JP7730744B2 - Controller and control method for power converter - Google Patents

Description

This disclosure relates to a controller and control method for a power converter.

In power supply systems such as microgrids, where an AC power supply system and a power storage facility are interconnected, a power converter is connected between the AC power supply system and the power storage facility to convert the AC power from the AC power supply system into DC power and convert the DC power from the power storage facility into AC power.

If a malfunction occurs in part of such a power supply system, there is a possibility that a momentary voltage drop will occur in the AC wiring between the AC power supply system and the energy storage equipment. Even if such a momentary voltage drop occurs, it is required to continue grid-connected operation and, when voltage in the AC power supply system recovers, to quickly restore the power output from the power converter to the AC wiring from the voltage drop state. For example, the Grid Interconnection Regulations (JEAC9701-2019), an approved standard by the Japan Electrotechnical Standards Committee, stipulates as one of the Fault Ride Through (FRT) requirements that, if the residual voltage is 20% or more, operation must continue for a voltage drop lasting 0.3 seconds without a phase change, and that output must be restored to 80% or more of the level prior to the voltage drop within 0.1 seconds after voltage recovery in the AC power supply system.

Meanwhile, the following Patent Document 1 proposes a power converter for use in such power supply systems that can disconnect the power storage equipment from the AC power supply system and switch to independent operation without changing the control method, and can also reconnect the power storage equipment to the AC power supply system to switch from independent operation to grid-connected operation.

Specifically, the power converter in Patent Document 1 is controlled by virtual generator model control, which, assuming that a virtual prime mover generator is connected instead of the power converter, estimates the current that should be output by the virtual prime mover generator and determines the estimated current as the target current value to be output by the power converter.

International Publication No. 2013/008413

In the control mode of a power converter that simulates a virtual prime mover generator, as in Patent Document 1, there is room for improvement in continuing grid-connected operation when a momentary voltage drop occurs in the AC wiring, and quickly recovering from the voltage drop state when the voltage in the AC power system recovers.

The present disclosure aims to solve the above-mentioned problems by providing a controller and control method for a power converter that can switch between stand-alone operation and grid-connected operation without changing the control method, that can continue grid-connected operation even if a momentary voltage drop occurs during grid-connected operation, and that can quickly recover from a voltage drop state when the voltage in the AC power system recovers.

A power converter controller according to one aspect of the present disclosure is a power converter controller that performs power conversion between a power storage facility and an AC power system, and acquires the voltage and frequency of AC wiring connecting the AC power system and the power converter, and the current output from the power converter to the AC wiring, calculates a frequency target value by a frequency target value calculation process that includes multiplying a value based on a first deviation, which is the deviation of the power converter active power from a predetermined active power command value, by a coefficient that indicates the first drooping characteristic so that the relationship between the frequency and the power converter active power output by the power converter to the AC wiring has a predetermined first drooping characteristic, calculates an internal phase difference angle by integrating a second deviation, which is the deviation of the frequency in the AC wiring from the frequency target value, and calculates a current output from the power converter to the AC wiring with respect to a predetermined reactive power command value. a power correction value is calculated from a fourth deviation, which is the deviation of the current command value from the current target value, and the voltage on the AC wiring; the power correction value is calculated from the fourth deviation, which is the deviation of the current command value from the current target value, and the voltage on the AC wiring; the frequency target value is corrected by correcting the first deviation using the power correction value; a limit voltage is generated by limiting the voltage on the AC wiring to within a predetermined limit range; and a drive signal for the power converter is generated by adding the limit voltage to a value based on a fifth deviation, which is the deviation of the current output to the AC wiring from the current command value, to generate a voltage command value.

A control method for a power converter according to another aspect of the present disclosure is a control method for a power converter that performs power conversion between a power storage facility and an AC power system, comprising: acquiring a voltage and frequency in AC wiring connecting the AC power system and the power converter, and a current output from the power converter to the AC wiring; calculating a frequency target value by a frequency target value calculation process including an operation of multiplying a value based on a first deviation, which is the deviation of the power converter active power from a predetermined active power command value, by a coefficient indicating the first drooping characteristic so that the relationship between the frequency and the power converter active power output by the power converter to the AC wiring has a predetermined first drooping characteristic; calculating an internal phase difference angle by integrating a second deviation, which is the deviation of the frequency in the AC wiring from the frequency target value; and outputting the power converter to the AC wiring with respect to a predetermined reactive power command value. a predetermined reference voltage is added to a value based on a third deviation, which is the deviation of the power converter reactive power output by the converter, to calculate an internal electromotive force target value; a current target value is calculated from the internal phase difference angle, the internal electromotive force target value, and the voltage in the AC wiring, and a current command value is generated by limiting the current target value to within a predetermined first range; a power correction value is calculated from a fourth deviation, which is the deviation of the current command value from the current target value, and the voltage in the AC wiring, and the frequency target value is corrected by correcting the first deviation using the power correction value; a limit voltage is generated by limiting the voltage in the AC wiring to within a predetermined second range; and a voltage command value is generated by adding the limit voltage to a value based on a fifth deviation, which is the deviation of the current output to the AC wiring from the current command value, to generate a drive signal for the power converter.

This disclosure makes it possible to switch between stand-alone operation and grid-connected operation without changing the control method, and it is possible to continue grid-connected operation even if a momentary voltage drop occurs during grid-connected operation, and to quickly recover from the voltage drop state when the voltage in the AC power system recovers.

FIG. 1 is a block diagram showing a schematic configuration of a power supply system including a controller for a power converter according to an embodiment of the present disclosure. FIG. 2 is a block diagram showing the configuration of a frequency target value calculation unit in the controller shown in FIG. FIG. 3 is a block diagram showing the configuration of an internal phase difference angle calculation unit in the controller shown in FIG. FIG. 4 is a block diagram showing the configuration of the internal electromotive voltage target value calculation unit in the controller shown in FIG. FIG. 5 is a block diagram showing the configuration of a current command value calculation unit in the controller shown in FIG. FIG. 6 is a block diagram showing the configuration of a drive signal generating unit in the controller shown in FIG. FIG. 7 is a graph showing the change in output power over time in the first simulation. FIG. 8 is a graph showing the change in frequency over time in the first simulation. FIG. 9 is a graph showing the change in the internal phase difference angle over time in the first simulation. FIG. 10 is a graph showing the time variation of the d-axis current command value and the d-axis current in the example in the first simulation. FIG. 11 is a graph showing the time variation of the d-axis current command value and the d-axis current of the comparative example in the first simulation. FIG. 12 is a graph showing the change over time in the output current from the power converter in the first simulation. FIG. 13 is a graph showing the time variation of AC voltage and frequency in the AC wiring based on the results of the second simulation for the example. FIG. 14 is a graph showing the time changes of AC voltage and frequency in the AC wiring based on the results of the second simulation for Comparative Example 1. FIG. 15 is a graph showing the time changes of AC voltage and frequency in the AC wiring based on the results of the second simulation for Comparative Example 2.

Embodiments of the present disclosure will be described below with reference to the drawings. Note that, throughout the following, elements that are identical or have the same function will be designated by the same reference numerals, and redundant explanations will be omitted.

[System configuration]
Fig. 1 is a block diagram showing a schematic configuration of a power supply system including a controller for a power converter according to an embodiment of the present disclosure. As shown in Fig. 1, the power supply system 1 according to the present embodiment includes an AC power supply system 2 such as a commercial power supply system, a power storage facility 3, and a power converter 4. The AC power supply system 2 and the power converter 4 are connected by AC wiring 5. The power storage facility 3 and the power converter 4 are connected by DC wiring 6. In the present embodiment, a case is illustrated in which the AC power supply system 2 is a three-phase AC system.

The power storage facility 3 includes a secondary battery, an electric double layer capacitor, a fuel cell, etc. The power converter 4 includes multiple power semiconductor elements and rapidly switches the power semiconductor elements on and off to convert DC power from the power storage facility 3 into AC power and output it to the AC wiring 5, or convert AC power from the AC power supply system 2 into DC power and store it in the power storage facility 3. A controller 10 is connected to the power converter 4 to control the on/off of the power semiconductor elements.

The power supply system 1 is equipped with a voltage detector 7 and a current detector 8. The voltage detector 7 detects the voltage in the AC wiring 5. The current detector 8 detects the current output from the power converter 4 to the AC wiring 5. For example, the voltage detector 7 is a transformer known as a PT (potential transformer), and the current detector 8 is a current transformer known as a CT (current transformer). The detected voltage and current are input to the controller 10.

A status detector 9 is connected to the power storage equipment 3 to detect the status of the power storage equipment 3, such as voltage, current, temperature, and pressure. The output of the status detector 9 is input to the status monitor 11. In addition to monitoring the status of the power storage equipment, the status monitor 11 calculates the SOC (State of Charge), which indicates the charging rate of the power storage equipment 3. The status monitor 11 is connected to the controller 10, and if it detects an abnormality in the status of the power storage equipment 3, it sends an abnormality signal to the controller 10. If the controller 10 receives an abnormality signal from the status monitor 11, it stops operation of the power converter 4.

The controller 10 includes a computer such as a microcontroller or a personal computer. For example, the controller 10 includes a CPU, main memory such as RAM, a communication interface, etc.

It should be noted that the functions of the elements disclosed herein can be performed using circuits or processing circuits, including general-purpose processors, special-purpose processors, integrated circuits, ASICs (Application Specific Integrated Circuits), conventional circuits, or combinations thereof, configured or programmed to perform the disclosed functions. A processor is considered a processing circuit or circuit because it includes transistors and other circuitry. In this specification, a circuit, unit, or means is hardware that performs the recited functions or hardware that is programmed to perform the recited functions. The hardware may be hardware disclosed herein or other known hardware that is programmed or configured to perform the recited functions. Where the hardware is a processor, which is considered a type of circuit, the circuit, unit, means, or section is a combination of hardware and software, and the software is used to configure the hardware or processor.

The controller 10 includes the following control blocks: a voltage calculation unit 71, a current calculation unit 72, a power calculation unit 73, a frequency target value calculation unit 80, an internal phase difference angle calculation unit 82, an internal electromotive voltage target value calculation unit 83, a current command value calculation unit 84, and a drive signal generation unit 85. As described above, each of these control blocks is considered to be a processing circuit or circuit. By executing processing in each control block, the controller 10 performs virtual generator model control, which simulates the power that a virtual generator would output to the AC wiring 5 if it were connected to the AC wiring 5 instead of the power converter 4, and controls the power that the power converter 4 outputs to the AC wiring 5. Each control block is described in detail below.

[Voltage calculation section]
The voltage calculation unit 71 calculates the AC voltage Vac from the instantaneous voltages v _a , v _b , and v _c of each phase detected by the voltage detector 7 using the following equation.

Furthermore, voltage calculation unit 71 calculates frequency fac and phase _φac in AC wiring 5 by known PLL (Phase Lock Loop) calculation. Furthermore, voltage calculation unit 71 calculates d-axis voltage _Vd and _q -axis voltage _Vq , which are voltages on each coordinate axis of a rotating coordinate system of AC voltage, i.e., a dq coordinate system, from the instantaneous voltages _{v a , v b , and v c and phase φac of each phase, using the following equation: As described above, in this embodiment, controller 10 obtains voltages Vd, Vq and frequency fac in AC wiring 5 from the instantaneous voltages v a} , v _b , and _vc detected by voltage detector 7.

[Current calculation section]
The current calculation unit 72 calculates the AC current Iac output from the power converter 4 to the AC wiring 5 from the instantaneous currents i _a , _ib , and i _c of each phase detected by the current detector 8 using the following equation.

Furthermore, the current calculation unit 72 calculates the d-axis current Id and the q-axis current Iq, which are currents on each coordinate axis of the rotating coordinate system of AC current, using the instantaneous currents i _a , _ib , and i _c of each phase detected by the current detector 8 and the phase φac calculated by the voltage calculation unit 71, using the following equation: As described above, in this embodiment, the controller 10 obtains the currents Id and Iq in the AC wiring 5 from the instantaneous currents i _a , _ib , and i _c detected by the current detector 8.

[Power calculation section]
The power calculation unit 73 calculates the corresponding power converter active power Pac and power converter reactive power Qac using the following equations from the voltages Vd, Vq calculated by the voltage calculation unit 71 and the currents Id, Iq calculated by the current calculation unit 72. Note that, hereinafter, the power converter active power Pac and power converter reactive power Qac may be simply referred to as active power Pac and reactive power Qac.

While the present embodiment has been described above as an example in which the active power Pac and reactive power Qac are calculated from the voltages Vd, Vq and currents Id, Iq obtained from the voltage detector 7 and current detector 8, this is not limiting. For example, instead, the power supply system 1 may be equipped with a power detector that detects the active power Pac and reactive power Qac in the AC wiring 5. That is, the controller 10 may obtain the active power Pac and reactive power Qac from an external source. For example, the power detector may be configured with a known wattmeter. Note that, hereinafter, the voltages Vd, Vq in the AC wiring 5 obtained by the voltage calculation unit 71 may be referred to as system voltages, and the currents Id, Iq output from the power converter 4 to the AC wiring 5 obtained by the current calculation unit 72 may be referred to as system currents.

[Frequency target value calculation section]
Fig. 2 is a block diagram showing the configuration of a frequency target value calculation unit in the controller shown in Fig. 1. As shown in Fig. 2, the frequency target value calculation unit 80 performs frequency target value calculation processing. In the frequency target value calculation processing, the frequency target value calculation unit 80 calculates a frequency target value fac_ref so that the relationship between the active power Pac and the frequency fac has a predetermined first drooping characteristic. Note that the first drooping characteristic means a characteristic in which the frequency fac decreases as the active power Pac increases, and the frequency fac increases as the active power Pac decreases.

Specifically, the frequency target value calculation unit 80 subtracts the power correction value Pac_cmp (described below) from the first deviation D1 = Pac_cmd - Pac, which is the deviation of the active power Pac from the predetermined active power command value Pac_cmd, and multiplies this value by a droop coefficient Dr_p that indicates the first drooping characteristic to calculate a value.

In this embodiment, the frequency target value calculation unit 80 inputs the calculated value to the first-order lag calculation unit 86, which performs a first-order lag calculation. This allows the moment of inertia generated in the actual generator to be simulated in the virtual generator model. Note that the moment of inertia generated in the generator may also be simulated using calculation processing other than first-order lag calculation.

Furthermore, the value output from the first-order lag calculation unit 86 is input to the upper and lower limiter 87. The upper and lower limiter 87 limits the value output from the first-order lag calculation unit 86 to a predetermined upper limit value and a predetermined lower limit value, and outputs the frequency reference value Δfac_ref. Note that the frequency reference value Δfac_ref may be calculated without providing the first-order lag calculation unit 86, the upper and lower limiter 87, or both, in the frequency target value calculation unit 80.

The frequency target value calculation unit 80 calculates the frequency target value fac_ref by adding a predetermined frequency command value fac_cmd to the frequency reference value Δfac_ref output from the upper and lower limiter 87.

When the power consumption of the load connected to the AC wiring 5 increases and the frequency fac in the AC wiring 5 decreases, the phase lead of the AC voltage in the AC wiring 5 increases relative to the AC voltage output by the power converter 4. In response, the controller 10 reduces the frequency target value fac_ref to offset this phase lead. As a result, the active power Pac output by the power converter 4 increases.

Conversely, when the power consumption of the load connected to the AC wiring 5 decreases and the frequency fac in the AC wiring 5 increases, the lag phase of the AC voltage in the AC wiring 5 increases relative to the AC voltage output by the power converter 4. In response to this, the controller 10 increases the frequency target value fac_ref to offset this lag phase. As a result, the active power Pac output by the power converter 4 decreases.

[Internal phase difference angle calculation section]
Fig. 3 is a block diagram showing the configuration of an internal phase difference angle calculation unit in the controller shown in Fig. 1. As shown in Fig. 3, an internal phase difference angle calculation unit 82 calculates a second deviation D2 = fac_ref - fac, which is the deviation of the frequency fac in the AC wiring 5 from the frequency target value fac_ref calculated by the frequency target value calculation unit 80, and inputs this to an integrator 88 for integration. The integrator 88 calculates an internal phase difference angle θ in the virtual generator by integrating the rotational speed of the virtual generator, which is obtained by multiplying the second deviation D2 by a coefficient Kw for unit conversion.

[Internal electromotive voltage target value calculation unit]
Fig. 4 is a block diagram showing the configuration of an internal electromotive voltage target value calculation unit in the controller shown in Fig. 1. As shown in Fig. 4, the internal electromotive voltage target value calculation unit 83 calculates an AC voltage target value Vac_ref based on the reactive power Qac calculated by the power calculation unit 73. Here, the internal electromotive voltage target value calculation unit 83 calculates the AC voltage target value Vac_ref so that the relationship between the reactive power Qac and the AC voltage Vac has a predetermined second drooping characteristic. Note that the second drooping characteristic means a characteristic such that as the reactive power Qac increases, the AC voltage Vac decreases, and as the reactive power Qac decreases, the AC voltage Vac increases.

Specifically, the internal electromotive force target value calculation unit 83 multiplies the third deviation D3 = Qac_cmd - Qac, which is the deviation of reactive power Qac from a predetermined reactive power command value Qac_cmd, by a droop coefficient Dr_q corresponding to the second drooping characteristic, and then adds the AC voltage command value Vac_cmd, which is a predetermined reference voltage, to this value to calculate the AC voltage target value Vac_ref.

The internal electromotive force target value calculation unit 83 generates the internal electromotive force target value Ef_ref by performing a proportional-integral calculation on the deviation of the AC voltage Vac from the AC voltage target value Vac_ref. The transfer function in the proportional-integral calculation is expressed as Kp + Ki/s, where Kp is the proportional gain and Ki is the integral gain. In this way, by applying a second drooping characteristic to the third deviation D3, which is the deviation of the reactive power Qac from the reactive power command value Qac_cmd, it is possible to easily realize a control mode that simulates automatic voltage regulation (AVR) control using reactive power Qac in a virtual generator.

In addition, like the frequency target value calculation unit 80, the internal electromotive force target value calculation unit 83 may perform a first-order lag calculation on the result of the proportional integral calculation of the deviation of the AC voltage Vac from the AC voltage target value Vac_ref. Furthermore, the internal electromotive force target value calculation unit 83 may apply upper and lower limiters to the result of the proportional integral calculation of the deviation of the AC voltage Vac from the AC voltage target value Vac_ref or the result of the first-order lag calculation.

[Current command value calculation unit]
Fig. 5 is a block diagram showing the configuration of a current command value calculation unit in the controller shown in Fig. 1. As shown in Fig. 5, in a current command value calculation unit 84, the internal phase difference angle θ calculated by the internal phase difference angle calculation unit 82, the internal electromotive voltage target value Ef_ref calculated by the internal electromotive voltage target value calculation unit 83, and the d-axis voltage Vd and q-axis voltage Vq calculated by the voltage calculation unit 71 are input to a function calculation unit 89. The function calculation unit 89 performs the calculation shown in the following equation to calculate current target values Id_ref and Iq_ref.

The current target values Id_ref and Iq_ref calculated by the above formula are the current values that flow through the overall impedance r+jx when it is assumed that this impedance is connected between a power supply that outputs the same voltage as the AC voltage Vac in the AC wiring 5 and a power supply that outputs the same voltage as the internal electromotive force target value Ef_ref.

Incidentally, the total impedance r+jx is defined as the sum of the internal impedance _rs + _jxs of the power storage equipment 3 and the external impedance _rl + _jxl between the power storage equipment 3 and the AC power supply system 2. However, the internal impedance _rs + _jxs in the actual power storage equipment 3 is approximately equal to zero, and the total impedance r+jx is approximately equal to the external impedance _rl + _jxl between the power storage equipment 3 and the AC wiring 5. However, as described above, in the present embodiment, the total impedance r+jx, which is the sum of the internal impedance _rs + _jxs of the power storage equipment 3 and the external impedance _rl + _jxl between the power storage equipment 3 and the AC power supply system 2, is used when calculating the current target values Id_ref and Iq_ref.

In particular, stable operation is possible by virtually increasing the internal impedance r _s +jx _s of the power storage equipment 3, determining the total impedance r+jx, and using this virtual impedance to calculate the current target values Id_ref and Iq_ref. This is because, when multiple power converters 4 are operated in parallel, a slight voltage difference between the power converters 4 can significantly disrupt the output balance because the impedance of the power converters 4 is low. By virtually increasing the internal impedance of the power storage equipment 3, the impedance of the power converters 4 increases, preventing the output balance from becoming unstable due to the voltage difference. For example, while the internal impedance r _s +jx _s is actually almost zero, considerable stabilization can be achieved by setting the resistance component of the total impedance to 0.1 PU and the reactance component to 0.4 PU.

In other words, the current command value calculation unit 84 assumes that a virtual generator is connected to the AC wiring 5 instead of the power converter 4, and estimates the current value that will be output to the AC wiring 5 when the virtual generator generates the internal electromotive voltage calculated by the internal electromotive voltage target value calculation unit 83 and the internal phase difference angle calculation unit 82. This prevents the apparent impedance of the power converter 4 from increasing and the system from becoming unstable during interconnected operation with the AC power supply system 2 or when power converters 4 are operated in parallel.

Furthermore, the current command value calculation unit 84 limits the current target values Id_ref and Iq_ref to within a predetermined first range to generate the current command values Id_cmd and Iq_cmd. That is, the current target values Id_ref and Iq_ref, which are the outputs of the function calculation unit 89, are input to limiters 90 and 91, respectively.

When the d-axis current target value Id_ref is equal to or greater than the upper limit of the first range, limiter 90 outputs the upper limit as the d-axis current command value Id_cmd. When the d-axis current target value Id_ref is equal to or less than the lower limit of the first range, limiter 90 outputs the lower limit as the d-axis current command value Id_cmd. When the d-axis current target value Id_ref is within the first range, limiter 90 outputs the d-axis current target value Id_ref as the d-axis current command value Id_cmd. Limiter 91 performs the same limiting process on the q-axis current target value Iq_ref as limiter 90. Note that the upper and lower limit values set by limiters 90 and 91 may be different values or may be the same values. The current command values Id_cmd and Iq_cmd output by limiters 90 and 91 are input to drive signal generation unit 85.

Furthermore, the current command value calculation unit 84 calculates the power correction value Pac_cmp from the system voltages Vd and Vq and the current deviations Id_cmp and Iq_cmp obtained as fourth deviations D4d and D4q, which are the deviations of the current command values Id_cmd and I_cmd from the current target values Id_ref and Iq_ref.

More specifically, the current command value calculation unit 84 subtracts the d-axis current command value Id_cmd, which is the output of limiter 90, from the d-axis current target value Id_ref, which is the input to limiter 90, to generate the d-axis current deviation Id_cmp = D4d = Id_ref - Id_cmd. Similarly, the current command value calculation unit 84 subtracts the q-axis current command value Iq_cmd, which is the output of limiter 91, from the q-axis current target value Iq_ref, which is the input to limiter 91, to generate the q-axis current deviation Iq_cmp = D4q = Iq_ref - Iq_cmd. The current deviations Id_cmp and Iq_cmp and the system voltages Vd and Vq are input to the power correction value calculation unit 92.

The power correction value calculation unit 92 calculates the power correction value Pac_cmp by adding the value obtained by multiplying the d-axis voltage Vd by the d-axis current deviation Id_cmp and the value obtained by multiplying the q-axis voltage Vq by the q-axis current deviation Iq_cmp. In other words, the power correction value Pac_cmp is expressed as Pac_cmp = Vd * Id_cmp + Vq * Iq_cmp.

The current command value calculation unit 84 limits the current command values Id_cmd and Iq_cmd to within a first range to prevent the power converter 4 from tripping relative to the current target values Id_ref and Iq_ref, which are the theoretical current outputs calculated from the virtual generator model. The power correction value Pac_cmp corresponds to the power that could not be output due to the above-mentioned constraints of the power converter 4 relative to the theoretical current output calculated from the virtual generator model.

As described above, the power correction value Pac_cmp is input to the frequency target value calculation unit 80. As shown in FIG. 2, the frequency target value calculation unit 80 subtracts the power correction value Pac_cmp from the first deviation D1, which is the deviation of the active power Pac from a predetermined active power command value Pac_cmd. As a result, the frequency target value calculation unit 80 corrects the frequency target value fac_ref by correcting the first deviation D1 = Pac_cmd - Pac using the power correction value Pac_cmp.

In this way, by limiting the current command values Id_cmd and Iq_cmd to the theoretical current output calculated from the virtual generator model, even if the power converter 4 does not output power according to the virtual generator model, the frequency target value calculation unit 80 calculates the frequency target value fac_ref on the assumption that the power converter 4 is outputting power according to the virtual generator model. Therefore, limiting the current command values Id_cmd and Iq_cmd can prevent the frequency target value fac_ref from becoming excessive.

[Drive signal generation unit]
Fig. 6 is a block diagram showing the configuration of a drive signal generation unit in the controller shown in Fig. 1. As shown in Fig. 6, the currents Id, Iq, phase φac, and current command values Id_cmd, Iq_cmd in the AC wiring 5 are input to the drive signal generation unit 85. The drive signal generation unit 85 generates a drive signal So that causes the grid currents Id, Iq to become the current command values Id_cmd, Iq_cmd, and outputs the drive signal So to the power converter 4.

Specifically, the drive signal generation unit 85 performs proportional-integral calculations on the fifth deviations D5d = Id_cmd - Id, D5q = Iq_cmd - Iq, which are the deviations of the grid currents Id and Iq from the current command values Id_cmd and Iq_cmd. Furthermore, the drive signal generation unit 85 generates a limit voltage that limits the grid voltage within a predetermined limit range. The limit voltage is a voltage at which at least the d-axis voltage Vd or the q-axis voltage Vq can change within the predetermined limit range.

In this embodiment, a d-axis voltage Vd is input to the drive signal generation unit 85. The d-axis voltage Vd input to the drive signal generation unit 85 is then input to the limiter 93. The limiter 93 outputs a d-axis limit voltage Vdl that limits the d-axis voltage Vd to a predetermined second range. In this embodiment, the magnitude of the d-axis limit voltage Vdl is between 0 and 1, where the rated voltage of the d-axis voltage Vd is 1. The drive signal generation unit 85 adds the d-axis limit voltage Vdl to the result of the proportional integral calculation for the fifth deviation D5d regarding the d-axis current Id. In other words, when the magnitude of the d-axis voltage Vd is equal to or less than the rated voltage, the d-axis limit voltage Vdl added to the result of the proportional integral calculation for the fifth deviation D5d regarding the d-axis current Id can change depending on the d-axis voltage Vd at that time.

In this embodiment, the q-axis limit voltage is not added to the result of the proportional-plus-integral calculation of the fifth deviation D5q related to the q-axis current Iq. This is equivalent to adding a fixed value of 0 as the conventional q-axis voltage addition term.

Furthermore, the drive signal generation unit 85 subtracts interference components from the results of proportional-plus-integral calculations of the fifth deviations D5d and D5q related to the current in the AC wiring 5. In the virtual generator, interference occurs between the d-axis current Id and the q-axis current Iq; when the d-axis current Id changes, the q-axis current Iq changes, and when the q-axis current Iq changes, the d-axis current Id changes. The drive signal generation unit 85 performs decoupling processing to remove such interference components from the voltage command values Vd_cmd and Vq_cmd, which are based on the current command values Id_cmd and Iq_cmd.

Specifically, the drive signal generation unit 85 subtracts the q-axis interference component XqIq, obtained by multiplying the q-axis current Iq by a predetermined gain Xq, from the result of the proportional integral calculation of the fifth deviation D5d related to the d-axis current Id. Similarly, the drive signal generation unit 85 adds the d-axis interference component XdId, obtained by multiplying the d-axis current Id by a predetermined gain Xd, to the result of the proportional integral calculation of the fifth deviation D5q related to the q-axis current Iq.

From the above, the drive signal generation unit 85 calculates the voltage command values Vd_cmd and Vq_cmd from the current command values Id_cmd and Iq_cmd using the following formula. Here, Kd, Kq, Xd, and Xq represent predetermined gains, and Tid and Tiq represent predetermined time constants.

The drive signal generation unit 85 calculates target values v _{a_ref} , v _{b_ref} , and v _{c_ref} of the instantaneous voltages v _a , v _b , and v _c of the respective phases of the three-phase AC wiring 5 from the voltage command values Vd_cmd and Vq_cmd. Specifically, the voltage command values Vd_cmd and Vq_cmd are input to a dq-abc conversion unit 94. The dq-abc conversion unit 94 calculates target values v _{a_ref} , v _{b_ref} , and v _{c_ref} of the instantaneous voltages of the respective phases of the AC wiring 5 from the voltage command values Vd_cmd and Vq_cmd based on the following equations:

The calculated target values v _{a_ref} , v _{b_ref} , and v _{c_ref} of the instantaneous voltages are input to the signal conversion unit 95. The signal conversion unit 95 generates a drive signal So for switching the power converter 4 based on the target values v _{a_ref} , v _{b_ref} , and v _{c_ref} of the instantaneous voltages. For example, the drive signal So is a PWM control signal. The drive signal generation unit 85 outputs the drive signal So generated in this manner. The drive signal So is input to the power converter 4, and the output voltage to the AC wiring 5 is controlled based on the drive signal So.

[effect]
According to this embodiment, in order to simulate the power that a virtual generator would output to the AC wiring 5 if it were connected to the AC wiring 5 instead of the power converter 4, the internal electromotive force target value Ef_ref and the internal phase difference angle θ of the virtual generator are calculated, and the current output from the virtual generator is estimated from the internal electromotive force target value Ef_ref, the internal phase difference angle θ, and the voltages Vd and Vq in the AC wiring 5. If a system fault or the like occurs in the power supply system 1 and an instantaneous voltage drop occurs, the estimated value of this current becomes excessively large. Therefore, if the power converter 4 is driven using the estimated current value as a current command value as is, the power converter 4 may trip or be damaged due to an overcurrent. As a result, it becomes impossible to continue operation during an instantaneous voltage drop and to quickly recover from the voltage drop state when the voltage in the AC power supply system 2 recovers.

Therefore, in this embodiment, the power converter 4 is driven using current command values Id_cmd and Iq_cmd, which are obtained by limiting the current target values Id_ref and Iq_ref estimated as the currents output from the virtual generators to within a first range using limiters 90 and 91. This makes it possible to suppress overcurrents during momentary voltage drops. Furthermore, because the d-axis limit voltage Vdl, which limits the d-axis voltage Vd to within a second range, is added to the feedback loop of the d-axis current command value Id_cmd, the responsiveness of current control during momentary voltage drops can be improved, preventing tripping due to overcurrent.

As described above, in this embodiment, the estimated value of the current output from the virtual generator is limited by limiters 90 and 91, and therefore the power output from the power converter 4 is also limited. In this embodiment, the frequency target value fac_ref in the virtual generator is estimated from the first deviation D1, which is the deviation of the active power Pac from a predetermined active power command value Pac_cmd, and the internal phase difference angle θ is calculated from this frequency target value fac_ref.

Therefore, if the active power Pac is limited during a momentary voltage drop, the first deviation D1 increases, and the frequency target value fac_ref also increases. As a result, the internal phase difference angle θ becomes larger than the theoretical value during a momentary voltage drop, causing large power fluctuations in the power supply system 1 when the power supply system 2 recovers from the voltage drop. This may prevent the AC power supply system 2 from recovering from the voltage drop quickly.

Therefore, in this embodiment, a power correction value Pac_cmp is calculated from the fourth deviation D4d = Id_cmp, D4q = Iq_cmp, which is the deviation of the current command values Id_cmd, I_cmd from the current target values Id_ref, Iq_ref, and the voltages Vd, Vq in the AC wiring 5. The calculated power correction value Pac_cmp is subtracted from the first deviation D1 used to calculate the frequency target value fac_ref. This suppresses the increase in the frequency target value fac_ref that accompanies a decrease in power output due to limitation by limiters 90, 91, and also suppresses an increase in the internal phase difference angle θ that occurs during a momentary voltage drop. Therefore, power oscillations are suppressed when voltage recovers in the AC power supply system 2, allowing the grid-connected operation of the power converter 4 to be quickly restored from a voltage drop state.

In conventional virtual generator model control, to enable switching between grid-connected operation and stand-alone operation without changing the control method, current feedback is performed using PI control in the drive signal generation unit, and the d-axis voltage Vd = 1 and q-axis voltage Vq = 0 are added as additive terms to the grid voltage to calculate the voltage command values Vd_cmd and Vq_cmd. While this makes it possible to switch between grid-connected operation and stand-alone operation without changing the control method, there is a risk of delayed response in the event of a momentary voltage drop, resulting in a transient excessive current flow.

In contrast, in this embodiment, the drive signal generator 85 adds a voltage that limits the voltage in the AC wiring 5 as an additive term to the grid voltage. That is, the drive signal generator 85 adds the d-axis limit voltage Vdl to the result of a proportional-plus-integral calculation of the fifth deviation D5d = Id_cmd - Id related to the d-axis current. This makes it possible to switch between grid-connected operation and stand-alone operation without changing the control method, as with conventional virtual generator model control, while also improving the responsiveness of current control when a momentary voltage drop occurs.

Another method for achieving continuity of operation during a momentary voltage drop and rapid recovery from the voltage drop state is to detect the occurrence of a momentary voltage drop and temporarily switch the control method. However, this requires additional equipment to detect the occurrence of a momentary voltage drop. On the other hand, according to this embodiment, there is no need to change the control method even when a momentary voltage drop occurs. Therefore, because there is no need to detect the occurrence of a momentary voltage drop, it is possible to achieve continuity of operation during a momentary voltage drop and rapid recovery from the voltage drop state when the voltage in the AC power system recovers without requiring additional equipment in the power supply system 1.

[Simulation results]
(1) Simulation of Occurrence of Momentary Voltage Drop Below, as a first simulation, the results of a simulation in which a momentary voltage drop occurs in the power supply system 1 of the above embodiment are shown below. In the first simulation, as an example, a simulation was performed on the power supply system 1 shown in FIG. 1 to examine the behavior when a momentary voltage drop of 20% and lasting for 0.3 seconds occurs in the AC power supply system 2, and the results were compared with a comparative example. In the first simulation, a momentary voltage drop occurs 5 seconds after the start of the simulation, and the voltage in the AC power supply system 2 recovers 0.3 seconds later.

The comparative example differs in that no correction is made using the power correction value Pac_cmp in the power supply system 1 shown in FIG. 1, and that in the current feedback control, the addition of the d-axis limit voltage Vdl, which is the limit voltage addition term, and the subtraction of the interference components XqIq and XdId are not made, but PI control is performed on the fifth deviations D5d and D5q related to the current in the AC wiring 5. Note that even in the comparative example, current limitation is performed using limiters 90 and 91 in the current command value calculation unit 84 shown in FIG. 5.

Figure 7 is a graph showing the change in output power over time in the first simulation. According to the graph of output power Pc in the comparative example, after voltage recovery in AC power supply system 2 5.3 seconds after the start of the simulation, the output power Pc of the power converter swings significantly into the negative region. This is thought to be because, as can be seen from the simulation results of the internal phase difference angle described below, the increase in the internal phase difference angle that occurred during the momentary voltage drop became excessive, causing a loss of synchronization. For this reason, it is not possible to satisfy, for example, one of the FRT requirements defined in the Grid Interconnection Regulations (JEAC9701), which requires that output be restored to 80% or more of the level before the voltage drop within 0.1 seconds after voltage recovery in the AC power supply system.

On the other hand, the graph of output power Pe in the example shows that no step-out phenomenon occurs, and power fluctuations are kept low after voltage recovery. As a result, the above FRT requirements are met. This is thought to be due to the fact that power compensation control using the power correction value Pac_cmp suppresses increases in the frequency target value fac_ref and the internal phase difference angle θ during momentary voltage drops.

Figure 8 is a graph showing the change in frequency over time in the first simulation. The frequency in the graph in Figure 8 is a value equivalent to the frequency fac obtained by the voltage calculation unit 71. According to the graph of frequency frc in the comparative example, during an instantaneous voltage drop, the current command value is limited, suppressing the output power and causing the frequency frc to increase. Furthermore, after the voltage recovers, a step-out phenomenon occurs, resulting in a further increase in frequency frc.

On the other hand, according to the graph of frequency fre in the example, power compensation control using the power correction value Pac_cmp suppresses the increase in frequency fre. Even after voltage recovery, no step-out phenomenon occurs, and although frequency fre temporarily decreases, it then returns to a value close to the original frequency in a short period of time.

Figure 9 is a graph showing the change in the internal phase difference angle over time in the first simulation. Comparing the graph of the internal phase difference angle θc in the comparative example with the graph of the internal phase difference angle θe in the example, the internal phase difference angle θc in the comparative example changes more significantly during an instantaneous voltage drop. Furthermore, in the comparative example, the internal phase difference angle θc changes instantaneously from a positive value to a negative value after the voltage recovers. This indicates that because the slope of the change in the internal phase difference angle θc in the comparative example during an instantaneous voltage drop is large, the phase difference cannot be restored when the voltage recovers, resulting in a loss of synchronization.

On the other hand, according to the graph of the internal phase difference angle θe in the example, after the voltage recovers, the internal phase difference angle θe returns to its original state. This is thought to be due to the effect of power compensation control using the power correction value Pac_cmp.

Figure 10 is a graph showing the time variation of the d-axis current command value and d-axis current for the example in the first simulation. Figure 11 is a graph showing the time variation of the d-axis current command value and d-axis current for the comparative example in the first simulation. Figure 12 is a graph showing the time variation of the output current from the power converter in the first simulation. The output current in the graph in Figure 12 is a value equivalent to the AC current Iac output from the power converter 4 to the AC wiring 5, obtained by the above equation (3).

The graph of the comparative example shown in Figure 11 shows that during a momentary voltage drop, the d-axis current Idc fails to track the d-axis current command value Idcc. That is, even though the d-axis current command value Idcc is limited by the limiter, the d-axis current Idc exceeds the d-axis current command value Idcc. In contrast, the graph of the example shown in Figure 10 shows that the d-axis current Ide can nearly track the d-axis current command value Idce. This is thought to be due to the addition of the d-axis limit voltage Vdl, which is a limit voltage addition term, to the current feedback loop.

Regarding the current command value shown in Figure 12, the graph of the current command value Ic in the comparative example shows that immediately after an instantaneous voltage drop, the current command value Ic exceeds 2.5 times the rated current. Therefore, at this point, there is a possibility that the power converter will trip due to an overcurrent. On the other hand, the graph of the current command value Ie in the example shows that the current command value Ie is appropriately limited even during an instantaneous voltage drop.

(2) Simulation of Transition from Grid-Connected Operation to Insulated Operation Below, as a second simulation, the results of a simulation in which switching from grid-connected operation to isolated operation is performed in the power supply system 1 of the above embodiment are shown below. In the second simulation, an example and a comparative example (hereinafter referred to as Comparative Example 1) similar to those in the first simulation were used to verify whether the switching from grid-connected operation to isolated operation achieved in Comparative Example 1 can be achieved in the example as well. Furthermore, a simulation in which switching from grid-connected operation to isolated operation is performed in the power supply system 1 of the above embodiment is also performed using Comparative Example 2, in which the voltage addition term added to the current feedback loop is not limited. In both examples, the simulation is performed in which switching from grid-connected operation to isolated operation is performed 5 seconds after the start of the simulation.

Figure 13 is a graph showing the time changes in AC voltage and frequency in AC wiring based on the results of the second simulation for the example. Also, Figure 14 is a graph showing the time changes in AC voltage and frequency in AC wiring based on the results of the second simulation for Comparative Example 1. The AC voltage in these graphs is a value equivalent to the AC voltage Vac obtained by the above formula (1). Also, the frequency in these graphs is a value equivalent to the frequency fac obtained by the voltage calculation unit 71.

According to the graph of AC voltage Vc1 and frequency fc1 of Comparative Example 1 shown in Figure 14, when switching from grid-connected operation to standalone operation, AC voltage Vc1 and frequency fc1 fluctuate slightly due to fluctuations in the power output from the power converter, etc. This is the response expected in virtual generator model control. The graph of AC voltage Ve and frequency fe of the example shown in Figure 13 also shows results similar to those of the graph of Comparative Example 1 shown in Figure 14. Therefore, it is believed that switching from grid-connected operation to standalone operation can be achieved without any problems in the power supply system 1 of the example, just like in Comparative Example 1.

Figure 15 is a graph showing the time variations in AC voltage and frequency in the AC wiring, based on the results of the second simulation for Comparative Example 2. In Comparative Example 2, after parallel-off between the AC power supply system 2 and the power converter 4 occurs 5 seconds after the start of the simulation, both the AC voltage Vc2 and frequency fc2 become abnormal values, indicating that autonomous operation is not possible.

(3) Summary of Simulations From the results of the above two simulations, it can be seen that the control mode of the power supply system 1 in the above embodiment makes it possible to switch between independent operation and grid-connected operation without changing the control method, and that even if a momentary voltage drop occurs during grid-connected operation, grid-connected operation can be continued and a quick recovery from the voltage drop state can be achieved when the voltage in the AC power supply system 2 recovers.

[Other embodiments]
Although the embodiments of the present disclosure have been described above, the present disclosure is not limited to the above-described embodiments, and various improvements, changes, and modifications are possible within the scope of the spirit of the present disclosure.

For example, in the above embodiment, the AC wiring 5 in the power supply system 1 is described as a three-phase system, but this is not limited to this. For example, even if the AC wiring 5 is a single-phase two-wire system or a single-phase three-wire system, a similar power supply system 1 can be constructed, except that the various calculation methods differ depending on the system type.

In addition, in the above embodiment, the drive signal generator 85 adds the d-axis limit voltage Vdl to the feedback loop of the d-axis current Id. However, in addition to or instead of this, the q-axis limit voltage Vql may be added to the feedback loop of the q-axis current Iq. That is, the drive signal generator 85 may generate the q-axis voltage command value Vq_ref by adding a q-axis limit voltage Vql, which limits the q-axis voltage Vq to a value based on the deviation D5q = Iq_cmd - Iq of the q-axis current Iq from the q-axis current command value Iq_cmd, as a limit voltage.

In the above embodiment, the second range, which is the limit range of the d-axis voltage Vd for generating the d-axis limit voltage Vd1, is set to 0 or more and 1 or less, where the rated voltage of the d-axis voltage Vd is 1. However, this is not limited to this. However, the second range is preferably a range of 1 or less, where the rated voltage of the d-axis voltage Vd is 1. Furthermore, when the q-axis limit voltage Vql is added to the feedback loop of the q-axis current Iq, the d-axis limit voltage Vdl may be a fixed value such as 1, where the rated voltage of the d-axis voltage Vd is 1. Furthermore, the third range, which is the limit range of the q-axis voltage Vq for generating the q-axis limit voltage Vq1, can be set appropriately depending on the power supply system 1. However, the third range is preferably a range that includes 0, where the rated voltage of the q-axis voltage Vq is 1.

In addition, in the above embodiment, an example was shown in which the drive signal generation unit 85 subtracts the interference component XqIq from the fifth deviation D5d regarding the d-axis current Id and adds the interference component XdId to the fifth deviation D5q regarding the q-axis current Iq to perform decoupling processing between the d-axis and q-axes, but the subtraction of the interference component XqIq and the addition of the interference component XdId do not have to be performed.

Summary of the Disclosure
A controller of a power converter according to one aspect of the present disclosure is a controller of a power converter that performs power conversion between an energy storage facility and an AC power supply system, and the controller acquires a voltage and a frequency in AC wiring that connects the AC power supply system and the power converter, and a current output from the power converter to the AC wiring, calculates a frequency target value by a frequency target value calculation process that includes an operation of multiplying a value based on a first deviation that is a deviation of the power converter active power from a predetermined active power command value by a coefficient that indicates the first drooping characteristic, so that a relationship of frequency with respect to power converter active power output by the power converter to the AC wiring has a predetermined first drooping characteristic, calculates an internal phase difference angle by integrating a second deviation that is a deviation of the frequency in the AC wiring from the frequency target value, and calculates a voltage and a frequency in AC wiring that connects the AC power supply system and the power converter, and a current output from the power converter to the AC wiring calculates an internal electromotive force target value by adding a predetermined reference voltage to a value based on a third deviation that is a deviation of the power converter reactive power output by the power converter; calculates a current target value from the internal phase difference angle, the internal electromotive force target value, and a voltage in the AC wiring; generates a current command value by limiting the current target value to a predetermined first range; calculates a power correction value from a fourth deviation that is a deviation of the current command value from the current target value and the voltage in the AC wiring; corrects the frequency target value by correcting the first deviation with the power correction value; generates a limit voltage that limits the voltage in the AC wiring to a predetermined limit range; and generates a voltage command value by adding the limit voltage to a value based on a fifth deviation that is a deviation of the current output to the AC wiring from the current command value.

With the above configuration, the power converter is driven using a current command value that limits the current target value estimated as the current output from the virtual generator to within a first range. This makes it possible to suppress overcurrent during momentary voltage drops. Furthermore, because a limit voltage that limits the voltage in the AC wiring to within a predetermined limit range is added to the feedback loop of the current command value, the responsiveness of current control during momentary voltage drops can be improved, making it possible to prevent tripping due to overcurrent.

Furthermore, in the above configuration, a power correction value is calculated from the fourth deviation, which is the deviation of the current command value from the current target value, and the voltage in the AC wiring. The calculated power correction value is subtracted from the first deviation used to calculate the frequency target value. This limits the current command value to the current target value estimated as the current output from the virtual generator, and suppresses an increase in the frequency target value associated with a decrease in power output due to the output power being determined based on this limited current command value. As a result, an increase in the internal phase difference angle that occurs during a momentary voltage drop is suppressed. Therefore, the occurrence of power oscillations is suppressed when voltage recovers in the AC power system, enabling rapid recovery from a voltage drop state.

In addition, in the above configuration, a voltage that limits the voltage in the AC wiring to within a predetermined limit range is added as an additive term to the voltage added to the current feedback loop to generate the drive signal. This makes it possible to switch between grid-connected operation and stand-alone operation without changing the control method, as with conventional virtual generator model control, while also improving the responsiveness of current control when a momentary voltage drop occurs.

The current command value may include a d-axis current command value and a q-axis current command value, and the controller may calculate a d-axis voltage and a q-axis voltage from the instantaneous value of the voltage in the AC wiring, calculate a d-axis current and a q-axis current from the instantaneous value of the current in the AC wiring, add a d-axis limit voltage that limits the d-axis voltage within a predetermined second range as the limit voltage to a value based on the deviation of the d-axis current from the d-axis current command value to generate a d-axis voltage command value, and generate a q-axis voltage command value based on the deviation of the q-axis current from the q-axis current command value.

The magnitude of the d-axis limit voltage may be greater than or equal to 0 and less than or equal to 1, where the rated voltage of the d-axis voltage is 1.

2 AC power supply system 3 Power storage equipment 4 Power converter 10 Controller 71 Voltage calculation unit 72 Current calculation unit 80 Frequency target value calculation unit 82 Internal phase difference angle calculation unit 83 Internal electromotive voltage target value calculation unit 84 Current command value calculation unit 85 Drive signal generation unit

Claims (4)

A controller for a power converter that performs power conversion between a power storage facility and an AC power supply system,
acquiring a voltage and a frequency in an AC wiring that connects the AC power supply system and the power converter, and a current output from the power converter to the AC wiring;
calculating a frequency target value by a frequency target value calculation process including a calculation of multiplying a value based on a first deviation, which is a deviation of the power converter active power from a predetermined active power command value, by a coefficient indicating the first drooping characteristic so that a relationship between the frequency and the power converter active power output by the power converter to the AC wiring has the predetermined first drooping characteristic;
calculating an internal phase difference angle by integrating a second deviation, which is a deviation of the frequency in the AC wiring from the frequency target value;
calculating an internal electromotive voltage target value by adding a predetermined reference voltage to a value based on a third deviation, which is a deviation of the power converter reactive power output by the power converter to the AC wiring from a predetermined reactive power command value;
calculating a current target value from the internal phase difference angle, the internal electromotive voltage target value, and a voltage in the AC wiring;
generating a current command value by limiting the current target value to within a predetermined first range;
calculating a power correction value from a fourth deviation, which is a deviation of the current command value from the current target value, and a voltage in the AC wiring;
correcting the first deviation using the power correction value to correct the frequency target value;
a controller that generates a limit voltage that limits the voltage in the AC wiring to a predetermined limit range, and generates a voltage command value by adding the limit voltage to a value based on a fifth deviation that is a deviation of the current output to the AC wiring from the current command value, thereby generating a drive signal for the power converter. the current command value includes a d-axis current command value and a q-axis current command value,
The controller
calculating a d-axis voltage and a q-axis voltage from the instantaneous value of the voltage in the AC wiring;
calculating a d-axis current and a q-axis current from the instantaneous value of the current in the AC wiring;
generating a d-axis voltage command value by adding a d-axis limit voltage, which limits the d-axis voltage within a predetermined second range as the limit voltage, to a value based on a deviation of the d-axis current from the d-axis current command value;
The controller according to claim 1 , wherein the controller generates a value based on a deviation of the q-axis current from the q-axis current command value as a q-axis voltage command value. The controller of claim 2, wherein the magnitude of the d-axis limit voltage is greater than or equal to 0 and less than or equal to 1, where the rated voltage of the d-axis voltage is 1. A control method for a power converter that performs power conversion between a power storage facility and an AC power supply system, comprising:
acquiring a voltage and a frequency in an AC wiring that connects the AC power supply system and the power converter, and a current output from the power converter to the AC wiring;
calculating a frequency target value by a frequency target value calculation process including a calculation of multiplying a value based on a first deviation, which is a deviation of the power converter active power from a predetermined active power command value, by a coefficient indicating the first drooping characteristic so that a relationship between the frequency and the power converter active power output by the power converter to the AC wiring has the predetermined first drooping characteristic;
calculating an internal phase difference angle by integrating a second deviation, which is a deviation of the frequency in the AC wiring from the frequency target value;
calculating an internal electromotive voltage target value by adding a predetermined reference voltage to a value based on a third deviation, which is a deviation of the power converter reactive power output by the power converter to the AC wiring from a predetermined reactive power command value;
calculating a current target value from the internal phase difference angle, the internal electromotive voltage target value, and a voltage in the AC wiring;
generating a current command value by limiting the current target value to within a predetermined first range;
calculating a power correction value from a fourth deviation, which is a deviation of the current command value from the current target value, and a voltage in the AC wiring;
correcting the first deviation using the power correction value to correct the frequency target value;
a limiting voltage that limits the voltage in the AC wiring to a predetermined second range is generated, and a voltage command value is generated by adding the limiting voltage to a value based on a fifth deviation that is a deviation of the current output to the AC wiring from the current command value, thereby generating a drive signal for the power converter.