Voltage control apparatus and voltage control method

Abstract

This voltage control apparatus is provided with: an inverter that converts DC power outputted from a battery into AC power and that supplies the AC power to a power system; and a control device that is connected to the inverter. The control device, when a voltage drop of the output voltage of the inverter is equal to or higher than a threshold due to fluctuation of the AC voltage of the power system, executes: a process for calculating a virtual impedance by using a variable including an active voltage command value and a reactive voltage command value calculated by a voltage command value, and an active current command value and a reactive current command value calculated by the output voltage of the inverter and the voltage command value; a process for calculating a virtual active voltage and a virtual reactive voltage by multiplying, by the virtual impedance, each of the active current and the reactive current calculated by the output current of the inverter; and a process for controlling the voltage so that the active current and the reactive current respectively approach the active current command value and the reactive current command value required for the power system, on the basis of the virtual active voltage and the virtual reactive voltage.

Description

Voltage control device and voltage control method

[0001] The present disclosure relates to a technique for controlling the output voltage of an inverter.

[0002] Patent Document 1 discloses a technique for stably continuing operation of a power converter (i.e., inverter) that executes control simulating a synchronous generator. In this conventional technique, when the AC voltage of the power grid fluctuates, voltage control is performed so that the active power input / output between the power grid and the power converter is reduced.

[0003] Japanese Patent No. 7183486

[0004] Consider a case where a voltage-controlled grid forming inverter (GFM) is used as an inverter that supplies AC voltage to a power grid. In this case, as disclosed in Patent Document 1, when the AC voltage of the power grid fluctuates, voltage control is performed so as to reduce the difference in active power input / output between the power grid and the power converter.

[0005] However, even if voltage control is performed to stabilize active power, the inverter output current may not reach the reference current required by the power grid, which may cause the power grid to become unstable. Therefore, in order to supply stable active power to the power grid, it is necessary to control the voltage so as to output the active current and reactive current required by the power grid.

[0006] One object of the present disclosure is to provide a technology that can control voltage so as to output active current and reactive current required by the power system when the AC voltage of the power system fluctuates.

[0007] A first aspect of the present disclosure relates to a voltage control device. The voltage control device includes an inverter that converts DC power output from a battery into AC power and supplies the AC power to a power grid. The voltage control device further includes a control device connected to the inverter. When a voltage drop in the inverter's output voltage due to fluctuations in the AC voltage of the power grid is equal to or greater than a threshold, the control device executes the following processes: calculating a virtual impedance using variables including an active voltage command value and a reactive voltage command value calculated from a voltage command value, and an active current command value and a reactive current command value calculated from the inverter's output voltage and the voltage command value; calculating a virtual active voltage and a virtual reactive voltage by multiplying each of the active current and reactive current calculated from the inverter's output current by the virtual impedance; and controlling the voltage based on the virtual active voltage and the virtual reactive voltage so that the active current and the reactive current approach the active current command value and the reactive current command value required by the power grid.

[0008] A second aspect of the present disclosure has the following feature in addition to the first aspect: the variables further include an active voltage and a reactive voltage calculated based on the output voltage. In the process of calculating the virtual impedance, the control device calculates the virtual impedance based on conditions that the reactive voltage is zero, the reactive voltage command value is zero, the voltage value obtained by subtracting the virtual active voltage and the active voltage from the active voltage command value is zero, and the voltage value obtained by subtracting the virtual reactive voltage and the reactive voltage from the reactive voltage command value is zero.

[0009] A third aspect of the present disclosure has the same features as the first aspect, but further includes the following: the virtual impedance includes a resistance and a reactance, and the control device calculates each of the resistance and the reactance according to the output voltage in the process of calculating the virtual impedance.

[0010] A fourth aspect of the present disclosure has the following characteristics in addition to the first aspect: the active current command value is calculated by multiplying a differential voltage, which is the difference between the voltage command value and the output voltage, by a constant value, and the reactive current command value is calculated so that the sum of the squares of the active current command value and the reactive current command value becomes 1.

[0011] A fifth aspect of the present disclosure has the same features as the fourth aspect, but further includes the following: the active current command value is expressed as a percentage, and when the calculation result of the active current command value exceeds 100%, the control device sets the active current command value to 100%.

[0012] A sixth aspect of the present disclosure relates to a voltage control method. The voltage control method includes: converting DC power output from a battery into AC power using an inverter, and supplying the AC power to a power grid; when a voltage drop in the inverter output voltage due to fluctuations in AC voltage in the power grid is equal to or greater than a threshold, calculating a virtual impedance using variables including an active voltage command value and a reactive voltage command value calculated from a voltage command value, and an active current command value and a reactive current command value calculated from the inverter output voltage and the voltage command value; calculating a virtual active voltage and a virtual reactive voltage by multiplying each of the active current and reactive current calculated from the inverter output current by the virtual impedance; and controlling the voltage based on the virtual active voltage and the virtual reactive voltage so that the active current and the reactive current approach the active current command value and the reactive current command value required by the power grid.

[0013] According to the present disclosure, when a voltage drop in the inverter output voltage due to fluctuations in the AC voltage of the power grid is equal to or greater than a threshold, a virtual impedance is calculated based on a voltage command value and the inverter output voltage. Then, a virtual active voltage and a virtual reactive voltage are generated based on the virtual impedance, and voltage control is performed based on the virtual active voltage and the virtual reactive voltage so that the active current and reactive current output from the inverter approach the active current command value and reactive current command value required by the power grid, respectively. This makes it possible to output the active current and reactive current required by the power grid even when the AC voltage of the power grid fluctuates. Therefore, it is possible to stabilize the power grid.

[0014] Fig. 1 is a diagram for explaining an overview of a power conversion system. Fig. 2 is a block diagram showing an example of the functions of a control device in a voltage control device according to an embodiment. Fig. 3 is an explanatory diagram showing a specific example of virtual impedance of a voltage control device according to an embodiment. Fig. 4 is a diagram for explaining an example of an output result of an inverter in a voltage control device according to an embodiment. Fig. 5 is a flowchart showing an example of processing by a control device in a voltage control device according to an embodiment.

[0015] A voltage control device and a voltage control method according to an embodiment of the present disclosure will be described with reference to the accompanying drawings. Elements common to the various drawings will be designated by the same reference numerals, and duplicated descriptions will be omitted.

[0016] 1. Overview of the Power Conversion System Fig. 1 is a diagram for explaining an overview of a power conversion system 1. The power conversion system 1 includes a voltage control device 10, a transformer 20, and a power grid 30. The voltage control device 10 includes a battery 11, an inverter 12, and a control device 100.

[0017] The inverter 12 is a device that converts DC power output from the battery 11 into AC power and supplies the AC power to the power grid 30 via a transformer 20. An example of the inverter 12 is a voltage-controlled GFM inverter.

[0018] The control device 100 is connected to the inverter 12 and controls the output power output from the inverter 12. Specifically, the control device 100 receives a detected value of the output voltage Vs (hereinafter referred to as the detected Vs value) and a detected value of the output current Io (hereinafter referred to as the detected Io value) output from the inverter 12. The detected Vs value and the detected Io value are detected by a detector (not shown) provided between the voltage control device 10 and the power grid 30. In the example shown in FIG. 1 , the control device 100 receives the detected Vs value and the detected Io value detected between the voltage control device 10 and the transformer 20. However, the detected Vs value and the detected Io value detected between the transformer 20 and the power grid 30 may also be input. In this way, the detected value of the AC voltage of the power grid 30 may be the detected Vs value. Furthermore, the detected value of the AC current of the power grid 30 may be the detected Io value.

[0019] The output voltage Vs output from the inverter 12 is made up of three-phase voltages (Vsu, Vsv, Vsw), and the output current Io output from the inverter 12 is made up of three-phase currents (Iou, Iov, Iow). That is, the above-mentioned Vs detection value includes the Vsu detection value, the Vsv detection value, and the Vsw detection value, and the above-mentioned Io detection value includes the Iou detection value, the Iov detection value, and the Iow detection value.

[0020] The control device 100 executes VSG (Virtual Synchronous Generator) control based on the input Vs and Io detected values. A VSG is a virtual synchronous generator that simulates the dynamic characteristics of a synchronous generator in the inverter 12. In other words, VSG control means controlling a virtual synchronous generator. The dynamic characteristics of a synchronous generator include an inertia constant M, a damping constant D, and the like. This makes it possible to prevent the power grid 30 from becoming unstable when a grid fault occurs, i.e., when a low voltage ride-through (LVRT) occurs.

[0021] Furthermore, based on the execution result of VSG control, control device 100 performs voltage control on inverter 12 during LVRT occurrence so that output current Io from inverter 12 becomes the current value required by power grid 30. The voltage control includes generating three-phase voltages (Vsu, Vsv, Vsw) and PWM control, which performs pulse width modulation on each of the three-phase voltages. Control device 100 then outputs the voltage control signal generated by PWM control to inverter 12. This allows inverter 12 to reduce the difference in AC power between power grid 30 and inverter 12 based on the voltage control signal.

[0022] 2. Specific Examples of Voltage Control Devices 2-1. Configuration Examples of Control Devices The control device 100 has hardware that realizes various functions. The hardware may be a processing circuit, or a computer that executes a program stored in a storage device using a CPU. Examples of processing circuits include FPGAs (Field Programmable Gate Arrays) and ASICs (Application Specific Integrated Circuits).

[0023] 2 is a block diagram showing an example of functions of the control device 100 in the voltage control device 10 according to the embodiment. The control device 100 includes an effective / reactive voltage calculation processing unit 101, an effective / reactive current calculation processing unit 102, a VSG control unit 103, a dq-axis voltage conversion unit 104, a dq-axis current command calculation unit 105, a virtual impedance generation unit 106, a virtual voltage generation unit 107, a first output selection circuit 108, a second output selection circuit 109, a switching determination circuit 110, an addition / subtraction circuit 111, and a voltage control circuit 112.

[0024] The effective and reactive voltage calculation processing unit 101 calculates an effective voltage Vd and a reactive voltage Vq based on the Vs detection value input to the control device 100. The effective voltage Vd and the reactive voltage Vq calculated by the effective and reactive voltage calculation processing unit 101 are expressed, for example, by the following equation (1).

[0025]

[0026] The active and reactive current calculation processing unit 102 calculates the active current Id and the reactive current Iq based on the Io detection value input to the control device 100. The active current Id and the reactive current Iq calculated by the active and reactive current calculation processing unit 102 are expressed, for example, by the following equation (2).

[0027]

[0028] The VSG control unit 103 generates a voltage command value Vref for the output voltage Vs of the inverter 12. The voltage command value Vref is a fixed value determined, for example, according to the power grid 30 connected to the inverter 12. The voltage command value Vref includes a d-axis voltage command value Vdref and a q-axis voltage command value Vqref.

[0029] The d-axis voltage command value Vdref is also referred to as an active voltage command value Vdref, and the q-axis voltage command value Vqref is also referred to as a reactive voltage command value Vqref. The dq-axis voltage converter 104 converts the voltage command value Vref generated by the VSG control unit 103 into the active voltage command value Vdref and the reactive voltage command value Vqref.

[0030] When LVRT occurs due to fluctuations in the AC voltage of the power grid 30, the dq-axis current command calculation unit 105 calculates a d-axis current command value Idref and a q-axis current command value Iqref required of the power grid 30 based on the voltage command value Vref and the Vs detection value. The d-axis current command value Idref is also referred to as an active current command value Idref, and the d-axis current command value Idref is also referred to as a reactive current command value Iqref. The active current command value Idref and the reactive current command value Iqref calculated by the dq-axis current command calculation unit 105 are expressed, for example, by the following equations (3) and (4), respectively. k is a constant value determined by the specifications of the inverter 12. ΔV is a differential voltage Vdiff obtained by subtracting the voltage command value Vref from the Vs detection value.

[0031]

[0032]

[0033] Here, when each variable used in Equation (3) and Equation (4) is expressed as a percentage, the active current command value Idref is calculated so as not to exceed 100%. Specifically, when the voltage drop of the Vs detection value decreases to 60%, ΔV is Vdref-Vs=100%-60%=40%. Furthermore, when k is a constant value of "2", Iqref is 2×40%=80%. In this case, Iqref does not exceed 100%, so it is 80%. The reactive current command value Iqref is calculated so that the sum of the square of the active current command value Idref and the square of the reactive current command value Iqref is 1. Therefore, when the active current command value Idref is 80%, the reactive current command value Iqref is 60% according to Equation (4).

[0034] Next, consider the case where the voltage drop of the Vs detection value drops to 10%. In this case, ΔV is Vdref-Vs=100%-10%=90%. Furthermore, if k is a constant value of "2", Iqref is 2×90%=180%. In this case, Iqref exceeds 100%, so it becomes the upper limit value of 100%. Note that when the active current command value Idref is 100%, the reactive current command value Iqref becomes 0% according to equation (4).

[0035] When the voltage drop of the Vs detection value is equal to or greater than a threshold, i.e., when a differential voltage Vdiff obtained by subtracting the Vs detection value from the voltage command value Vref is equal to or greater than a threshold, the virtual impedance generator 106 calculates a virtual impedance to increase the output voltage Vs of the inverter 12. The virtual impedance includes a resistance Rv and a reactance Xv. The resistance Rv and the reactance Xv are calculated based on an active voltage command value Vdref, a reactive voltage command value Vqref, an active current command value Idref, a reactive current command value Iqref, an active voltage Vd, and a reactive voltage Vq, respectively. The calculation of the virtual impedance (resistance Rv and reactance Xv) will be described in detail later.

[0036] The virtual voltage generator 107 generates a virtual effective voltage Vud and a virtual reactive voltage Vuq for increasing the output voltage Vs of the inverter 12 based on the virtual impedance (resistance Rv and reactance Xv), the active current Id, and the reactive current Iq. The virtual effective voltage Vud and the virtual reactive voltage Vuq calculated by the virtual voltage generator 107 are each expressed by, for example, the following equation (5):

[0037]

[0038] The first output selection circuit 108 is a changeover switch having two input terminals and one output terminal. The first output selection circuit 108 switches to select one of the two input terminals. Specifically, when the switching determination result input to the first output selection circuit 108 is “0,” the first output selection circuit 108 switches to select the first input terminal. On the other hand, when the switching determination result input to the first output selection circuit 108 is “1,” the first output selection circuit 108 switches to select the second input terminal. In the example shown in FIG. 2 , a fixed value (zero) is input to the first input terminal of the first output selection circuit 108, and a virtual effective voltage Vud is input to the second input terminal of the first output selection circuit 108.

[0039] The switching determination result is generated by the switching determination circuit 110. If the differential voltage Vdiff between the Vs detection value and the voltage command value Vref is less than the threshold, i.e., if the AC voltage of the power grid 30 is not in the LVRT state, the switching determination circuit 110 outputs "0" as the switching determination result. On the other hand, if the differential voltage Vdiff between the Vs detection value and the voltage command value Vref is equal to or greater than the threshold, i.e., if the AC voltage of the power grid 30 is in the LVRT state, the switching determination circuit 110 outputs "1" as the switching determination result. The first output selection circuit 108 outputs the voltage input to the selected input terminal. The voltage output from the first output selection circuit 108 is called the selected active voltage Vdsel.

[0040] The second output selection circuit 109 is a changeover switch having two input terminals and one output terminal. The second output selection circuit 109 switches to select one of the two input terminals. Specifically, when the switching determination result input to the second output selection circuit 109 is "0," the second output selection circuit 109 switches to select the first input terminal. On the other hand, when the switching determination result input to the second output selection circuit 109 is "1," the second output selection circuit 109 switches to select the second input terminal. In the example shown in FIG. 2 , a fixed value (zero) is input to the first input terminal of the second output selection circuit 109, and a virtual reactive voltage Vuq is input to the second input terminal of the second output selection circuit 109.

[0041] As described above, the switching determination result is generated by the switching determination circuit 110. The second output selection circuit 109 outputs the voltage input to the selected input terminal. The voltage output from the second output selection circuit 109 is called a selected reactive voltage Vqsel.

[0042] The adding / subtracting circuit 111 includes four adding / subtracting circuits (a first adding / subtracting circuit 111a, a second adding / subtracting circuit 111b, a third adding / subtracting circuit 111c, and a fourth adding / subtracting circuit 111d). The first adding / subtracting circuit 111a is an adding / subtracting circuit that adds an active voltage command value Vdref and subtracts a selected active voltage Vdsel. The second adding / subtracting circuit 111b is an adding / subtracting circuit that adds a reactive voltage command value Vqref and subtracts a selected reactive voltage Vqsel. The third adding / subtracting circuit 111c is an adding / subtracting circuit that adds the output of the first adding / subtracting circuit 111a and subtracts an active voltage Vd. The fourth adding / subtracting circuit 111d is an adding / subtracting circuit that adds the output of the second adding / subtracting circuit 111b and subtracts a reactive voltage Vq.

[0043] If the effective voltage output from the third adding / subtracting circuit 111c is the abnormal effective voltage Errd and the reactive voltage output from the fourth adding / subtracting circuit 111d is the abnormal reactive voltage Errq, the following relational expression (6) holds true.

[0044]

[0045] Furthermore, when equation (5) is substituted into equation (6), the following equation (7) is obtained.

[0046]

[0047] In this case, if the abnormal active voltage Errd and the abnormal reactive voltage Errq are both zero, it means that the virtual active voltage Vud and the virtual reactive voltage Vuq are both appropriate voltage values. In other words, by setting the abnormal active voltage Errd and the abnormal reactive voltage Errq to zero in advance, it is possible to calculate the optimal values ​​of the virtual active voltage Vud and the virtual reactive voltage Vuq.

[0048] Furthermore, it is desirable that the power factor, which is the percentage of the active voltage Vd relative to the output voltage Vs of the inverter 12, is high. For this reason, the reactive voltage Vq and the reactive voltage command value Vqref are both set to zero. Therefore, when Errd = 0, Errq = 0, the reactive voltage Vq = 0, and the reactive voltage command value Vqref = 0 in equation (7), the resistance Rv and reactance Xv of the virtual impedance are each expressed, for example, by the following equation (8).

[0049]

[0050] As a result, since the calculation formula for the virtual impedance includes the active current command value Idref and the reactive current command value Iqref, the voltage control device 10 can output appropriate active current Id and reactive current Iq to the power system 30.

[0051] The voltage control circuit 112 is a circuit that controls voltage based on a virtual active voltage Vud and a virtual reactive voltage Vuq calculated based on the virtual impedance so that the active current Id and the reactive current Iq approach an active current command value Idref and a reactive current command value Iqref, respectively, required by the power grid 30. Then, the voltage control circuit 112 outputs a voltage control signal Vsc generated by the voltage control.

[0052] As described above, the voltage control includes the process of generating three-phase voltages (Vsu, Vsv, Vsw) and the PWM control of performing pulse width modulation on each of the three-phase voltages. In the PWM control, the generated three-phase voltages are controlled to have a predetermined pulse width and a predetermined frequency. In addition, for example, PI control is used to generate the three-phase voltages by the voltage control.

[0053] 2-3. Example of Calculation Results of Virtual Impedance Figure 3 is an explanatory diagram showing a specific example of the virtual impedance of the voltage control device 10 according to the embodiment. Specifically, Figure 3 shows the waveforms of the resistance Rv and reactance Xv in the virtual impedance relative to the Vs detection value. The horizontal axis of the graph shown in Figure 3 represents the Vs detection value, and the vertical axis represents the values ​​of the resistance Rv and reactance Xv. In the example shown in Figure 3, the value of each variable is expressed in PU (Per Unit), but it may also be expressed as a percentage.

[0054] In the example shown in FIG. 3 , the waveforms of the resistance Rv and reactance Xv are those obtained when the active voltage command value Vdref is set to "1" and the constant k is set to "2" in the above-mentioned equation (7). For example, consider the resistance Rv and reactance Xv when the Vs detection value drops to 0.4 (40%). In this case, the active current command value Idref required for the power grid 30 is 0% based on the above-mentioned equation (3), and the reactive current command value Iqref required for the power grid 30 is 100% based on the above-mentioned equation (4). To achieve this, as shown in FIG. 3 , the resistance Rv is 0.06 and the reactance Xv is 0.6.

[0055] 2-4. Example of Inverter Output Result Figure 4 is a diagram illustrating an example of the output result of the inverter 12 in the voltage control device 10 according to the embodiment. Figure 4(A) shows an example of the waveform of the active voltage Vd and the waveform of the reactive voltage Vq when the Vs detection value drops to 65%. Figure 4(B) shows an example of the waveform of the active current Id and the reactive current Iq output from the inverter 12 to the power grid 30 based on the voltage control signal Vsc generated by the voltage control circuit 112 when the Vs detection value drops to 65%. In the example shown in Figure 4(B), when the Vs detection value drops to 65%, the active current Id is output at approximately 70% and the reactive current Iq is output at approximately -70%.

[0056] 4(C) shows an example of the waveform of the active voltage Vd and the waveform of the reactive voltage Vq when the Vs detection value drops to 40%. FIG. 4(D) shows an example of the waveform of the active current Id and the waveform of the reactive current Iq output from the inverter 12 to the power grid 30 based on the voltage control signal Vsc generated by the voltage control circuit 112 when the Vs detection value drops to 40%. In the example shown in FIG. 4(D), when the Vs detection value drops to 40%, the active current Id is output at approximately 0% and the reactive current Iq is output at approximately -100%.

[0057] In this way, when the AC voltage of the power grid 30 fluctuates, the active current Id and reactive current Iq required by the power grid 30 are output based on the virtual impedance so as to reduce the difference in AC power between the power grid 30 and the inverter 12.

[0058] 3. Processing Example Fig. 5 is a flowchart showing a processing example of the control device 100 in the voltage control device 10 according to the embodiment.

[0059] In step S100, the control device 100 determines whether the voltage drop of the output voltage of the inverter 12 is equal to or greater than a threshold value. If the voltage drop of the output voltage is equal to or greater than the threshold value (step S100; Yes), the process proceeds to step S110. Otherwise (step S100; No), the control device 100 ends the process.

[0060] In step S110, the control device 100 calculates a virtual impedance based on the voltage command value Vref and the output voltage Vs of the inverter 12. After that, the process proceeds to step S120.

[0061] The virtual impedance is calculated based on predetermined conditions, which mean the conditions that the reactive voltage Vq is zero, the reactive voltage command value Vqref is zero, the voltage value obtained by subtracting the virtual effective voltage Vud and the effective voltage Vd from the active voltage command value Vdref is zero, and the voltage value obtained by subtracting the virtual reactive voltage Vuq and the reactive voltage Vq from the reactive voltage command value Vqref is zero.

[0062] In step S120, the control device 100 calculates a virtual effective voltage Vud and a virtual reactive voltage Vuq by multiplying the effective current Id and the reactive current Iq, which are calculated based on the output current Io of the inverter 12, by a virtual impedance. Then, the process proceeds to step S130.

[0063] In step S130, the control device 100 performs voltage control based on the virtual effective voltage Vud and the virtual reactive voltage Vuq so that the active current Id and the reactive current Iq output from the inverter 12 approach the active current command value Idref and the reactive current command value Iqref, respectively, required by the power grid 30.

[0064] 4. Effects According to the voltage control device 10 according to the embodiment, when a voltage drop in the output voltage Vs of the inverter 12 due to fluctuations in the AC voltage of the power grid 30 is equal to or greater than a threshold, a virtual impedance is calculated based on the voltage command value Vref and the output voltage Vs of the inverter 12. Then, the voltage control device 10 generates a virtual active voltage Vud and a virtual reactive voltage Vuq based on the virtual impedance. Furthermore, the voltage control device 10 performs voltage control based on the virtual active voltage Vud and the virtual reactive voltage Vuq so that the active current Id and the reactive current Iq output from the inverter 12 approach the active current command value Idref and the reactive current command value Iqref, respectively, required for the power grid 30. As a result, even when the AC voltage of the power grid 30 fluctuates, the active current Id and the reactive current Iq required for the power grid 30 can be output. Therefore, the power grid 30 can be stabilized.

[0065] 1...power conversion system, 10...voltage control device, 11...battery, 12...inverter, 20...transformer, 30...power system, 100...control device, 101...effective and reactive voltage calculation processing unit, 102...effective and reactive current calculation processing unit, 103...VSG control unit, 104...dq axis voltage conversion unit, 105...dq axis current command calculation unit, 106...virtual impedance generation unit, 107...virtual voltage generation unit, 108...first output selection circuit, 109...second output selection circuit, 110...switching determination circuit, 111...addition and subtraction circuit, 111a...first addition and subtraction circuit, 111b...second addition and subtraction circuit, 111c...third addition and subtraction circuit, 111d...fourth addition and subtraction circuit, 112...voltage control circuit

Claims

1. A voltage control device comprising: an inverter that converts DC power output from a battery into AC power and supplies the AC power to a power grid; and a control device connected to the inverter and configured to execute the following processes: when a voltage drop in the output voltage of the inverter due to fluctuations in the AC voltage of the power grid is equal to or greater than a threshold value, a process of calculating a virtual impedance using variables including an active voltage command value and a reactive voltage command value calculated from a voltage command value, and an active current command value and a reactive current command value calculated from the output voltage and the voltage command value; a process of calculating a virtual effective voltage and a virtual reactive voltage by multiplying each of an active current and a reactive current calculated from the output current of the inverter by the virtual impedance; and a process of performing voltage control based on the virtual effective voltage and the virtual reactive voltage so that each of the active current and the reactive current approaches the active current command value and the reactive current command value required by the power grid.

2. A voltage control device as claimed in claim 1, wherein the variables further include an active voltage and a reactive voltage calculated from the output voltage, and the control device is configured to calculate the virtual impedance in a process of calculating the virtual impedance based on the following conditions: the reactive voltage is zero, the reactive voltage command value is zero, a voltage value obtained by subtracting the virtual effective voltage and the effective voltage from the effective voltage command value is zero, and a voltage value obtained by subtracting the virtual reactive voltage and the reactive voltage from the reactive voltage command value is zero.

3. A voltage control device as claimed in claim 1, characterized in that the virtual impedance includes a resistance and a reactance, and the control device is configured to calculate each of the resistance and the reactance according to the output voltage in a process of calculating the virtual impedance.

4. A voltage control device as claimed in claim 1, characterized in that the active current command value is calculated by multiplying a differential voltage, which is the difference between the voltage command value and the output voltage, by a constant value, and the reactive current command value is calculated so that the sum of the square of the active current command value and the square of the reactive current command value is 1.

5. A voltage control device according to claim 4, wherein the active current command value is expressed as a percentage, and when the calculated active current command value exceeds 100%, the control device sets the active current command value to 100%.

6. A voltage control method comprising: using an inverter to convert DC power output from a battery into AC power, and supplying the AC power to a power grid; when a voltage drop in the output voltage of the inverter due to fluctuations in the AC voltage of the power grid is equal to or greater than a threshold value, calculating a virtual impedance using variables including an active voltage command value and a reactive voltage command value calculated from a voltage command value, and an active current command value and a reactive current command value calculated from the output voltage and the voltage command value; calculating a virtual effective voltage and a virtual reactive voltage by multiplying each of an active current and a reactive current calculated from the output current of the inverter by the virtual impedance; and performing voltage control based on the virtual effective voltage and the virtual reactive voltage so that each of the active current and the reactive current approaches the active current command value and the reactive current command value required for the power grid.