Control device and control method

Abstract

To provide a control device capable of reducing harmonic waves.SOLUTION: A control device (40) includes: a first control signal calculation part (43) for calculating a first control signal for controlling a power conversion device (20) so as to suppress a harmonic wave voltage; a harmonic wave current extraction part (44) for extracting a harmonic wave current; a virtual drop voltage calculation part (45) for calculating a virtual drop voltage; and a second control signal calculation part (46a, 46b) for calculating a second control signal by subtracting the virtual drop voltage from the first control signal.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a control device and a control method for suppressing harmonic currents. [Background technology]

[0002] Patent Document 1 discloses a converter device that can reduce harmonic components of 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 the DC power to a line connecting the PWM converter main circuit and a load. The converter device includes a PWM signal generating unit that generates a PWM signal for controlling the PWM converter main circuit. The PWM signal generating unit generates a PWM signal so as to compensate for the harmonic components contained in the AC power from the AC power supply. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 3323759 Summary of the Invention [Problem to be solved by the invention]

[0004] An object of one aspect of the present invention is to provide a control device or the like that can stably reduce harmonics in a system that converts DC power into AC power and supplies the AC power to an AC line. [Means for solving the problem]

[0005] In order 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 a line voltage and a line current in 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. The control device feedback controls the power conversion device so as to suppress the harmonic voltages contained in the line voltage based on the second control signal.

[0006] Furthermore, a control method according to one aspect of the present invention is 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 controlling the power conversion device by referring to a line voltage and a line current in the AC line, the control method including: a first control signal calculation step of calculating a first control signal indicating a target harmonic voltage, for feedback controlling the power conversion device so as to suppress a harmonic voltage contained in the line voltage; a harmonic current extraction step of extracting a harmonic current 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, and feedback controlling the power conversion device so as to suppress the harmonic voltage based on the second control signal.

[0007] The control device according to each aspect of the present invention may be realized by a computer. In this case, the control program of the control device that realizes the control device by the computer by making the computer operate as each part (software element) of the control device, and the computer-readable recording medium on which the control program is recorded, also fall within the scope of the present invention. [Effects of the Invention]

[0008] According to one aspect of the present invention, harmonics can be reduced in a system that converts DC power into AC power and supplies the AC power to an AC line. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a diagram showing the configuration of a power supply system including a control device according to the present invention. [Figure 2] 1 is a diagram showing an image of control of a power conversion device by a control device according to the present invention; [Figure 3] 1 is a block diagram showing an example of the configuration of a control device according to the present invention; [Figure 4] FIG. 2 is a block diagram showing an example of the configuration of a virtual voltage drop calculation unit. [Figure 5] 10 is a flowchart illustrating an example of a process performed by a control unit to calculate a control signal. [Figure 6] 6 is a graph showing an example of the effect of control by the control device according to the present invention. [Figure 7] FIG. 7 is a partially enlarged view of the graph shown in FIG. 6. [Figure 8] FIG. 7 is a partially enlarged view of the graph shown in FIG. 6. DETAILED DESCRIPTION OF THE INVENTION

[0010] [Embodiment 1] Hereinafter, one embodiment of the present invention will be described in detail.

[0011] (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 the 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.

[0012] 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.

[0013] The power conversion device 20 converts DC power supplied from the renewable energy source 10 into AC power and outputs it to an AC line 30. The AC power output by the power conversion device 20 is controlled by a control device 40, which will be described later.

[0014] 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 include an interconnection reactor 31 for reducing harmonics of the line current and line voltage in the AC line 30.

[0015] The control device 40 controls the power conversion device 20 by referring to the line voltage and line current in the AC line 30. A specific configuration of the control device 40 will be described later.

[0016] The power supply system 1 further includes a voltage detector 51 and a current detector 52. The voltage detector 51 detects a three-phase line voltage V u ,V v ,V wThe current detector 52 detects the three-phase line current I u ,I v ,I w 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.

[0017] 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. Furthermore, the virtual impedance unit 22 calculates a virtual impedance value R+jω for the n-th harmonic. n L(R is the resistance value, j is the imaginary unit, ω n is the angular frequency of the n-th harmonic, and L is the reactance. That is, the control device 40 controls the value of the output voltage of the power conversion device 20 to a value that is lower than the value corresponding to the line voltage of the AC line 30 by the amount of the virtual impedance that the virtual impedance unit 22 has.

[0018] The virtual impedance value may be set as appropriate by the designer or user of the power supply system 1. Regarding the magnitudes of the resistance value R and reactance ωL of the virtual impedance value, the absolute value of the virtual impedance value R+jωL preferably satisfies %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 may be set to 0.04 Ω or greater. 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. When 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 the output voltage from the power conversion device 20 due to the virtual impedance is referred to as the virtual voltage drop.

[0019] (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.

[0020] The harmonic voltage extraction unit 41 extracts n-th order harmonic voltages (n is any integer) included 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 three-phase line voltage V indicated by the signal output from the voltage detector 51 into a harmonic voltage of n-th order (n is any integer). u ,V v ,V w The nth harmonic voltage contained in is expressed as the angular frequency ω n Using the complex voltage V d0 +jV q0 The low-pass filter 41b converts the complex voltage V d0 +jV q0 The high frequency components of the complex voltage V are blocked, and only the low frequency components are generated. d +jV q Allow the light to pass through.

[0021] The subtraction units 42a and 42b calculate the control input V ref_d -V d +j(V ref_q -V q Specifically, the subtractor 42a calculates the real part V of the complex voltage. d is the real command value V ref_d The subtractor 42b subtracts the imaginary part V of the complex voltage. q is the imaginary command value V ref_q Subtract from the real command value V ref_d and the imaginary command value V ref_qis a target value in controlling harmonic voltages, and is set by the designer of the control device 40 or the administrator of the power supply system 1. ref_d and the imaginary command value V ref_q The value of is usually 0, aiming for no high frequency voltage components.

[0022] The first control signal calculation unit 43 calculates the control input V ref_d -V d +j(V ref_q -V q ) based on the first control signal V d_out1 +jV q_out1 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 the harmonic voltage 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 that controls the output voltage of the power conversion unit 21. The first control signal calculation unit 43 calculates a control signal by, for example, PI (Proportional-Integral) control or PID (Proportional-Integral-Differential) control.

[0023] The harmonic current extraction unit 44 extracts n-th order harmonic currents (n is the same as the value in the harmonic voltage extraction unit 41) contained in the line current in the AC line 30. 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 three-phase line current I u ,I v ,I w The nth harmonic current contained in is expressed as the angular frequency ω n Using the complex current I d0 +jI q0 The low-pass filter 44b converts the complex current I d0 +jI q0 The high frequency components of the complex current I are blocked, and only the low frequency components are generated. d +jI qAllow the light to pass through.

[0024] The virtual voltage drop calculation unit 45 calculates the complex current I d +jI q is multiplied by the virtual impedance to calculate a 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).

number

[0025] The second control signal calculation units 46a and 46b calculate the virtual voltage drop V d_z +jV q_z is the first control signal V d_out1 +jV q_out1 The second control signal V is obtained by correcting the first control signal by subtracting it from d_out2 +jV q_out2 Specifically, the second control signal calculation unit 46a calculates the real part V of the virtual voltage drop. d_z The real part V of the first control signal output by the first control signal calculation unit 43 d_out1 The real part of the second control signal, V, is subtracted from d_out2 The second control signal calculation unit 46b calculates the imaginary part V of the virtual voltage drop. q_z is calculated as the imaginary part V of the first control signal output by the first control signal calculation unit 43. q_out1 The imaginary part of the second control signal, V, is subtracted from q_out2 Calculate.

[0026] The coordinate inverse transformation unit 47 converts the second control signal V d_out2 +jV q_out2 , the three-phase control signal V u_out ,V v_out ,V w_out The control device 40 converts the three-phase control signal V u_out ,V v_out ,V w_out In other words, the control device 40 controls the power conversion device 20 based on the second control signal in a feedback manner so as to suppress harmonic voltages.

[0027] (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.

[0028] The virtual voltage drop calculation unit 45 calculates the complex current I d +jI q , and the virtual impedance R+jω n The first multiplier 45a calculates the real part I of the complex current. d The second multiplier 45b multiplies the real part I of the complex current d The imaginary part of the virtual impedance ω n The third multiplier 45c multiplies the imaginary part I of the complex current. q The fourth multiplier 45d multiplies the real part R of the virtual impedance by the imaginary part I of the complex current. q The imaginary part of the virtual impedance ω n Multiply by L.

[0029] 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 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 the above calculations, the virtual voltage drop calculation unit 45 calculates the virtual voltage drop as shown in the above-mentioned equation (1).

[0030] 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 for calculating the second control signal may be different 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. Alternatively, the power supply system 1 may calculate a different second control signal for reducing a harmonic current of an order other than the fifth or seventh order. In this 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.

[0031] (Processing in the control device 40) 5 is a flowchart showing an example of a process for calculating a control signal by the control device 40. The process shown in FIG. 5 is a 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 three-phase line voltage V u ,V v ,V w The coordinates of the complex voltage V d +jV q Next, the low-pass filter 41b converts the complex voltage V d +jV q The subtraction units 42a and 42b subtract the complex voltage V d +jV q The command value V ref_d +jV ref_q Subtract from the control input V ref_d -V d +j(V ref_q -V q ) (S3). The first control signal calculation unit 43 calculates the control input V ref_d -V d +j(V ref_q -V q ) based on the first control signal V d_out1 +jV q_out1 is calculated (S4, first control signal calculation step).

[0032] In parallel with steps S1 to S4, the coordinate conversion unit 44a converts the three-phase line current I u ,I v ,I w , the complex current I d0 +jI q0 (S5, harmonic current extraction step). Next, the low-pass filter 44b converts the complex current I d0 +jI q0 The virtual voltage drop calculation unit 45 cuts off the high frequency components of the complex current I d +jI q is multiplied by the virtual impedance to calculate a virtual voltage drop (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.

[0033] After steps S1 to S7 are completed, the second control signal calculation units 46a and 46b calculate the second control signal V d_out2 +jV q_out2 (S8, second control signal calculation step). The coordinate inverse conversion unit 47 calculates the second control signal V d_out2 +jV q_out2 , the three-phase control signal V u_out ,V v_out ,V w_out (S9).

[0034] The three-phase control signal V obtained by the above processing u_out ,V v_out ,V w_out In other words, the control device 40 controls the power conversion device 20 based on the second control signal V d_out2 +jV q_out2 It can be said that the power conversion device 20 is feedback controlled by this.

[0035] (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 by 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.

[0036] Within the period T1, the period T11 is a period during which the control device 40 does not execute control using the virtual impedance. In other words, the period T11 is a period during which the control device 40 controls the power conversion device 20 using the first control signal. The period T12 is a period during which the control device 40 executes control using the virtual impedance. In other words, the period T12 is a period during which the control device 40 controls the power conversion device 20 using the second control signal.

[0037] Figures 7 and 8 are partial enlarged views 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.

[0038] 6 to 8, the AC system has a resonance point in the frequency band of the fifth harmonic due to the impedance value of AC load 100. Therefore, the fifth harmonic diverges during period T11 when control device 40 is not executing 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 frequency band of the fifth harmonic.

[0039] On the other hand, by controlling the control device 40 using a virtual impedance having a resistance component, excessive signal components of the control signal in the harmonic frequency band can be suppressed. Therefore, the fifth harmonic converges during the period T12 when the control device 40 is performing control using the virtual impedance.

[0040] As described above, according to the control device 40, the output voltage from the power conversion device 20 is reduced 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 0, the imaginary part ω of the virtual impedance derived from the reactance L n L can shift the resonance point of the impedance including the AC load 100. Therefore, harmonics of the current supplied to the AC line 30 can be reduced without divergence.

[0041] Furthermore, since the control device 40 executes control using a virtual impedance, there is no need to insert impedance into the actual circuit, and therefore the control for reducing harmonics does not affect the control of the fundamental wave.

[0042] 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, the control conditions for reducing the harmonic voltage may change, for example, when the AC load 100 connected to the power supply system 1 is changed. Even in such a case, 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.

[0043] 〔summary〕 A control device according to one embodiment 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 controls 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 that corrects the first control signal by subtracting the virtual voltage drop from the first control signal. The control device feedback controls the power conversion device so as to suppress the harmonic voltages contained in the line voltage based on the second control signal.

[0044] According to the above configuration, in a power supply system in which a power conversion device converts DC power into AC power and supplies the AC power 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, 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 so as to suppress harmonic voltages included in the line voltage. The virtual voltage drop calculation unit calculates a virtual voltage drop at the virtual impedance by multiplying a harmonic current included in the line current by a virtual impedance. The second control signal calculation unit calculates the second control signal by subtracting the virtual voltage drop from the first control signal. The power conversion device outputs a voltage lower than the target harmonic voltage by the virtual voltage drop.

[0045] This allows the control device to damp harmonics and shift the impedance resonance point, thereby reducing harmonics in the current supplied to the AC line.

[0046] In the control device according to an aspect of the present invention, it is preferable that the absolute value of the virtual impedance satisfies %Z≧10% with respect to a system capacity of the power supply system.

[0047] According to the above configuration, by appropriately damping the harmonics, the harmonics can be reduced without diverging.

[0048] Furthermore, the control device according to an aspect of the present invention preferably calculates the second control signal for each order of the harmonic voltage to be reduced.

[0049] According to the above configuration, the control device can calculate an appropriate second control signal for each order of the harmonic voltage to be reduced.

[0050] Furthermore, a control method according to one aspect of the present invention is 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 controlling the power conversion device by referring to a line voltage and a line current in the AC line, the control method including: a first control signal calculation step of calculating a first control signal indicating a target harmonic voltage, for feedback controlling the power conversion device so as to suppress a harmonic voltage contained in the line voltage; a harmonic current extraction step of extracting a harmonic current 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, and feedback controlling the power conversion device so as to suppress the harmonic voltage based on the second control signal.

[0051] According to the above configuration, the same effects as those of the above-mentioned control device are achieved.

[0052] [Software implementation example] The functions of the control device 40 (hereinafter referred to as "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.

[0053] 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 control device and storage device execute the program, thereby realizing the functions described in each of the above embodiments.

[0054] The program may be non-transitory and may be recorded on one or more computer-readable recording media. The recording 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.

[0055] Furthermore, some or all of the functions of the control blocks can be realized by logic circuits. For example, an integrated circuit in which a logic circuit that functions as each of the control blocks is formed is also included in the scope of the present invention. In addition, the functions of the control blocks can also be realized by, for example, a quantum computer.

[0056] 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).

[0057] 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. [Explanation of symbols]

[0058] 1 Power System 10 Renewable energy sources (DC power) 20 Power conversion device 30 AC line 40 Control device 43 First control signal calculation unit 44 Harmonic current extraction section 45 Virtual voltage drop calculation section 46a, 46b Second control signal calculation unit

Claims

1. 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.

2. The control device according to claim 1 , wherein the absolute value of the virtual impedance satisfies %Z≧10% with respect to a system capacity of the power supply system.

3. The control device according to claim 1 or 2, wherein the second control signal is calculated for each order of the harmonic voltage to be reduced.

4. 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;

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