JP7765700B2 - Control device and control method - Google Patents

Description

The present invention relates to a control device and control method for suppressing harmonic currents.

Patent Document 1 discloses a converter device that can reduce harmonic components in power supply current, etc., even when there is distortion in the power supply voltage, etc. The converter device converts AC power from an AC power supply into DC power using a PWM (Pulse Width Modulation) converter main circuit and supplies it to a line connecting the PWM converter main circuit to a load. The converter device includes a PWM signal generating unit that generates a PWM signal to control the PWM converter main circuit. The PWM signal generating unit generates a PWM signal to compensate for harmonic components contained in the AC power from the AC power supply.

Patent No. 3323759

One aspect of the present invention aims to realize a control device or the like that can stably reduce harmonics in a system that converts DC power to AC power and supplies it to an AC line.

To solve the above problem, one aspect of the present invention provides a control device for a power supply system in which DC power supplied from a DC power source is converted into AC power by a power conversion device and supplied to an AC line. The control device controls the power conversion device by referring to the line voltage and line current of the AC line. The control device includes: a first control signal calculation unit that calculates a first control signal indicating a target harmonic voltage for feedback control of the power conversion device to suppress harmonic voltages contained in the line voltage; a harmonic current extraction unit that extracts harmonic currents contained in the line current; a virtual voltage drop calculation unit that calculates a virtual voltage drop at a virtual impedance by multiplying the harmonic current by a virtual impedance; and a second control signal calculation unit that calculates a second control signal obtained by correcting the first control signal by subtracting the virtual voltage drop from the first control signal. The control device feedback controls the power conversion device to suppress the harmonic voltages based on the second control signal.

Furthermore, one aspect of the present invention provides a control method for a power supply system in which DC power supplied from a DC power source is converted into AC power by a power conversion device and supplied to an AC line. The control method controls the power conversion device by referring to the line voltage and line current of the AC line, and includes the following steps: a first control signal calculation step of calculating a first control signal indicating a target harmonic voltage for feedback-controlling the power conversion device to suppress harmonic voltages contained in the line voltage; a harmonic current extraction step of extracting harmonic currents contained in the line current; a virtual voltage drop calculation step of calculating a virtual voltage drop at the virtual impedance by multiplying the harmonic current by a virtual impedance; and a second control signal calculation step of calculating a second control signal obtained by correcting the first control signal by subtracting the virtual voltage drop from the first control signal. The power conversion device is feedback-controlled to suppress the harmonic voltages based on the second control signal.

The control device according to each aspect of the present invention may be realized by a computer. In this case, the control program for the control device, which causes the computer to operate as each unit (software element) of the control device, thereby realizing the control device on the computer, and the computer-readable recording medium on which the control program is recorded, also fall within the scope of the present invention.

One aspect of the present invention makes it possible to reduce harmonics in a system that converts DC power into AC power and supplies it to an AC line.

1 is a diagram showing the configuration of a power supply system including a control device according to the present invention. 1 is a diagram showing an image of control of a power conversion device by a control device according to the present invention; 1 is a block diagram showing an example of the configuration of a control device according to the present invention; FIG. 2 is a block diagram showing an example of the configuration of a virtual voltage drop calculation unit. 10 is a flowchart illustrating an example of a process performed by a control unit to calculate a control signal. 6 is a graph showing an example of the effect of control by the control device according to the present invention. FIG. 7 is a partially enlarged view of the graph shown in FIG. 6. FIG. 7 is a partially enlarged view of the graph shown in FIG. 6.

[Embodiment 1]
Hereinafter, one embodiment of the present invention will be described in detail.

(Configuration of power supply system 1)
FIG. 1 is a diagram showing the configuration of a power supply system 1 including a control device 40 according to the present invention. As shown in FIG. 1, the power supply system 1 includes a renewable energy source 10, a power conversion device 20, and a control device 40. The power supply system 1 is connected to an AC load 100 via an AC line 30. The power supply system 1 converts DC power supplied from the renewable energy source 10 into AC power using the power conversion device 20 and supplies the AC power to the AC line 30. In other words, the power supply system 1 is a system that supplies AC power to the AC load 100 via the AC line 30. A specific example of the power supply system 1 is a backup system that operates autonomously and supplies power to the AC load 100 when the AC load 100 is no longer able to receive power from a commercial power source.

The renewable energy source 10 is a DC power supply that supplies DC power to the power conversion device 20 as needed. The power supply system 1 may include another DC power supply instead of the renewable energy source 10.

The power conversion device 20 converts the DC power supplied from the renewable energy source 10 into AC power and outputs it to the AC line 30. The AC power output by the power conversion device 20 is controlled by the control device 40, which will be described later.

The AC line 30 is a line for supplying AC power from the power supply system 1 to the AC load 100. The power supply system 1 may be equipped with an interconnection reactor 31 for reducing harmonics in the line current and line voltage on the AC line 30.

The control device 40 controls the power conversion device 20 by referring to the line voltage and line current on the AC line 30. The specific configuration of the control device 40 will be described later.

The power supply system 1 further includes a voltage detector 51 and a current detector 52. The voltage detector 51 detects three-phase line voltages _Vu , _Vv , and _Vw in the AC line 30. The current detector 52 detects three-phase line currents _Iu , _Iv , and _Iw in the AC line 30. A signal indicating the line voltage detected by the voltage detector 51 and a signal indicating the line current detected by the current detector 52 are output to the control device 40.

2 is a diagram illustrating an image of control of the power conversion device 20 by the control device 40. The control device 40 controls the output voltage from the power conversion device 20 as if the power conversion device 20 were equipped with a power conversion unit 21 and a virtual impedance unit 22. Here, the power conversion unit 21 converts DC power from the renewable energy source 10 into AC power having a voltage corresponding to the line voltage of the AC line 30. The virtual impedance unit 22 has a virtual impedance value R+jω _n L (R is the resistance value, j is the imaginary unit, ω _n is the angular frequency of the nth harmonic, and L is the reactance) for the nth harmonic. In other words, the control device 40 controls the output voltage value of the power conversion device 20 to a value lower than the value corresponding to the line voltage of the AC line 30 by the amount of the virtual impedance of the virtual impedance unit 22.

The virtual impedance value may be set as appropriate by the designer or user of the power supply system 1. Regarding the magnitude of the resistance value R and reactance ωL of the virtual impedance value, it is preferable that the absolute value of the virtual impedance value R + jωL satisfy %Z ≥ 10% of the system capacity of the power supply system 1. Here, %Z indicates percent impedance. For example, if the system capacity of the power supply system 1 is 100 kW, the rated voltage is 200 V, and the rated frequency is 50 Hz, the real part of the virtual impedance value R should be set to ≥ 0.04 Ω. The value of the reactance L of the virtual impedance value is set as appropriate when it is necessary to shift the resonance point of the impedance including the AC load 100. If it is not necessary to shift the resonance point of the impedance, reactance L may be set to 0. In the following description, the amount of drop in output voltage from the power conversion device 20 due to the virtual impedance is referred to as the virtual voltage drop.

(Configuration of control device 40)
Fig. 3 is a block diagram showing an example of the configuration of the control device 40. As shown in Fig. 3, the control device 40 includes a harmonic voltage extraction unit 41, subtraction units 42a and 42b, a first control signal calculation unit 43, a harmonic current extraction unit 44, a virtual voltage drop calculation unit 45, second control signal calculation units 46a and 46b, and a coordinate inverse conversion unit 47. The control device 40 also includes a block (not shown) for controlling the fundamental wave output from the power conversion device 20.

The harmonic voltage extraction unit 41 extracts an nth-order harmonic voltage (n is any integer) contained in the line voltage of the AC line 30. The harmonic voltage extraction unit 41 includes a coordinate conversion unit 41a and a low-pass filter 41b. The coordinate conversion unit 41a converts the nth-order harmonic voltage contained in the three-phase line voltages _Vu , _Vv , and _Vw indicated by the signal output from the voltage detector 51 into a complex voltage _Vd0 + _jVq0 using the angular frequency _ωn of the nth-order harmonic. The low-pass filter 41b blocks the high-frequency components of the complex voltage _Vd0 + _jVq0 and transmits the complex voltage _Vd + _jVq , which is composed only of low-frequency components.

Subtraction units 42a and 42b calculate a control input V _{ref — d} − _{V d} + j (V _{ref — q} − V _q ) to be input to first control signal calculation unit 43. Specifically, subtraction unit 42a subtracts the real part V _d of the complex voltage from real command value V _{ref — d} . Furthermore, subtraction unit 42b subtracts the imaginary part V _q of the complex voltage from imaginary command value V _{ref — q} . The real command value V _{ref — d} and the imaginary command value V _{ref — q} are target values for controlling harmonic voltages, and are set by the designer of control device 40 or the administrator of power supply system 1, for example. The values of real command value V _{ref — d} and imaginary command value V _{ref — q} are normally 0, aiming for a state in which there are no high-frequency voltage components.

The first control signal calculation unit 43 calculates a first control signal V _{d — out1} + jV _{q — out1} based on a control input V _ref — d − _{V d} + j(V _{ref — q} − V _q ). The first control signal V d — out1 + jV q _{— out1} is a signal indicating a target harmonic voltage for feedback control of the power conversion device 20 so as to suppress harmonic voltages contained in the line voltage of the AC line 30. In the image shown in FIG. 2 , the first control signal V _{d — out1} + jV _{q — out1} is a control signal for controlling the output voltage of the power conversion unit 21. The first control signal calculation unit 43 calculates a control signal using, for example, PI (Proportional-Integral) control or PID (Proportional-Integral-Differential) control.

The harmonic current extraction unit 44 extracts n-th order harmonic currents contained in the line currents in the AC line 30 (n is the same value as in the harmonic voltage extraction unit 41). The harmonic current extraction unit 44 includes a coordinate conversion unit 44a and a low-pass filter 44b. The coordinate conversion unit 44a converts the n-th order harmonic currents contained in the three-phase line currents _Iu , _Iv , and _Iw, which are indicated by the signals output from the current detector 52, into the form of a complex current _Id0 + _jIq0 using the angular frequency _ωn of the n-th order harmonic. The low-pass filter 44b blocks the high-frequency components of the complex current _Id0 + _jIq0 and passes the complex current _Id + _jIq , which is composed of only low-frequency components.

The virtual voltage drop calculation unit 45 multiplies the complex current I _d +jI _q by the virtual impedance to calculate the virtual voltage drop at the virtual impedance. The calculation formula used by the virtual voltage drop calculation unit 45 is as shown in the following formula (1).
The specific configuration of the virtual voltage drop calculation unit 45 will be described later.

The second control signal calculators 46a and 46b calculate a second control signal _{Vd_out2} + _{jVq_out2 obtained by correcting the first control signal by subtracting the virtual voltage drop Vd_z+jVq_z from the first control signal Vd_out1+jVq_out1 . Specifically , the} second control signal calculator 46a calculates a real part Vd_out2 of the second control signal by subtracting the real part _{Vd_z} of the virtual voltage drop from the real part _{Vd_out1} of the first control signal output by the first control signal calculator 43. The second control signal calculator 46b calculates an imaginary part _{Vq_out2} of the second control signal by subtracting the imaginary part _{Vq_z} of the virtual voltage drop from the imaginary part _{Vq_out1} of the first control signal output by the first control signal calculator ₄₃ .

The coordinate inverse transformation unit 47 converts the second control signal _{Vd_out2} + _{jVq_out2} , which is in the form of a complex number, into the form of three-phase control signals _{Vu_out} , _{Vv_out} , and _{Vw_out} . The control device 40 controls the power conversion device 20 using the three-phase control signals _{Vu_out} , _{Vv_out} , and _{Vw_out} . In other words, it can be said that the control device 40 feedback-controls the power conversion device 20 based on the second control signal so as to suppress harmonic voltages.

(Configuration of virtual voltage drop calculation unit 45)
Fig. 4 is a block diagram showing an example of the configuration of the virtual voltage drop calculation unit 45. As shown in Fig. 4, the virtual voltage drop calculation unit 45 includes a first multiplication unit 45a, a second multiplication unit 45b, a third multiplication unit 45c, a fourth multiplication unit 45d, a real part subtraction unit 45e, and an imaginary part addition unit 45f.

The virtual voltage drop calculation unit 45 performs the following calculations for the complex current _Id + _jIq and the virtual impedance R+ _jωnL . The first multiplication unit 45a multiplies the real part _Id of the complex current by the real part R of the virtual impedance. The second multiplication unit 45b multiplies the real part _Id of the complex current by the imaginary part _ωnL of the virtual impedance. The third multiplication unit 45c multiplies the imaginary part _Iq of the complex current by the real part R of the virtual impedance. The fourth multiplication unit 45d multiplies the imaginary part _Iq of the complex current by the imaginary part _ωnL of the virtual impedance.

The real part subtraction unit 45e subtracts the calculation result of the third multiplication unit 45c from the calculation result of the first multiplication unit 45a. Because the calculation result of the third multiplication unit 45c is the product of the imaginary parts, this calculation result is subtracted from the calculation result of the first multiplication unit 45a. The imaginary part addition unit 45f adds the calculation result of the fourth multiplication unit 45d to the calculation result of the second multiplication unit 45b. Through these calculations, the virtual voltage drop calculation unit 45 calculates the virtual voltage drop as shown in equation (1) above.

It is preferable that the control device 40 calculates the second control signal for each order of the harmonic voltage to be reduced. In this case, the value of the virtual impedance used to calculate the second control signal may differ for each harmonic voltage. For example, the power supply system 1 may calculate a second control signal for reducing a fifth-order harmonic voltage and a second control signal for reducing a seventh-order harmonic current. The power supply system 1 may also calculate a different second control signal for reducing a harmonic current of an order other than either the fifth or seventh order. In that case, the power supply system 1 does not need to calculate a second control signal for reducing either the fifth or seventh order harmonic current.

(Processing in the control device 40)
FIG. 5 is a flowchart showing an example of a process by which the control device 40 calculates a control signal. The process shown in FIG. 5 is part of a control method for the power conversion device 20 by the control device 40. As shown in FIG. 5, first, the coordinate conversion unit 41a converts the coordinates of the three-phase line voltages _Vu , _Vv , and _Vw indicated by the signal output from the voltage detector 51 into the form of a complex voltage _Vd + _jVq (S1). Next, the low-pass filter 41b blocks high-frequency components of the complex voltage _Vd + _jVq (S2). The subtraction units 42a and 42b subtract the complex voltage _Vd + _{jVq that} passed through the low-pass filter 41b from the command value _{Vref_d} + _{jVref_q} to calculate a control input _{Vref_d} - _Vd +j( _{Vref_q} - _Vq ) (S3). The first control signal calculation unit 43 calculates a first control signal V _{d — out1} +jV q _{— out1} based on the control input V _{ref — d} −V _d +j(V _{ref —} q −V _q ) (S 4 , first control signal calculation step).

In parallel with steps S1 to S4, the coordinate conversion unit 44a converts the three-phase line currents _Iu , _Iv , and _Iw indicated by the signal output from the current detector 52 into the form of a complex current _Id0 + _jIq0 (S5, harmonic current extraction step). Next, the low-pass filter 44b blocks the high-frequency components of the complex current _Id0 + _jIq0 (S6, harmonic current extraction step). The virtual voltage drop calculation unit 45 calculates a virtual voltage drop by multiplying the complex current _Id + _jIq by a virtual impedance (S7, virtual voltage drop calculation step). Note that the control device 40 may execute steps S5 to S7 after steps S1 to S4. Alternatively, the control device 40 may execute steps S1 to S4 after executing steps S5 to S7.

After steps S1 to S7 are completed, the second control signal calculation units 46a and 46b calculate the second control signal _{Vd_out2} + _{jVq_out2} (S8, second control signal calculation step). The coordinate inverse transformation unit 47 inversely transforms the second control signal _{Vd_out2} + _{jVq_out2} into three-phase control signals _{Vu_out} , _{Vv_out} , and _{Vw_out} (S9).

The control device 40 controls the power conversion device 20 using the three-phase control signals _{Vu_out} , _{Vv_out} , and _{Vw_out} obtained by the above processing. That is, it can be said that the control device 40 feedback-controls the power conversion device 20 using the second control signal _{Vd_out2} + _{jVq_out2} .

(Effect of control by the control device 40)
Fig. 6 is a graph showing an example of the effect of control by the control device 40. In the graph shown in Fig. 6, the horizontal axis represents time, and the vertical axis represents the voltage applied to the AC load 100 due to the fifth harmonic. In Fig. 6, period T0 is a period during which control to reduce the harmonic voltage is not executed. Period T1 is a period during which control to reduce the harmonic voltage is executed.

Within period T1, period T11 is a period during which the control device 40 is not performing control using virtual impedance. In other words, period T11 is a period during which the control device 40 controls the power conversion device 20 using a first control signal. Period T12 is a period during which the control device 40 performs control using virtual impedance. In other words, period T12 is a period during which the control device 40 controls the power conversion device 20 using a second control signal.

Figures 7 and 8 are enlarged views of portions of the graph shown in Figure 6. Specifically, Figure 7 is an enlarged view of the vicinity of the boundary between periods T0 and T11 in the graph shown in Figure 6. Also, Figure 8 is an enlarged view of the vicinity of the boundary between periods T11 and T12 in the graph shown in Figure 6.

In the examples shown in Figures 6 to 8, the impedance value of AC load 100 causes the AC system to have a resonance point in the fifth harmonic frequency band. Therefore, the fifth harmonic diverges during period T11, when control device 40 is not performing control using the virtual impedance. In other words, the signal component of the control signal for controlling the fifth harmonic becomes excessive, resulting in unstable control in the fifth harmonic frequency band.

On the other hand, by having the control device 40 perform control using a virtual impedance with a resistive component, it is possible to suppress excess signal components of the control signal in the harmonic frequency band. As a result, the fifth harmonic converges during period T12 when the control device 40 is performing control using a virtual impedance.

As described above, according to the control device 40, the output voltage from the power conversion device 20 drops by the virtual voltage drop due to the virtual impedance compared to the output voltage based on the first control signal. Therefore, even if the resistance value of the AC load 100 is small, the real part R of the virtual impedance can damp harmonics. Furthermore, when the reactance L of the virtual impedance is not zero, the imaginary part ω _n L of the virtual impedance derived from the reactance L can shift the resonance point of the impedance including the AC load 100. Therefore, the harmonics of the current supplied to the AC line 30 can be reduced without divergence.

In addition, because the control device 40 performs control using virtual impedance, there is no need to insert impedance into the actual circuit. As a result, control to reduce harmonics does not affect control of the fundamental wave.

Furthermore, as described above, the control device 40 may calculate the second control signal for each order of the harmonic voltage to be reduced. Calculating the second control signal using a different value of virtual impedance for each order of the harmonic voltage allows for further reduction of the harmonic voltage. Furthermore, there may be cases where the control conditions for reducing the harmonic voltage change, such as when the AC load 100 connected to the power supply system 1 is changed. Even in such cases, an appropriate second control signal can be calculated by simply adjusting the value of the virtual impedance for each order of the harmonic voltage, allowing the power supply system 1 to easily adapt to the changed conditions.

〔summary〕
A control device according to one aspect of the present invention is a control device for a power supply system in which DC power supplied from a DC power source is converted into AC power by a power conversion device and supplied to an AC line, the control device controlling the power conversion device by referring to the line voltage and line current of the AC line, and includes: a first control signal calculation unit that calculates a first control signal indicating a target harmonic voltage, for feedback controlling the power conversion device so as to suppress harmonic voltages contained in the line voltage; a harmonic current extraction unit that extracts harmonic currents contained in the line current; a virtual voltage drop calculation unit that calculates a virtual voltage drop at a virtual impedance by multiplying the harmonic current by a virtual impedance; and a second control signal calculation unit that calculates a second control signal obtained by correcting the first control signal by subtracting the virtual voltage drop from the first control signal, and feedback controlling the power conversion device so as to suppress the harmonic voltages based on the second control signal.

In accordance with the above configuration, in a power supply system in which a power conversion device converts DC power into AC power and supplies it to an AC line, the control device controls the power conversion device by referencing the line voltage and line current of the AC line. The control device includes a first control signal calculation unit, a virtual voltage drop calculation unit, and a second control signal calculation unit. The first control signal calculation unit calculates a first control signal indicating a target harmonic voltage for feedback control of the power conversion device to suppress harmonic voltages contained in the line voltage. The virtual voltage drop calculation unit calculates a virtual voltage drop at the virtual impedance by multiplying the harmonic current contained in the line current by a virtual impedance. The second control signal calculation unit calculates a second control signal by subtracting the virtual voltage drop from the first control signal. The power conversion device outputs a voltage that is lower than the target harmonic voltage by the virtual voltage drop.

This allows the control device to apply damping to harmonics. The control device can also shift the impedance resonance point. Therefore, the control device can reduce harmonics in the current supplied to the AC line.

Furthermore, in a control device according to one aspect of the present invention, it is preferable that the absolute value of the virtual impedance satisfies %Z≧10% with respect to the system capacity of the power supply system.

With the above configuration, harmonics can be appropriately damped, reducing them without dissipating them.

Furthermore, it is preferable that the control device according to one aspect of the present invention calculates the second control signal for each order of the harmonic voltage to be reduced.

With the above configuration, the control device can calculate an appropriate second control signal for each order of the harmonic voltage to be reduced.

Furthermore, one aspect of the present invention provides a control method for a power supply system in which DC power supplied from a DC power source is converted into AC power by a power conversion device and supplied to an AC line. The control method controls the power conversion device by referring to the line voltage and line current of the AC line, and includes the following steps: a first control signal calculation step of calculating a first control signal indicating a target harmonic voltage for feedback-controlling the power conversion device to suppress harmonic voltages contained in the line voltage; a harmonic current extraction step of extracting harmonic currents contained in the line current; a virtual voltage drop calculation step of calculating a virtual voltage drop at the virtual impedance by multiplying the harmonic current by a virtual impedance; and a second control signal calculation step of calculating a second control signal obtained by correcting the first control signal by subtracting the virtual voltage drop from the first control signal. The power conversion device is feedback-controlled to suppress the harmonic voltages based on the second control signal.

The above configuration achieves the same effects as the control device described above.

[Software implementation example]
The functions of the control device 40 (hereinafter referred to as the "device") can be realized by a program that causes a computer to function as the device, and a program that causes a computer to function as each control block of the device.

In this case, the device includes a computer having at least one control device (e.g., a processor) and at least one storage device (e.g., a memory) as hardware for executing the program. The functions described in each of the above embodiments are realized by executing the program using this control device and storage device.

The above program may be stored non-transitory on one or more computer-readable storage media. These storage media may or may not be included in the device. In the latter case, the program may be supplied to the device via any wired or wireless transmission medium.

Furthermore, some or all of the functions of each of the above control blocks can be realized by logic circuits. For example, integrated circuits incorporating logic circuits that function as each of the above control blocks are also included in the scope of the present invention. In addition, the functions of each of the above control blocks can also be realized by, for example, a quantum computer.

Furthermore, each process described in each of the above embodiments may be executed by AI (Artificial Intelligence). In this case, the AI may run on the control device, or on another device (for example, an edge computer or a cloud server).

The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the claims. Embodiments obtained by appropriately combining the technical means disclosed in different embodiments are also included in the technical scope of the present invention.

1 Power supply system 10 Renewable energy source (DC power supply)
20 Power conversion device 30 AC line 40 Control device 43 First control signal calculation unit 44 Harmonic current extraction unit 45 Virtual voltage drop calculation unit 46a, 46b Second control signal calculation unit

Claims (4)

1. A control device for controlling a power conversion device in a power supply system in which DC power supplied from a DC power source is converted into AC power by a power conversion device and the AC power is supplied to an AC line, the control device comprising:
a first control signal calculation unit that calculates a first control signal indicating a target harmonic voltage for feedback control of the power conversion device so as to suppress a harmonic voltage included in the line voltage;
a harmonic current extraction unit that extracts harmonic currents contained in the line current;
a virtual voltage drop calculation unit that calculates a virtual voltage drop at a virtual impedance by multiplying the harmonic current by the virtual impedance;
a second control signal calculation unit that calculates a second control signal obtained by correcting the first control signal by subtracting the virtual voltage drop from the first control signal,
a control device that feedback controls the power conversion device based on the second control signal so as to suppress the harmonic voltage. The control device described in claim 1, wherein the absolute value of the virtual impedance satisfies %Z≧10% of the system capacity of the power supply system. The control device described in claim 1 or 2, wherein the second control signal is calculated for each order of the harmonic voltage to be reduced. 1. A control method for a power supply system in which DC power supplied from a DC power source is converted into AC power by a power conversion device and the AC power is supplied to an AC line, the control method comprising: controlling the power conversion device by referring to a line voltage and a line current in the AC line, the method comprising:
a first control signal calculation step of calculating a first control signal indicating a target harmonic voltage for feedback control of the power conversion device so as to suppress a harmonic voltage included in the line voltage;
a harmonic current extraction step of extracting harmonic currents contained in the line current;
a virtual voltage drop calculation step of multiplying the harmonic current by a virtual impedance to calculate a virtual voltage drop at the virtual impedance;
a second control signal calculation step of calculating a second control signal obtained by correcting the first control signal by subtracting the virtual voltage drop from the first control signal,
a control method for feedback-controlling the power conversion device so as to suppress the harmonic voltage based on the second control signal;