Electric power conversion device and control method for same

Abstract

Provided is a control circuit (100) that controls a main circuit unit (20) of an electric power conversion device (10), wherein an operation phase generation unit (110B) generates a fluctuation amount (ΔF) of an operation frequency by performing feedback control calculation on the deviation between an active electric power command value (Pd*) and a detection value of an active electric power output (Pd), and generates an operation frequency (Fop) by adding a base value to the fluctuation amount (ΔF). The base value is generated by extracting a low-frequency component (F1) of the operation frequency (Fop).

Description

Power Conversion Device and Control Method Thereof

[0001] The present disclosure relates to a power conversion device and a control method thereof.

[0002] The introduction of a power conversion device that converts energy stored in renewable energy, energy storage devices, etc. into electric power and supplies it to an AC power system is progressing. In recent years, a control function for operating the power conversion device as a voltage source has attracted attention. Such a control function is also called a virtual generator function because the power conversion device itself controls to behave like a virtual generator when it functions as a voltage source by generating an output voltage and an operating phase. The introduction of the virtual generator function is expected to play roles such as providing inertia to the AC power system and forming an autonomous system.

[0003] Generally, a power conversion device having a virtual generator function calculates a frequency variation based on the deviation between a given active power command value and the detected active power output. Then, the power conversion device generates its own operating frequency by adding a reference frequency of a power system such as 50 Hz or 60 Hz to the calculated frequency variation (see, for example, Japanese Unexamined Patent Application Publication No. 2019-80476 (Patent Document 1)).

[0004] Japanese Unexamined Patent Application Publication No. 2019-80476

[0005] By the way, the power system cannot always strictly maintain the reference frequency. It is known that frequency variations of about ±0.2 Hz around the reference frequency always occur within a day.

[0006] In the case of a power conversion device that generates its own operating frequency by the method as described in the above Japanese Unexamined Patent Application Publication No. 2019-80476 (Patent Document 1), if the frequency of the power system does not match the reference frequency, the active power output cannot follow the active power command value. As a result, the power conversion device repeats useless active power output or input to the power system within a day.

[0007] In the case of power converters equipped with energy storage devices, unnecessary charging and discharging of the energy storage device occurs repeatedly throughout the day, making it difficult to manage the charge state of the energy storage device. If such unnecessary charging and discharging is left unchecked, and the charge state of the energy storage device reaches 0% or 100%, the power converter will have to be disconnected from the power grid.

[0008] To solve the above problems, one possible method is to generate the operating frequency of the power converter by adding the actual frequency of the power system detected by a phase synchronous circuit, rather than a reference frequency, to the calculated frequency fluctuation. However, since phase synchronous circuits generally generate frequency based on the voltage detection value of the power system, if the voltage of the power system contains unwanted components such as harmonic components, torsional resonance components, or out-of-phase components, the operation of the power converter may become unstable. Furthermore, in the event of a power system accident, such as a generator becoming disconnected from the power system or the power system experiencing a temporary ground fault or short circuit, the system voltage becomes unstable, preventing the phase synchronous circuit from accurately detecting the actual frequency.

[0009] This disclosure has been made in consideration of the above-mentioned problems, and its purpose is to provide a power converter and a control method thereof that can make the active power output follow the active power command value even when the frequency of the power system deviates from the reference frequency.

[0010] A power converter according to one embodiment is connected between a DC power source and an AC power system and comprises a main circuit unit that performs power conversion between DC power and AC power by switching a plurality of switching elements on and off, and a control circuit that generates on / off command signals to control the on / off switching of the plurality of switching elements. The control circuit includes an instantaneous voltage command value generation unit that generates a command value of the instantaneous voltage that the main circuit unit outputs to the AC power system from a given voltage amplitude and the operating phase of the power converter, a command signal generation unit that generates the above on / off command signals based on the command value of the instantaneous voltage, and an operating phase generation unit that generates the operating frequency of the power converter and the operating phase which is the integral value of the operating frequency. Here, the operating phase generation unit includes a feedback controller that generates a variation in the operating frequency by performing a feedback control calculation to reduce the deviation between the detected value of the active power output from the main circuit unit to the AC power system and the active power command value, an adder that generates the operating frequency by adding a base value to the variation in the operating frequency, and a low-frequency component extraction unit that extracts the low-frequency component of the operating frequency in order to generate the base value.

[0011] According to the power converter of the above embodiment, when generating the operating frequency, a low-frequency component extracted from the operating frequency, rather than a predetermined reference frequency, is added as a base value to the fluctuation of the operating frequency. As a result, even if the frequency of the power system deviates from the reference frequency, the active power output can be made to follow the active power command value.

[0012] This is a block diagram illustrating the schematic configuration of a power converter according to Embodiment 1. This is a circuit diagram illustrating an example of the configuration of the main circuit section in Figure 1. This is a block diagram illustrating an example of the hardware configuration of the control circuit in Figure 1. This is a block diagram illustrating an example of the configuration of the operating phase generation section of a comparative example. This is a block diagram illustrating an example of the configuration of the operating phase generation section in the power converter of Embodiment 1. This is a diagram showing the simulation results for a power converter having the operating phase generation section of the comparative example in Figure 5. This is a diagram showing the simulation results for a power converter of Embodiment 1 having the operating phase generation section in Figure 6. This is a block diagram illustrating an example of the configuration of the operating phase generation section of the control circuit in a power converter of Embodiment 2. This is a block diagram illustrating an example of the configuration of a power converter equipped with a switch. This is a block diagram illustrating an example of the configuration of the control circuit in Figure 10. This is a flowchart illustrating the control method of a power converter according to Embodiment 2.

[0013] Each embodiment will be described in detail below with reference to the drawings. Note that identical or corresponding parts will be denoted by the same reference numerals, and their descriptions will generally not be repeated.

[0014] Embodiment 1. [Outline Configuration of Power Conversion Device] Figure 1 is a block diagram illustrating the schematic configuration of a power conversion device 10 according to Embodiment 1. As shown in Figure 1, the power conversion device 10 comprises a main circuit unit 20, a current detector 40, a voltage detector 50, and a control circuit 100.

[0015] It goes without saying that the power converter 10 may include other components besides those shown in Figure 1. For example, the power converter 10 may further have a communication unit that can exchange signals with an external device. This allows the power converter 10 to receive various command values ​​from an external device and transmit actual data based on detected values ​​to the external device.

[0016] The main circuit section 20 is connected between the energy storage element 30, which is an example of a "DC power source," and the transmission line 7 of the AC power system 5, and performs power conversion between DC voltage and AC voltage. The "DC power source" may be a renewable energy source such as a solar cell or a DC power system using DC transmission lines, in addition to the energy storage element 30, and is not particularly limited. The transmission line 7 has an equivalent wiring impedance. In the following embodiment 1, an example in which the AC power system 5 is configured as three-phase AC will be described, but the AC power system 5 can also be configured as single-phase AC.

[0017] Figure 2 is a circuit diagram illustrating an example of the configuration of the main circuit section 20 shown in Figure 1. Figure 2 also shows an example of the configuration of the energy storage element 30 along with the main circuit section 20.

[0018] The energy storage element 30 in the example shown in Figure 2 includes capacitors 31 and 32 connected in series. In addition to capacitors, any DC power storage element, such as a secondary battery, can be arbitrarily applied as the energy storage element 30.

[0019] The main circuit section 20 of the example shown in Figure 2 has three-level inverters 21u, 21v, and 21w as three-level converters. Each of the three-level inverters 21u, 21v, and 21w has a known configuration having four switching elements composed of IGBTs (Insulated Gate Bipolar Transistors) or SiC (Silicon Carbide)-MOSFETs (Metal-oxide-semiconductor Field Effect Transistors). Each of the three-level inverters 21u, 21v, and 21w converts the DC voltage of the capacitor connected in parallel with the energy storage element 30 into a sinusoidal AC voltage by PWM (Pulse Modulation Control) control of the four switching elements.

[0020] The three-level inverters 21u, 21v, and 21w are each input to an on / off command signal Sgu, Sgv, and Sgw from the control circuit 100. Each of the on / off command signals Sgu, Sgv, and Sgw represents the on / off command signals (for all four) generated by the PAM control of the four switching elements in the corresponding three-level inverter.

[0021] The three-level inverters 21u, 21v, and 21w output sinusoidal AC voltages with a phase difference of 120 degrees to the three-phase transmission lines 7 (Figure 1) on the AC side. As a result, the main circuit unit 20 operates as a three-phase three-level converter. Although not shown in Figure 2, the main circuit unit 20 may further include a filter circuit to remove high-frequency components between the three-level inverters 21u, 21v, and 21w and the transmission lines 7 of the AC power system 5.

[0022] The main circuit section 20 is not particularly limited as long as it has a bidirectional power conversion function between DC power and AC power. For example, the main circuit section 20 can be composed of a 3-level converter, a 2-level converter, or a self-excited converter such as a modular multilevel converter.

[0023] Referring again to Figure 1, the current detector 40 detects the three-phase alternating currents of the transmission line 7 of the AC power system 5. Specifically, the current detector 40 detects the U-phase alternating current Iu, the V-phase alternating current Iv, and the W-phase alternating current Iw of the AC power system 5 (more specifically, the interconnection point on the transmission line 7). The alternating currents Iu, Iv, and Iw are input to the control circuit 100. Hereinafter, the alternating currents Iu, Iv, and Iw will also be collectively referred to as the alternating current Isys.

[0024] The voltage detector 50 detects the three-phase AC voltages of the AC power system 5 (more specifically, the interconnection point on the transmission line 7). For example, the voltage detector 50 detects the U-phase AC voltage Vu, the V-phase AC voltage Vv, and the W-phase AC voltage Vw of the AC power system 5. The AC voltages Vu, Vv, and Vw are input to the control circuit 100. Hereinafter, the AC voltages Vu, Vv, and Vw will also be collectively referred to as the AC voltage Vsys. In this way, the current detector 40 and the voltage detector 50 can obtain the detected values ​​of the AC output from the main circuit section 20 of the power converter 10 to the AC power system 5 (i.e., current, voltage, and power obtained by multiplying the two).

[0025] The control circuit 100 controls the power conversion operation of the main circuit 20 by generating on / off command signals for the main circuit 20, for example, on / off command signals Sgu, Sgv, and Sgw for the three-level inverters 21u, 21v, and 21w shown in Figure 2.

[0026] [Example of Control Circuit Hardware Configuration] Figure 3 is a block diagram showing an example of the hardware configuration of the control circuit 100 in Figure 1. Figure 3 shows an example of configuring the control circuit 100 using a computer.

[0027] Referring to Figure 3, the control circuit 100 includes one or more input converters 70, one or more sample-and-hold (S / H) circuits 71, a multiplexer (MUX) 72, an A / D converter 73, one or more CPUs (Central Processing Units) 74, a RAM (Random Access Memory) 75, a ROM (Read Only Memory) 76, one or more input / output interfaces (I / Fs) 77, and an auxiliary storage device 78. The control circuit 100 also includes a bus 79 that connects the components to each other.

[0028] The input converter 70 has an auxiliary transformer for each input channel. For example, each auxiliary transformer converts the detection signals from the current detector 40 and voltage detector 50 in Figure 1 into signals with a voltage level suitable for subsequent signal processing.

[0029] A sample-and-hold circuit 71 is provided for each input converter 70. The sample-and-hold circuit 71 samples and holds the signal representing the quantity of electricity received from the corresponding input converter 70 at a specified sampling frequency.

[0030] The multiplexer 72 sequentially selects signals held by multiple sample-and-hold circuits 71. The A / D converter 73 converts the signals selected by the multiplexer 72 into digital values. Note that by providing multiple A / D converters 73, A / D conversion may be performed in parallel for detection signals from multiple input channels.

[0031] The CPU 74 controls the entire control circuit 100 and executes arithmetic processing according to the program. The RAM 75 as volatile memory and the ROM 76 as non-volatile memory are used as the main memory of the CPU 74. The ROM 76 stores the program and setting values ​​for signal processing. The auxiliary storage device 78 is a non-volatile memory with a larger capacity than the ROM 76 and stores the program and data of detected electrical quantities.

[0032] The input / output interface 77 is an interface circuit for communication between the CPU 74 and external devices. As described above, the input / output interface 77 can be configured to have a communication interface function for data communication with the outside of the power converter 10.

[0033] Unlike the example in Figure 2, it is also possible to configure at least a portion of the control circuit 100 using digital circuits such as FPGAs (Field Programmable Gate Arrays) and / or ASICs (Application Specific Integrated Circuits). Furthermore, at least a portion of the control circuit 100 may be configured using analog circuits.

[0034] [Functional Configuration of the Control Circuit] Figure 4 is a block diagram showing the functional configuration of the control circuit 100 in Figure 1. The functions of the block diagrams in Figure 4 and Figures 5, 6, 9, and 11, which will be described later, are realized by hardware processing and / or software processing by the control circuit 100.

[0035] Referring to Figure 4, the control circuit 100 functionally includes a coordinate transformation unit 105, an operating phase generation unit 110, a voltage amplitude output unit 120, an instantaneous voltage command value generation unit 130, and a PWM calculator 140 as an example of a "command signal generation unit".

[0036] The coordinate transformation unit 105 performs a coordinate transformation on the AC current Isys (Iu, Iv, Iw) detected by the current detector 40 and the AC voltage Vsys (Vu, Vv, Vw) detected by the voltage detector 50 using the operating phase θop described later. As a result, the AC current Isys and AC voltage Vsys in UVW coordinates are converted into active and inactive components in dq coordinates.

[0037] The coordinate transformation unit 105 uses the coordinate transformation to calculate the active component Id and reactive component Iq of the output current (hereinafter referred to as active current output Id and reactive current output Iq), the active component Vd and reactive component Vq of the output voltage (hereinafter referred to as active voltage output Vd and reactive voltage output Vq), and the active component Pd and reactive component Pq of the output power (hereinafter referred to as active power output Pd and reactive power output Pq), which are output from the main circuit unit 20 of the power converter 10 to the AC power system 5.

[0038] In Embodiment 1, the active current output Id, reactive current output Iq, active voltage output Vd, reactive voltage output Vq, active power output Pd, and reactive power output Pq are expressed in pu units.

[0039] The operating phase generation unit 110 generates the operating frequency Fop of the power converter 10 and further generates the operating phase θop based on the active power command value Pd* determined for the power converter 10 and the detected active power output Pd. The detailed configuration of the operating phase generation unit 110 will be described later with reference to Figures 5 and 6.

[0040] The voltage amplitude output unit 120 outputs a command value Ve of the voltage amplitude that the power converter 10 outputs to the AC power system 5. Strictly speaking, the voltage amplitude command value Ve is expressed as a vector sum of the active and reactive components, but in Embodiment 1, it is basically assumed that the voltage amplitude command value Ve is set as the value of the active component without including the reactive component. Here, the voltage amplitude command value Ve may be a predetermined fixed value (i.e., a system nominal value), or it may be generated according to the reactive power to be output. In other words, in Embodiment 1, the method of generating the voltage amplitude command value Ve by the voltage amplitude output unit 120 is not particularly limited.

[0041] The instantaneous voltage command value generation unit 130 generates command values ​​Vu*, Vv*, and Vw* for the instantaneous voltages of each phase (U, V, W phases) output from the main circuit unit 20 to the AC power system 5, using the voltage amplitude command value Ve output from the voltage amplitude output unit 120 and the operating phase θop output from the operating phase generation unit 110. More specifically, the instantaneous voltage command values ​​Vu*, Vv*, and Vw* can be calculated by transforming the voltage amplitude command value Ve on the dq axis to UVW coordinates using the operating reference phase θop from the integrator 116. Alternatively, the instantaneous AC voltage command values ​​Vu*, Vv*, and Vw* may be generated by further considering the values ​​of the passive elements constituting the filter circuit, or by incorporating feedback control such as voltage control and / or current control into the instantaneous voltage command value generation unit 130. In other words, in Embodiment 1, the method for generating the instantaneous AC voltage command values ​​Vu*, Vv*, and Vw* by the instantaneous voltage command value generation unit 130 is not particularly limited.

[0042] The command signal generation unit (PWM calculator 140 as an example) generates on / off command signals Sgu, Sgv, and Sgw for controlling the switching elements constituting the main circuit unit 20, based on the three-phase instantaneous voltage command values ​​Vu*, Vv*, and Vw* output from the instantaneous voltage command value generation unit 130. Figure 4 shows a PWM calculator 140 as an example of a command signal generation unit, which generates on / off command signals Sgu, Sgv, and Sgw by PWM control. Note that the method for generating the on / off command signals Sgu, Sgv, and Sgw may be pulse frequency control instead of PWM control, and is not particularly limited.

[0043] Specifically, the PWM calculator 140 in the case of Embodiment 1 executes a PWM calculation that compares each of the three-phase instantaneous voltage command values Vu*, Vv*, Vw* output from the instantaneous voltage command value generation unit 130 with a carrier signal. As a result, the PWM calculator 140 generates on / off command signals Sgu, Sgw for each of the four switching elements of the three-level inverters 21u, 21v, 21w shown as an example of the main circuit unit 20 in FIG. 2. As a result, the actual operating phase θop of the power conversion device 10 depends on the phases of the instantaneous voltage command values Vu*, Vv*, Vw*. In Embodiment 1, which exemplifies three-phase alternating current, the voltage phase of the U phase (that is, the phase of the instantaneous voltage command value Vu*) corresponds to the actual operating phase θop of the power conversion device 10.

[0044] [Detailed Configuration of Operating Phase Generation Unit] Hereinafter, the detailed configuration of the operating phase generation unit 110 in FIG. 4 will be described. First, the configuration of the operating phase generation unit 110A of the comparative example will be described with reference to FIG. 5, and then the configuration of the operating phase generation unit 110B in the power conversion device 10 of Embodiment 1 will be described with reference to FIG. 6.

[0045] FIG. 5 is a block diagram for explaining a configuration example of the operating phase generation unit 110A of the comparative example. As shown in FIG. 5, the operating phase generation unit 110A of the comparative example includes a subtractor 111, a feedback controller 112, a constant multiplier 113, an adder 114, a constant multiplier 115, and an integrator 116.

[0046] The subtractor 111 generates an active power deviation by subtracting the detected value of the active power Pd from the active power command value Pd*.

[0047] The feedback controller 112 performs a control operation to reduce the active power deviation with respect to the active power deviation and outputs the control operation result. As the control operation, various methods such as virtual synchronous generator (VSG: Virtual Synchronous Generator) control, proportional control, droop control, virtual synchronous machine (VSM: Virtual Synchronous Machine) control, etc. can be applied and are not particularly limited. In the example of FIG. 5, an example of VSG control using the unit inertia constant M and the damping coefficient D is shown. In this case, the transfer function of the feedback controller 112 is represented by 1 / (Ms + D) and is equivalent to the transfer function of a first-order lag.

[0048] The constant multiplier 113 multiplies the output of the feedback controller 112 by a predetermined reference frequency Fr to generate a variation ΔF in the operating frequency of the power conversion device 10. The reference frequency Fr is the rated frequency of the power system such as 50 Hz or 60 Hz.

[0049] The adder 114 generates the operating frequency Fop of the power conversion device 10 by adding the above variation ΔF with the reference frequency Fr as a base value.

[0050] The constant multiplier 115 multiplies the operating frequency Fop by 2π. The integrator 116 integrates the operating frequency Fop multiplied by 2π to generate the operating phase θop of the power conversion device 10.

[0051] FIG. 6 is a block diagram showing a configuration example of the operating phase generation unit 110B in the power conversion device 10 of Embodiment 1. As shown in FIG. 6, the operating phase generation unit 110B in the case of Embodiment 1 includes a subtractor 111, a feedback controller 112, a constant multiplier 113, an adder 114, a constant multiplier 115, an integrator 116, and a low-frequency component extraction unit 117. That is, the operating phase generation unit 110B in FIG. 6 is different from the operating phase generation unit 110A of the comparative example shown in FIG. 5 in that it further includes a low-frequency component extraction unit 117.

[0052] In the operating phase generation unit 110B of Figure 6, the procedure for generating the operating frequency variation ΔF is the same as in Figure 5. That is, the subtractor 111 generates the active power deviation by subtracting the detected value of the active power Pd from the active power command value Pd*. The feedback controller 112 performs a control calculation on the generated active power deviation and outputs the control calculation result. The constant multiplier 113 generates the operating frequency variation ΔF of the power converter 10 by multiplying the output of the feedback controller 112 by the reference frequency Fr.

[0053] The low-frequency component extraction unit 117 extracts the low-frequency component F1 from the operating frequency Fop output from the adder 114. For example, if the operating frequency Fop is composed of the sum of the reference frequency Fr and the variation ΔF, then the low-frequency component F1 of the operating frequency Fop is equal to the reference frequency Fr.

[0054] The adder 114 generates the operating frequency Fop by adding the low-frequency component F1 extracted by the low-frequency component extraction unit 117 to the fluctuation ΔF, instead of the reference frequency Fr.

[0055] The subsequent operation of the operating phase generation unit 110B is the same as in the comparative example shown in Figure 5. That is, the constant multiplier 115 multiplies the operating frequency Fop by 2π. The integrator 116 generates the operating phase θop of the power converter 10 by integrating the operating frequency Fop multiplied by 2π.

[0056] [Operation of the Operating Phase Generation Unit] Next, we will explain the difference in operation of the operating phase generation units 110A and 110B in Figures 5 and 6 when the reference frequency Fr is 60 Hz and the frequency Fs of the power system changes from 60 Hz to 60.1 Hz.

[0057] First, referring to Figure 5, the operation of the operating phase generation unit 110A in the comparative example will be explained. In order for the power converter 10 to maintain synchronization with the power system when the frequency of the power system changes from 60 Hz to 60.1 Hz, the operating frequency Fop of the power converter 10 must also be equal to the frequency of the power system, which is 60.1 Hz. For this reason, the fluctuation ΔF obtained by subtracting the reference frequency Fr from the operating frequency Fop is 0.1 Hz, and not 0 Hz.

[0058] As shown in Figure 5, the operating frequency Fop is generated by multiplying the deviation between the active power command value Pd* and the detected value of the active power output Pd by 1 / (Ms+D), which is a transfer function equivalent to a first-order lag, and then by the reference frequency Fr. Therefore, if the deviation between the active power command value Pd* and the detected value of the active power output Pd is 0, the operating frequency fluctuation ΔF converges to 0. On the other hand, if the operating frequency fluctuation ΔF is not 0 Hz, it is not possible to maintain a deviation of 0 between the active power command value Pd* and the active power output Pd.

[0059] Therefore, when the power system frequency is 60.1 Hz, in order to maintain the operating frequency fluctuation ΔF at 0.1 Hz, the active power deviation, obtained by subtracting the active power output Pd from the active power command value Pd*, must remain a positive value. In other words, the active power output Pd must remain lower than the active power command value Pd*. Conversely, when the power system frequency is lower than the reference frequency Fr, the operating frequency fluctuation ΔF must be maintained at a negative value, so the active power output Pd must remain higher than the active power command value d*.

[0060] Thus, in the case of the configuration of the operating phase generation unit 110A in the comparative example shown in Figure 5, when the frequency of the power system changes from the reference frequency Fr, a phenomenon occurs in which the active power output Pd cannot follow the active power command value Pd*.

[0061] Next, with reference to Figure 6, the operation of the operating phase generation unit 110B in the power converter 10 of Embodiment 1 will be described. In order for the power converter 10 to maintain synchronization with the AC power system 5 when the frequency of the power system changes from 60 Hz to 60.1 Hz, the operating frequency Fop of the power converter 10 must also be equal to the frequency of the AC power system 5, which is 60.1 Hz. This point is the same as in the comparative example in Figure 5.

[0062] In the first embodiment shown in Figure 6, the low-frequency component F1 of the operating frequency Fop of the power converter 10 extracted by the low-frequency component extraction unit 117 is 60 Hz while the power system frequency is 60 Hz. When the power system frequency changes from 60 Hz to 60.1 Hz, the fluctuation ΔF of the operating frequency becomes 0.1 Hz, so the operating frequency Fop of the power converter 10 changes to 60.1 Hz. As a result, the low-frequency component F1 of the operating frequency Fop of the power converter 10 gradually transitions from 60 Hz to 60.1 Hz. If the fluctuation ΔF of the operating frequency does not change at 0.1 Hz, the operating frequency Fop of the power converter 10 will increase further. As the operating frequency Fop of the power converter 10 increases, the operating phase θop of the power converter 10 gradually increases, so the active power output Pd, which was lower than the active power command value Pd*, gradually approaches the active power command value Pd*. In other words, as the deviation between the active power command value Pd* and the active power output Pd decreases, the fluctuation in operating frequency ΔF gradually transitions from 0.1 Hz to 0 Hz. Therefore, after a sufficient amount of time has elapsed, the operating frequency Fop of the power converter 10 becomes 60.1 Hz, the low-frequency component F1 of the operating frequency Fop of the power converter 10 also becomes 60.1 Hz, and the fluctuation in operating frequency ΔF becomes 0 Hz. As a result, the deviation between the active power command value Pd* and the active power output Pd becomes 0, and the active power output Pd follows the active power command value Pd*.

[0063] As shown in Figure 6, a low-pass filter is used as the low-frequency component extraction unit 117. By setting the time constant of the low-pass filter, the frequency range over which low-frequency components are extracted can be changed. Therefore, if you want the active power output Pd to immediately follow the active power command value Pd* when the frequency of the power system changes, you should set the time constant as small as possible. If you want the active power output Pd to follow the active power output Pd* with a delay of a few seconds when the frequency of the power system changes, you should set the time constant to a few seconds.

[0064] [Simulation Results] The results of the simulation verification of the operation of the power converter 10 of Embodiment 1 will be described below with reference to Figures 7 and 8.

[0065] The simulation model is as shown in Figure 1. The power system 5 is simulated with an ideal three-phase voltage source, and the transmission line 7 between the AC power system 5 and the power converter 10 is simulated with a simplified model using resistance and reactor components. Here, the reactor component represents AC reactors, transformer leakage reactance, and transmission / distribution line reactance, and is assumed to be sufficiently larger than the resistance component.

[0066] In the simulation, the active power command value Pd* is set to 0.2pu, and the frequency of the power system is assumed to change in a step from 60Hz to 60.1Hz. Note that the active power pu represents a percentage based on the rated capacity of the power converter 10. 0.2pu is 20% of the rated capacity.

[0067] Figure 7 shows the simulation results for a power converter 10 having an operating phase generation unit 110A, which is a comparative example of Figure 5. Figure 7(A) shows the relationship between output power (pu) and simulation time (s). The detected value of the active power output Pd is shown by a solid line, and the active power command value Pd* is shown by a dashed line. Figure 7(B) shows the relationship between frequency (Hz) and simulation time (s). The operating frequency Fop is shown by a solid line, and the power system frequency Fs is shown by a dashed line.

[0068] Referring to Figures 7(A) and (B), in the initial state, the active power output Pd is 0.2pu, following the active power command value Pd*. Therefore, it is estimated that the operating phase θop of the power converter 10 leads the phase θs of the power system.

[0069] At a simulation time of 10 seconds, the power system frequency Fs is changed in a stepwise manner from 60 Hz to 60.1 Hz. As a result, the phase θs of the power system increases and advances beyond the operating phase θop of the power converter 10, causing the active power output Pd to become a negative value. Consequently, the active power deviation obtained by subtracting the active power output Pd from the active power command value Pd* becomes a positive value. Therefore, the operating frequency variation ΔF obtained by multiplying the result of the feedback control calculation applied to the active power deviation by the reference frequency Fr becomes a positive value. As a result, the operating frequency Fop of the power converter 10, obtained by adding the reference frequency Fr to the operating frequency variation ΔF, increases from 60 Hz and converges to 60.1 Hz.

[0070] After the operating frequency Fop of the power converter 10 converges to 60.1 Hz, the deviation between the active power command value Pd* and the active power output Pd remains at a constant interval and does not decrease. In the case of VSG control, the deviation between the active power command value Pd* and the active power output Pd increases in proportion to the damping coefficient D.

[0071] In the example shown in Figure 7, the active power command value Pd* is a positive value, while the active power output Pd is a negative value, and the active power is input from the AC power system 5 to the power converter 10. Therefore, if the energy storage element 30 is connected to the power converter 10, even if a command is issued to discharge from the energy storage element 30, if the frequency Fs of the power system becomes greater than the reference frequency Fr, the energy storage element 30 will continue to be charged, which is a problem.

[0072] Figure 8 shows the simulation results for the power converter 10 of Embodiment 1, which has the operating phase generation unit 110B shown in Figure 6. Figure 8(A) shows the relationship between output power (pu) and simulation time (s). The detected value of the active power output Pd is shown by a solid line, and the active power command value Pd* is shown by a dashed line. Figure 8(B) shows the relationship between frequency (Hz) and simulation time (s). The operating frequency Fop is shown by a solid line, and the power system frequency Fs is shown by a dashed line.

[0073] Referring to Figures 8(A) and (B), in the initial state, the active power output Pd is 0.2pu, following the active power command value Pd*. Therefore, it is estimated that the operating phase θop of the power converter 10 leads the phase θs of the power system.

[0074] At a simulation time of 10 seconds, the frequency Fs of the power system is changed in a stepwise manner from 60 Hz to 60.1 Hz. As a result, the phase θs of the power system increases and leads the operating phase θop of the power converter 10, causing the active power output Pd to become a negative value. Consequently, the active power deviation obtained by subtracting the active power output Pd from the active power command value Pd* becomes a positive value, and the operating frequency fluctuation ΔF obtained by multiplying the result of the feedback control calculation applied to the active power deviation by the reference frequency Fr becomes a positive value. As a result, as described above, the low-frequency component F1 of the operating frequency Fop of the power converter 10 gradually transitions from 60 Hz to 60.1 Hz. As a result, the operating frequency fluctuation ΔF gradually transitions from 0.1 Hz to 0 Hz, and as shown in Figure 8(A), the deviation between the active power command value Pd* and the active power output Pd gradually decreases. Furthermore, as shown in Figure 8(B), the operating frequency Fop of the power converter 10 may overshoot the frequency Fs of the power system during transient states, but ultimately converges to 60.1 Hz, the same as the frequency Fs of the power system. As a result, the power converter 10 maintains synchronization with the AC power system 5.

[0075] [Effects of Embodiment 1] As described above, according to the power converter 10 of Embodiment 1, when generating the operating frequency Fop of the power converter 10, the low-frequency component F1 of the operating frequency Fop is added as the base value to the fluctuation amount ΔF of the operating frequency, rather than a predetermined fixed reference frequency Fr. As a result, even if the frequency Fs of the power system deviates from the reference frequency Fr, the active power output Pd of the power converter 10 can be made to follow the active power command value Pd*.

[0076] When the frequency Fs of the power system deviates from the reference frequency Fr, another method for making the active power output Pd of the power converter 10 follow the active power command value Pd* is to use the actual frequency Fs of the power system detected by the phase synchronous controller to control the power converter 10. Specifically, in the operating phase generation unit 110A of the comparative example in Figure 5, the detected actual frequency Fs of the power system is added to the operating frequency fluctuation ΔF instead of the reference frequency Fr.

[0077] However, since phase synchronous generators generally generate frequencies based on the detected voltage of the power system, if the power system voltage contains unwanted components such as harmonic components, torsional resonance components, or out-of-phase components, the phase synchronous generator will detect the system voltage containing these unwanted components. As a result, these unwanted components are superimposed on the operating phase θop of the power converter 10, causing vibrations or instability in the operation of the power converter 10. In contrast, the control circuit 100 of the power converter 10 in Embodiment 1 generates the operating phase θop of the power converter 10 based only on the deviation between the active power command value Pd* and the detected value of the active power output Pd, thus reducing the risk of being affected by unwanted components in the system voltage.

[0078] Embodiment 2. Embodiment 2 describes a case in which a power converter 10 having a virtual generator function operates autonomously when disconnected from the power grid and supplies power to a load connected to the power converter 10. In Embodiment 1, the operating frequency Fop of the power converter 10 operated in synchronization with the frequency Fs of the power grid. However, when the power converter 10 is disconnected from the power grid, the power converter 10 needs to generate its own output voltage and operating frequency. For this reason, in the power converter 10 of Embodiment 2, the operating phase generation unit 110C of the control circuit 100 has a configuration that combines the operating phase generation unit 110A of the comparative example in Figure 5 and the operating phase generation unit 110B of Embodiment 1 in Figure 6. This will be explained in detail below with reference to the drawings.

[0079] [Example of configuration of the operating phase generation unit] Figure 9 is a block diagram showing an example of the configuration of the operating phase generation unit 110C of the control circuit 100 in the power converter 10 of Embodiment 2. The operating phase generation unit 110C in Figure 9 differs from the operating phase generation unit 110B in Figure 6 in that it further includes a selection unit 118.

[0080] The selection unit 118 selects either the low-frequency component F1 of the operating frequency Fop or the reference frequency Fr as the base value according to the selection command SC and outputs it to the adder 114. The adder 114 generates the operating frequency Fop by adding the operating frequency variation ΔF to the base value selected by the selection unit 118. Other aspects of Figure 9 are the same as in Figure 6, so the same or corresponding parts are given the same reference numerals and the explanation will not be repeated. Note that the selection by the selection unit 118 may be switched manually by the operator, not by the selection command SC.

[0081] When the power converter 10 is connected to the AC power system 5, the selection unit 118 selects the low-frequency component F1 of the operating frequency Fop as the base value. As a result, the power converter 10 of Embodiment 2 operates in the same way as the power converter 10 of Embodiment 1, which has the operating phase generation unit 110B shown in Figure 6. On the other hand, when the power converter 10 is disconnected from the AC power system 5, the selection unit 118 selects the reference frequency Fr as the base value for autonomous operation. This avoids a situation where the operating frequency Fop of the power converter 10 is not uniquely determined, making it difficult for the power converter 10 to continue operating.

[0082] [Example of Power Conversion Device Configuration] Figure 10 is a block diagram showing an example of the configuration of a power conversion device 10 equipped with a switch 60. As shown in Figure 10, the switch 60 is connected between the main circuit section 20 of the power conversion device 10 and the transmission line 7 of the AC power system 5. A status signal SS representing the open / closed state of the switch 60 is input to the control circuit 100.

[0083] Furthermore, a load 80 is connected to the AC line between the main circuit section 20 and the switch 60. When the switch 60 is closed, AC power is supplied to the load 80 from the AC power system 5. When the switch 60 is open, AC power is supplied to the load 80 from the power converter 10.

[0084] Other aspects of Figure 10 are the same as those in Figure 1, so the same or corresponding parts are denoted by the same reference numerals and their descriptions are not repeated.

[0085] Figure 11 is a block diagram showing an example configuration of the control circuit 100 of Figure 10. The control circuit 100 of Figure 10 differs from the control circuit 100 of Figure 4 in that it further includes a selection control unit 150.

[0086] The selection control unit 150 determines the open / closed state of the switch 60 based on the state signal SS, which is output from the switch 60 and represents the open / closed state of the switch 60. The selection control unit 150 then outputs a selection command SC based on the open / closed state of the switch 60 to the selection unit 118 of the operating phase generation unit 110C shown in Figure 9. Specifically, if the switch 60 is in the open state, the selection control unit 150 instructs the selection unit 118 to select a reference frequency Fr, and if the switch 60 is in the closed state, it instructs the selection unit 118 to select the low-frequency component F1 of the operating frequency Fop. This allows the operation of the control circuit 100 of the power converter 10 to be automatically changed according to the open / closed state of the switch 60.

[0087] [Summary and Effects of Embodiment 2] Figure 12 is a flowchart illustrating the control method of the power converter according to Embodiment 2. The above explanation will be summarized below with reference to Figures 9 to 12.

[0088] In step S110 of Figure 12, the subtractor 111 of the operating phase generation unit 110C generates a deviation between the detected value of the active power Pd output from the AC power system 5 from the main circuit unit 20 and the active power command value Pd*.

[0089] In the next step S120, the feedback controller 112 performs a feedback control calculation on the deviation of the active power to reduce the deviation of the active power, and further multiplies the control calculation result by a reference frequency Fr to generate the variation ΔF of the operating frequency Fop of the power converter 10.

[0090] In the next step S130, the low-frequency component extraction unit 117 extracts the low-frequency component F1 of the operating frequency Fop in order to generate the base value of the operating frequency Fop.

[0091] In the next step S140, the selection control unit 150 of the control circuit 100 determines the open / closed state of the switch 60 based on the state signal SS from the switch 60. If the switch 60 is in the closed state (YES in step S140), the control circuit 100 proceeds to step S150A, and if the switch 60 is in the open state (NO in step S140), the control circuit 100 proceeds to step S150B.

[0092] In step S150A, the selection unit 118 selects the low-frequency component F1 of the operating frequency Fop as the base value according to the selection command SC from the selection control unit 150. In step S150B, the selection unit 118 selects the reference frequency Fr of the AC power system 5 as the base value according to the selection command SC from the selection control unit 150.

[0093] In the next step S160, the adder 114 of the operating phase generation unit 110C generates the operating frequency Fop by adding the base value to the variation ΔF of the operating frequency.

[0094] In the next step S170, the constant multiplier 115 multiplies the generated operating frequency Fop by 2π, and the integrator 116 generates the operating phase θop by integrating the operating frequency Fop multiplied by 2π.

[0095] In the next step S180, the instantaneous voltage command value generation unit 130 of the control circuit 100 generates instantaneous voltage command values ​​Vu*, Vv*, and Vw* that the main circuit unit 20 outputs to the AC power system 5, from the given voltage amplitude command value Ve and the generated operating phase θop.

[0096] In the next step S190, the command signal generation unit of the control circuit 100 (PWM calculator 140 as an example) generates on / off command signals Sgu, Sgv, and Sgw to control the on / off switching of the multiple switching elements constituting the main circuit unit 20, based on the command value of the instantaneous voltage. The above procedure is then repeated.

[0097] According to the control method for the power converter 10 of the second embodiment described above, when the switch 60 is closed, the low-frequency component F1 of the operating frequency Fop is added to the fluctuation ΔF of the operating frequency as a base value. This makes it possible to make the active power output Pd from the main circuit section 20 follow the active power output Pd* even when the frequency Fs of the power system changes from the reference frequency Fr. On the other hand, when the switch 60 is opened and the power converter 10 is disconnected from the AC power system 5, a predetermined reference frequency Fr is added to the fluctuation ΔF of the operating frequency as a base value. This makes it possible to independently generate the AC voltage and operating frequency that the power converter 10 supplies to the load, thus avoiding a situation where the operating frequency Fop of the power converter 10 is not uniquely determined and it becomes difficult for the power converter 10 to continue operating.

[0098] The embodiments disclosed herein should be considered in all respects to be illustrative and not restrictive. The scope of this application is indicated by the claims and not by the foregoing description, and all modifications within the meaning and scope equivalent to the claims are intended to be included.

[0099] 5 AC power system, 7 transmission line, 10 power converter, 20 main circuit section, 21u, 21v, 21w 3-level inverter, 30 energy storage element, 31, 32 capacitor, 40 current detector, 50 voltage detector, 60 switch, 70 input converter, 71 sample-and-hold circuit, 72 multiplexer, 73 A / D converter, 75 RAM, 76 ROM, 77 input / output interface, 78 auxiliary storage device, 79 bus, 80 load, 100 control circuit, 105 coordinate transformation section, 110, 110A, 110B, 110C operating phase generation section, 111 subtractor, 112 feedback controller, 113, 115 constant multiplier, 114 adder, 116 integrator, 117 low-frequency component extraction section, 118 selection section, 120 Voltage amplitude output unit, 130 Instantaneous voltage command value generation unit, 140 PWM calculator (command signal generation unit), 150 Selection control unit, F1 Low frequency component, Fop Operating frequency, Fr Reference frequency, θop Operating phase, Isys, Iu, Iv, Iw AC current, SC Selection command, SS Status signal, Sgu, Sgv, Sgw On / off command signal, Ve Voltage amplitude, Vsys, Vu, Vv, Vw AC voltage, Vu*, Vv*, Vw* Instantaneous voltage command value.

Claims

1. A power converter comprising: a main circuit unit connected between a DC power source and an AC power system, which performs power conversion between DC power and AC power by switching a plurality of switching elements on and off; and a control circuit that generates on / off command signals for controlling the on / off switching of the plurality of switching elements, wherein the control circuit includes: an instantaneous voltage command value generation unit that generates a command value for the instantaneous voltage output by the main circuit unit to the AC power system from a given voltage amplitude and the operating phase of the power converter; a command signal generation unit that generates the on / off command signal based on the command value of the instantaneous voltage; and an operating phase generation unit that generates the operating frequency and the operating phase which is the integral value of the operating frequency, wherein the operating phase generation unit includes: a feedback controller that generates a variation in the operating frequency by performing a feedback control calculation to reduce the deviation between the detected value of the active power output from the main circuit unit to the AC power system and the active power command value; and an adder that generates the operating frequency by adding a base value to the variation in the operating frequency, A power conversion device comprising a low-frequency component extraction unit that extracts the low-frequency components of the operating frequency in order to generate the base value.

2. The power conversion device according to claim 1, wherein the operating phase generation unit further includes a selection unit that selects one of a predetermined reference frequency of the AC power system and the low-frequency component of the operating frequency as the base value.

3. The power conversion device according to claim 2, further comprising a switch provided in a transmission line between the main circuit unit and the AC power system, wherein when the switch is closed, the main circuit unit is disconnected from the AC power system and independently supplies AC power to the load, and the control circuit further comprises a selection control unit that controls the selection by the selection unit according to the open / closed state of the switch.

4. The power conversion device according to any one of claims 1 to 3, wherein the feedback controller performs virtual synchronous generator control using a given unit inertia constant and braking coefficient.

5. A control method for a power converter including a main circuit unit connected between a DC power source and an AC power system, which performs power conversion between DC power and AC power by switching a plurality of switching elements on and off, comprising: a step of generating a variation in the operating frequency of the power converter by performing a feedback control calculation to reduce the deviation between a detected value of the active power output from the main circuit unit to the AC power system and an active power command value; a step of extracting the low-frequency component of the operating frequency in order to generate a base value of the operating frequency; a step of generating the operating frequency by adding the base value to the variation in the operating frequency; a step of generating the operating phase by integrating the operating frequency; a step of generating a command value of the instantaneous voltage output by the main circuit unit from a given voltage amplitude and the operating phase; and a step of generating an on / off command signal for controlling the on / off of the plurality of switching elements based on the command value of the instantaneous voltage.

6. A control method for a power converter according to claim 5, further comprising the step of selecting one of a predetermined reference frequency of the AC power system and the low-frequency component of the operating frequency as the base value.

7. The power conversion device further includes a switch provided on a transmission line between the main circuit and the AC power system, wherein when the switch is closed, the main circuit is disconnected from the AC power system and supplies AC power to a load independently, and the step of selecting as the base value includes selecting the reference frequency as the base value when the switch is open, and selecting the low-frequency component as the base value when the switch is closed, the control method for the power conversion device according to claim 6.

8. The control method for a power converter according to any one of claims 5 to 7, wherein the feedback control calculation is based on virtual synchronous generator control using a given unit inertia constant and damping coefficient.

Images