POWER CONVERSION DEVICE

A power conversion device includes a power converter connected to a power storage element, and a control device. The control device includes a generator simulating unit to generate a voltage command value, and a signal generating unit to generate a control signal for the power converter based on the voltage command value. The generator simulating unit includes a first characteristics simulating unit to generate a first command value by simulating characteristics of a first synchronous generator, a second characteristics simulating unit to generate a second command value by simulating characteristics of a second synchronous generator different from the characteristics of the first synchronous generator, an adder to perform addition of the first command value and the second command value, and a voltage command generating unit to generate the voltage command value, based on an addition result of the first command value and the second command value.

TECHNICAL FIELD

The present disclosure relates to a power conversion device.

BACKGROUND ART

In recent years, many distributed power sources using renewable energy such as photovoltaic power systems have been introduced to power grids. Distributed power sources are often connected to a power grid through a power converter. Therefore, as the number of distributed power sources connected to a power grid increases, the proportion of synchronous power generators connected to the power grid decreases and the inertia energy in the power grid decreases, which increases a change in frequency at a time of load abrupt change. Virtual synchronous machine control has been proposed, which allows a power converter to behave similarly to a synchronous generator and thereby makes up for the decreased inertia energy. The power converter with virtual synchronous machine control is controlled so as to simulate the behavior of a synchronous generator to be simulated being connected to a power grid.

For example, WO2019/187411 (PTL 1) discloses a control device for a distributed power source. The control device calculates a virtual inertia value based on the specifications and operating state of a distributed power source and sets a virtual inertia in a power conversion device based on the virtual inertia value and a request inertia value requested from a grid operator.

CITATION LIST

Patent Literature

SUMMARY OF INVENTION

Technical Problem

A plurality of synchronous generators are connected to a power grid and the synchronous generators often have different characteristics. There is a need for improving the stability of the power grid by simulating the characteristics of a plurality of synchronous generators using a power converter. PTL 1 neither teaches nor suggests techniques for such a need.

An object in an aspect of the present disclosure is to provide a power conversion device capable of improving the stability of a power grid by simulating the characteristics of a plurality of synchronous generators.

Solution to Problem

A power conversion device according to an embodiment includes a power converter connected to a power storage element, and a control device to control the power converter. The power converter converts a DC power output from the power storage element into an AC power and outputs the AC power to a power grid. The control device includes a generator simulating unit to generate a voltage command value for the power converter by simulating characteristics of a plurality of synchronous generators, and a signal generating unit to generate a control signal for the power converter, based on the voltage command value generated by the generator simulating unit. The generator simulating unit includes a first characteristics simulating unit to generate a first command value by simulating characteristics of a first synchronous generator, a second characteristics simulating unit to generate a second command value by simulating characteristics of a second synchronous generator different from the characteristics of the first synchronous generator, an adder to perform addition of the first command value and the second command value, and a voltage command generating unit to generate the voltage command value, based on an addition result of the first command value and the second command value.

Advantageous Effects of Invention

According to the present disclosure, the stability of a power grid can be improved by simulating the characteristics of a plurality of synchronous generators.

DESCRIPTION OF EMBODIMENTS

Embodiments will be described below with reference to the drawings. In the following description, like parts are denoted by like signs. Their names and functions are also the same. A detailed description thereof will not be repeated.

First Embodiment

FIG.1is a diagram showing an overall configuration of a power conversion system according to a first embodiment. The power conversion system includes a power grid2, a transformer3, a power conversion device6, a current detector7, a voltage detector8, and a power storage element60. Power grid2is, for example, a three phase AC power source. Power conversion device6includes a control device100and a power converter20. Power converter20is connected to an interconnection point4of power grid2through transformer3. Instead of transformer3, an interconnection reactor may be connected to power converter20.

Power converter20is a power converter connected to power storage element60to perform power conversion between power storage element60and power grid2. Specifically, power converter20converts a DC power output from power storage element60into an AC power and outputs the AC power to power grid2through transformer3. Power converter20also converts an AC power from power grid2into a DC power and outputs the DC power to power storage element60. Power converter20thus charges and discharge power of power storage element60. Power converter20is, for example, a self-commutated converter such as a two-level converter, a three-level converter, or a modular multilevel converter. Power storage element60is, for example, an energy storage element such as an electric double layer capacitor or a secondary battery.

Current detector7detects three phase AC current at interconnection point4of power grid2. Specifically, current detector7detects an a-phase AC current Ia, a b-phase AC current Ib, and a c-phase AC current Ic flowing between interconnection point4and power converter20. AC currents Ia, Ib, and Ic are input to control device100. Hereinafter AC currents Ia, Ib, and Ic may be collectively referred to as AC current Isys.

Voltage detector8detects three phase AC voltage at interconnection point4of power grid2. Specifically, voltage detector8detects an a-phase AC voltage Va, a b-phase AC voltage Vb, and a c-phase AC voltage Vc at interconnection point4. AC voltages Va, Vb, and Vc are input to control device100. Hereinafter AC voltages Va, Vb, and Vc may be collectively referred to as AC voltage Vsys.

Control device100is a device that controls the operation of power converter20. Specifically, control device100includes, as a main functional configuration, a generator simulating unit101and a signal generating unit103. The functions of generator simulating unit101and signal generating unit103are implemented by a processing circuit. The processing circuit may be dedicated hardware or may be a CPU that executes a program stored in an internal memory of control device100. When the processing circuit is dedicated hardware, the processing circuit is configured with, for example, an FPGA, an ASIC, or a combination thereof.

Generator simulating unit101generates a voltage command value for power converter20by simulating the characteristics of a plurality of synchronous generators based on AC voltage Vsys and AC current Isys at interconnection point4. Specifically, generator simulating unit101includes a first characteristics simulating unit11, a second characteristics simulating unit12, an adder13, a reactive power command generating unit14, and a voltage command generating unit15.

First characteristics simulating unit11simulates characteristics of a first synchronous generator to output an active power command value P1xindicating a target value of active power to be output to simulate the characteristics. Second characteristics simulating unit12simulates characteristics of a second synchronous generator different from the characteristics of the first synchronous generator to output an active power command value P2xindicating a target value of active power to be output to simulate the characteristics. A specific configuration of first characteristics simulating unit11and second characteristics simulating unit12will be described later. Hereinafter first characteristics simulating unit11and second characteristics simulating unit12may be collectively referred to as “characteristics simulating unit10”.

Adder13adds active power command value P1xand active power command value P2xto generate an active power command value Pref (=P1x+P2x) as the addition result. Active power command value Pref is a target value of active power to be output by power converter20in order to simulate both of the characteristics of the first synchronous generator and the characteristics of the second synchronous generator.

Reactive power command generating unit14performs automatic AC voltage regulation for bringing a detection value of AC voltage Vsysy to a rated value, based on a detection value of AC voltage Vsys. In addition, reactive power command generating unit14performs automatic reactive power regulation for bringing a reactive power measurement value to a target value, based on a reactive power measurement value calculated from detection values of AC voltage Vsys and AC current Isys. Reactive power command generating unit14generates a reactive power command value Qref by performing automatic AC voltage regulation and automatic reactive power regulation.

Voltage command generating unit15generates a voltage command value Vref based on the addition result of adder13(that is, active power command value Pref). Specifically, voltage command generating unit15calculates an active current component and a reactive current component by variable transformation of detection values of three phase AC current Isys. Further, voltage command generating unit15calculates an active voltage component and a reactive voltage component by variable transformation of detection values of three phase AC voltage Vsys. Voltage command generating unit15generates voltage command value Vref of each phase of power converter20such that an active power is output in accordance with active power command value Pref and a reactive power is output in accordance with reactive power command value Qref, based on the active current component, the reactive current component, the active voltage component, and the reactive voltage component.

Signal generating unit103generates a control signal for power converter20, based on voltage command value Vref generated by generator simulating unit101, and outputs the generated control signal to power converter20. Specifically, signal generating unit103includes a three phase voltage generating unit17and a pulse width modulation (PWM) control unit19.

Three phase voltage generating unit17generates three phase sinusoidal voltages Va*, Vb*, and Vc* based on the absolute value |Vref| and the phase θref of voltage command value Vref. Specifically, Va *=|Vref|×sin(θref), Vb*=|Vref|×sin(θref+2π/3), and Vc*=|Vref|×sin(θref+4π/3) are generated.

PWM control unit19performs pulse width modulation for each of three phase sinusoidal voltages Va*, Vb*, and Vc* and generates a control signal as a PWM signal. PWM control unit19outputs the control signal to power converter20. Typically, the control signal is a gate control signal for controlling the on/off of each switching element included in power converter20.

FIG.2is a diagram showing a hardware configuration example of control device100.FIG.2shows an example in which control device100is configured with a computer.

Referring toFIG.2, control device100includes one or more input converters70, one or more sample and hold (S/H) circuits71, a multiplexer (MUX)72, an A/D converter73, one or more central processing units (CPU)74, a random access memory (RAM)75, a read only memory (ROM)76, one or more input/output interfaces77, and an auxiliary storage device78. Control device100also includes a bus79connecting the components to each other.

Input converter70has an auxiliary transformer for each input channel. Each auxiliary transformer converts a detection signal by current detector7and voltage detector8inFIG.1into a signal with a voltage level suitable for the subsequent signal processing.

Sample and hold circuit71is provided for each input converter70. Sample and hold circuit71samples and holds a signal indicating the electrical quantity received from the corresponding input converter70at a preset sampling frequency.

Multiplexer72sequentially selects signals held by a plurality of sample and hold circuits71. A/D converter73converts a signal selected by multiplexer72into a digital value. A plurality of A/D converters73may be provided to perform A/D conversion for detection signals of a plurality of input channels in parallel.

CPU74controls the entire control device100and performs computational processing under instructions of a program. RAM75as a volatile memory and ROM76as a nonvolatile memory are used as a main memory of CPU74. ROM76stores a program and setting values for signal processing. Auxiliary storage device78is a nonvolatile memory having a larger capacity than ROM76and stores a program and data such as electrical quantity detection values.

Input/output interface77is an interface circuit for communication between CPU74and an external device.

Unlike the example ofFIG.2, at least a part of control device100may be configured using circuitry such as a field programmable gate array (FPGA) and an application specific integrated circuit (ASIC).

<Configuration of Characteristics Simulating Unit>

FIG.3is a block diagram showing a specific functional configuration of the characteristics simulating unit according to the first embodiment. Referring toFIG.3, characteristics simulating unit10includes a divider30, a subtractor31, proportional elements32and34, integrators33and36, adders35and37, and a computing element38.

Divider30calculates a torque output value Te (=Px/ω) by dividing an active power command value Px by angular frequency ω. Subtractor31calculates a difference ΔT (=Te−Td) between torque output value Te and damping torque Td. In this way, the damping force of a synchronous generator is simulated in the control of power converter20. Proportional element32multiplies difference ΔT by “1/M”. “M” is the moment of inertia (also referred to as inertia constant) of the rotor of the synchronous generator (hereinafter also referred to as “virtual synchronous generator”) to be simulated by characteristics simulating unit10. Active power command value Px corresponds to a target value of active power to be output from power converter20to simulate the characteristics equivalent to the synchronous generator.

Integrator33outputs angular frequency deviation Δω by integrating the multiplication value of proportional element32(that is, ΔT/M) with respect to time. Angular frequency deviation Δω corresponds to the difference between angular frequency ω of the rotor of the virtual synchronous generator and a reference angular frequency ω0of power grid2. Reference angular frequency ω0is the angular frequency of a reference frequency (for example, 50 Hz or 60 Hz) of power in power grid2. Proportional element34calculates damping torque Td by multiplying angular frequency deviation Δω by “D”. “D” is a damping coefficient of the virtual synchronous generator. Adder35calculates angular frequency ω by adding angular frequency deviation Δω and reference angular frequency ω0.

Integrator36outputs a phase deviation Δθm by integrating angular frequency deviation Δω with respect to time. Phase deviation Δθm corresponds to the difference between the phase of AC voltage Vsys at interconnection point4and the phase of the rotor of the virtual synchronous generator. Adder37calculates a phase deviation Δθ (=Δθm+Δθo) by adding phase deviation Δθm and phase deviation Δθo. Phase deviation400corresponds to the difference between the phase of AC voltage Vsys at interconnection point4and the reference phase of AC voltage to from power converter20. Phase deviation Δθ corresponds to the difference between the phase of AC voltage Vsys at interconnection point4and the phase of AC voltage to be output from power converter20.

Computing element38calculates active power command value Px, based on power supply voltage Vs of power grid2, AC voltage Vsys at interconnection point4, phase deviation Δθ, and inductance Xg of power converter20. It is assumed that power supply voltage Vs is the rated voltage. Computing element38calculates active power command value Px by dividing the multiplication value of power supply voltage Vs, AC voltage Vsys, and phase deviation Δθ (that is, Vs×Vsys×Δθ) by inductance Xg.

First characteristics simulating unit11and second characteristics simulating unit12each simulate the corresponding synchronous generator, based on the corresponding moment of inertia M and damping coefficient D. Specifically, first characteristics simulating unit11generates an active power command value P1xfor simulating the characteristics of the first synchronous generator using the moment of inertia M1and damping coefficient D1. Second characteristics simulating unit12generates an active power command value P2xfor simulating the characteristics of the second synchronous generator using the moment of inertia M2and damping coefficient D2. The characteristics of the first synchronous generator are dependent on the values of the moment of inertia M1and damping coefficient D1, and the characteristics of the second synchronous generator are dependent on the values of the moment of inertia M2and damping coefficient D2. The moment of inertia M1is different from the moment of inertia M2. In addition, damping coefficient D1may be different from damping coefficient D2.

Typically, due to a large inertia and loss, a synchronous generator has a characteristic of suppressing a frequency component due to resonance between the connected power grid and the synchronous generator (for example, frequency component due to the impedance of the power grid, and the moment of inertia and the internal impedance of the synchronous generator). Therefore, when a plurality of synchronous generators with different moments of inertia and internal impedances are connected to the power grid, the frequency component due to resonance between the power grid and each synchronous generator is suppressed. Generator simulating unit101according to the first embodiment simulates the characteristics of both of the first synchronous generator and the second synchronous generator. As a result, the frequency component due to resonance between each of the first synchronous generator and the second synchronous generator and power grid2is suppressed, thereby improving the stability of the frequency range corresponding to each frequency component.

Second Embodiment

FIG.4is a diagram showing an overall configuration of a power conversion system according to a second embodiment. In the power conversion system inFIG.4, control device100of the power conversion system inFIG.1is replaced by a control device100A. In control device100A, generator simulating unit101of control device100is replaced by a generator simulating unit101A. The configuration is similar to the configuration ofFIG.1except for generator simulating unit101A and will not be further elaborated.

Generator simulating unit101A includes a first characteristics simulating unit11A, a second characteristics simulating unit12A, an adder13A, and a voltage command generating unit15A.

First characteristics simulating unit11A simulates characteristics of a first synchronous generator to output a phase command value θ1xindicating a target value of the phase of voltage to be output to simulate the characteristics. Second characteristics simulating unit12simulates characteristics of a second synchronous generator to output a phase command value θ2xindicating a target value of the phase of voltage to be output to simulate the characteristics. Hereinafter first characteristics simulating unit11A and second characteristics simulating unit12A may be collectively referred to as “characteristics simulating unit10A”.

Adder13A adds phase command value θ1xand phase command value θ2xto generate a phase command value θref (=θ1x+θ2x) as the addition result. Phase command value θref is a target value of the phase of voltage to be output by power converter20to simulate both of the characteristics of the first synchronous generator and the characteristics of the second synchronous generator.

Voltage command generating unit15A generates a voltage command value Vref for power converter20, based on the phase in accordance with the addition result of adder13A (that is, phase command value θref). Specifically, voltage command generating unit15A sets the absolute value |Vo| of a target value Vo of AC voltage Vsys (hereinafter also referred to as target voltage) as the magnitude of voltage command value Vref (that is, absolute value |Vref|). Furthermore, voltage command generating unit15A sets phase command value θref as the phase of voltage command value Vref.

As explained inFIG.1, signal generating unit103generates a control signal for power converter20, based on the absolute value |Vref| and the phase θref of voltage command value Vref generated by generator simulating unit101, and outputs the generated control signal to power converter20.

<Configuration of Characteristics Simulating Unit>

FIG.5is a block diagram showing a specific functional configuration of a characteristics simulating unit according to the second embodiment. Referring toFIG.5, characteristics simulating unit10A includes subtractors50and51, proportional elements52and54, integrators53and56, and an adder55.

Subtractor50calculates a difference ΔP between active power command value Px and active power P. Active power P is an active power at the present time supplied to interconnection point4and calculated based on AC current Isys detected by current detector7and AC voltage Vsys detected by voltage detector8. Active power command value Px is set as appropriate by a grid operator.

Subtractor51subtracts an output value of proportional element54from difference ΔP. The output value of proportional element54is the multiplication value “D*×Δω” of angular frequency deviation Δω and damping coefficient D+. The damping force of a synchronous generator is simulated in the control of power converter20by subtracting the multiplication value “D*×Δω” from difference ΔP. Proportional element52multiplies difference ΔP by “1/M*”.

“M*” is the moment of inertia of the rotor of a synchronous generator to be simulated by characteristics simulating unit10A, and “D*” is the damping coefficient of the synchronous generator. The moment of inertia M and damping coefficient D inFIG.3are values used to calculate angular frequency deviation Aw based on torque, and the moment of inertia M* and damping coefficient D* inFIG.5are values used to calculate the angular frequency deviation Δω based on active power.

Integrator53outputs angular frequency deviation Δω by integrating the multiplication value of proportional element52(that is, ΔP/M*) with respect to time. Adder55calculates angular frequency ω by adding angular frequency deviation Δω and reference angular frequency ω0. Integrator56outputs a phase command value θx by integrating angular frequency ω with respect to time.

First characteristics simulating unit11A and second characteristics simulating unit12A each simulate the corresponding synchronous generator, based on the corresponding moment of inertia M* and damping coefficient D*. Specifically, first characteristics simulating unit11A generates a phase command value θ1xfor simulating the characteristics of the first synchronous generator using the moment of inertia M1* and damping coefficient D1*. Second characteristics simulating unit12generates a phase command value θ2xfor simulating the characteristics of the second synchronous generator using the moment of inertia M2* and damping coefficient D2*. The characteristics of the first synchronous generator are dependent on the values of the moment of inertia M1* and damping coefficient D1*, and the characteristics of the second synchronous generator are dependent on the values of the moment of inertia M2* and damping coefficient D2*. The moment of inertia M1* is different from the moment of inertia M2*. In addition, damping coefficient D1* may be different from damping coefficient D2*.

Generator simulating unit101A according to the second embodiment simulates the characteristics of both of the first synchronous generator and the second synchronous generator. As a result, the frequency component due to resonance between each of the first synchronous generator and the second synchronous generator and power grid2is suppressed, thereby improving the stability of the frequency range corresponding to each frequency component.

Other Embodiments

(1) A modification of generator simulating unit101and generator simulating unit101A will be described.FIG.6is a diagram for explaining a modification of a generator simulating unit. Specifically,FIG.6(a)shows a modification of generator simulating unit101according to the first embodiment.FIG.6(b)shows a modification of generator simulating unit101A according to the second embodiment.

Referring toFIG.6(a), compared to the configuration inFIG.1, a proportional element41is added between first characteristics simulating unit11and adder13. Further, a proportional element42is added between second characteristics simulating unit12and adder13. With this configuration, adder13outputs an active power command value Pref by adding a value obtained by multiplying active power command value P1xby a gain G1(that is, P1x×G1) and a value obtained by multiplying active power command value P2xby a gain G2(that is, P2x×G2).

Referring toFIG.6(b), compared to the configuration inFIG.4, a proportional element41is added between first characteristics simulating unit11A and adder13A, and a proportional element42is added between second characteristics simulating unit12A and adder13A. With this configuration, adder13A outputs a phase command value θref by adding a value obtained by multiplying phase command value θ1xby a gain G1(that is, θ1x×G1) and a value obtained by multiplying phase command value θ2xby a gain G2(that is, θ2x×G2).

By adjusting gains G1and G2, the oscillation of power grid2can be suppressed more effectively. For example, when the oscillation of power grid2can be suppressed more by making the moment of inertia of the first synchronous generator larger than the moment of inertia of the second synchronous generator, gain G1is set to be larger than gain G2. On the other hand, when the oscillation of power grid2can be suppressed more by making the moment of inertia of the second synchronous generator larger than the moment of inertia of the first synchronous generator, gain G2is set to be larger than gain G1.

Specifically, power grid2includes a first oscillation frequency component corresponding to the first synchronous generator and a second oscillation frequency component corresponding to the second synchronous generator. The first oscillation frequency component is a frequency component due to resonance between power grid2and the first synchronous generator and has a frequency and an amplitude resulting from the impedance of power grid2and the moment of inertia and the internal impedance of the first synchronous generator. The second oscillation frequency component is a frequency component due to resonance between the second synchronous generator and power grid2and has a frequency and an amplitude resulting from the impedance of power grid2and the moment of inertia and the internal impedance of the second synchronous generator.

Gain G1and gain G2are set based on the magnitudes (for example, amplitudes) of the first oscillation frequency component and the second oscillation frequency component. For example, when the first oscillation frequency component is larger than the second oscillation frequency component, gain G1is set to be larger than gain G2to suppress the oscillation of power grid2more effectively. On the other hand, when the second oscillation frequency component is larger than the first oscillation frequency component, gain G2is set to be larger than gain G1to suppress the oscillation of power grid2more effectively.

(2) In the foregoing embodiments, a configuration in which the generator simulating unit simulates two synchronous generators: a first synchronous generator and a second synchronous generator has been described. However, embodiments are not limited to this configuration. The generator simulating unit may simulate three or more synchronous generators.

(3) The above configurations described as embodiments are examples of the configuration of the present disclosure, can be combined with other known techniques, and are susceptible to modifications such as partial omission without departing from the spirit of the present disclosure. In the foregoing embodiments, the processing and configuration described in other embodiments may be employed and carried out, if necessary.

Embodiments disclosed here should be understood as being illustrative rather

than being limitative in all respects. The scope of the present disclosure is shown not in the foregoing description but in the claims, and it is intended that all modifications that come within the meaning and range of equivalence to the claims are embraced here.

REFERENCE SIGNS LIST