Control device and power converter

Abstract

The control device (10) includes a generator simulation unit (101) that generates a first voltage command value and a signal generation unit (103) that generates a control signal based on the first voltage command value. The generator simulation unit (101) includes an angular frequency generation unit (11) that outputs a first angular frequency, a phase generation unit (13) that outputs a first phase, and a voltage command generation unit (15) that generates a first voltage command value that suppresses overcurrent of the power converter. The voltage command generation unit (15) generates a second voltage command value based on a target value of AC voltage and a first phase, and generates a first voltage command value based on a first suppression method when the power converter (20) is outputting power to the power system (2) when the suppression condition is met that current suppression of the power converter (20) is required, and generates a first voltage command value based on a second suppression method different from the first suppression method when the power converter (20) is receiving power input from the power system (2) when the suppression condition is met.

Description

Technical Field

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

Background Art

[0002] In recent years, many distributed power sources using renewable energy such as solar power generation facilities have been introduced to the power system. Distributed power sources are often connected to the power system via a power converter. Therefore, as the number of distributed power sources connected to the power system increases, the ratio of synchronous generators connected to the power system decreases, and the inertial energy in the power system decreases. Thus, a virtual synchronous machine control has been proposed to supplement the reduced inertial energy by making the power converter behave in the same way as a synchronous machine.

[0003] A power converter equipped with a voltage control type virtual synchronous machine control (hereinafter also referred to as "virtual synchronous machine") is controlled to simulate the behavior when a synchronous generator to be simulated is connected to the power system. The behavior of the synchronous generator to be simulated is realized by solving the swing equation. For example, Japanese Patent Application Laid-Open No. 2025-18647 (Patent Document 1) examines stabilizing the system frequency or voltage while suppressing overcurrent in a power conversion device including an inverter operating as a virtual synchronous generator.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] Voltage-controlled virtual synchronous machines operate as a voltage source by controlling the voltage, but they cannot control the current. Therefore, overcurrents may occur during grid disturbances, potentially causing the power converter to shut down. Patent Document 1 describes a system in which, in order to suppress overcurrents flowing into the power grid, the current limiting mode (e.g., apparent current limiting mode, active current limiting mode, and reactive current limiting mode) is automatically or manually switched according to fluctuations in the amplitude, frequency, etc., of the inverter's output voltage.

[0006] However, when suppressing overcurrents, the impact of the power converter on the power system due to the application of the current limiting mode changes depending on whether the power converter is in an operating state that is outputting power to the power system or in an operating state that is receiving power from the power system. Specifically, if control is performed according to the current limiting mode without considering the operating state of the power converter, the power system will become unstable.

[0007] In some aspects of this disclosure, the objective is to provide a control device and a power converter capable of stabilizing a power system while suppressing overcurrents. [Means for solving the problem]

[0008] A control device according to one embodiment controls a power converter connected to a power system. The control device includes a generator simulation unit that generates a first voltage command value for the power converter by simulating the characteristics of a synchronous generator based on the AC voltage in the power system, and a signal generation unit that generates a control signal for the power converter based on the first voltage command value. The generator simulation unit includes an angular frequency generation unit that outputs a first angular frequency by integrating over time the first difference between a first active power and an active power target value, a phase generation unit that outputs a first phase by integrating over time the sum of the first angular frequency and a reference angular frequency, and a voltage command generation unit that generates a first voltage command value that suppresses overcurrent of the power converter based on the AC voltage, a target value of the AC voltage, and the first phase. The voltage command generation unit generates a second voltage command value based on the target value of the AC voltage and the first phase, and determines whether the suppression condition that requires current suppression of the power converter has been met based on the second voltage command value. If the power converter is outputting power to the power system when the suppression condition is met, it generates a first voltage command value based on a first suppression method for suppressing the current of the power converter. If the power converter is receiving power input from the power system when the suppression condition is met, it generates a first voltage command value based on a second suppression method different from the first suppression method.

[0009] A power converter according to another embodiment comprises a power converter connected to a power grid and a control device for controlling the power converter. The control device includes a generator simulation unit that generates a first voltage command value for the power converter by simulating the characteristics of a synchronous generator based on the AC voltage in the power grid, and a signal generation unit that generates a control signal for the power converter based on the first voltage command value. The generator simulation unit includes an angular frequency generation unit that outputs a first angular frequency by integrating over time the first difference between a first active power and an active power target value, a phase generation unit that outputs a first phase by integrating over time the sum of the first angular frequency and a reference angular frequency, and a voltage command generation unit that generates a first voltage command value that suppresses overcurrent of the power converter based on the AC voltage, a target value of the AC voltage, and the first phase. The voltage command generation unit generates a second voltage command value based on the target value of the AC voltage and the first phase, and determines whether the suppression condition that requires current suppression of the power converter has been met based on the second voltage command value. If the power converter is outputting power to the power system when the suppression condition is met, it generates a first voltage command value based on a first suppression method for suppressing the current of the power converter. If the power converter is receiving power input from the power system when the suppression condition is met, it generates a first voltage command value based on a second suppression method different from the first suppression method. [Effects of the Invention]

[0010] According to this disclosure, it is possible to stabilize the power system while suppressing overcurrents. [Brief explanation of the drawing]

[0011] [Figure 1] This is a diagram illustrating an example of the overall configuration of a power conversion system. [Figure 2] This figure shows an example of the hardware configuration of a control device. [Figure 3] This is a block diagram showing an example of the functional configuration of the angular frequency generation unit. [Figure 4] This is a block diagram showing an example of the functional configuration of the phase generation unit. [Figure 5] This figure shows the relationship between the phase of a power converter and its output active power. [Figure 6] This flowchart shows an example of the processing procedure for the voltage command generation unit. [Figure 7] This diagram illustrates an example of the overall configuration of a power conversion system according to another embodiment. [Figure 8] This block diagram shows an example of the functional configuration of an angular frequency generation unit according to another embodiment. [Modes for carrying out the invention]

[0012] This embodiment will be described below with reference to the drawings. In the following description, identical parts are denoted by the same reference numerals. Their names and functions are also the same. Therefore, detailed descriptions of them will not be repeated.

[0013] <Overall Structure> Figure 1 is a diagram illustrating an example of the overall configuration of the power conversion system 1000. The power conversion system 1000 includes a power grid 2, a transformer 3, a power conversion device 6, a current detector 7, a voltage detector 8, and a DC circuit 30. The power conversion device 6 includes a control device 10 and a power converter 20.

[0014] The power converter 20 is a power converter that performs power conversion between the DC circuit 30 and the power system 2. Specifically, the power converter 20 is connected to the power system 2 via the busbar 5 and the transformer 3. The power converter 20 converts the DC power from the DC circuit 30 into AC power and outputs the AC power to the power system 2. Alternatively, the power converter 20 converts the AC power from the power system 2 into DC power and outputs the DC power to the DC circuit 30. The power converter 20 can be, for example, a two-level converter, a three-level converter, a modular multilevel converter, etc.

[0015] The power system 2 is, for example, a three-phase AC system. The DC circuit 30 is, for example, a power storage element connected to the DC terminals of the power converter 20. The power storage element is a power storage device including an electric double layer capacitor or a storage battery such as a lithium ion battery. Also, the DC circuit 30 may be, for example, a DC power system including a DC power transmission network or the DC terminals of another power converter connected to the DC terminals of the power converter 20.

[0016] The current detector 7 detects the three-phase AC current of the power system 2. Specifically, the current detector 7 detects the AC current Ia of phase a, the AC current Ib of phase b, and the AC current Ic of phase c of the power system 2. The AC currents Ia, Ib, and Ic are input to the control device 10. Hereinafter, the AC currents Ia, Ib, and Ic are also collectively referred to as the AC current Isys.

[0017] The voltage detector 8 detects the three-phase AC voltage of the power system 2. Specifically, the voltage detector 8 detects the AC voltage Va of phase a, the AC voltage Vb of phase b, and the AC voltage Vc of phase c of the power system 2. The AC voltages Va, Vb, and Vc are input to the control device 10. Hereinafter, the AC voltages Va, Vb, and Vc are also collectively referred to as the AC voltage Vsys.

[0018] The control device 10 is a device that controls the operation of the power converter 20. Specifically, the control device 10 mainly includes a generator simulation unit 101 and a signal generation unit 103 as functional configurations. Each function of the generator simulation unit 101 and the signal generation unit 103 is realized by a processing circuit. The processing circuit may be dedicated hardware or one or more CPUs (Central Processing Units) that execute a program stored in the internal memory of the control device 10. When the processing circuit is dedicated hardware, the processing circuit is composed of, for example, an FPGA (Field Programmable Gate Array), an ASIC (Application Specific Integrated Circuit), or a combination thereof.

[0019] The generator simulation unit 101 generates a voltage command value for the power converter 20 by simulating the characteristics of a synchronous generator based on the AC voltage Vsys in the power system 2. Typically, the generator simulation unit 101 simulates the behavior of a synchronous generator by solving the equation of motion shown in the following equation (1).

[0020]

Number

[0021] “Pо” is the target value of the active power to be output to the power converter 20 (hereinafter, also referred to as the “active power target value”), “Pe” is the active power output from the power converter 20, “M” is the moment of inertia of the simulated generator, “Δω” is the difference between the angular frequency ω of the rotor of the simulated generator and the reference angular frequency ω0 of the power system 2, and “D ” is the braking coefficient of the simulated generator. Also, assuming that the voltage phase angle of the simulated generator (hereinafter, also simply referred to as the “phase”) is “θ”, the following equation (2) regarding the phase θ holds.

[0022]

Number

[0023] Specifically, the generator simulation unit 101 includes an angular frequency generation unit 11, a phase generation unit 13, and a voltage command generation unit 15.

[0024] The angular frequency generation unit 11 generates an angular frequency deviation Δω based on the AC current Isys and the AC voltage Vsys. The specific configuration of the angular frequency generation unit 11 will be described later.

[0025] The phase generation unit 13 generates a phase θ based on the angular frequency deviation Δω generated by the angular frequency generation unit 11 and the reference angular frequency ω0. The phase generation unit 13 generates the phase θ by time integration according to equation (2). The specific configuration of the phase generation unit 13 will be described later. The reference angular frequency ω0 is the angular frequency of the power reference frequency in the power system 2 (for example, 50Hz or 60Hz).

[0026] The voltage command generation unit 15 generates a voltage command value Vref that suppresses overcurrent in the power converter 20, based on the AC voltage Vsys of the power system 2, the target value of the AC voltage Vsys (hereinafter also referred to as the target voltage) Vo, and the phase θ. In the following description, the magnitude (i.e., absolute value) of the voltage command value Vref will also be written as |Vref|. This is the same for other voltages or currents.

[0027] Furthermore, the voltage command generation unit 15 generates a voltage command value Vk based on the target voltage Vo and phase θ. The voltage command value Vk is a voltage vector represented by the magnitude and phase of the voltage. Specifically, the magnitude of the voltage command value Vk (i.e., the absolute value |Vk|) is the absolute value |Vo| of the target voltage Vo, and the phase of the voltage command value Vk is the phase θ. Here, the voltage command value Vref mentioned above is a voltage command value for current suppression that takes into account the upper limit of the output current of the power converter 20. In contrast, the voltage command value Vk is a voltage command value that does not take into account the said upper limit. The method for generating the voltage command value Vref for current suppression will be described later.

[0028] The signal generation unit 103 generates a control signal for the power converter 20 based on the absolute value |Vref| and phase θref of the voltage command value Vref generated by the generator simulation unit 101, and outputs it to the power converter 20. Specifically, the signal generation unit 103 includes a three-phase voltage generation unit 17 and a PWM (Pulse Width Modulation) control unit 19.

[0029] The three-phase voltage generation unit 17 generates three-phase sinusoidal voltages Va*, Vb*, and Vc* based on the absolute value |Vref| and phase θref from the voltage command generation unit 15. Specifically, Va* = |Vref| × sin(θref), Vb* = |Vref| × sin(θref + 2π / 3), and Vc* = |Vref| × sin(θref + 4π / 3) are generated.

[0030] The PWM control unit 19 performs pulse width modulation on each of the three-phase sinusoidal voltages Va*, Vb*, and Vc* to generate a control signal as a PWM signal. The PWM control unit 19 outputs this control signal to the power converter 20. Typically, the control signal is a gate control signal for controlling the on and off states of each switching element included in the power converter 20.

[0031] <Hardware Configuration> Figure 2 shows an example of the hardware configuration of the control device 10. Figure 2 shows an example in which the control device 10 is configured by a computer.

[0032] Referring to Figure 2, the control device 10 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 77, and an auxiliary storage device 78. The control device 10 also includes a bus 79 that connects the components to each other.

[0033] The input converter 70 has an auxiliary transformer for each input channel. Each auxiliary transformer converts the detection signals from the current detector 7 and voltage detector 8 in Figure 1 into signals with voltage levels suitable for subsequent signal processing.

[0034] 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.

[0035] 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.

[0036] The CPU 74 controls the entire control unit 10 and performs arithmetic processing according to the program. The RAM 75, which is volatile memory, and the ROM 76, which is 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.

[0037] The input / output interface 77 is an interface circuit for communication between the CPU 74 and external devices.

[0038] Unlike the example in Figure 2, it is also possible to configure at least a part of the control device 10 using FPGAs and circuits such as ASICs.

[0039] <Specific configuration of the generator simulation section> The specific configurations of the angular frequency generation unit 11, phase generation unit 13, and voltage command generation unit 15 of the generator simulation unit 101 described in Figure 1 will now be explained.

[0040] (Angular frequency generation unit) Figure 3 is a block diagram showing an example of the functional configuration of the angular frequency generation unit 11. Referring to Figure 3, the angular frequency generation unit 11 includes an active power calculation unit 201 and an angular frequency calculation unit 202.

[0041] The active power calculation unit 201 calculates the current active power Pe of the power system 2 based on the AC current Isys detected by the current detector 7 and the AC voltage Vsys detected by the voltage detector 8.

[0042] The angular frequency calculation unit 202 includes a subtractor 211, an integrator (labeled "1 / MS" in the figure) 212, and a proportionalizer (labeled "D" in the figure) 213.

[0043] The subtractor 211 subtracts the output value of the proportionalizer 213 from the difference ΔP between the active power target value Po and the active power Pe. The active power target value Po is set as appropriate by the grid operator. The output value of the proportionalizer 213 is the product of the angular frequency deviation Δω and the braking coefficient D, "D × Δω". By subtracting the product "D × Δω" from the difference ΔP, the angular frequency calculation unit 202 simulates the braking force of the synchronous generator in the control of the power converter 20. Note that the output value of the proportionalizer 213 may not be input to the subtractor 211.

[0044] The integrator 212 outputs the angular frequency deviation Δω by integrating the output value of the subtractor 211 over time. If the output value of the proportionalizer 213 is not input to the subtractor 211, the integrator 212 outputs the angular frequency deviation Δω by integrating the difference ΔP between the active power target value Po and the active power Pe over time.

[0045] (Phase generation section) Figure 4 is a block diagram showing an example of the functional configuration of the phase generation unit 13. Referring to Figure 4, the phase generation unit 13 includes an adder 302 and an integrator 304.

[0046] The adder 302 outputs the angular frequency ω (=Δω+ω0), which is the sum of the angular frequency deviation Δω generated by the angular frequency generator 11 and the reference angular frequency ω0. The integrator 304 integrates the angular frequency ω over time and outputs the phase θ of the AC voltage Vsys output from the power converter 20.

[0047] (Voltage command generation unit) The voltage command generation unit 15 primarily performs the following processes: determining whether or not to suppress the current flowing through the power converter 20; determining the operating state of the power converter 20; and generating a voltage command value Vref for current suppression. First, the method for determining whether or not current suppression is necessary will be explained.

[0048] The voltage command generation unit 15 generates a voltage command value Vk based on the target voltage Vo of the AC voltage Vsys and the phase θ output from the phase generation unit 13. Based on the voltage command value Vk, the voltage command generation unit 15 determines whether the current suppression condition, which indicates that current suppression of the power converter 20 is necessary, has been met.

[0049] Specifically, first, the voltage command generation unit 15 calculates the differential voltage ΔVk by subtracting the AC voltage Vsys from the voltage command value Vk. The differential voltage ΔVk is a differential voltage vector obtained by subtracting the AC voltage vector representing the AC voltage Vsys from the voltage command value vector representing the voltage command value Vk. Based on the differential voltage ΔVk and the leakage impedance Zt of the transformer 3, the voltage command generation unit 15 calculates the current Ik. Specifically, the current Ik is a current vector calculated by "Ik = ΔVk / jZt", where "j" is the imaginary unit. Furthermore, the current Ik represents a hypothetical current output from the power converter 20 to the transformer 3, assuming that the power converter 20 is controlled based on the voltage command value Vk without considering current suppression.

[0050] Next, the voltage command generation unit 15 determines whether the current suppression condition is met based on the absolute value |Ik|, which indicates the magnitude of the current Ik, and the threshold Ith, which indicates the upper limit of the output current of the power converter 20. Specifically, the current suppression condition is that the absolute value |Ik| is greater than or equal to the threshold Ith. Therefore, the voltage command generation unit 15 determines that it will suppress the current if the absolute value |Ik| is greater than or equal to the threshold Ith (i.e., the current suppression condition is met), and determines that it will not suppress the current if |Ik| is less than the threshold Ith (i.e., the current suppression condition is not met).

[0051] If the current suppression condition is not met, the voltage command generation unit 15 generates a voltage command value Vk based on the absolute value |Vo| and phase θ as the voltage command value Vref. On the other hand, if the current suppression condition is met, the voltage command generation unit 15 executes a process to suppress the current (hereinafter also referred to as the "current suppression process"). The voltage command generation unit 15 executes different current suppression processes depending on the operating state of the power converter 20 when the current suppression condition is met.

[0052] Next, the method for determining the operating state of the power converter 20 will be explained. The operating state of the power converter 20 includes a power transmission operating state Ro, in which the power converter 20 outputs power to the power system 2 (i.e., transmits power), and a power reception operating state Ri, in which the power converter 20 receives power input from the power system 2 (i.e., receives power). The power transmission operating state Ro is, for example, a state in which the power converter 20 is operating to convert DC power from the DC circuit 30 into AC power and discharge said AC power to the power system 2. The power reception operating state Ri is, for example, a state in which the power converter 20 is operating to convert AC power from the power system 2 into DC power and charge said DC power to the DC circuit 30.

[0053] The power converter 20 is connected to the power system 2 via a transmission line, and the direction of the output power of the power converter 20 is generally determined by the sign of the phase θref of the voltage command value Vref. Therefore, the voltage command generation unit 15 determines the operating state of the power converter 20 based on the sign of the phase θref of the current (for example, previously generated in the last control cycle) voltage command value Vref. Typically, the voltage command generation unit 15 determines that the power converter 20 is in the power transmission operating state Ro if the phase θref of the voltage command value Vref is positive, and determines that the power converter 20 is in the power reception operating state Ri if the phase θref is negative.

[0054] Alternatively, the voltage command generation unit 15 may determine the operating state of the power converter 20 based on the sign of the active power Pe of the power system 2 at the present time. Typically, the voltage command generation unit 15 determines that the power converter 20 is in transmission operation state Ro if the active power Pe is positive (for example, active power Pe is output from the power converter 20), and determines that the power converter 20 is in power receiving operation state Ri if the active power Pe is negative (for example, active power Pe is input to the power converter 20). The voltage command generation unit 15 may also calculate the active power Pe based on the AC current Isys and AC voltage Vsys, similar to the active power calculation unit 201 of the angular frequency generation unit 11. Alternatively, the voltage command generation unit 15 may obtain the active power Pe calculated by the active power calculation unit 201.

[0055] Next, the method for generating the voltage command value Vref for current suppression will be explained. When the power suppression condition is met and the power converter 20 is outputting power to the power system 2 (i.e., the power converter 20 is in the power transmission operation state Ro), the voltage command generation unit 15 generates the voltage command value Vref based on the current suppression method M1 for suppressing the current of the power converter 20. On the other hand, when the power suppression condition is met and the power converter 20 is receiving power input from the power system (i.e., the power converter 20 is in the power receiving operation state Ri), the voltage command generation unit 15 generates the voltage command value Vref based on a current suppression method M2 that is different from the current suppression method M1.

[0056] Figure 5 shows the relationship between the phase of the power converter and the output active power. The vertical axis of the graph in Figure 5 represents the active power Pe output from the power converter 20. When the active power Pe is positive, the power converter 20 outputs active power Pe to the power system 2. When the active power Pe is negative, the power converter 20 receives active power Pe as input from the power system 2. The horizontal axis of the graph in Figure 5 represents the voltage phase angle (i.e., phase θ) of the power converter 20.

[0057] Referring to Figure 5, Graph 310 shows the relationship between phase θ and active power Pe when the voltage command generation unit 15 does not suppress the current of the power converter 20 (i.e., when the voltage command value Vk is generated as the voltage command value Vref). Graph 310 shows a sinusoidal wave that is symmetrical in the positive and negative directions with respect to zero phase θ.

[0058] When the current of the power converter 20 is suppressed, the output characteristics of the active power Pe differ depending on the output direction and the selected current suppression method. Graph 320 shows a characteristic graph when current suppression method M1, one example of various current suppression methods, is selected. Specifically, graph 320 shows the relationship between phase θ and active power Pe when the voltage command generation unit 15 suppresses the current of the power converter 20 using current suppression method M1. Graph 320 has a shape that is generally symmetrical in the positive and negative directions with respect to zero phase θ. Referring to graph 320, it can be understood that when phase θ is greater than or equal to phase θ1, the active power Pe decreases as phase θ increases, and when phase θ is less than phase θ2, the active power Pe increases as phase θ decreases.

[0059] Graph 330 shows a characteristic graph when current suppression method M2, one example of various current suppression methods, is selected. Specifically, Graph 330 shows the relationship between phase θ and active power Pe when the voltage command generation unit 15 suppresses the current of the power converter 20 using current suppression method M2. Graph 330 has an asymmetric shape in the positive and negative directions with zero as the reference point for phase θ. Referring to Graph 330, it can be seen that when phase θ is greater than or equal to phase θ1, the active power Pe decreases significantly as phase θ increases, and when phase θ is less than phase θ2, the active power Pe remains constant even if phase θ decreases.

[0060] Here, we focus on the change in active power Pe when the phase θ is "positive". In this case, the active power Pe in graph 330 decreases sharply when the phase θ is greater than or equal to 1. On the other hand, the active power Pe in graph 320 decreases slightly when the phase θ is greater than or equal to 1. From this, it can be seen that when the phase θ is "positive", the active power Pe on the positive side is larger in graph 320 than in graph 330. Therefore, the current suppression method M1 corresponding to graph 320 can stabilize the power system more than the current suppression method M2 corresponding to graph 330. This makes it possible to stabilize power systems that have low transient stability and are prone to instability (for example, prone to losing synchronism).

[0061] Therefore, when it is necessary to suppress the current of the power converter 20, and the active power Pe is output from the power converter 20 to the power system 2 (for example, when the phase θ is "positive"), it is preferable from the viewpoint of stabilizing the power system (for example, stabilizing the voltage and frequency of the power system) to perform current suppression using the current suppression method M1 corresponding to graph 320, where the positive active power Pe is large, rather than using the current suppression method M2 corresponding to graph 330.

[0062] Next, let's look at the change in active power Pe when the phase θ is "negative". In this case, the active power Pe in graph 320 increases when the phase θ is less than 2. On the other hand, the active power Pe in graph 330 remains at an almost constant value when the phase θ is less than 2. From this, it can be seen that when the phase θ is "negative", the negative active power Pe is larger in graph 330 than in graph 320. Therefore, for example, even in power systems with low transient stability and prone to instability, the current suppression method M2 corresponding to graph 330 can stabilize the power system more than the current suppression method M1 corresponding to graph 320.

[0063] Therefore, when it is necessary to suppress the current of the power converter 20, and active power Pe is input to the power converter 20 from the power system 2 (for example, when the phase θ is "negative"), it is preferable from the standpoint of stabilizing the power system to use the current suppression method M2, which corresponds to graph 330 where the negative active power Pe is large, rather than using the current suppression method M1, which corresponds to graph 320.

[0064] Based on the above, the voltage command generation unit 15 generates a voltage command value Vref based on the current suppression method M1 when the power suppression condition is met and the power converter 20 is outputting power to the power system 2, and generates a voltage command value Vref based on the current suppression method M2 when the power suppression condition is met and the power converter 20 is receiving power input from the power system.

[0065] Here, we will explain the generation method of the voltage command value Vref based on the current suppression methods M1 and M2. First, we will explain a specific example of the generation method of the voltage command value Vref based on the current suppression method M1.

[0066] The voltage command generation unit 15 generates a voltage command value Vref according to the current suppression method M1 based on the voltage command value Vk, the difference voltage ΔVk between the AC voltage Vsys and the voltage command value Vk, and the virtual impedance obtained by adding the virtual reactance jXc to the leakage impedance Zt.

[0067] Specifically, the voltage command generation unit 15 calculates the current Ika1 based on the current Ik calculation formula "Ik = ΔVk / jZt", the virtual impedance obtained by adding a small virtual reactance jXc to the leakage impedance Zt in the denominator of the calculation formula, and the differential voltage ΔVk. In this case, the current Ika1 is expressed as "Ika1 = ΔVk / (jXc + jZt)".

[0068] The voltage command generation unit 15 compares the absolute value of current Ika1| |Ika1| with the threshold Ith (for example, the upper limit of the output current of the power converter 20). If the absolute value |Ika1| is greater than or equal to the threshold Ith, the voltage command generation unit 15 adds an even smaller virtual reactance jXc to the denominator of the formula for calculating current Ika1 to calculate current Ika2. Specifically, current Ika2 is expressed as "Ika2 = ΔVk / (jXc + jXc + jZt)". The voltage command generation unit 15 compares the absolute value of current Ika2| |Ika2| with the threshold Ith. In this way, the voltage command generation unit 15 repeats the comparison process between current Ikam and threshold Ith until current Ikam (where m is a natural number greater than or equal to 1) becomes less than the threshold Ith.

[0069] Then, when the current Ikan (where n is a natural number greater than or equal to 1) becomes greater than or equal to the threshold Ith, the voltage command generation unit 15 calculates a voltage command value Vkm1 based on the current suppression method M1 using the current Ikan. Specifically, the voltage command value Vkm1 is calculated using the calculation formula "Vkm1 = Vk - jXc × Ikan". The voltage command generation unit 15 generates the voltage command value Vkm1 as a voltage command value Vref that suppresses the overcurrent of the power converter 20. The absolute value |Vref| of the voltage command value Vref is the absolute value |Vkm1| of the voltage command value Vkm1, and the phase θref of the voltage command value Vref is the phase θkm1 of the voltage command value Vkm1.

[0070] Next, a specific example of the method for generating the voltage command value Vref based on the current suppression method M2 will be described. The voltage command generation unit 15 generates the voltage command value Vref according to the current suppression method M2 based on the voltage command value Vk, the difference voltage ΔVk between the AC voltage Vsys and the voltage command value Vk, and the virtual impedance obtained by adding the virtual resistance Rc to the leakage impedance Zt.

[0071] Specifically, the voltage command generation unit 15 calculates the current Ikb1 based on the current Ik calculation formula "Ik = ΔVk / jZt", the virtual impedance obtained by adding a small virtual resistance Rc to the leakage impedance Zt in the denominator of the calculation formula, and the differential voltage ΔVk. In this case, the current Ikb1 is expressed as "Ikb1 = ΔVk / (Rc + jZt)".

[0072] The voltage command generation unit 15 compares the absolute value of current Ikb1| |Ikb1| with the threshold Ith. If the absolute value |Ikb1| is greater than or equal to the threshold Ith, the voltage command generation unit 15 adds an even smaller virtual resistance Rc to the denominator of the formula for calculating current Ikb1 to calculate current Ikb2. Specifically, current Ikb2 is expressed as "Ikb2 = ΔVk / (Rc + Rc + jZt)". The voltage command generation unit 15 compares the absolute value of current Ikb2| |Ikb2| with the threshold Ith. In this way, the voltage command generation unit 15 repeats the comparison process between current Ikbm and threshold Ith until current Ikbm becomes less than threshold Ith.

[0073] Then, when the current Ikbn exceeds the threshold Ith, the voltage command generation unit 15 calculates a voltage command value Vkm2 based on the current suppression method M2 using the current Ikbn. Specifically, the voltage command value Vkm2 is calculated using the formula "Vkm2 = Vk - Rc × Ikbn". The voltage command generation unit 15 generates the voltage command value Vkm2 as a voltage command value Vref that suppresses the overcurrent of the power converter 20. The absolute value |Vref| of the voltage command value Vref is the absolute value |Vkm2| of the voltage command value Vkm2, and the phase θref of the voltage command value Vref is the phase θkm2 of the voltage command value Vkm2.

[0074] The above-described method for calculating the voltage command value Vref based on current suppression methods M1 and M2 is merely an example, and a configuration in which the voltage command value Vref is calculated based on other current suppression methods is also possible. Specifically, instead of current suppression method M1 corresponding to graph 320 in Figure 5, a current suppression method having a similar trend to graph 320 (i.e., a characteristic graph similar to graph 320) from among various current suppression methods may be used. Also, instead of current suppression method M2 corresponding to graph 330 in Figure 5, a current suppression method having a similar trend to graph 330 (i.e., a characteristic graph similar to graph 330) from among various current suppression methods may be used.

[0075] Figure 6 is a flowchart showing an example of the processing procedure of the voltage command generation unit 15. Each step in Figure 6 is typically performed by the processing circuit of the control device 10. Referring to Figure 6, the voltage command generation unit 15 determines whether or not to suppress the current of the power converter 20 (step S10). Specifically, the voltage command generation unit 15 determines to suppress the current if the absolute value |Ik| is greater than or equal to the threshold Ith, and determines not to suppress the current if the absolute value |Ik| is less than the threshold Ith.

[0076] If the current is not suppressed (NO in step S10), the voltage command generation unit 15 generates a voltage command value Vk based on the absolute value |Vo| and phase θ as the voltage command value Vref (step S18).

[0077] On the other hand, if the current is to be suppressed (YES in step S10), the voltage command generation unit 15 determines whether the power converter 20 is in the power transmission operation state Ro (step S12). If the power converter 20 is in the power transmission operation state Ro (YES in step S12), the voltage command generation unit 15 generates a voltage command value Vref based on the current suppression method M1 described above (step S14). Specifically, the voltage command generation unit 15 generates a voltage command value Vkm1 as the voltage command value Vref.

[0078] If the power converter 20 is not in the power transmission operation state Ro (i.e., in the power reception operation state Ri) (NO in step S12), the voltage command generation unit 15 generates a voltage command value Vref based on the current suppression method M2 described above (step S16). Specifically, the voltage command generation unit 15 generates a voltage command value Vkm2 as the voltage command value Vref.

[0079] <Advantages> According to this embodiment, the control device 10 can suppress overcurrent in the output current of the power converter 20 by using a voltage command value Vref to suppress overcurrent while causing the power converter 20 to perform control that simulates a synchronous generator. Furthermore, when the control device 10 determines that it is necessary to suppress the current of the power converter 20, it generates a voltage command value Vref using an appropriate current suppression method according to the output power direction of the power converter 20. This makes it possible to stabilize the power system.

[0080] Other embodiments. (1) In the embodiment described above, a configuration in which active power Pe is input to the subtractor 211 of the angular frequency calculation unit 202 has been described, but the technical concept of this disclosure is not limited to this configuration. For example, instead of active power Pe, a virtual active power Pk output from the power converter 20, assuming that the power converter 20 is controlled based on a voltage command value Vk, may be input to the subtractor 211.

[0081] Figure 7 is a diagram illustrating an example of the overall configuration of a power conversion system 1000A according to another embodiment. The power conversion system 1000A in Figure 7 differs from the power conversion system 1000 in Figure 1 in that the control device 10 in Figure 1 is replaced with a control device 10A. The control device 10A differs from the control device 10 in Figure 1 in that the generator simulation unit 101 in Figure 1 is replaced with a generator simulation unit 101A. The generator simulation unit 101A includes an angular frequency generation unit 11A, a phase generation unit 13, and a voltage command generation unit 15A. Here, only the differences from the generator simulation unit 101 will be explained.

[0082] The angular frequency generation unit 11A generates an angular frequency deviation Δω based on the active power Pk from the voltage command generation unit 15A. The active power Pk is calculated in the voltage command generation unit 15A. The angular frequency generation unit 11A replaces "Pe" in equation (1) with "Pk" and generates the angular frequency deviation Δω by time integration according to equation (1). The phase generation unit 13 performs the above-described functions and processes to generate a phase θ based on the angular frequency deviation Δω and the reference angular frequency ω0.

[0083] The voltage command generation unit 15A calculates the active power Pk based on the AC voltage Vsys and the voltage command value Vk. Specifically, the voltage command generation unit 15A calculates the differential voltage ΔVk by subtracting the AC voltage Vsys from the voltage command value Vk, and calculates the current Ik based on the differential voltage ΔVk and the leakage impedance Zt of the transformer 3. Then, the voltage command generation unit 15A calculates the active power Pk based on the voltage command value Vk and the current Ik. If the complex conjugate of the current Ik is "Ik*", then "Pk = Vk × Ik*". Note that "Vk = |Vo| × (cosθ + jsinθ)". The active power Pk represents the virtual active power output from the power converter 20, assuming that the power converter 20 is controlled based on the voltage command value Vk. The active power Pk is output to the angular frequency generation unit 11.

[0084] The voltage command generation unit 15A is the same as the voltage command generation unit 15 except for the calculation of the active power Pk and the output of the active power Pk to the angular frequency generation unit 11A. The voltage command generation unit 15A may also determine the operating state of the power converter 20 based on the sign of the active power Pk.

[0085] Figure 8 is a block diagram showing an example of the functional configuration of the angular frequency generation unit 11A according to another embodiment. Referring to Figure 8, the angular frequency generation unit 11A includes a subtractor 211, an integrator 212, and a proportionalizer 213. That is, the angular frequency generation unit 11A differs from the angular frequency generation unit 11 of Figure 3 in that the active power calculation unit 201 is omitted, and the active power Pe input to the subtractor 211 is changed to active power Pk.

[0086] The subtractor 211 subtracts the output value of the proportionalizer 213 (i.e., "D × Δω") from the difference ΔP1 between the target active power value Po and the active power Pk. The integrator 212 integrates the output value of the subtractor 211 over time and outputs the angular frequency deviation Δω.

[0087] According to the modified version described above, the angular frequency deviation Δω and phase θ are generated using the active power Pk produced by the voltage command value Vk according to equations (1) and (2). Therefore, the voltage phase as a synchronous generator can be maintained at an appropriate value. Consequently, even if the period during which overcurrent is suppressed is extended, unstable responses such as loss of synchronism can be prevented when performing control that simulates a synchronous generator. Furthermore, when the disturbance in power system 2 subsides, it becomes easy to return the power converter 20 to control that simulates a synchronous generator.

[0088] (2) In the above-described embodiment, if the operating state of the power converter 20 switches while the current suppression condition is met, the current suppression method may not be changed. For example, the voltage command generation unit 15 generates a voltage command value Vref based on the current suppression method M1 when the power converter 20 is in the power transmission operation state Ro when the current suppression condition is met. Then, suppose the power converter 20 changes to the power receiving operation state Ri while the current suppression condition remains met. As explained in Figure 5, the output of active power Pe differs depending on the current suppression method, so if the current suppression method M1 is changed to the current suppression method M2 in conjunction with the change from the power transmission operation state Ro to the power receiving operation state Ri, the active power Pe may change rapidly.

[0089] Therefore, even if the operating state of the power converter 20 changes from the power transmission operating state Ro to the power reception operating state Ri while the current suppression conditions are met, the voltage command generation unit 15 generates a voltage command value Vref based on the current suppression method M1. In other words, the voltage command generation unit 15 does not switch from the current suppression method M2 to the current suppression method M1. Similarly, even if the operating state of the power converter 20 changes from the power reception operating state Ri to the power transmission operating state Ro while the current suppression conditions are met, the voltage command generation unit 15 generates a voltage command value Vref based on the current suppression method M2. This prevents abrupt changes in active power Pe and stabilizes the power system.

[0090] Referring to graphs 320 and 330 in Figure 5, for example, within the range where the phase θ is between θ2 and θ1, the value of the active power Pe is the same for both current suppression methods M1 and M2. Therefore, the voltage command generation unit 15 may be configured to change the current suppression method when the operating state of the power converter 20 switches while the current suppression conditions are met. For example, if the power transmission operating state Ro changes to the power reception operating state Ri while the current suppression conditions are met, the voltage command generation unit 15 switches from current suppression method M1 to current suppression method M2 and generates a voltage command value Vref based on current suppression method M2. Similarly, for example, if the power reception operating state Ri changes to the power transmission operating state Ro while the current suppression conditions are met, the voltage command generation unit 15 generates a voltage command value Vref based on the current suppression method M1, which was switched from current suppression method M2.

[0091] (3) The configurations illustrated above as embodiments are examples of the configurations of the Disclosure and can be combined with other known technologies, and can be modified, such as by omitting parts, without departing from the gist of the Disclosure. Furthermore, in the embodiments described above, processes and configurations described in other embodiments may be adopted as appropriate.

[0092] The embodiments disclosed herein should be considered in all respects to be illustrative and not restrictive. The scope of this disclosure is indicated by the claims rather than by the foregoing description, and all modifications within the meaning and scope of the claims are intended to be included. [Explanation of Symbols]

[0093] 2 Power system, 3 Transformer, 5 Busbar, 6 Power converter, 7 Current detector, 8 Voltage detector, 10, 10A Control device, 11, 11A Angular frequency generator, 13 Phase generator, 15, 15A Voltage command generator, 17 Three-phase voltage generator, 19 PWM control unit, 20 Power converter, 30 DC circuit, 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, 101, 101A Generator simulation unit, 103 Signal generation unit, 201 Active power calculation unit, 202 Angular frequency calculation unit, 211 Subtractor, 212, 304 Integrator, 213 Proportional converter, 302 Adder, 1000, 1000A Power conversion system.

Claims

1. A control device for controlling a power converter connected to a power grid, A generator simulation unit generates a first voltage command value for the power converter by simulating the characteristics of a synchronous generator based on the AC voltage in the power system, The system includes a signal generation unit that generates a control signal for the power converter based on the first voltage command value, The aforementioned generator simulation unit is, An angular frequency generation unit that outputs a first angular frequency by integrating the first difference between the first active power and the target active power value over time, A phase generation unit that outputs a first phase by time-integrating the sum of the first angular frequency and the reference angular frequency, The system includes a voltage command generation unit that generates a first voltage command value that suppresses overcurrent in the power converter based on the AC voltage, a target value of the AC voltage, and the first phase, The voltage command generation unit, A target value for the AC voltage and a second voltage command value based on the first phase are generated. Based on the second voltage command value, it is determined whether or not the suppression condition that requires suppression of the current of the power converter has been met. If the power converter is outputting power to the power system when the suppression condition is met, the first voltage command value is generated based on the first suppression method for suppressing the current of the power converter. A control device that, when the suppression condition is met and the power converter is receiving power input from the power system, generates the first voltage command value based on a second suppression method different from the first suppression method.

2. The voltage command generation unit, Based on the difference voltage between the AC voltage and the second voltage command value, and the leakage impedance of the transformer installed in the power system, the first current is calculated. The control device according to claim 1, which determines that the suppression condition has been met when the first current is equal to or greater than a threshold.

3. The control device according to claim 2, wherein the voltage command generation unit generates the second voltage command value as the first voltage command value when the suppression condition is not met.

4. The control device according to claim 2 or 3, wherein the voltage command generation unit generates the first voltage command value according to the first suppression method based on the second voltage command value, the differential voltage, and the first virtual impedance obtained by adding a virtual reactance to the leakage impedance.

5. The control device according to claim 2 or 3, wherein the voltage command generation unit generates the first voltage command value according to the second suppression method based on the second voltage command value, the differential voltage, and the second virtual impedance obtained by adding a virtual resistance to the leakage impedance.

6. The control device according to any one of claims 1 to 3, wherein the angular frequency generation unit calculates the first active power based on the AC voltage and the AC current in the power system.

7. The control device according to any one of claims 1 to 3, wherein the voltage command generation unit calculates the first active power based on the AC voltage and the second voltage command value.

8. The control device according to any one of claims 1 to 3, wherein, while the suppression condition is met, the state of the power converter changes from a state in which the power converter outputs power to the power system to a state in which the power converter receives power input from the power system, and the voltage command generation unit generates the first voltage command value based on the first suppression method.

9. A power converter connected to the power grid, The system includes a control device for controlling the power converter, The control device is A generator simulation unit generates a first voltage command value for the power converter by simulating the characteristics of a synchronous generator based on the AC voltage in the power system, Includes a signal generation unit that generates a control signal for the power converter based on the first voltage command value, The aforementioned generator simulation unit is, An angular frequency generation unit that outputs a first angular frequency by integrating the first difference between the first active power and the target active power value over time, A phase generation unit that outputs a first phase by time-integrating the sum of the first angular frequency and the reference angular frequency, The system includes a voltage command generation unit that generates a first voltage command value that suppresses overcurrent in the power converter based on the AC voltage, a target value of the AC voltage, and the first phase, The voltage command generation unit, A target value for the AC voltage and a second voltage command value based on the first phase are generated. Based on the second voltage command value, it is determined whether or not the suppression condition that requires suppression of the current of the power converter has been met. If the power converter is outputting power to the power system when the suppression condition is met, the first voltage command value is generated based on the first suppression method for suppressing the current of the power converter. A power converter that, when the suppression condition is met and the power converter is receiving power input from the power system, generates the first voltage command value based on a second suppression method different from the first suppression method.

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