Power conversion device

Abstract

A power conversion device (50) comprises a power converter (20) and a control device (10). The control device (10): generates a first voltage command value for the power converter (20) by simulating the characteristics of a synchronous generator on the basis of an AC voltage and an AC current in a power system (30); generates a first current command value for the power converter (20) on the basis of the first voltage command value; sets a limit value so that a second current range according to the limit value is smaller than a first current range according to the current capacity of the power converter (20) when an accident is not detected in the power system (30); generates a second current command value by limiting the first current command value within the second current range; generates a second voltage command value on the basis of the second current command value; and generates a control signal for the power converter (20) on the basis of the second voltage command value.

Description

Power Conversion Device

[0001] The present disclosure relates to a power conversion device.

[0002] In recent years, many distributed power sources using renewable energy, such as solar power generation facilities, have been introduced into power grids. As a result, an increase in the rate of change of frequency (RoCoF), which indicates the magnitude of frequency change per unit time, has become an issue in the event of an accident such as a generator tripping. Frequency stabilization devices stabilize power grids by controlling the active power output from power sources such as solar panels and storage batteries using power converters and applying a pseudo-inertial force to the power grid.

[0003] For example, Japanese Patent Laid-Open Publication No. 2019-176584 (Patent Document 1) discloses a control device for a distributed power source. This control device sets a virtual inertia for a power conversion device that connects a distributed power source to a power grid, calculates a virtual inertia value based on the specifications and operating state of the distributed power source, and sets the virtual inertia for the power conversion device based on either the calculated virtual inertia value or a required inertia value requested by a grid operator.

[0004] JP 2019-176584 A

[0005] In the above-described power conversion device, during normal operation, active power is output up to the upper limit. However, in this case, even if the power conversion device attempts to increase the active power output to the power grid in the event of a grid fault (e.g., a generator trip), the power conversion device does not have the capacity to do so. Therefore, the power conversion device cannot apply inertial force to the power grid and cannot stabilize the grid frequency.

[0006] An object in one aspect of the present disclosure is to provide a power conversion device that can provide inertial force when an accident occurs in the power system while supplying active power to the power system under normal circumstances.

[0007] A power conversion device according to an embodiment includes a power converter that performs power conversion between a DC circuit and an electric power grid, and a control device that controls the power converter. The control device includes: a generator simulation unit that generates a first voltage command value for the power converter by simulating characteristics of a synchronous generator based on an AC voltage and an AC current in the electric power grid, a current command generation unit that generates a first current command value for the power converter based on the first voltage command value generated by the generator simulation unit, a setting unit that sets a limit value so that a second current range based on the limit value is smaller than a first current range based on a current capacity of the power converter when no fault is detected in the electric power grid, a limiter that limits the first current command value within the second current range to generate a second current command value, a voltage command generation unit that generates the second voltage command value based on the second current command value, and a signal generation unit that generates a control signal for the power converter based on the second voltage command value.

[0008] A power conversion device according to another embodiment includes a power converter that performs power conversion between a DC circuit and a power grid, and a control device that controls the power converter. The control device includes: a generator simulation unit that generates a first voltage command value for the power converter by simulating characteristics of a synchronous generator based on an AC voltage and an AC current in the power grid; a current command generation unit that generates a first current command value for the power converter based on the first voltage command value generated by the generator simulation unit; a limiter that generates a second current command value by limiting the first current command value within a current range determined by a current capacity of the power converter; a voltage command generation unit that generates a second voltage command value based on the second current command value; and a signal generation unit that generates a control signal for the power converter based on the second voltage command value. When no fault is detected in the power grid, the generator simulation unit generates a phase of the first voltage command value based on a second active power that is smaller than a first active power calculated based on the current capacity of the power converter.

[0009] According to the power conversion device according to the present disclosure, it is possible to provide an inertial force in the event of a fault in the power grid while supplying active power to the power grid during normal times.

[0010] FIG. 1 is a diagram for explaining an example of the overall configuration of a power conversion system. FIG. 2 is a diagram showing an example of time variation of active power output from a power converter. FIG. 3 is a diagram showing an example of the hardware configuration of a control device. FIG. 4 is a block diagram showing an example of the functional configuration of a command generation unit according to a first embodiment. FIG. 5 is a block diagram showing an example of the configuration of a voltage amplitude generation unit. FIG. 6 is a flowchart showing an example of processing by a fault detection unit and a setting unit. FIG. 7 is a diagram showing an example of time variation of active power output from a power converter according to a first embodiment. FIG. 8 is a diagram for explaining an example of the configuration of a setting unit according to a modification of the first embodiment. FIG. 9 is a diagram for explaining another example of the configuration of the setting unit according to a modification of the first embodiment. FIG. 10 is a diagram showing an example of time variation of active power output from a power converter according to a modification of the first embodiment. FIG. 11 is a block diagram showing an example of the configuration of a command generation unit according to a second embodiment. FIG. 12 is a block diagram showing an example of the configuration of a command generation unit according to a modification of the second embodiment. FIG. 13 is a diagram for explaining the relationship between an active power command value and a power margin.

[0011] Hereinafter, the present embodiment will be described with reference to the drawings. In the following description, the same components are denoted by the same reference numerals. The names and functions of these components are also the same. Therefore, detailed description thereof will not be repeated.

[0012] 1 is a diagram illustrating an example of the overall configuration of a power conversion system. The power conversion system 1000 includes a power grid 30, a transformer 34, a DC circuit 40, a power conversion device 50, a current detector 91, and a voltage detector 93. The power conversion device 50 includes a control device 10 and a power converter 20.

[0013] Power converter 20 is a power converter that performs power conversion between DC circuit 40 and power grid 30. Specifically, power converter 20 is connected to power grid 30 via transformer 34, converts DC power from DC circuit 40 into AC power, and outputs the AC power to power grid 30.

[0014] The power converter 20 is, for example, a self-excited converter such as a two-level converter, a three-level converter, or a modular multilevel converter. A self-excited converter is a converter that uses self-arc-suppressing elements and can freely control the magnitude and phase of the output voltage. Furthermore, a self-excited converter can convert AC and DC power even without a power grid power source. The power grid 30 is, for example, a three-phase AC grid. The DC circuit 40 is, for example, a renewable energy power source such as a solar cell or wind power generator, a power storage element, a DC transmission system, or the DC terminal of another power converter.

[0015] The current detector 91 detects three-phase AC currents at the interconnection point 32 between the power grid 30 and the power converter 20. Specifically, the current detector 91 detects a U-phase AC current Isysu, a V-phase AC current Isysv, and a W-phase AC current Isysw that flow between the interconnection point 32 and the transformer 34. The AC currents Isysu, Isysv, and Isysw (hereinafter also collectively referred to as "AC current Isys") are input to the control device 10.

[0016] The voltage detector 93 detects three-phase AC voltages at the interconnection point 32 of the power grid 30. Specifically, the voltage detector 93 detects the U-phase AC voltage Vsysu, the V-phase AC voltage Vsysv, and the W-phase AC voltage Vsysw at the interconnection point 32. The AC voltages Vsysu, Vsysv, and Vsysw (hereinafter collectively referred to as "AC voltages Vsys") are input to the control device 10.

[0017] The control device 10 is a device that controls the operation of the power converter 20. Specifically, the control device 10 includes, as its main functional components, a command generating unit 100 and a signal generating unit 200. The functions of the command generating unit 100 and the signal generating unit 200 are realized by processing circuits. The processing circuit may be dedicated hardware, or may be a CPU (Central Processing Unit) that executes a program stored in the internal memory of the control device 10. When the processing circuit is dedicated hardware, the processing circuit is configured, for example, by a field programmable gate array (FPGA), an application specific integrated circuit (ASIC), or a combination thereof.

[0018] The command generating unit 100 mainly has a function of simulating the characteristics of a synchronous generator, and generates a phase θ of the voltage output from the power converter 20 (i.e., the phase of the voltage command value) and an amplitude V of the voltage (i.e., the amplitude of the voltage command value). The phase θ is a reference phase used for controlling the power converter 20. The command generating unit 100 will be described in detail later.

[0019] The signal generating unit 200 generates a control signal for the power converter 20 based on the voltage command value (i.e., the amplitude V and the phase θ) generated by the command generating unit 100, and outputs the control signal to the power converter 20. Specifically, the signal generating unit 200 includes a three-phase voltage generating unit 202 and a PWM control unit 204.

[0020] The three-phase voltage generator 202 generates three-phase sinusoidal voltages Vu*, Vv*, and Vw* by two-phase / three-phase conversion based on the phase θ and the amplitude V.

[0021] The PWM control unit 204 performs pulse width modulation on each of the three-phase sinusoidal wave voltages Vu*, Vv*, and Vw* to generate a control signal as a PWM signal. The PWM control unit 204 outputs the control signal to the power converter 20. Typically, the control signal is a gate control signal for controlling the on / off of each switching element included in the power converter 20.

[0022] The power conversion device 50 as described above has a function of simulating the characteristics of a synchronous generator and adjusts the active power output to the power grid 30 in order to stabilize the system frequency. Here, if the power conversion device 50 is configured to be able to output active power up to an upper limit determined by the hardware performance during normal operation when no fault occurs in the power grid 30, a problem such as that shown in FIG. 2 occurs.

[0023] Fig. 2 is a diagram showing an example of the change over time in active power output from the power converter. The horizontal axis of Fig. 2 represents time, and the vertical axis represents the output active power of the power converter 20. The direction in which active power is output from the power converter 20 to the power grid 30 is defined as the positive direction, and the direction in which active power is input (i.e., absorbed) from the power grid 30 to the power converter 20 is defined as the negative direction. In the example of Fig. 2, the DC circuit 40 is configured from a renewable energy power source that supplies power to the power converter 20.

[0024] 2 indicates an active power upper limit defined by the upper limit of the positive polarity of the current capacity (i.e., the allowable current) determined by the hardware performance of the power converter 20. The “lower limit” in Fig. 2 indicates an active power lower limit defined by the upper limit of the negative polarity of the current capacity of the power converter 20 (i.e., the lower limit of the current capacity).

[0025] 2 , the power converter 20 continues to output active power at an upper limit value under normal circumstances. When a system fault occurs (e.g., a generator in the power system 30 trips) and the system frequency drops, the power converter 20 attempts to increase the active power output to the power system 30 in order to increase the system frequency, but is unable to increase the active power due to limitations in hardware performance. In other words, it is unable to impart a pseudo-inertial force to the power system 30, and is therefore unable to maintain the system frequency at a specified value. Therefore, it is preferable that the power conversion device 50 be configured to be able to increase the active power output when the system frequency drops.

[0026] Furthermore, when a grid fault occurs (for example, a load in the power grid 30 drops) and the grid frequency increases, the power converter 20 reduces the active power output to the power grid 30 in order to lower the grid frequency. However, in the example of Fig. 2, the renewable energy power source serving as the DC circuit 40 cannot absorb active power. Therefore, it is necessary to appropriately set the lower limit of the active power output by the power conversion device 50.

[0027] The power conversion device 50 according to this embodiment is configured so that it can increase the effective power output to the power system 30 and apply an inertial force when the system frequency drops. The specific configuration will be described later.

[0028] <Hardware Configuration> Fig. 3 is a diagram showing an example of the hardware configuration of the control device 10. Fig. 3 shows an example in which the control device 10 is configured by a computer.

[0029] 3, the control device 10 includes one or more input converters 70, one or more sample-and-hold circuits 71, a multiplexer 72, an A / D converter 73, one or more central processing units (CPUs) 74, a random access memory (RAM) 75, a read-only memory (ROM) 76, one or more input / output interfaces 77, and an auxiliary storage device 78. The control device 10 includes a bus 79 that interconnects the components.

[0030] The input converter 70 has an auxiliary transformer for each input channel, which converts the signals detected by the current detector 91 and the voltage detector 93 shown in FIG. 1 into signals of a voltage level suitable for subsequent signal processing.

[0031] A sample-and-hold circuit 71 is provided for each input converter 70. The sample-and-hold circuit 71 samples and holds a signal representing an electrical quantity received from the corresponding input converter 70 at a specified sampling frequency.

[0032] The multiplexer 72 sequentially selects the signals held in the plurality of sample-and-hold circuits 71. The A / D converter 73 converts the signal selected by the multiplexer 72 into a digital value. Note that by providing a plurality of A / D converters 73, A / D conversion may be performed in parallel on detection signals of a plurality of input channels.

[0033] The CPU 74 controls the entire control device 10 and executes arithmetic processing according to a program. A RAM 75 as a volatile memory and a ROM 76 as a nonvolatile memory are used as the main memory of the CPU 74. The ROM 76 stores programs, setting values ​​for signal processing, etc. The auxiliary storage device 78 is a nonvolatile memory with a larger capacity than the ROM 76, and stores programs, data on detected values ​​of electricity, etc.

[0034] The input / output interface 77 is an interface circuit for communication between the CPU 74 and an external device.

[0035] Unlike the example of FIG. 3, at least a part of the control device 10 can be configured using circuits such as FPGA and ASIC.

[0036] <Command Generation Unit> Fig. 4 is a block diagram showing an example of a functional configuration of a command generation unit according to embodiment 1. Referring to Fig. 4, command generation unit 100 includes a generator simulation unit 110, a fault detection unit 120, a setting unit 130, a current command generation unit 140, a current limiter 142, a voltage command generation unit 144, a coordinate conversion unit 150, an AC power calculation unit 152, and a voltage amplitude generation unit 160. It should be noted that in Fig. 4, it is assumed that DC circuit 40 is configured from a renewable energy power supply.

[0037] The coordinate converter 150 performs a three-phase / two-phase conversion on the AC currents Isysu, Isysv, and Isysw using the phase θ to calculate the d-axis current Id and the q-axis current Iq. The coordinate converter 150 also performs a three-phase / two-phase conversion on the AC voltages Vsysu, Vsysv, and Vsysw using the phase θ to calculate the d-axis voltage Vd and the q-axis voltage Vq.

[0038] The AC power calculation unit 152 calculates the active power P and reactive power Q at the interconnection point 32 based on the AC current Isys detected by the current detector 91 and the AC voltage Vsys detected by the voltage detector 93.

[0039] The voltage amplitude generator 160 generates an amplitude Vref of the voltage command value based on the d-axis voltage Vd, the q-axis voltage Vq, the reactive power Q, and the reactive power command value Qref. The amplitude Vref corresponds to the amplitude command value of the output voltage of the power converter 20.

[0040] 5 is a block diagram showing an example of the configuration of the voltage amplitude generation unit 160. Referring to FIG. 5, the voltage amplitude generation unit 160 includes a positive-phase voltage calculation unit 161, subtractors 162 and 163, a voltage adjustment unit 164, coordinate conversion units 165 and 167, and an adder 166.

[0041] A positive-sequence voltage calculation unit 161 calculates a positive-sequence voltage Vpos based on the d-axis voltage Vd and the q-axis voltage Vq. A subtractor 162 calculates a deviation ΔQ (=Qref−Q) between a reactive power command value Qref and a reactive power Q. The subtractor 162 calculates a deviation ΔVpos (=Vacref−Vpos) between a grid voltage command value Vacref and a positive-sequence voltage Vpos.

[0042] The voltage adjustment unit 164 selects either an automatic reactive power adjustment mode or an automatic voltage adjustment mode, and generates a voltage amplitude adjustment amount ΔVacref based on the selected mode. Specifically, when the automatic reactive power adjustment mode is selected, the voltage adjustment unit 164 generates the voltage amplitude adjustment amount ΔVacref by feedback control for keeping the deviation ΔQ below a specified value (for example, 0). When the automatic voltage adjustment mode is selected, the voltage adjustment unit 164 generates the voltage amplitude adjustment amount ΔVacref by feedback control for keeping the deviation ΔVpos below a specified value (for example, 0). The voltage adjustment unit 164 is composed of a PI controller, a first-order lag element, etc.

[0043] The coordinate converter 165 converts the d-axis component (i.e., the specified d-axis voltage command value Vdx) and the q-axis component (i.e., the specified q-axis voltage command value Vqx) of the specified voltage command value into an amplitude |Vx| and a phase φv. The specified d-axis voltage command value Vdx and the specified q-axis voltage command value Vqx are values ​​that are set in advance by a system operator or the like. The adder 166 adds the amplitude |Vx| and the voltage amplitude adjustment amount ΔVacref. The coordinate converter 167 performs dq-axis transformation on the sum of the amplitude |Vx| and the voltage amplitude adjustment amount ΔVacref and the phase φv to generate a d-axis voltage amplitude Vdref (i.e., the d-axis component of the amplitude Vref) and a q-axis voltage amplitude Vqref (i.e., the q-axis component of the amplitude Vref).

[0044] 4 again, generator simulation unit 110 generates a voltage command value V1* for power converter 20 by simulating the characteristics of a synchronous generator based on the AC voltage Vsys and AC current Isys in power system 30. Specifically, generator simulation unit 110 includes a subtractor 111, an integrator 112, an adder 113, an integrator 114, and a voltage command generation unit 115.

[0045] The subtractor 111 outputs the difference ΔP (=Pref−P) between the active power P and the active power command value Pref to the integrator 112. The active power command value Pref is a target value of the active power P and is determined by the system operator.

[0046] The integrator 112 outputs the angular frequency deviation Δω by time-integrating the difference ΔP, which is the output value of the subtractor 111. This simulates the braking force of the synchronous generator in the control of the power converter 20. "M" in the integrator 112 is the inertia constant of the synchronous generator. The angular frequency deviation Δω corresponds to the difference between the angular frequency of the rotor in the virtual synchronous generator and the reference angular frequency ω0. The reference angular frequency ω0 is the angular frequency of the reference frequency of power in the power grid 30 (for example, 50 Hz or 60 Hz).

[0047] An adder 113 adds the angular frequency deviation Δω and a reference angular frequency ω0 to output an angular frequency ω (= Δω + ω0). An integrator 114 calculates a phase θ by time-integrating the angular frequency ω. A voltage command generator 115 generates a voltage command value V1* based on the amplitude Vref and the phase θ.

[0048] The fault detection unit 120 detects a fault in the power grid 30 based on the angular frequency deviation Δω. For example, if the angular frequency deviation Δω is equal to or greater than a set value, the fault detection unit 120 determines that a fault has occurred in the power grid 30 (i.e., detects a grid fault), and if the angular frequency deviation Δω is less than the set value, the fault detection unit 120 determines that no fault has occurred in the power grid 30. The fault detection unit 120 outputs a signal Tr according to the detection result. For example, the value of the signal Tr indicates "1" if a grid fault has been detected, and indicates "0" if no grid fault has been detected.

[0049] However, the above detection method is merely an example, and other known detection methods may be used. For example, the fault detection unit 120 may detect a fault in the power grid 30 based on the difference between the active power command value Pref and the active power P, or on other signals in the generator simulation unit 110. The fault detection unit 120 may also detect a fault in the power grid 30 based on the frequency fluctuation results of the AC voltage Vsys at the interconnection point 32.

[0050] The setting unit 130 sets the limit value of the current limiter 142. Specifically, the setting unit 130 includes a selector 131, a current calculation unit 132, and a subtractor 133.

[0051] The selector 131 outputs "Pmar" to the current calculation unit 132 when no grid fault is detected (i.e., when the value of the signal Tr is "0"), and outputs "0" to the current calculation unit 132 when a grid fault is detected (i.e., when the value of the signal Tr is "1"). Pmar indicates the margin of active power output from the power converter 20 to the power grid 30 (hereinafter also referred to as "power margin"). The power margin Pmar is a value determined in advance by the grid operator.

[0052] When no grid fault is detected, the current calculation unit 132 calculates the current margin Imar required for the active power output of the power margin Pmar. Here, the upper limit value of the current capacity of the power converter 20 (i.e., the upper limit value of the positive polarity) is referred to as the "current capacity Ica." The subtractor 133 sets the subtracted value (i.e., Ica - Imar) obtained by subtracting the current margin Imar from the current capacity Ica (where Ica > 0) of the power converter 20 as an upper limit value Imax indicating the upper limit value of the limit value of the current limiter 142.

[0053] On the other hand, when a grid fault is detected, the current calculation unit 132 outputs “0” to the subtractor 133. The subtractor 133 sets the current capacity Ica of the power converter 20 to the upper limit value Imax of the current limiter 142.

[0054] Since the renewable energy power supply serving as the DC circuit 40 cannot absorb active power, the setting unit 130 sets the lower limit value Imin, which indicates the lower limit of the limit value of the current limiter 142, to zero.

[0055] As described above, when no fault is detected in the power system 30, the setting unit 130 sets the upper limit value Imax to a value obtained by subtracting the current margin Imar from the upper limit value of the current capacity (i.e., Ica), and sets the lower limit value Imin to 0. In this case, since the upper limit value Imax is smaller than the current capacity Ica, the current range R2 according to the limit values ​​is smaller than the current range R1 according to the current capacity.

[0056] Furthermore, when a fault is detected in the power system 30, the setting unit 130 sets the upper limit value Imax to "Ica" and the lower limit value Imin to "0." In this case, the current capacity Ica and the upper limit value max are the same.

[0057] 6 is a flowchart showing an example of the processing of the fault detection unit and the setting unit. Referring to FIG. 6, the fault detection unit 120 determines whether a fault has been detected in the power grid 30 (step S10). If a fault has been detected (YES in step S10), the setting unit 130 sets the current capacity Ica of the power converter 20 as the upper limit value Imax (step S12). In this case, the power margin Pmar is not taken into consideration. On the other hand, if a fault has not been detected (NO in step S10), the setting unit 130 sets a value that reflects the power margin Pmar in the current capacity Ica (i.e., Ica - Imar) as the upper limit value Imax (step S14).

[0058] 4 again, current command generating unit 140 generates current command value I1* for power converter 20 based on voltage command value V1* generated by generator simulator 110. Here, the relationship between AC voltage Vsys at interconnection point 32, output voltage Vi of power converter 20, output current I of power converter 20, and leakage reactance X of transformer 34 is expressed by the following equation (1).

[0059] I=(Vsys-Vi) / X (1) Therefore, the current command value I1* is calculated as "(Vsys-V1*) / X" using the formula (1).

[0060] The current limiter 142 limits the current command value I1* to within a current range R2 to generate a current command value I2*. Specifically, the current limiter 142 outputs a value obtained by limiting the current command value I1* to a value equal to or greater than a lower limit value Imin and equal to or less than an upper limit value Imax as the current command value I2*. For example, when no grid fault is detected (i.e., when the lower limit value Imin is set to "0" and the upper limit value Imax is set to "Ica-Imar"), the current command value I2* is limited to a value equal to or greater than 0 and equal to or less than "Ica-Imar".

[0061] The voltage command generating unit 144 generates a voltage command value V2* for the power converter 20 based on the current command value I2*. Specifically, the voltage command generating unit 144 generates the voltage command value V2* (specifically, V2*=Vsys-(X×I2*)) by converting the current command value I2* using equation (1). The voltage command value V2* corresponds to the voltage command value having the phase θ and the amplitude V output from the command generating unit 100 in FIG. 1. Therefore, the signal generating unit 200 in FIG. 1 generates a control signal for the power converter 20 based on the voltage command value V2* generated by the command generating unit 100, and outputs the control signal to the power converter 20.

[0062] According to the configuration of the command generating unit 100 described above, the change over time in the active power output from the power converter 20 is as shown in FIG.

[0063] Fig. 7 is a diagram showing an example of a change over time in the active power output from the power converter according to the first embodiment. The horizontal axis of Fig. 7 represents time, and the vertical axis represents the output active power of power converter 20. The direction in which active power is output from power converter 20 to power grid 30 is defined as a positive direction, and the direction in which active power is absorbed by power converter 20 from power grid 30 is defined as a negative direction.

[0064] Referring to FIG. 7, before a fault occurs in power system 30, power converter 20 outputs active power corresponding to the upper limit value in normal times (that is, active power corresponding to current "Ica-Imar").

[0065] When a grid fault occurs (for example, a generator in the power grid 30 trips) and the grid frequency drops, the power converter 20 increases the active power output to the power grid 30 by Pmar in order to increase the grid frequency, and outputs active power corresponding to the upper limit value at the time of the fault (i.e., active power corresponding to the current "Ica"). This makes it possible to impart a pseudo-inertial force to the power grid 30, thereby achieving stabilization of the grid frequency.

[0066] On the other hand, when a grid fault occurs (for example, a load in the power grid 30 drops) and the grid frequency increases, the power converter 20 reduces the active power to the power grid 30 in order to lower the grid frequency, and outputs the lower limit value of active power (i.e., active power corresponding to a current of "0"). That is, the power converter 20 sets the active power output to zero. This allows the lower limit value of the active power output of the power converter 20 to be appropriately controlled.

[0067] <Modification> In the above-described first embodiment, an example in which the DC circuit 40 is configured with a renewable energy power supply has been described. However, in a modification of the first embodiment, an example in which the DC circuit 40 is configured with a power supply (e.g., a power storage element) capable of supplying and receiving power to and from the power converter 20 will be described. The power storage element differs from the renewable energy power supply in that it is capable of supplying and absorbing active power. For example, the power storage element is a power storage device including a storage battery such as an electric double layer capacitor or a lithium ion battery.

[0068] In the following description, the DC circuit 40 is described as a power storage element, but the DC circuit 40 may be, for example, a DC power system including a DC transmission network or a DC terminal of another power conversion device. In the latter case, two power converters are linked together to form a BTB (Back To Back) system for connecting AC power systems with different rated frequencies, etc.

[0069] Fig. 8 is a diagram illustrating an example of a configuration of a setting unit according to a modification of embodiment 1. Referring to Fig. 8, setting unit 130A has a configuration in which polarity inversion unit 134 is added to setting unit 130 in Fig. 4.

[0070] When no grid fault is detected, the subtractor 133 subtracts the current margin Imar from the current capacity Ica (where Ica > 0) of the power converter 20, and sets the resulting value (i.e., Ica - Imar) as the upper limit value Imax of the current limiter 142. The subtractor 133 also outputs the subtracted value to the polarity inversion unit 134. The polarity inversion unit 134 inverts the polarity of the subtracted value (i.e., -Ica + Imar), and sets the lower limit value Imin of the current limiter 142.

[0071] When a grid fault is detected, the subtractor 133 sets the current capacity Ica of the power converter 20 to the upper limit value Imax of the current limiter 142. The subtractor 133 also outputs the current capacity Ica to the polarity inversion unit 134. The polarity inversion unit 134 sets the value obtained by inverting the polarity of the current capacity Ica (i.e., −Ica) as the lower limit value Imin of the current limiter 142.

[0072] As described above, when no system fault is detected, the setting unit 130A sets the value obtained by subtracting the current margin Imar from the upper limit value of the current capacity (i.e., Ica) as the upper limit value Imax, and sets the value obtained by adding the current margin Imar to the lower limit value of the current capacity (i.e., −Ica) as the lower limit value Imin.

[0073] On the other hand, when a grid fault is detected, the setting unit 130A sets the upper limit value of the current capacity (i.e., Ica) as the upper limit value Imax, and sets the lower limit value of the current capacity (i.e., −Ica) as the lower limit value Imin.

[0074] Fig. 9 is a diagram illustrating another example of the configuration of the setting unit according to the modification of embodiment 1. Referring to Fig. 9, setting unit 130B has a configuration in which selector 135, current calculation unit 136, subtractor 137, and polarity inversion unit 138 are added to setting unit 130 in Fig. 4 .

[0075] The method for setting the upper limit value Imax of the current limiter 142 is the same as that described with reference to FIG. 4 or FIG.

[0076] The selector 135 outputs "Pmar2" to the current calculation unit 132 when a grid fault is not detected (i.e., when the value of the signal Tr is "0"), and outputs "0" to the current calculation unit 132 when a grid fault is detected (i.e., when the value of the signal Tr is "1"). Pmar2 indicates a margin of active power output from the power converter 20 to the DC circuit 40 (e.g., absorbed by the power storage element). The power margin Pmar2 is a value determined in advance by the grid operator. Typically, the power margin Pmar2 is a value different from the power margin Pmar of the active power output from the power converter 20 to the power grid 30.

[0077] When no grid fault is detected, the current calculation unit 136 calculates the current margin Imar2 required for the active power output of the power margin Pmar2. The subtractor 137 subtracts the current margin Imar2 from the current capacity Ica of the power converter 20, and outputs the result (i.e., Ica - Imar2) to the polarity inversion unit 138. The polarity inversion unit 138 sets the value obtained by inverting the polarity of the subtraction value (i.e., -Ica + Imar2) as the lower limit value Imin of the current limiter 142.

[0078] On the other hand, when a grid fault is detected, the current calculation unit 136 outputs "0" to the subtractor 137. The subtractor 137 outputs the current capacity Ica to the polarity inversion unit 138. The polarity inversion unit 138 sets the value obtained by inverting the polarity of the current capacity Ica (i.e., -Ica) as the lower limit value Imin of the current limiter 142.

[0079] As described above, when no system fault is detected, the setting unit 130B sets the value obtained by subtracting the current margin Imar from the upper limit value of the current capacity (i.e., Ica) as the upper limit value Imax, and sets the value obtained by adding the current margin Imar2 to the lower limit value of the current capacity (i.e., -Ica) as the lower limit value Imin.

[0080] On the other hand, when a grid fault is detected, the setting unit 130B sets the upper limit value of the current capacity (i.e., Ica) as the upper limit value Imax, and sets the lower limit value of the current capacity (i.e., −Ica) as the lower limit value Imin.

[0081] As described above, when no grid fault is detected, the setting method of setting units 130A and 130B causes upper limit value Imax to be smaller than the upper limit value of the current capacity (i.e., Ica) and lower limit value Imin to be larger than the lower limit value of the current capacity (i.e., −Ica). Therefore, current range R2 according to the limit values ​​(i.e., upper limit value Imax and lower limit value Imin) is smaller than current range R1 according to the current capacity of power converter 20.

[0082] According to the configuration of the above-described modified example, the time variation of the active power output from the power converter 20 is shown in FIG. 10 or FIG.

[0083] Fig. 10 is a diagram showing an example of the change over time in the active power output from the power converter according to the modification of the first embodiment. The horizontal axis of Fig. 10 represents time, and the vertical axis represents the output active power of the power converter 20. The direction in which active power is output from the power converter 20 to the power grid 30 is defined as the positive direction, and the direction in which active power is absorbed by the power converter 20 from the power grid 30 is defined as the negative direction. This also applies to Fig. 11 below. In the example of Fig. 10, the power converter 20 transmits active power to the power grid 30 during normal operation.

[0084] Referring to FIG. 10, before a fault occurs in power system 30, power converter 20 outputs active power corresponding to the upper limit value in normal times (that is, active power corresponding to current "Ica-Imar").

[0085] When a grid fault occurs and the grid frequency drops, the power converter 20 increases the active power output to the power grid 30 in order to increase the grid frequency, and outputs active power corresponding to the upper limit value at the time of the fault (i.e., active power corresponding to the current "Ica"). On the other hand, when a grid fault occurs and the grid frequency increases, the power converter 20 reduces the active power to the power grid 30 in order to lower the grid frequency, and causes the storage element to absorb the active power corresponding to the lower limit value at the time of the fault (i.e., active power corresponding to the current "-Ica").

[0086] Fig. 11 is a diagram showing another example of the change over time in the active power output from the power converter according to the modification of embodiment 1. In the example of Fig. 11, power converter 20 receives active power from power grid 30 during normal operation.

[0087] Referring to FIG. 11, before a fault occurs in power system 30, power converter 20 absorbs active power corresponding to the lower limit value under normal conditions (that is, active power corresponding to current "-Ica+Imar").

[0088] When a grid fault occurs and the grid frequency drops, the power converter 20 increases the active power output to the power grid 30 in order to increase the grid frequency, and outputs active power corresponding to the upper limit value at the time of the fault (i.e., active power corresponding to the current "Ica"). On the other hand, when a grid fault occurs and the grid frequency increases, the power converter 20 causes the storage element to absorb the active power from the power grid 30 (i.e., active power corresponding to the current "-Ica") in order to lower the grid frequency.

[0089] As described above, according to the modified example, the power converter 20 outputs active power to the power grid 30 when the system frequency drops, and absorbs active power from the power grid 30 when the system frequency increases, thereby achieving stabilization of the system frequency.

[0090] <Advantages> According to the first embodiment, when the DC circuit 40 is configured using a power source that can only output active power but cannot absorb it (for example, a renewable energy power source), the power conversion device 50 can supply active power to the power grid 30 during normal operation, and can also apply a pseudo-inertial force by outputting appropriate active power when an inertial force is required in the event of an accident or the like in the power grid 30.

[0091] Furthermore, when DC circuit 40 is configured using a power source that can both output and absorb active power (for example, a power storage element, a DC transmission system, or a DC terminal of another power converter), an active power margin for imparting inertial force can be secured both when operating during normal operation to transmit active power to power system 30 and when operating during normal operation to receive active power from power system 30. As a result, during normal operation of power conversion device 50, active power can be supplied to power system 30, and when inertial force is required due to an accident or the like in power system 30, appropriate active power can be output or absorbed to impart a pseudo-inertial force.

[0092] Second Embodiment In the first embodiment, a configuration has been described in which an active power margin is ensured by appropriately setting the limit value of the current limiter 142. In the second embodiment, a configuration will be described in which a generator simulation unit outputs a voltage command value V1* that reflects the active power margin.

[0093] Fig. 12 is a block diagram showing an example configuration of a command generating unit according to the second embodiment. Referring to Fig. 12, command generating unit 100A includes a generator simulator 110A, a fault detector 120, a current command generating unit 140, a current limiter 142, a voltage command generating unit 144, a coordinate converter 150, an AC power calculator 152, and a voltage amplitude generating unit 160. Detailed description of the same configuration as in Fig. 4 will not be repeated. Note that in Fig. 12, it is assumed that DC circuit 40 is configured using a renewable energy power supply.

[0094] The generator simulation unit 110A includes a subtractor 111, an integrator 112, an adder 113, an integrator 114A, a voltage command generation unit 115, a phase calculation unit 116, and a limit setting unit 117. The functional configurations of the subtractor 111, the integrator 112, and the adder 113 are similar to those in FIG. 4 .

[0095] The phase calculation unit 116 generates the phase θmarS based on the active power PmarS that takes into account an active power margin (e.g., Pmar). For example, the active power PmarS is a value obtained by subtracting the power margin Pmar from the active power upper limit value (e.g., Pref). Here, the relationship between the active power PmarS, the AC voltage Vsys, the output voltage Vi of the power converter 20, the phase θmarS, and the leakage reactance X of the transformer 34 is expressed by the following equation (2).

[0096] PmarS={Vsys×Vi×sin(θmarS)} / X (2) Therefore, the phase θmarS is calculated using equation (2).

[0097] When a system fault is detected (i.e., when the value of signal Tr is "1"), limit setting unit 117 sets the upper limit value of integrator 114A to +∞ and the lower limit value to -∞. In other words, limit setting unit 117 does not limit the output value of integrator 114A.

[0098] On the other hand, when no system fault is detected (i.e., when the value of the signal Tr is "0"), the limit setting unit 117 sets both the upper limit value and the lower limit value to the phase θmarS. That is, the limit setting unit 117 limits the output value of the integrator 114A to "θmarS".

[0099] When the upper limit value and the lower limit value are not limited by the limit setting unit 117 (i.e., when a system fault is detected), the integrator 114A outputs the value obtained by integrating the angular frequency ω over time as the phase θ. On the other hand, when the upper limit value and the lower limit value are limited to "θmarS" by the limit setting unit 117 (i.e., when a system fault is not detected), the integrator 114A outputs "θmarS" as the phase θ regardless of the result of the time integration.

[0100] The voltage command generator 115 generates a voltage command value V1* based on the amplitude Vref and the phase θ. Specifically, when no grid fault is detected, the voltage command generator 115 sets the phase θmar calculated based on the active power PmarS as the phase θ of the voltage command value V1*. When a grid fault is detected, the voltage command generator 115 sets the value obtained by integrating the angular frequency ω with respect to time as the phase θ of the voltage command value V1*.

[0101] The functional configurations of the current command generating unit 140, the current limiter 142, and the voltage command generating unit 144 are the same as those described in Fig. 4. However, the upper limit value Imax of the current limiter 142 is fixed to the current capacity Ica, and the lower limit value Imin is fixed to zero.

[0102] Fig. 13 is a block diagram showing an example configuration of a command generating unit according to a modification of the second embodiment. Referring to Fig. 13, a command generating unit 100B corresponds to a configuration in which the generator simulation unit 110A of the command generating unit 100A is replaced with a generator simulation unit 110B. Detailed description of the same configuration as in Fig. 12 will not be repeated. Note that in Fig. 13, it is assumed that the DC circuit 40 is configured from a renewable energy power supply.

[0103] The generator simulation unit 110B includes subtractors 111A and 111B, an integrator 112, an adder 113, an integrator 114, a voltage command generation unit 115, and a selector 118. The functional configurations of the integrator 112, the adder 113, the integrator 114, and the voltage command generation unit 115 are similar to those described in FIG.

[0104] The selector 118 outputs "Pmar" to the subtractor 111A when no system fault is detected (i.e., when the value of the signal Tr is "0"), and outputs "0" to the subtractor 111A when a fault is detected in the power system 30 (i.e., when the value of the signal Tr is "1").

[0105] When no system fault is detected (i.e., when the value of signal Tr is "0"), subtractor 111A outputs the difference ΔP1 between the active power command value Pref and the power margin Pmar (i.e., ΔP1 = Pref - Pmar) to subtractor 111B.

[0106] 14 is a diagram for explaining the relationship between the active power command value and the power margin. Referring to Fig. 14, the active power command value Pref corresponds to the active power upper limit value defined by the upper limit value of the positive polarity of the current capacity determined by the hardware performance of the power converter 20.

[0107] The active power PmarS corresponds to the upper limit of the active power during normal operation. For example, the active power PmarS corresponds to the active power corresponding to the current "Ica-Imar" described in the first embodiment. From this, it can be understood that the difference ΔP1 corresponds to the active power PmarS.

[0108] Referring again to FIG. 13 , when no grid fault is detected, the subtractor 111B outputs the difference ΔP2 (i.e., ΔP2=PmarS−P) between the active power PmarS (i.e., the difference ΔP1) and the active power P to the integrator 112. The integrator 112 outputs the angular frequency deviation Δω by integrating the difference ΔP2 over time. The integrator 114 calculates the phase θ by integrating the angular frequency ω, which is the sum of the angular frequency deviation Δω and the reference angular frequency ω0, over time. The voltage command generator 115 generates a voltage command value V1* based on the amplitude Vref and the phase θ. In this way, the voltage command generator 115 sets the phase calculated based on the difference between the active power PmarS and the active power P as the phase θ of the voltage command value V1*.

[0109] The method for generating the phase θ when a system fault is detected is the same as that described with reference to FIG.

[0110] As described above, when no grid fault is detected, the generator simulator 110A generates the phase of the voltage command value V1* based on an active power (e.g., PmarS) that is smaller than the active power (e.g., active power command value Pref) calculated based on the current capacity of the power converter 20. This makes it possible to output the voltage command value V1* that takes into account the active power margin.

[0111] <Advantages> The second embodiment has the same advantages as the first embodiment.

[0112] Other Embodiments. The configurations exemplified as the above-described embodiments are examples of the configurations of the present disclosure, and may be combined with other known technologies, or may be modified, such as by omitting some parts, within the scope of the gist of the present disclosure. Furthermore, the above-described embodiments may be implemented by appropriately adopting the processes and configurations described in other embodiments.

[0113] The embodiments disclosed herein should be considered to be illustrative in all respects and not restrictive. The scope of the present disclosure is defined by the claims, not by the above description, and is intended to include all modifications within the meaning and scope of the claims.

[0114] 10 Control device, 20 Power converter, 30 Power system, 32 Interconnection point, 34 Transformer, 40 DC circuit, 50 Power conversion device, 70 Input converter, 71 Sample and hold circuit, 72 Multiplexer, 73 A / D converter, 74 CPU, 75 RAM, 76 ROM, 77 Input / output interface, 78 Auxiliary storage device, 79 Bus, 91 Current detector, 93 Voltage detector, 100, 100A, 100B Command generation unit, 110, 110A, 110B Generator simulation unit, 112, 114, 114A Integrator, 115, 144 Voltage command generation unit, 116 Phase calculation unit, 117 Limit setting unit, 118, 131, 135 Selector, 120 Fault detection unit, 130, 130A, 130B Setting unit, 132, 136 Current calculation unit, 134, 138 Polarity inversion unit, 140 Current command generation unit, 142 Current limiter, 150, 165, 167 Coordinate conversion unit, 152 AC power calculation unit, 160 Voltage amplitude generation unit, 161 Positive sequence voltage calculation unit, 164 Voltage adjustment unit, 200 Signal generation unit, 202 Three-phase voltage generation unit, 204 PWM control unit, 1000 Power conversion system.

Claims

1. A power conversion device comprising: a power converter that performs power conversion between a DC circuit and a power system; and a control device that controls the power converter, wherein the control device includes: a generator simulation unit that generates a first voltage command value for the power converter by simulating characteristics of a synchronous generator based on the AC voltage and AC current in the power system; a current command generation unit that generates a first current command value for the power converter based on the first voltage command value generated by the generator simulation unit; a setting unit that sets the limit value so that a second current range according to a limit value is smaller than a first current range according to a current capacity of the power converter when no fault is detected in the power system; a limiter that limits the first current command value within the second current range to generate a second current command value; a voltage command generation unit that generates a second voltage command value based on the second current command value; and a signal generation unit that generates a control signal for the power converter based on the second voltage command value.

2. The power conversion device according to claim 1, wherein the DC circuit includes a power source capable of supplying and receiving power to and from the power converter, and when no fault is detected in the power system, the setting unit sets a value obtained by subtracting a margin from the upper limit value of the current capacity as the upper limit value of the limit value, and sets a value obtained by adding the margin to the lower limit value of the current capacity as the lower limit value of the limit value.

3. The power conversion device according to claim 2, wherein the DC circuit includes a power source capable of supplying and receiving power to and from the power converter, and when no fault is detected in the power system, the setting unit sets a value obtained by subtracting a first margin from the upper limit value of the current capacity as the upper limit value of the limit value, and sets a value obtained by adding a second margin to the lower limit value of the current capacity as the lower limit value of the limit value.

4. A power conversion device as described in claim 2 or claim 3, wherein, when an accident is detected in the power system, the setting unit sets an upper limit value of the current capacity as the upper limit value of the limit value, and sets a lower limit value of the current capacity as the lower limit value of the limit value.

5. The power conversion device according to claim 1, wherein the DC circuit includes a renewable energy power source that supplies power to the power converter, and when no fault is detected in the power system, the setting unit sets a value obtained by subtracting a margin from the upper limit value of the current capacity as the upper limit value of the limit value, and sets a lower limit value of the limit value to zero.

6. The power conversion device according to any one of claims 1 to 5, wherein the generator simulation unit calculates a first angular frequency by integrating over time the difference between an active power calculated based on the AC voltage and the AC current and a target value of the active power, and the control device further includes an accident detection unit that detects an accident in the power system based on the first angular frequency.

7. A power conversion device comprising: a power converter that performs power conversion between a DC circuit and a power system; and a control device that controls the power converter, wherein the control device includes: a generator simulation unit that generates a first voltage command value for the power converter by simulating characteristics of a synchronous generator based on an AC voltage and an AC current in the power system; a current command generation unit that generates a first current command value for the power converter based on the first voltage command value generated by the generator simulation unit; a limiter that generates a second current command value by limiting the first current command value within a current range in accordance with a current capacity of the power converter; a voltage command generation unit that generates a second voltage command value based on the second current command value; and a signal generation unit that generates a control signal for the power converter based on the second voltage command value, wherein when no fault is detected in the power system, the generator simulation unit generates the phase of the first voltage command value based on a second active power that is smaller than a first active power calculated based on the current capacity of the power converter.

8. A power conversion device as described in claim 7, wherein when no fault is detected in the power system, the generator simulation unit sets a phase calculated based on the second active power as the phase of the first voltage command value.

9. The power conversion device according to claim 7, wherein, when no fault is detected in the power system, the generator simulation unit calculates a difference between the first active power and a specified active power as the second active power, calculates a first angular frequency by integrating over time the difference between the active power calculated based on the AC voltage and the AC current and the second active power, calculates a first phase by integrating over time the first angular frequency, and sets the first phase as the phase of the first voltage command value.

10. A power conversion device according to any one of claims 7 to 9, wherein, when an accident is detected in the power system, the generator simulation unit calculates a second angular frequency by integrating over time a difference between an active power calculated based on the AC voltage and the AC current and the first active power, calculates a second phase by integrating over time the second angular frequency, and sets the second phase as the phase of the first voltage command value.

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