of the system voltage Vps and converting the voltage waveform into the frequency domain using FFT (Fast Fourier Transform). If the intensity of the fundamental frequency (50 Hz or 60 Hz) of the system voltage is 10 times or more greater than the intensity of the harmonics, it determines that the system voltage Vps is stable, and if it is less than 10 times greater, it determines that the system voltage Vps is unstable.

[0106] <First inverter control unit> When the determination unit 81 determines that the grid voltage Vps is unstable after the detection unit 21 detects an instantaneous voltage drop, the first inverter control unit 23 controls the bidirectional inverter 11 based on a third current target value Itg3 that is smaller than the first current target value Itg1. On the other hand, when the determination unit 81 determines that the grid voltage Vps is stable, the first inverter control unit 23 controls the bidirectional inverter 11 based on the second current target value Itg2. The third current target value is, for example, a value obtained by multiplying the acquired first current target value Itg1 by a coefficient, as in the second embodiment. The coefficient is, for example, 0.1. For example, when the first current target value Itg1 in normal mode is 20 A, the third current target value Itg3 is 2 A. The third current target value Itg3 may be smaller than the second current target value Itg2. Furthermore, the power conversion device 1 can improve the stability of the power grid 2.

[0107] [3-3. Control of power conversion equipment] 9 is a flowchart showing an example of control of a power conversion device in embodiment 3. The control flow of embodiment 3 differs from the control flow of embodiment 1 in that steps S91 and S92 are added after step S02 in embodiment 1.

[0108] <Step S91> After the detection unit 21 detects an instantaneous voltage drop (NO in step S02), the control device 14 proceeds to step S91. In step S91, the determination unit 81 determines whether the system voltage is stable. For example, if the intensity of the fundamental frequency (50 Hz or 60 Hz) of the system voltage is 10 times or more greater than the intensity of the harmonics, the determination unit 81 determines that the system voltage Vps is stable, and if it is less than 10 times greater, the determination unit 81 determines that the system voltage Vps is unstable (NO in step S91). Then, the control device 14 proceeds to step S92.

[0109] On the other hand, if the determination unit 81 determines that the system voltage is stable (YES in step S91), the control device 14 proceeds to step S05 to determine the second current target value Itg2, and controls the bidirectional inverter 11 based on the determined second current target value Itg2 (step S06).

[0110] <Step S92> The control device 14 proceeds to step S92, where it controls the bidirectional inverter 11 based on the third current target value Itg3, which is smaller than the first current target value Itg1, using the first inverter control unit 23 of the third embodiment (step S92). After operating the bidirectional inverter 11 based on the third current target value Itg3, the control device 14 proceeds to step S07.

[0111] 3-4. Operation of the power conversion device The changes in voltage and current at each part before and after the occurrence of an instantaneous voltage drop are the same as those in the second embodiment, so the operation of the power conversion device 1 of the third embodiment will be described with reference to FIG.

[0112] <Period 1 and Period 2> Period 1 and Period 2 in the third embodiment are the same as Period 1 and Period 2 in the first embodiment, respectively.

[0113] <Period 3-1> After the detection unit 21 determines that an instantaneous voltage drop has occurred (NO in step S02), the process enters period 3-1, and the control device 14 executes step S91. In step S91, the determination unit 81 monitors the grid voltage Vps and determines whether the grid voltage Vps has stabilized. Since the determination unit 81 determines that the grid voltage Vps is not stable in period 3-1, the first inverter control unit 23 of the third embodiment obtains, for example, data indicating the first current target value Itg1 from the memory 14b, and controls the bidirectional inverter 11 based on the third current target value Itg3 obtained by multiplying the obtained first current target value Itg1 by a coefficient.

[0114] At the same time, the first converter control unit 25 controls the bidirectional converter 12 so that the DC bus voltage Vbus converges to the target voltage Vtg. As a result, the DC bus voltage Vbus of the DC bus PL3 decreases toward the target voltage Vtg and is maintained at the target voltage Vtg.

[0115] <Period 3-2> When the period 3-2 begins, the determination unit 81 determines that the grid voltage Vps has stabilized, and the first inverter control unit 23 of the third embodiment controls the bidirectional inverter 11 based on the second current target value Itg2, which is the current target value in the FRT mode.

[0116] As a result, the control device 14 controls the bidirectional inverter 11 to the third current target value Itg3, which is smaller than the first current target value Itg1, from the time when the occurrence of an instantaneous voltage drop is detected until the grid voltage Vps stabilizes. As a result, for example, the first current target value Itg1 in normal mode is changed from 20 A to 2 A from the time when the instantaneous voltage drop is detected until the grid voltage stabilizes, and the power conversion device 1 operates at a current target value that is extremely small relative to the power grid 2. As a result, the power conversion device 1 can improve the stability of the power grid 2.

[0117] Note that the second embodiment and the third embodiment may be implemented simultaneously. The instantaneous voltage drop of the system voltage Vps varies, and even if the system voltage Vps becomes stable once, it may become unstable again. Even in such a case, by implementing the second embodiment and the third embodiment simultaneously, the power conversion device 1 can more reliably improve the stability of the power system 2.

[0118] 4. Embodiment 4 Hereinafter, a power conversion device according to a fourth embodiment, which is an example of the present disclosure, will be described with reference to the drawings. FIG. 10 is a functional block diagram showing functions of the control device 14 in the fourth embodiment. The difference between the first and fourth embodiments is that the control device 14 further includes a second determination unit 101. The same components as those in the first embodiment are denoted by the same reference numerals, and descriptions of the same components, functions, and operations will be omitted.

[0119] 4-1. Problems to be Solved by the Present Embodiment In the first embodiment, after an instantaneous voltage drop occurs, the control device 14 controls the bidirectional inverter 11 based on the second current target value Itg2. However, because instantaneous voltage drops occur in various ways, the system voltage Vps may drop to 0 V. In such a case, the system voltage Vps is 0 V, so it is possible to temporarily stop the bidirectional inverter 11. The fourth embodiment aims to enable the bidirectional inverter to be stopped when the system voltage Vps drops to 0 V.

[0120] [4-2. Functions of the control device] <Second Decision Section> In the fourth embodiment, the control device 14 further includes a second decision unit 101. When the detection unit 21 detects an instantaneous voltage drop, the second decision unit 101 determines whether the first inverter control unit 23 controls the bidirectional inverter 11 based on the second current target value Itg2 or executes a stop control process to stop the bidirectional inverter 11.

[0121] The stop control process, for example, cuts off signals applied to control electrodes of switching elements included in the bidirectional inverter 11, and controls the bidirectional inverter 11 to maintain the switching elements in the off state. This stops the bidirectional inverter. Note that the control of cutting off signals applied to control electrodes of switching elements and maintaining the switching elements in the off state is also called gate blocking.

[0122] [4-3. Control of power conversion equipment] 11 is a flowchart showing an example of control of the power conversion device in embodiment 4. The control flow in embodiment 4 differs from the control flow in embodiment 1 in that steps S111 and S112 are added.

[0123] <Step S111> After the detection unit 21 detects an instantaneous voltage drop (NO in step S02), the control device 14 proceeds to step S111. The control device 14 determines, via the second decision unit 101, whether the first inverter control unit 23 controls the bidirectional inverter 11 based on the second current target value Itg2 or whether to execute a stop control process to stop the bidirectional inverter 11 (step S111). If the system voltage Vps is, for example, 0 V, the second decision unit 101 decides to execute the stop control process (stop control in step S111), and the control device 14 proceeds to step S112. If the system voltage Vps is, for example, greater than 0 V, the second decision unit 101 decides that the first inverter control unit 23 controls the bidirectional inverter 11 based on the second current target value Itg2 (current target value control in step S111), and the control device 14 proceeds to step S05.

[0124] <Step S112> When the second decision unit 101 decides to execute the stop control process, the control device 14 proceeds to step S112 and executes the stop control process. In the stop control process, the first inverter control unit 23 of the control device 14 controls the bidirectional inverter 11 to, for example, cut off a signal applied to a control electrode of a switching element included in the bidirectional inverter 11 and maintain the switching element in an off state.

[0125] After executing the stop control process, for example, the control device 14 proceeds to step S07 and controls the bidirectional converter 12 so that the DC bus voltage Vbus converges to the target voltage Vtg. Alternatively, the control device 14 may control the bidirectional converter 12 to stop in the same manner as the bidirectional inverter 11. In this case, the control device 14 may proceed to step S01.

[0126] As a result, the control device 14 can stop the bidirectional inverter 11 when the system voltage Vps drops to 0V.

[0127] The embodiments disclosed herein are illustrative in all respects and are not restrictive. The scope of the present invention is not limited to the above-described embodiments, but includes all modifications within the scope of equivalents to the configurations described in the claims. [Explanation of symbols]

[0128] 1 Power conversion device 2 Power system 3 EV (distributed power source) 4. Load 11 Bidirectional inverter 11a 1st end 11b 2nd end 12 Bidirectional Converter 12a 1st end 12b 2nd end 13 Capacitor 14 Control device 14a processor 14b memory 14c input / output I / F 14d ADC 15 Relay 16 Voltage detection signal line 20 Memory section 21 Detection unit 22 First Decision Section 23 First inverter control unit 24 Second inverter control unit 25 First converter control section 26 First converter control section 81 Judgment section 101 Second Decision Section Itg1 First current target value Itg2 Second current target value Itg3 Third current target value PL1a, PL1b, PL1c, PL2, PL4, power line PL3 DC bus Pra Rated output of power converter RP receiving end Vps system voltage Vps1 1st system voltage Vps2 Second system voltage Vtg target voltage Vbus DC bus voltage Vn Nominal voltage< / adc>

Claims

1. A power conversion device for connecting a distributed power source to a power grid, a bidirectional converter having a first end connected to the distributed power source and a second end connected to a DC bus; a bidirectional inverter having a first end connected to a power grid and a second end connected to the DC bus; a control device that controls the bidirectional converter and the bidirectional inverter; Equipped with The control device A detection unit that detects an instantaneous voltage drop in a system voltage that is a voltage of the power system; a first determination unit that, when the detection unit detects the instantaneous voltage sag, determines a second current target value that is a target value of the output current of the bidirectional inverter after the instantaneous voltage sag, based on a ratio between a first system voltage before the instantaneous voltage sag and a second system voltage after the instantaneous voltage sag, and a first current target value that is a target value of the output current of the bidirectional inverter before the instantaneous voltage sag; a first inverter control unit that controls the bidirectional inverter while the instantaneous voltage sag occurs, based on the second current target value determined by the first determination unit, Power conversion device.

2. The first inverter control unit After the detection unit detects the instantaneous voltage drop, the bidirectional inverter is controlled based on a third current target value that is smaller than the first current target value, and after the control based on the third current target value, the bidirectional inverter is controlled based on the second current target value. The power conversion device according to claim 1 .

3. The control device Further comprising a determination unit that determines whether the system voltage is stable; The first inverter control unit After the detection unit detects the instantaneous voltage drop, the bidirectional inverter is controlled based on a third current target value that is smaller than the first current target value, and after the determination unit determines that the grid voltage is stable, the bidirectional inverter is controlled based on the second current target value. The power conversion device according to claim 1 .

4. The power conversion device according to claim 2 or 3, wherein the third current target value is smaller than the second current target value.

5. The control device a first converter control unit that controls the bidirectional converter so that a voltage of the DC bus converges to a target value when the detection unit detects the instantaneous voltage drop; The power conversion device according to claim 1 , further comprising:

6. The control device a second inverter control unit that controls the bidirectional inverter so that the voltage of the DC bus converges to a target value when the detection unit does not detect the instantaneous voltage drop; Further including: The power conversion device according to any one of claims 1 to 3.

7. The control device a second converter control unit that controls the bidirectional converter so that, when the detection unit does not detect the instantaneous voltage drop, the bidirectional converter outputs to the DC bus power corresponding to power consumption of a load connected to the first end of the bidirectional inverter; The power converter of claim 5 further comprising:

8. The control device a second determination unit that determines whether the first inverter control unit controls the bidirectional inverter based on the second current target value or executes a stop control process to stop the bidirectional inverter when the detection unit detects the instantaneous voltage drop; Further including: The power conversion device according to any one of claims 1 to 3.

9. A control method for a power conversion device for connecting a distributed power source to a power grid, comprising: The power conversion device is a bidirectional converter having a first end connected to the distributed power source and a second end connected to a DC bus; a bidirectional inverter having a first end connected to a power grid and a second end connected to the DC bus; a control device for controlling the bidirectional converter and the bidirectional inverter, The control method includes: detecting an instantaneous voltage drop in a system voltage, which is a voltage of the power system; determining, when the instantaneous voltage sag is detected, a second current target value which is a target value of the output current of the bidirectional inverter after the instantaneous voltage sag, based on a ratio between a first system voltage before the instantaneous voltage sag and a second system voltage after the instantaneous voltage sag, and a first current target value which is a target value of the output current of the bidirectional inverter before the instantaneous voltage sag; and controlling the bidirectional inverter during the voltage sag based on the determined second current target value. Method for controlling a power conversion device

10. A computer program for operating a control device that controls a power conversion device for connecting a distributed power source to a power grid, The power conversion device is a bidirectional converter having a first end connected to the distributed power source and a second end connected to a DC bus; a bidirectional inverter having a first end connected to a power grid and a second end connected to the DC bus; a control device for controlling the bidirectional converter and the bidirectional inverter, The computer program comprises: detecting an instantaneous voltage drop in a system voltage, which is a voltage of the power system; determining, when the instantaneous voltage sag is detected, a second current target value which is a target value of the output current of the bidirectional inverter after the instantaneous voltage sag, based on a ratio between a first system voltage before the instantaneous voltage sag and a second system voltage after the instantaneous voltage sag, and a first current target value which is a target value of the output current of the bidirectional inverter before the instantaneous voltage sag; and controlling the bidirectional inverter during the voltage sag based on the determined second current target value. Computer program.

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