Power Conversion Systems

Abstract

A self-excited power converter (110) is connected between an AC system (1) and a DC system (3). In a first operation mode in which the power converter (110) operates as a voltage source, a control device (200) sets current command values ​​for AC output currents (Iacu, Iacv, Iacw) based on fundamental wave components of the AC output currents (Iacu, Iacv, Iacw) measured by an AC current detector (9). In the first operation mode, the control device (200) generates on / off commands (Sul, Suu, Svl, Svu, Swl, Swu) for semiconductor switching elements of the power converter (110) so that the AC voltages at AC terminals (Nu, Nv, Nw) are controlled in accordance with the sum of a feedback component for compensating for deviations of the output AC currents (Iacu, Iacv, Iacw) from the AC current command values ​​and an AC component according to the frequency of the fundamental wave component.

Description

Technical Field

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

Background Art

[0002] As a system that connects two or more AC systems with a DC circuit, a high voltage direct current (HVDC) system is widely known. In an HVDC system, a power converter that performs AC-DC conversion is provided between each AC system and the DC circuit, and each power converter has an independent control system. Also, each power converter has a common or independent power storage element (capacitor).

[0003] In an HVDC system using a self-excited converter as a power converter for AC-DC conversion, since the self-excited converter can operate as a voltage source, when the connected AC system experiences a blackout, it is possible to perform an operation responsible for starting up (black start) the AC system.

[0004] Japanese Patent No. 6730946 (Patent Document 1) describes control for continuing the operation of a power converter (self-excited converter) during such a black start. Specifically, during a black start, it is described that an AC voltage command value for AC current control is set based on a preset AC voltage component and a voltage component for making the harmonic current other than the fundamental wave extracted from the AC current measurement value zero.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] When a self-excited converter is responsible for black starting an AC system, there is a concern that a large current may be generated as the output current increases in accordance with the connection of generators and loads included in the AC system when the AC system starts up by generating an AC voltage with the self-excitation converter. In this case, since self-excitation converters using self-extinguishing semiconductor elements cannot operate with a large current of two to three times the element's rated current, even for a short time, it is important to suppress overcurrent.

[0007] However, while the AC current control during black start described in Patent Document 1 has a suppressive effect against a sudden current increase that differs from the frequency of the fundamental wave component, it is difficult to obtain an appropriate suppressive effect when considering cases where the current of the fundamental wave component increases in response to an increase in load. As a result, there are concerns that the operation of the power conversion system, which acts as a voltage source, will be restricted, and the performance of continuous operation will deteriorate.

[0008] This disclosure was made to solve these problems, and the purpose of this disclosure is to improve the continuous operation performance of a power conversion system that operates as a voltage source. [Means for solving the problem]

[0009] In certain aspects of this disclosure, a power conversion system is provided. The power conversion system comprises a power converter connected between an AC circuit and a DC circuit, an AC current detector, and a control device. The power converter is configured to include at least one self-extinguishing semiconductor switching element. The control device controls the power converter by generating on / off commands for the semiconductor switching element. The AC current detector detects the output AC current from the power converter to the AC circuit. The control device includes a current command value setting unit and an AC current control unit. The current command value setting unit sets an AC current command value. The AC current control unit generates on / off commands according to an AC voltage command value calculated such that the output AC current detected by the AC current detector follows the AC current command value. In a first operating mode in which the power converter operates as a voltage source, the current command value setting unit sets the AC current command value based on the fundamental wave component of the measured output AC current. In the first operating mode, the AC current control unit calculates the AC voltage command value according to the sum of a feedback component to compensate for the deviation between the AC current command value and the detected output AC current, and an AC component according to the frequency of the fundamental wave component. [Effects of the Invention]

[0010] According to this disclosure, in a first operating mode in which the power converter operates as a voltage source, the current command value of the output AC current of the power converter is set based on the fundamental wave component of the measured output AC current, thereby suppressing the shutdown of the power converter due to overcurrent protection and improving the continuous operation performance of the power conversion system. [Brief explanation of the drawing]

[0011] [Figure 1] This is a schematic diagram of the power conversion system according to this embodiment. [Figure 2] Figure 1 is a block diagram showing an example of the hardware configuration of the control device. [Figure 3] This is a functional block diagram illustrating an example of the functional configuration of the control device according to Embodiment 1. [Figure 4]It is a functional block diagram for explaining the functional configuration example of the voltage FF control unit shown in FIG. 3. [Figure 5] It is a functional block diagram for explaining the first functional configuration example of the current command value setting unit according to Embodiment 1. [Figure 6] It is a functional block diagram for explaining the second functional configuration example of the current command value setting unit according to Embodiment 1. [Figure 7] It is a conceptual diagram for explaining the function of the current upper limit value adjustment unit shown in FIG. 6. [Figure 8] It is a functional block diagram for explaining the third functional configuration example of the current command value setting unit according to Embodiment 1. [Figure 9] It is a functional block diagram for explaining the fourth functional configuration example of the current command value setting unit according to Embodiment 1. [Figure 10] It is a functional block diagram for explaining the first functional configuration example of the current command value setting unit according to Embodiment 2. [Figure 11] It is a conceptual waveform diagram for explaining the operation of the lamp rate limiter (RRL) shown in FIG. 10. [Figure 12] It is a functional block diagram for explaining the second functional configuration example of the current command value setting unit according to Embodiment 2. [Figure 13] It is a functional block diagram for explaining the third functional configuration example of the current command value setting unit according to Embodiment 2. [Figure 14] It is a functional block diagram for explaining the functional configuration example of the voltage FF control unit according to Embodiment 2. [Figure 15] It is a functional block diagram for explaining the functional configuration example of the current command value setting unit according to Embodiment 3. [Figure 16] It is a functional block diagram for explaining the functional configuration example of the AC voltage command value calculation unit according to Embodiment 3. [Figure 17] It is a functional block diagram for explaining the functional configuration example of the voltage FF control unit according to Embodiment 3. [Figure 18] It is a conceptual diagram for explaining the function of the gain setting unit in FIG. 15. [Figure 19]It is a conceptual diagram for explaining the frequency characteristics of the band-pass filter in FIG. 15. [Figure 20] It is a functional block diagram for explaining a functional configuration example of a current command value setting unit according to a combination of Embodiment 3 and Embodiment 2. [Figure 21] It is a schematic configuration diagram for explaining a modified example of the power converter. [Figure 22] It is a circuit diagram showing a first configuration example of the converter cell shown in FIG. 21. [Figure 23] It is a circuit diagram showing a second configuration example of the converter cell shown in FIG. 21.

Embodiments for Carrying Out the Invention

[0012] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. In the following, the same or corresponding parts in the drawings are denoted by the same reference numerals, and the description thereof will not be repeated in principle.

[0013] Embodiment 1. [Configuration Example of Power Conversion System] FIG. 1 is a schematic configuration diagram of a power conversion system 100 according to the present embodiment.

[0014] The power conversion system 100 is a system for controlling the power of a DC power transmission system with a single-pole configuration, and performs bidirectional DC / AC power conversion between the DC system 3 and the AC system 1. The AC system 1 is a three-phase AC system, but in FIG. 1, it is shown by a single line for ease of illustration. The AC system 1 corresponds to an "AC circuit", and the DC system 3 corresponds to a "DC circuit".

[0015] The DC system 3 is a DC power transmission line. When the power conversion system 100 corresponds to an HVDC system, the length of the DC power transmission line is, for example, several tens of km to several hundreds of km. When the power conversion system 100 corresponds to a BTB (Back To Back) system, the length of the DC power transmission line is, for example, several meters to several tens of meters.

[0016] Furthermore, by connecting DC system 3 to another AC system (not shown) via another DC / AC converter (not shown) similar to the power conversion system, a system (HVDC system) for transmitting and receiving power between AC system 1 and the other AC system can be configured.

[0017] As shown in Figure 1, the power conversion system 100 according to Embodiment 1 comprises a power converter 110 and a control device 200. The power converter 110 is connected between the AC system 1 and the DC system 3.

[0018] The power converter 110 is a self-commutated power converter that functions as a forward converter, converting AC power to DC power, and a reverse converter, converting DC power to AC power. The power converter 110 performs power conversion between AC system 1 and DC system 3. The power converter 110 includes multiple leg circuits 4u, 4v, and 4w. The multiple leg circuits 4u, 4v, and 4w are connected in parallel to each other between the positive DC terminal (i.e., the high-potential DC terminal) Np and the negative DC terminal (i.e., the low-potential DC terminal) Nn. In the following description, when referring to the multiple leg circuits 4u, 4v, and 4w collectively, they will be referred to as "leg circuit 4".

[0019] The leg circuits 4 are provided for each of the multiple phases that make up the alternating current. The leg circuits 4 are connected between the AC system 1 and the DC system 3 and perform power conversion between the two systems. Figure 2 shows the case where the AC system 1 is a three-phase AC system, and three leg circuits 4u, 4v, and 4w are provided corresponding to the U phase, V phase, and W phase, respectively.

[0020] The AC terminals Nu, Nv, and Nw, provided on the leg circuits 4u, 4v, and 4w respectively, are connected to the AC system 1 via the transformer 13. In Figure 1, for the sake of illustration, the connection between the AC terminals Nv and Nw and the transformer 13 is not shown.

[0021] Instead of using the transformer 13 in Figure 1, the system may be configured to connect to the AC system 1 via an interconnection reactor (not shown). Furthermore, instead of the AC terminals Nu, Nv, Nw, primary windings may be provided on the leg circuits 4u, 4v, 4w, and the leg circuits 4u, 4v, 4w may be AC-connected to the transformer 13 or the interconnection reactor via secondary windings that are magnetically coupled to these primary windings. In other words, the leg circuits 4 are electrically (i.e., DC-wise or AC-wise) connected to the AC system 1 via connection points provided on each leg circuit 4u, 4v, 4w, such as the AC terminals Nu, Nv, Nw or the primary windings mentioned above.

[0022] Furthermore, the high-potential DC terminal Np and the low-potential DC terminal Nn, which are commonly connected to each leg circuit 4, are connected to the DC system 3.

[0023] In Embodiment 1, the power converter 110 has a configuration referred to as a two-level converter. Specifically, in the power converter 110 shown in Figure 1, each of the U-phase, V-phase, and W-phase leg circuits 4u, 4v, and 4w includes an upper arm 5 and a lower arm 6. The upper arm 5 and the lower arm 6 are configured by connecting a freewheeling diode (FWD) in antiparallel to a self-extinguishing switching element 31 capable of controlling both on and off operation. For example, the switching element 31 can be an IGBT (Insulated Gate Bipolar Transistor), a GCT (Gate Commutated Turn-off) thyristor, a MOSFET (Metal-Oxide-Semiconductor Field Effect Transistor), etc.

[0024] The energy storage element 12 is connected between the high-potential DC terminal Np and the low-potential DC terminal Nn. A capacitor is used for the energy storage element 12. In the following explanation, the energy storage element 12 may also be referred to as a "capacitor".

[0025] The power conversion system 100 further includes AC current detectors 9, 16, AC voltage detector 10, DC voltage detectors 11A, 11B, and DC current detector 17 as detectors for detecting the electrical quantities (current, voltage, etc.) used for control. The signals detected by these detectors are input to the control device 200.

[0026] The AC current detector 9 detects the output AC currents Iacu, Iacv, and Iacw of the U-phase, V-phase, and W-phase, which correspond to the currents output from the AC terminals Nu, Nv, and Nw to the transformer 13, i.e., the output currents from the power converter 110 to the AC system 1. In the following description, Iacu, Iacv, and Iacw will be collectively referred to as the output AC current Iac.

[0027] The AC voltage detector 10 detects the U-phase AC voltage Vsysu, the V-phase AC voltage Vsysv, and the W-phase AC voltage Vsysw of the AC system 1. In the following description, Vsysu, Vsysv, and Vsysw will be collectively referred to as the system AC voltage Vsys.

[0028] The AC current detector 16 detects the U-phase AC current Isysu, V-phase AC current Isysv, and W-phase AC current Isysw of the AC system 1. In the following description, Isysu, Isysv, and Isysw will be collectively referred to as the system AC current Isys.

[0029] The DC voltage detector 11A detects the DC voltage Vdcp at the high-potential DC terminal Np connected to the DC system 3. The DC voltage detector 11B detects the DC voltage Vdcn at the low-potential DC terminal Nn connected to the DC system 3. The DC current detector 17 detects the DC current Idc flowing through either the high-potential DC terminal Np or the low-potential DC terminal Nn.

[0030] The control device 200 controls the operation of the power converter 110. Specifically, the control device 200 controls the power conversion by the power converter 110 by outputting on / off control signals Suu, Sul, Svu, Svl, Swu, Swl for the switching elements 31 of the upper arm 5 and lower arm 6 of the leg circuits 4u~4w.

[0031] [Control device configuration] (Example of hardware configuration for a control device) Figure 2 shows a block diagram illustrating an example of the hardware configuration of the control device 200. Figure 2 also shows an example of the control device 200 being configured by a computer.

[0032] As shown in Figure 2, the control device 200 includes one or more input converters 20, one or more sample-and-hold (S / H) circuits 21, a multiplexer (MUX) 22, and an A / D (Analog to Digital) converter 23. Furthermore, the control device 200 includes one or more CPUs (Central Processing Units) 24, RAM (Random Access Memory) 25, and ROM (Read Only Memory) 26. Furthermore, the control device 200 includes one or more input / output interfaces 27, an auxiliary storage device 28, and a bus 29 that interconnects the above components.

[0033] The input converter 20 has an auxiliary transformer (not shown) for each input channel. Each auxiliary transformer converts the detection signals from each electrical quantity detector in Figure 1 into signals with a voltage level suitable for subsequent signal processing.

[0034] A sample-and-hold circuit 21 is provided for each input converter 20. The sample-and-hold circuit 21 samples and holds the signal representing the quantity of electricity received from the corresponding input converter 20 at a specified sampling frequency.

[0035] The multiplexer 22 sequentially selects signals held by multiple sample-and-hold circuits 21. The A / D converter 23 converts the signals selected by the multiplexer 22 into digital values. Note that by providing multiple A / D converters 23, A / D conversion may be performed in parallel for detection signals from multiple input channels.

[0036] The CPU 24 controls the entire control unit 200 and executes arithmetic processing according to the program. The RAM 25, which is volatile memory, and the ROM 26, which is non-volatile memory, are used as the main memory of the CPU 24. The ROM 26 stores programs and setting values ​​for signal processing. The auxiliary storage device 28 is a non-volatile memory with a larger capacity than the ROM 26, and stores programs and data for detected electrical quantities. The input / output interface 27 is an interface circuit for communication between the CPU 24 and external devices.

[0037] Unlike the example in Figure 2, it is also possible to configure at least a portion of the control device 200 using circuits such as FPGAs (Field Programmable Gate Arrays) and ASICs (Application Specific Integrated Circuits). In other words, the mechanisms of each functional block described in Figure 3 and later can be configured based on the computer exemplified in Figure 2, or at least a portion of it can be configured using circuits such as FPGAs and ASICs. Furthermore, at least a portion of the functions of each functional block can also be configured using analog circuits.

[0038] (Example of control device function configuration) Referring again to Figure 1, the self-commutated power converter 110 can operate as a voltage source, and, similar to Patent Document 1, can perform a black start operation by outputting a predetermined AC voltage to the AC system 1 when the AC system 1 blacks out. Furthermore, the power converter 110 can also control the power input and output from the power converter 110 to the AC system 1 after being connected to the AC system 1.

[0039] In this embodiment, the power converter 110 performs power conversion by selectively applying a plurality of operating modes, and these plurality of operating modes include at least a "voltage source mode" applied during black start and a "grid connection mode" applied when the AC system 1 is connected to the grid. The voltage source mode corresponds to the "first operating mode," and the grid connection mode corresponds to the "second operating mode."

[0040] As will be described later, the control device 200 switches the content of the control calculations for generating the on / off control signals Suu, Sul, Svu, Svl, Swu, Swl for each switching element 31 shown in Figure 1, between the voltage source mode and the grid connection mode. Selection commands between multiple operating modes, including the voltage source mode and the grid connection mode, are input from a higher-level system (not shown).

[0041] In grid-connected mode, the control device 200 controls the output AC current of the power converter 110 so that the power conversion system 100 operates in synchronization with the voltage and frequency of the AC grid 1.

[0042] Embodiment 1 primarily demonstrates the control function in voltage source mode. When there are no (or few) generators to support the voltage and frequency of AC system 1, for example, when AC system 1 blacks out, or when the components of AC system 1 consist only of renewable power sources or loads, equipment is needed to generate the voltage of AC system 1. In such cases, the operating mode of the power conversion system 100 is set to voltage source mode by a command from a higher level (not shown). Thus, the voltage source mode according to this embodiment is applied at least during a black start, but can also be applied in the above-mentioned situations other than a black start.

[0043] When the voltage source mode is commanded, the control device 200 controls the operation of the power converter 110 so that it takes on the role of controlling the system voltage (voltage and frequency) of the AC system 1. In the voltage source mode, the output AC current of the power converter 110 is determined depending on the components connected to the AC system 1, but in this embodiment, the control device 200 controls the output AC current so as to suppress overcurrent in the switching element 31 and the like.

[0044] Figure 3 is a functional block diagram illustrating an example of the functional configuration of the control device 200 according to Embodiment 1.

[0045] As shown in Figure 3, the control device 200 includes an AC current control unit 205, coordinate transformation units 210 and 220, and a current command value setting unit 230. The AC current control unit 205 includes an AC voltage command value calculation unit 240, a three-phase voltage command generation unit 260, and a PWM (Pulse Width Modulation) control unit 270.

[0046] The coordinate transformation unit 210 converts the output AC currents Iacu, Iacv, and Iacw from the power converter 110 to the AC system 1, detected by the AC current detector 9, into d-axis current Id and q-axis current Iq by a known three-phase to two-phase (dq axis) rotational coordinate transformation. The rotational coordinate transformation in the coordinate transformation unit 210 is performed by the following equation (1).

[0047]

number

[0048] In equation (1), θ is the phase synchronized with the grid voltage phase in grid-connected mode and is controlled by known PLL control (phase-synchronous control). In voltage source mode, θ is calculated by self-oscillation based on the fundamental frequency fsys (e.g., 50 Hz or 60 Hz). Therefore, the d-axis current Id and q-axis current Iq are calculated as the frequency components of the fundamental frequency fsys in the output AC currents Iacu, Iacv, and Iacw, i.e., the amplitude and phase of the fundamental wave component.

[0049] The coordinate transformation unit 220 takes the grid AC voltages Vsysu, Vsysv, and Vsysw as input and outputs the grid AC voltages Vsysd and Vsysq for the d and q axes by performing a rotation coordinate transformation from three phases (uvw phases) to two phases (dq axis) similar to the coordinate transformation unit 210. In equation (1), the grid AC voltages Vsysd and Vqsys for the d and q axes can be calculated by replacing the output AC currents Iacu, Iacv, and Iacw with the grid AC voltages Vsysu, Vsysv, and Vsysw detected by the AC voltage detector 10.

[0050] The current command value setting unit 230 sets the d-axis current command value Idref and the q-axis current command value Iqref for the d-axis current Id and q-axis current Iq output from the coordinate transformation unit 210. In Embodiment 1, the d-axis current command value Idref and the q-axis current command value Iqref correspond to the "AC current command value".

[0051] The AC voltage command value calculation unit 240 includes a subtractor 242, a control calculation unit 245, an adder 247, and a voltage feedforward (FF) control unit 250.

[0052] The subtractor 242 subtracts the d-axis current Id and q-axis current Iq from the coordinate transformation unit 210 from the d-axis current command value Idref and q-axis current command value Iqref from the current command value setting unit 230, thereby outputting the d-axis current deviation ΔId (ΔId = Idref - Id) and the q-axis current deviation ΔIq (ΔIq = Iqref - Iq).

[0053] The control calculation unit 245 takes the d-axis current deviation ΔId and q-axis current deviation ΔIq, which are calculated for each control cycle, as input and outputs calculated values ​​Vd* and Vq* through a predetermined control calculation (typically proportional (P) control or proportional-integral (PI) control, etc.).

[0054] The adder 247 generates the d and q axis voltage command values ​​Vdref and Vqref by adding the d and q axis voltage FF terms Vdff and Vqff output from the voltage FF control unit 250 with the calculated values ​​Vd* and Vq* from the control calculation unit 245.

[0055] Thus, it is understood that the voltage command values ​​Vdref and Vqref for the d and q axes are calculated according to the sum of Vd* and Vq*, which correspond to the feedback components for making the d-axis current Id and q-axis current Iq (coordinate transformation unit 210) follow the d-axis and q-axis current command values ​​Idref and Iqref (current command value setting unit 230), and the AC components shown by the voltage FF terms Vdff and Vqff.

[0056] The three-phase voltage command generation unit 260 converts the voltage command values ​​Vdref and Vqref of the dq axis into three-phase voltage command values ​​Vu*, Vv*, and Vw* using a predetermined two-phase / three-phase coordinate transformation. The voltage command values ​​Vu*, Vv*, and Vw* are sinusoidal voltages with fundamental frequencies fsys, each shifted in phase by 120 degrees.

[0057] The PWM control unit 270 generates on / off control signals Suu, Sul, Svu, Svl, Swu, Swl for each switching element 31 of the U-phase to V-phase upper and lower arms of the power converter 110 shown in Figure 1, by performing PWM control that compares each of the three-phase voltage command values ​​Vu*, Vv*, Vw* with the carrier signal, so that the voltages of the AC terminals Nu, Nv, Nw in Figure 1 (i.e., the AC output voltages Vacu, Vaccv, Vacw of the power converter 110) approximate the sinusoidal voltages represented by the three-phase voltage command values ​​Vu*, Vv*, Vw*.

[0058] Figure 4 is a functional block diagram illustrating an example of the functional configuration of the voltage FF control unit 250. As shown in Figure 4, the voltage FF control unit 250 includes a low-pass filter (LPF) 251, a coordinate transformation unit 252, selectors 253 and 254, and a coordinate transformation unit 255.

[0059] The LPF251 filters the system AC voltages Vsysd and Vsysq output from the coordinate transformation unit 220 to remove high-frequency components such as noise. The coordinate transformation unit 252 performs polar coordinate transformation on the filtered system AC voltages Vsysd and Vsysq in the d and q axes, and calculates the amplitude Vdqmag and phase Vdqph according to the following equations (2) and (3).

[0060]

number

[0061] Selectors 253 and 254 have their outputs switched according to the mode selection signal SMD. The mode selection signal SMD is set to "1" when the voltage source mode is commanded, and to "0" when the grid connection mode is commanded. For example, the mode selection signal SMD is input from a higher-level system that manages the power conversion system 100.

[0062] Selector 253 outputs either the amplitude Vdqmag calculated by the coordinate transformation unit 252 or the predefined amplitude Vmagref for the voltage source mode (e.g., Vmagref = 1.0 [pu]) according to the mode selection signal SMD. Similarly, selector 254 outputs either the phase Vdqph calculated by the coordinate transformation unit 252 or the predefined phase Vphref for the voltage source mode (e.g., Vphref = 0) according to the mode selection signal SMD.

[0063] In voltage source mode, where SMD is set to "1", selectors 253 and 254 output predetermined amplitude Vmagref and phase Vphref, respectively. In contrast, in grid connection mode, where SMD is set to "0", selectors 253 and 254 output amplitude Vdqmag and phase Vdqph, respectively, calculated by the coordinate transformation unit 252.

[0064] The coordinate transformation unit 255 uses the amplitude output from selector 253 and the phase output from selector 254 to perform a transformation from polar coordinates to a rotating coordinate system (dq axis) and calculates the voltage FF terms Vdff and Vqff for the d axis and q axis.

[0065] As a result, the voltage FF terms Vdff and Vqff exhibit AC (sine wave) components with predetermined amplitude and phase in the voltage source mode (SMD = "1"). On the other hand, in the grid-connected mode (SMD = "0"), the voltage FF terms Vdff and Vqff exhibit AC (sine wave) components with amplitude and phase similar to the grid AC voltage Vsys.

[0066] Referring again to Figure 3, in the current command value setting unit 230, the calculation of the d-axis current command value Idref and the q-axis current command value Iqref is switched according to the mode selection according to the mode selection signal SMD from the higher level.

[0067] In grid-connected mode, where SMD is set to "0", the current command value setting unit 230 sets current command values ​​Idref and Iqref for current control in accordance with commands from the higher-level system. For example, a command for power control is given from the higher-level system, and the current command values ​​Idref and Iqref are generated so that the active current component (Iq) and reactive current component (Id) are controlled in accordance with that command.

[0068] As a result, in grid-connected mode, the power conversion system 100 operates as a current source that synchronizes with the grid AC voltage Vsys of the AC system 1 and follows commands from the higher-level system.

[0069] On the other hand, in this embodiment, the current command value setting unit 230 generates current command values ​​Idref and Iqref based on the fundamental wave component of the output AC current Iac detected by the AC current detector 9 in the voltage source mode where SMD = "1". That is, in the voltage source mode applied during black start, the current command values ​​Idref and Iqref are basically generated in such a way that they suppress abrupt fluctuations while suppressing frequency components other than the positive-sequence fundamental wave of the output AC current Iac.

[0070] As a result, in voltage source mode, the power conversion system 100 operates as a voltage source that outputs an AC voltage with a predetermined amplitude and phase and a fundamental frequency fsys, determined by the voltage FF terms Vdff, Vqff, while limiting the output AC current Iac according to the current command values ​​Idref, Iqref.

[0071] The following describes a specific example of the functional configuration of the current command value setting unit 230 for generating current command values ​​Idref and Iqref in the voltage source mode in Embodiment 1.

[0072] Figure 5 is a functional block diagram illustrating a first example of the functional configuration of the current command value setting unit according to Embodiment 1.

[0073] As shown in Figure 5, in the first functional configuration example, the current command value setting unit 230 has a low-pass filter (LPF) 231 with a limiter function. The LPF 231 filters the d-axis current Id and q-axis current Iq that are sequentially output from the coordinate transformation unit 210, and generates d-axis and q-axis current command values ​​Idref and Iqref according to the filtered d-axis current Id and q-axis current Iq, respectively.

[0074] In this process, the LPF231 performs limit processing so that the current command values ​​Idref and Iqref each fall within a predetermined range of -Idqmax to +Idqmax (-Idqmax ≤ Idref, Iqref ≤ +Idqmax).

[0075] For example, when the filtered d-axis current Id is greater than Idqmax (Id > Idqmax), Idref = Idqmax is set by the limiter process. Similarly, when the filtered d-axis current Id is less than -Idqref (Id < -Idqmax), Idref = -Idqmax is set by the limiter process. On the other hand, when the filtered d-axis current Id is between -Idqref and +Idqmax (-Idqref < Id < +Idqmax), Idref = Id is set. The same limiter process is also executed for the filtered q-axis current Iq.

[0076] The upper limit values +Idqmax and -Idqmax in the limiter process in LPF231 are set based on the allowable upper limit value of the current amplitude. The allowable upper limit value can be set, for example, corresponding to the rated current of the switching element 31 constituting the power converter 110 so that overcurrent protection of the power converter 110 does not activate. Note that the upper limit values in the limiter process may be set such that the absolute values are the same on the positive and negative sides, or may be set such that the absolute values are different on the positive and negative sides.

[0077] Thereby, the current command values Idref and Iqref are set based on the current d-axis current Id and q-axis current Iq so that the output AC current Iac does not fluctuate rapidly. Furthermore, the limiter process can avoid the occurrence of overcurrent in the power converter 110.

[0078] FIG. 6 shows a second functional configuration example of the current command value setting unit according to Embodiment 1. Referring to FIG. 6, the current command value setting unit 230 includes, in the first functional configuration example, low-pass filters (LPFs) 232 and 233 with a limiter function, and a current upper limit value adjustment unit 233X.

[0079] LPF232 filters the d-axis current Id output from the coordinate transformation unit 210 and generates a d-axis current command value Idref according to the filtered d-axis current Id. LPF233 filters the q-axis current Iq output from the coordinate transformation unit 210 and generates a q-axis current command value Iqref according to the filtered q-axis current Iq.

[0080] In Figure 6, the current upper limits for the limiter processing of LPF232 and 233 are set individually. In LPF232, limit processing is performed so that the current command value Idref falls within a predetermined range from -Idmax to +Idmax. The current upper limit value Idmax also corresponds to the "first upper limit value" in one embodiment.

[0081] In contrast, the current limit value Iqmax used for limiter processing of LPF233 is variably set by the current limit value adjustment unit 233X according to the output value (Idref) of LPF232. The current limit value Iqmax corresponds to one embodiment of the "second limit value".

[0082] Figure 7 shows a conceptual diagram illustrating the function of the current upper limit adjustment unit 233X. The current amplitude Irefmag, given current command values ​​Idref and Iqref, is given by Irefmag = √(Idref) using Idref and Iqref on the dq plane. 2 +Iqref 2 This is shown by ). Therefore, the current limit adjustment unit 233X calculates the current limit Iqmax of the q-axis current command value Iqref according to the following equation (4) so ​​that the current amplitude Irefmag, which is determined by the current command value Idref set in LPF232 and the current command value Iqref set in LPF233, does not exceed the current limit Idqmax. Note that the current limit Idqmax in equation (4) is set in accordance with the allowable upper limit of the current amplitude as described above.

[0083]

number

[0084] The current limit values ​​in the limiting process of the LPF233 can be set to +Iqmax and -Iqmax using the current limit value Iqmax calculated by the current limit adjustment unit 233X. Alternatively, the current limit values ​​in the limiting process of the LPF233 may be set to have different absolute values ​​for the positive and negative sides.

[0085] According to the second functional configuration example in Figure 6, it becomes easier to ensure a current amplitude Irefmag within the range that does not exceed the current upper limit Idqmax. On the other hand, the current command values ​​Idref and Iqref are more likely to be set so that the current phase changes due to the current d-axis current Id and q-axis current Iq.

[0086] Furthermore, in Figure 6, it is also possible to configure the system so that the d-axis current Id and the q-axis current Iq are swapped. In this case, the LPF232 performs limit processing to keep the q-axis current Iq within the range of -Iqmax to +Iqmax, and sets the d-axis current command value Iqref. In addition, the current upper limit adjustment unit 233X can set the current upper limit Idmax of the d-axis current command value Idref by substituting Idref with Iqref in equation (4). Then, the LPF233 sets the d-axis current command value Idref by performing limit processing to keep the d-axis current Id within the range of -Idmax to +Idmax using the current upper limit Idmax calculated by the current upper limit adjustment unit 233X. That is, in this case, the current upper limit Iqmax corresponds to the "first upper limit," and the current upper limit Idmax corresponds to the "second upper limit."

[0087] Figure 8 is a functional block diagram illustrating a third example of the functional configuration of the current command value setting unit 230.

[0088] As shown in Figure 8, in the third functional configuration example, the current command value setting unit 230 includes a low-pass filter (LPF) 234, coordinate transformation units 235 and 237, and a limiter 236.

[0089] The LPF234 filters the d-axis current Id and q-axis current Iq output from the coordinate transformation unit 210, and outputs the filtered d-axis current Idf and q-axis current Iqf.

[0090] The coordinate transformation unit 235 performs polar coordinate transformation on the filtered d-axis current Idf and q-axis current Iqf, similar to the coordinate transformation unit 252, to calculate the amplitude Idqmag and phase Idqph.

[0091] The limiter 236 performs limiter processing on the amplitude Idqmag calculated by the coordinate transformation unit 235, according to the current upper limit value Idqmax. Therefore, the output value of the limiter 236 is updated to Idqmag = Idqmax when Idqmag > +Idqmax. On the other hand, when Idqmag < +Idqmax, the amplitude Idqmag calculated by the coordinate transformation unit 235 becomes the output value of the limiter 236 as is. The current upper limit value Idqmax in the limiter 236 is set in accordance with the allowable upper limit of the current amplitude described above, as in Figure 6.

[0092] The coordinate transformation unit 237 uses the amplitude Idqmag that has passed through the limiter 236 and the phase Idqph and phase calculated by the coordinate transformation unit 235 to perform a coordinate transformation similar to that of the coordinate transformation unit 255, thereby generating the d-axis and q-axis current command values ​​Idref and Iqref.

[0093] In the third functional configuration example, the current command values ​​Idref and Iqref are set so that amplitude limiting is performed on the grid AC current Isys while maintaining the current current phase, based on the current d-axis current Id and q-axis current Iq.

[0094] Figure 9 shows a functional block diagram illustrating a fourth example of the functional configuration of the current command value setting unit according to Embodiment 1.

[0095] The fourth functional configuration example shown in Figure 9 differs from the third functional configuration example (Figure 8) in that the filter 234 is omitted and a low-pass filter (LPF) 238 with limiter functionality is placed in place of the limiter 236.

[0096] Therefore, the coordinate transformation unit 235 transforms the d-axis current Id and q-axis current Iq output from the coordinate transformation unit 210 into polar coordinates to calculate the amplitude Idqmag and phase Idqph. The LPF 238 filters the amplitude Idqmag calculated sequentially by the coordinate transformation unit 235 and performs limit processing on the filtered amplitude Idqmag in the same way as the limiter 236.

[0097] Then, the coordinate transformation unit 237 generates the d-axis and q-axis current command values ​​Idref and Iqref using the amplitude Idqmag that has passed through the LPF 238 and the phase Idqph calculated by the coordinate transformation unit 235.

[0098] The fourth functional configuration example also allows current command values ​​Idref and Iqref to be set so that amplitude limiting is performed while maintaining the current current phase of the grid AC current Isys. Furthermore, since the phase Idqph is calculated without filtering, the current command values ​​Idref and Iqref can be quickly adjusted to respond to changes in the phase of the output AC current Iac.

[0099] Thus, according to the power conversion system of Embodiment 1, in the voltage source mode applied at least during black start, the output AC current Iac is controlled according to current command values ​​Idref and Iqref set based on the current d-axis current Id and q-axis current Iq, and the system can operate as a voltage source that outputs an AC voltage with a predetermined amplitude and phase and a fundamental frequency fsys.

[0100] This allows the output AC current Iac from the power converter 110 to be controlled in a way that suppresses sharp fluctuations in the output AC current Iac. Therefore, the power conversion system 100 can be operated as a voltage source in such a way that the overcurrent protection function of the power converter 110 does not activate due to a sharp increase in the output AC current Iac (fundamental wave component) in response to an increase in the load current in the AC system 1.

[0101] Furthermore, by generating current command values ​​Idref and Iqref with limiter processing, the effect of avoiding overcurrent in the power converter 110 is enhanced, thereby further improving the continuous operation performance of the power conversion system 100, which operates as a voltage source.

[0102] Embodiment 2. Embodiment 2 describes the control when switching operating modes. In Embodiment 2, the power conversion system differs from Embodiment 1 in that the current command value setting unit 230X and voltage FF control unit 250X, which will be described later, are arranged in place of the current command value setting unit 230 and voltage FF control unit 250. In the power conversion system according to Embodiment 2, the configuration and operation other than the parts described below are the same as the power conversion system 100 described in Embodiment 1.

[0103] Figure 10 is a functional block diagram illustrating a first example of the functional configuration of the current command value setting unit 230X.

[0104] As shown in Figure 10, the current command value setting unit 230X according to Embodiment 2, in the first functional configuration example, further includes a coordinate transformation unit 301, inverters 302, 303, multipliers 305a, 305b, 306a, 306b, adders 307, 308, and ramp limiters (RRL) 311-314, in addition to the coordinate transformation units 235, 237 and low-pass filter (LPF) 238 with limiter function, similar to the current command value setting unit 230 shown in Figure 9.

[0105] The coordinate transformation unit 301 transforms the current command values ​​Idref0 and Iqref0, which are set in grid connection mode, into polar coordinates in the same way as the coordinate transformation unit 235, and calculates the amplitude Idq0mag and phase Idq0ph. As described above, the current command values ​​Idref0 and Iqref0 are generated according to commands from the higher-level system.

[0106] RRL311 and 313 receive the mode selection signal SMD, while RRL312 and 314 receive the inverted signal / SMD of the mode selection signal SMD from inverters 302 and 303. As described above, the mode selection signal SMD is set to "1" in voltage source mode and to "0" in grid-connected mode, according to a command from the higher-level system.

[0107] Figure 11 shows a conceptual waveform diagram to explain the operation of RRL311~314 in Figure 10.

[0108] Referring to Figure 11, the RRL output value is set to the same value as the input value SMD, / SMD under normal circumstances. On the other hand, when the input value changes from "0" to "1", the RRL output value gradually changes from 0 to 1.0 according to a predetermined ramp rate kr. On the other hand, when the input value changes from "1" to "0", the RRL output value gradually changes from 1.0 to 0 according to a predetermined ramp rate -kr.

[0109] Referring again to Figure 10, multiplier 305a multiplies the amplitude Idqmgf (for voltage source mode) after filtering by LPF238 with the output value of RRL311. Multiplier 305b multiplies the amplitude Idq0mag (for grid connection mode) from coordinate transformation unit 301 with the output value of RRL312. Adder 307 adds the output value of multiplier 305a and the output value of multiplier 305b to output the amplitude Idqmgx of the current command value.

[0110] In this voltage source mode (SMD = "1"), the output value of RRL311 is 1.0, while the output value of RRL312 is 0. Therefore, the output values ​​of adder 307 are Idqmgx = Idqmgf.

[0111] When switching from voltage source mode to grid-connected mode, the mode selection signal SMD changes from "1" to "0". As a result, the output value of RRL311 changes from 1.0 to 0 according to the ramp rate -kr, while the output value of RRL312 changes from 0 to 1.0 according to the ramp rate kr. Therefore, the sum of the output values ​​of RRL311 and RRL312 is always maintained at 1.0.

[0112] Therefore, the output value Idqmgx of adder 307 is given by equation (5) below, using the amplitudes Idqmgf (for voltage source mode) and Idq0mag (for grid connection mode), the output value α of RRL311, and the output value (1-α) of RRL312. Equation (5) is valid in common when the mode selection signal SMD is maintained at "0" or "1", and when it switches between "0" and "1".

[0113] Idqmgx=α·Idqmgf+(1-α)·Idq0mag …(5) Similarly, multiplier 306a multiplies the phase Idqph (for voltage source mode) from coordinate transformation unit 235 by the output value of RRL313. Multiplier 306b multiplies the phase Idq0ph (for grid connection mode) from coordinate transformation unit 301 by the output value of RRL314. Adder 308 adds the output value of multiplier 306a and the output value of multiplier 306b to output the phase Idqphx of the current command value.

[0114] Here, with respect to the mode selection signal SMD, the output value of PPL313 is the same as the output value of RRL311, and the output value of PPL314 is the same as the output value of RRL312. Therefore, the output value Idqphx of adder 308 is given by equation (6) below, using the phase Idqph (for voltage source mode) and phase Idq0ph (for grid connection mode), and α, (1-α) similar to equation (5). Equation (6) also holds true when the mode selection signal SMD is maintained at "0" or "1", and when it switches between "0" and "1".

[0115] Idqphx=α·Idqph+(1-α)·Idq0ph …(6) The coordinate transformation unit 237 generates current command values ​​Idref and Iqref for the d axis and q axis using the amplitude Idqmgx and phase Idqphx output from the adders 307 and 308, respectively. In Embodiment 2, the output AC current Iac of the power converter 110 is controlled by the AC current control unit 205 shown in Figure 3, which generates voltage command values ​​Vdref and Vqref according to the current command values ​​Idref and Iqref set by the current command value setting unit 230X.

[0116] As a result, when SMD = "1", the current command values ​​Idref and Iqref can be set according to the amplitude Idqmagf and phase Idqph for the voltage source mode, and when SMD = "0", the current command values ​​Idref and Iqref can be set according to the amplitude Idq0mag and phase Idq0ph for the grid connection mode. In other words, the current command values ​​Idref and Iqref can be appropriately switched according to the change in the operating mode. Furthermore, when SMD changes from "1" to "0" or from "0" to "1", the values ​​of the current command values ​​Idref and Iqref can be changed while limiting the amount of change per unit time according to the ramp rates kr and -kr.

[0117] As a result, in the power conversion system according to Embodiment 2, when transitioning from voltage source mode to grid-connected mode, it is possible to prevent fluctuations in the output AC current Iac from the power converter 110 due to a sudden change in the current command values ​​Idref and Iqref. Similarly, when transitioning from grid-connected mode to voltage source mode, a sudden change in the current command values ​​Idref and Iqref can be avoided. This stabilizes the operation of the power conversion system when the operating mode is switched.

[0118] Furthermore, in the configuration shown in Figure 10, it is also possible to configure the coordinate transformation unit 235 to receive the d-axis current Idf and q-axis current Iqf filtered by the LPF 234, similar to the configuration shown in Figure 8 (third functional configuration example of Embodiment 1), and to replace the LPF 238 with the limiter 236.

[0119] Figure 10 illustrates an example of applying Embodiment 2 to the functional configuration example shown in Figure 8 or Figure 9 of Embodiment 1. However, Embodiment 2 can also be applied to each of the other configuration examples described in Embodiment 1.

[0120] Figure 12 is a functional block diagram illustrating a second example of the functional configuration of the current command value setting unit 230X according to Embodiment 2.

[0121] As shown in Figure 12, in the second functional configuration example, the current command value setting unit 230X further includes inverters 302, 303, multipliers 309-312, adders 313, 314, and ramp plate limiters (RRL) 321-324, in addition to the LPFs 232, 233 similar to those of the current command value setting unit 230 shown in Figure 6. Similar to Figure 6, the current upper limit value used for the limiter function of the LPF 233 is set by the current upper limit value adjustment unit 233X (Figure 6) according to equation (4).

[0122] The output values ​​of RRL321 and 323 are the same as those of RRL311 and 313 in Figure 10, and the output values ​​of RRL322 and 324 are the same as those of RRL312 and 314 in Figure 10.

[0123] Multiplier 309 multiplies the d-axis current Idf after filtering and limiting by LPF232 by the output value of RRL321. Multiplier 310 multiplies the d-axis current command value Idref0 for grid connection mode by the output value of RRL322. Adder 313 adds the output value of multiplier 309 and the output value of multiplier 310 to output the d-axis current command value Idref.

[0124] Therefore, the current command value Idref output from the adder 313 is given by equation (7) below, using the d-axis current Idf output from the LPF 232, the d-axis current command value Idref0 for grid connection mode, the output value α of RRL321, and the output value (1-α) of RRL322. Equation (7) also holds true when the mode selection signal SMD is maintained at "0" or "1", and when it switches between "0" and "1".

[0125] Idref=α·Idf+(1-α)·Idref0 …(7) Similarly, multiplier 311 multiplies the q-axis current Iqf after filtering and limiting by LPF233 by the output value of RRL323. Multiplier 312 multiplies the q-axis current command value Iqref0 for grid-connected mode by the output value of RRL324. Adder 312 adds the output value of multiplier 311 and the output value of multiplier 312 to output the q-axis current command value Iqref.

[0126] Therefore, the current command value Iqref output from adder 314 is given by equation (8) below, using the q-axis current Iqf output from LPF233, the q-axis current command value Iqref0 for grid connection mode, the output value α of RRL323, and the output value (1-α) of RRL324. Equation (8) also holds true when the mode selection signal SMD is maintained at "0" or "1", and when it switches between "0" and "1".

[0127] Iqref=α·Iqf+(1-α)·Iqref0 …(8) As described above, even when the current command values ​​Idref and Iqref in voltage source mode are set according to the second functional configuration example of Embodiment 1 (Figure 6), abrupt changes in the current command values ​​Idref and Iqref can be prevented when the operating mode transitions between voltage source mode and grid-connected mode. Similarly, abrupt changes in the current command values ​​Idref and Iqref can be avoided when transitioning from grid-connected mode to voltage source mode. This stabilizes the operation of the power conversion system when the operating mode is switched.

[0128] Furthermore, in Figure 12, as explained in Figure 6, it is possible to swap the limit processing for the d-axis current Id and the q-axis current. That is, while Iqf may be calculated by setting the limit processing for the q-axis current Iq within the range of -Iqmax to +Idmax, Iqf may also be calculated by using limit processing with the upper limit value (Iqmax) set by the current upper limit adjustment unit 233X (Figure 6).

[0129] Figure 13 is a functional block diagram illustrating a third example of the functional configuration of the current command value setting unit 230X according to Embodiment 2.

[0130] As shown in Figure 13, in the third functional configuration example, the current command value setting unit 230X further includes inverters 302, 303, multipliers 309-312, adders 313, 314, and ramp limiters (RRL) 321-324, in addition to the LPF 231 similar to the current command value setting unit 230 shown in Figure 5.

[0131] In Figure 13, the LPF231 in Figure 5 is shown separately as LPF231d, which filters the d-axis current Id, and LPF231q, which filters the q-axis current Iq. The upper limits +Idqmax and -Idqmax in the limiting process of LPF231d and 231q are set to the same values ​​as LPF231 in Figure 5, based on the allowable upper limit of the current amplitude.

[0132] The configuration in Figure 13 corresponds to the configuration in Figure 12, but with LPF232 and 233 replaced by LPF231d and 231q. Therefore, in the third configuration example shown in Figure 13, the current command values ​​Idref and Iqref for the d and q axes are given by equations (7) and (8) described above. This allows for stabilization of the operation of the power conversion system when the operating mode is switched, as explained in Figures 10 and 12.

[0133] Next, the voltage FF control unit 250X in the power conversion unit according to Embodiment 2 will be described.

[0134] Figure 14 is a functional block diagram illustrating an example of the functional configuration of the voltage FF control unit 250X according to Embodiment 2.

[0135] As shown in Figure 14, the voltage FF control unit 250X has an LPF 251, coordinate transformation units 252 and 255 similar to the current FF control unit 250 shown in Figure 4, as well as inverters 304 and 305, multipliers 315 to 317, adder 318, and ramp plate limiters (RRLs) 325 to 327, which are arranged in place of selectors 253 and 254 (Figure 4). In the configuration example in Figure 14, the phase in voltage source mode is Vphref = 0.

[0136] The coordinate transformation unit 252 performs polar coordinate transformation on the filtered grid AC voltages Vsysd and Vsysq of the d and q axes to calculate the amplitude Vdqmag and phase Vdqph for grid connection mode.

[0137] RRL325 and 327 receive the inverted signal ( / SMD) of the mode selection signal SMD from inverters 304 and 305. RRL326 receives the mode selection signal SMD. Therefore, the output values ​​of RRL325 and 327 are the same as RRL312 and 314 in Figure 10, and the output value of RRL326 is the same as RRL311 and 313 in Figure 10.

[0138] Multiplier 315 multiplies the amplitude Vdqmag (for grid connection mode) from the coordinate transformation unit 252 by the output value of RRL325. Multiplier 316 multiplies the amplitude Vmagref (e.g., 1.0[pu]) for voltage source mode by the output value of RRL326. Adder 318 adds the output value of multiplier 315 and the output value of multiplier 316 to output the amplitude Vdqmgx of the voltage FF term.

[0139] Furthermore, the multiplier 317 multiplies the phase Vdqph (for grid-connected mode) from the coordinate transformation unit 252 by the output value of the RRL 327. The multiplier 317 outputs the phase Vdqphx of the voltage FF term.

[0140] Therefore, the amplitude Vdqmgx and phase Vdqphx of the voltage FF term are given by equations (9) and (10) below, using the output value α of RRL326 and the output values ​​(1-α) of RRL325 and RRL327. Equations (9) and (10) also hold true when the mode selection signal SMD is maintained at "0" or "1", and when it switches between "0" and "1".

[0141] Vdqmgx=α·Vmagref+(1-α)·Vdqmag …(9) Vdqphx = (1-α)·Vdqph …(10) In voltage source mode (SMD="1"), α=1.0, so Vdqmgx=Vmgref and Vdqphx=0. Conversely, in grid-connected mode (SMD="0"), α=0, so Vdqmgx=Vdqmag and Vdqphx=Vdqph. In other words, the value of the voltage FF term can be appropriately switched according to the change in operating mode.

[0142] The coordinate transformation unit 255 uses the amplitude Vdqmgx and phase Vdqmag, which are set as described above, to transform from polar coordinates to a rotating coordinate system (dq axis) and calculate the voltage FF terms Vdff and Vqff for the d axis and q axis. As a result, the voltage FF terms Vdff and Vqff are set in both the voltage source mode and the grid connection mode, similar to Figure 4.

[0143] Furthermore, when the mode selection signal SMD changes from "1" to "0" or from "0" to "1", the values ​​of the voltage FF terms Vdff and Vqff can be changed while limiting the amount of change per unit time according to the ramp rates kr and -kr.

[0144] As a result, in the power conversion system according to Embodiment 2, when transitioning from voltage source mode to grid-connected mode, the voltage FF terms Vdff and Vqff change rapidly, which in turn causes a rapid change in voltage command values ​​Vdref and Vqref, preventing fluctuations in the output AC current Iac from the power converter 110. Similarly, when transitioning from grid-connected mode to voltage source mode, a rapid change in voltage command values ​​Vdref and Vqref can be avoided. This stabilizes the operation of the power conversion system when the operating mode is switched.

[0145] Furthermore, when switching operating modes, the timing of changing the current command value and the timing of changing the voltage command value may be staggered. Specifically, by providing a timer that delays the off timing ("1" → "0") for the mode selection signal SMD supplied to the voltage FF control unit 250X, the current command value setting unit 230X can change the mode selection signal SMD from "1" to "0" before the voltage FF control unit 250X when transitioning from voltage source mode to grid connection mode. This allows the voltage command value to be changed after the current command value has been changed, thereby reducing the impact of voltage fluctuations caused by the change in operating mode.

[0146] Embodiment 3. Embodiment 1 described AC current control involving coordinate transformation to a rotating coordinate system, but Embodiment 3 describes AC current control on a stationary coordinate system.

[0147] In the power conversion system according to Embodiment 3, in the example of functional configuration shown in Figure 3, a current command value setting unit 230Y (Figure 15) is provided in place of the current command value setting unit 230, and an AC voltage command value calculation unit 240Y (Figure 16) is provided in place of the AC voltage command value calculation unit 240, thereby controlling the output AC current Iac by the AC current control unit 205. Furthermore, in the AC voltage command value calculation unit 240Y, a voltage F control unit 250Y (Figure 17) is provided in place of the voltage FF control unit 250.

[0148] The following describes the functional configurations of the current command value setting unit 230Y, the AC voltage command value calculation unit 240Y, and the voltage FF control unit 250Y used in the power conversion unit according to Embodiment 3.

[0149] Figure 15 shows an example of a functional configuration for generating a current command value in voltage source mode of the current command value setting unit according to Embodiment 3.

[0150] In Embodiment 3, which uses stationary coordinates, the current command value setting unit 230Y sets the current command values ​​Iacuref, Iacvref, and Iacvref for the U-phase, V-phase, and W-phase, respectively. Hereinafter, the current command values ​​Iacuref, Iacvref, and Iacvref will be collectively referred to as the current command value Ircref. The output AC currents Iacu, Iacv, and Iacw for the U-phase, V-phase, and W-phase are controlled according to the current command values ​​Iacuref, Iacvref, and Iacvref, respectively.

[0151] Figure 16 shows a functional block diagram illustrating an example of the functional configuration of the AC voltage command value calculation unit according to Embodiment 3.

[0152] As shown in Figure 16, the AC voltage command value calculation unit 240Y calculates the current deviation ΔIac for each of the U-phase, V-phase, and W-phase by subtracting the measured output AC current Iac from the current command value Iacref set by the current command value setting unit 230Y.

[0153] In the AC voltage command value calculation unit 240Y, the control calculation unit 245 calculates the feedback component (Vacfb) of the AC voltage command value Vacref for each phase by performing a predetermined control calculation (for example, proportional (P) control or proportional resonance (PR) control, etc.) using the current deviation ΔIac, which is calculated for each control cycle, as input for each of the U-phase, V-phase, and W-phase. Furthermore, the adder 247 calculates the AC voltage command value Vacref by adding the feedback component Vacfb from the control calculation unit 245 and the voltage FF term Vacff (AC component) from the voltage FF control unit 250Y for each of the U-phase, V-phase, and W-phase. Note that the voltage FF term Vacff is a general term for the voltage FF terms calculated for each of the U-phase, V-phase, and W-phase.

[0154] Figure 17 shows a functional block diagram illustrating an example of the functional configuration of the voltage FF control unit 250Y according to Embodiment 3.

[0155] As shown in Figure 17, the voltage FF control unit 250Y has a selector 256 whose output is switched according to the mode selection signal SMD. The selector 256 receives an AC voltage command value Vacref predetermined for the voltage source mode and the system AC voltage Vsys (Vsysu, Vsysv, Vsysw) detected by the AC voltage detector 10 as input.

[0156] The AC voltage command value Vacref is set in advance for each of the U-phase, V-phase, and W-phase as a sinusoidal voltage with a predetermined amplitude (e.g., 1.0 [pu]) and a fundamental frequency fsys with a phase difference of 120 degrees between phases.

[0157] In voltage source mode, where SMD is set to "1", selector 256 outputs the AC voltage command value Vacref for the voltage source mode as the voltage FF term Vacff for each of the U, V, and W phases.

[0158] On the other hand, in grid connection mode where SMD is set to "0", the selector 256 outputs the grid AC voltage Vsys (Vsysu, Vsysv, Vsysw) detected by the AC voltage detector 10 as the voltage FF term Vacff for each of the U, V, and W phases.

[0159] Referring again to Figure 16, the AC voltage command value Vacref generated by the adder 247 corresponds to the collective term for the three-phase voltage command values ​​Vu*, Vv*, Vw* in Figure 3. Similar to Figure 3, the on / off control signals Suu, Sul, Svu, Svl, Swu, Swl for each switching element 31 of the U-phase to V-phase upper and lower arms of the power converter 110 are generated from the voltage command values ​​Vu*, Vv*, Vw*.

[0160] Next, the setting of the current command value Iacref in Embodiment 3 will be described in detail. Referring again to Figure 15, the current command value setting unit 230Y has a band-pass filter 400 and a gain setting unit 410. The band-pass filter 400 outputs the fundamental wave component of the output AC current Iac(Iacu,Iacv,Iacw) measured sequentially by the AC current detector 9. The transfer function G(s) of the band-pass filter 400 is given by the following equation (11).

[0161]

number

[0162] In equation (11), the angular frequency ωf is given by ωf = 2π·fsys, where fsys is the fundamental frequency. Therefore, the band-pass filter 400 outputs the fundamental component (fundamental frequency fs) of the measured output AC current Iac multiplied by Kp. The output value of the band-pass filter 400 sets the current command value Ircref(Iacuref,Iacvref,Iacvref). Thus, the current command value Iacref is set in each phase based on the amplitude and phase of the fundamental component of the current output AC current Iac.

[0163] The gain setting unit 410 has the function of setting the gain Kp in equation (11) to be variable according to the amplitude of the output AC current Iac(Iacu,Iacv,Iacw).

[0164] Figure 18 shows a conceptual diagram illustrating the function of the gain setting unit 410. As shown in Figure 18, the gain setting unit 410 sets Kp to 1.0 when the amplitude Iacmag of the output AC current Iac is less than or equal to a predetermined current upper limit Iacmax. The current upper limit Iacmax is an upper limit set in stationary coordinates, similar to the current upper limit Idqmax in the rotating coordinate system, so that the overcurrent protection of the power converter 110 is not activated.

[0165] Therefore, when Iacmag ≤ Iacmax, the band-pass filter 400 sets the current command value Iacref according to the amplitude and phase of the fundamental wave component of the current output AC current Iac.

[0166] In response to this, the gain setting unit 410 sets Kp = Iacmax / Iacmag when the amplitude Iacmag of the output AC current Iac is greater than the current upper limit Iacmax. As a result, the amplitude of the fundamental wave component of the AC current Iac output from the band-pass filter 400 is limited to less than or equal to the current upper limit Iacmax, so that the amplitude of the current command value Iacref can be limited in the same way as the limiter processing using the current upper limit Idqmax in Embodiment 1.

[0167] Figure 19 shows the frequency characteristics of the band-pass filter 400 with respect to the gain Kp set by the gain setting unit 410. Figure 19 shows the characteristics when Kp = 1, 0.1, 0.01, and 0.001.

[0168] As shown in Figure 19, the sharpness of the bandpass decreases as the gain Kp decreases, but it is understood that when the gain Kp is around 0.1 or less, i.e., in the range of Iacmag ≤ 10·Iacmax, filtering can be performed with sufficient sharpness to extract the fundamental wave component (fundamental frequency fsys). Note that the sharpness of the frequency response changes depending on the value of "ξ" in equation (11). Specifically, the smaller ξ is, the higher the sharpness, and sharpness can be ensured even for smaller gains Kp. Thus, the frequency response of the bandpass filter 400 changes depending on the constant ξ and the gain Kp, so the frequency response shown in Figure 19 is just one example.

[0169] Thus, according to the power conversion system of Embodiment 3, by controlling the output current in a stationary coordinate system, it is possible to operate as a voltage source that outputs an AC voltage with a predetermined amplitude and phase and a fundamental frequency fsys, while limiting the output AC current Iac according to the current value, at least in the voltage source mode applied during black start. In other words, the same effects as in Embodiment 1 can be achieved by controlling the output current in a stationary coordinate system.

[0170] Furthermore, by adjusting the gain Kp of the band-pass filter 400 according to the amplitude of the output AC current Iac using the gain setting unit 410 (Figure 18), limiter processing similar to that in Embodiment 1 can be achieved.

[0171] It is also possible to combine the power conversion unit according to Embodiment 3 with Embodiment 2.

[0172] Figure 20 shows a functional block diagram illustrating an example of the functional configuration of the current command value setting unit 230Z in a combination of Embodiment 3 and Embodiment 2.

[0173] As shown in Figure 20, the current command value setting unit 230Z includes, in addition to the band-pass filter 400 and gain setting unit 410 similar to those in Figure 16, an inverter 306, ramp plate limiters (RRLs) 328, 329, multipliers 420, 422, and adder 425.

[0174] The mode selection signal SMD is input to RRL328. The inverted signal ( / SMD) of the mode selection signal SMD from inverter 306 is input to RRL329. Therefore, the output value of RRL328 is the same as RRL311 and 313 in Figure 10, and the output value of RRL329 is the same as RRL312 and 314 in Figure 10.

[0175] Multiplier 420 multiplies the output value of the bandpass filter 400, i.e., the current command value Iacref1 for voltage source mode, by the output value of RRL328. Multiplier 422 multiplies the current command value Iacref0 for grid-connected mode by the output value of RRL329. The current command value Iacref0 for grid-connected mode is generated according to a command from a higher-level system, similar to the current command values ​​Idref0 and Iqref0 in Embodiment 1.

[0176] The adder 425 adds the output value of the multiplier 420 and the output value of the multiplier 422 to output the current command value Iacref in the stationary coordinate system. Therefore, the current command value Iacref is given by equation (12) below, using the current command Iacref1 for the voltage source mode, the current command value Iacref0 for the grid connection mode, the output value α of RRL328, and the output value (1-α) of RRL329. Equation (12) also holds true when the mode selection signal SMD is maintained at "0" or "1", and when it switches between "0" and "1".

[0177] Iacref=α·Iacref1+(1-α)·Iacref0 …(12) Thus, even in the power conversion system according to Embodiment 3, which performs AC current control on a stationary coordinate system, by applying Embodiment 2, it is possible to prevent the current command value Iacref from changing abruptly when the operating mode transitions between the voltage source mode and the grid connection mode, thereby stabilizing the operation of the power conversion system.

[0178] Although not shown in the diagram, by adding a ramp plate limiter (RRL) to the voltage FF control unit 250Y shown in Figure 17, as described in Embodiment 2, the voltage FF term Vacff can be changed gradually when the operating mode transitions between the voltage source mode and the grid connection mode. This stabilizes the operation of the power conversion system when the operating mode is switched.

[0179] <Variable configuration of a power converter> The circuit configuration of the power converter 110 is not limited to the two-level converter exemplified in Figure 1; for example, the power converter 110 can also be configured using a modular multilevel converter (MMC).

[0180] Figure 21 shows a schematic diagram illustrating a modified version of the power converter 110. Referring to Figure 21, the modified power converter 110 is composed of a modular multilevel converter (MMC) which includes multiple converter cells connected in series with each other. Note that "converter cells" are also called "submodules," "SMs," or "unit converters."

[0181] The power converter 110 shown in Figure 21 has a different configuration of leg circuits 4u, 4v, and 4w compared to the circuit configuration in Figure 1. Since leg circuits 4u, 4v, and 4w have similar configurations, the configuration of leg circuit 4u will be described as representative below.

[0182] The upper arm 5 includes a plurality of cascaded transducer cells 7 and a reactor 8A. The plurality of transducer cells 7 and reactor 8A are connected in series. Similarly, the lower arm 6 includes a plurality of cascaded transducer cells 7 and a reactor 8B. The plurality of transducer cells 7 and reactor 8B are connected in series. The number of transducer cells 7 included in each of the upper arm 5 and the lower arm 6 is two or more.

[0183] The reactor 8A may be inserted at any position on the upper arm 5 of the leg circuit 4u, and the reactor 8B may be inserted at any position on the lower arm 6 of the leg circuit 4u. Multiple reactors 8A and 8B may be provided. The inductance values ​​of each reactor may be different from each other. Furthermore, only reactor 8A on the upper arm 5 or only reactor 8B on the lower arm 6 may be provided. Alternatively, the transformer connection may be designed to cancel the magnetic flux of the DC current and the leakage inductance of the transformer may act on the AC current, thus serving as a substitute for the reactor. By providing reactors 8A and 8B, a rapid increase in fault current during a fault in AC system 1 or DC system 3 can be suppressed.

[0184] The arm current detectors 9A and 9B provided in the leg circuit 4u for the U phase detect the upper arm current Ipu flowing through the upper arm 5 and the lower arm current Inu flowing through the lower arm 6, respectively. The arm current detectors 9A and 9B provided in the leg circuit 4v for the V phase detect the upper arm current Ipv and the lower arm current Inv, respectively. The arm current detectors 9A and 9B provided in the leg circuit 4w for the W phase detect the upper arm current Ipw and the lower arm current Inw flowing through the upper arm 5, respectively. In the following description, the upper arm currents Ipu, Ipv, and Ipw will be collectively referred to as "upper arm current Iarmp," and the lower arm currents Inu, Inv, and Inw will be collectively referred to as "lower arm current Iarmn." In each phase, the output AC current Iac can be determined from the difference between the upper arm current Iarmp and the lower arm current Iarmn, similar to the AC current detector 9 in Figure 1.

[0185] Figure 22 is a circuit diagram showing a first configuration example of the converter cell 7 shown in Figure 21. As shown in Figure 22, the converter cell 7 according to the first configuration example has a circuit configuration called a half-bridge configuration.

[0186] The converter cell 7 comprises a series unit formed by connecting two switching elements 31p and 31n in series, a storage element 32, a voltage detector 33, and input / output terminals P1 and P2. The series unit of switching elements 31p and 31n and the storage element 32 are connected in parallel. The voltage detector 33 detects the voltage Vc across the ends of the storage element 32.

[0187] Both terminals of the switching element 31n are connected to the input / output terminals P1 and P2, respectively. The converter cell 7 outputs the voltage Vc of the energy storage element 32 or zero voltage between the input / output terminals P1 and P2 by the switching operation of the switching elements 31p and 31n. When the switching element 31p is on and the switching element 31n is off, the converter cell 7 outputs the voltage Vc of the energy storage element 32. When the switching element 31p is off and the switching element 31n is on, the converter cell 7 outputs zero voltage.

[0188] Figure 23 is a circuit diagram showing a second configuration example of the converter cell 7 shown in Figure 21. As shown in Figure 23, the converter cell 7 in the second configuration example has a circuit configuration called a full-bridge configuration.

[0189] The converter cell 7 comprises a first series unit formed by connecting two switching elements 31p1 and 31n1 in series, a second series unit formed by connecting two switching elements 31p2 and 31n2 in series, an energy storage element 32, a voltage detector 33, and input / output terminals P1 and P2. The first series unit, the second series unit, and the energy storage element 32 are connected in parallel. The voltage detector 33 detects the voltage Vc across the ends of the energy storage element 32.

[0190] The midpoints of switching elements 31p1 and 31n1 are connected to input / output terminal P1. Similarly, the midpoints of switching elements 31p2 and 31n2 are connected to input / output terminal P2. The converter cell 7 outputs the voltage Vc, -Vc, or zero voltage of the energy storage element 32 between input / output terminals P1 and P2 through the switching operation of switching elements 31p1, 31n1, 31p2, and 31n2.

[0191] In Figures 22 and 23, the switching elements 31p, 31n, 31p1, 31n1, 31p2, and 31n2 are configured by connecting FWD in antiparallel to self-extinguishing semiconductor switching elements such as IGBTs and GCT thyristors, as described in Embodiment 1. In addition, capacitors such as film capacitors are mainly used for the energy storage element 32.

[0192] As shown in Figure 21, the converter cells 7 are cascaded. In Figures 22 and 23, in the converter cell 7 located on the upper arm 5, the input / output terminal P1 is connected to the input / output terminal P2 or the high-potential DC terminal Np of the adjacent converter cell 7, and the input / output terminal P2 is connected to the input / output terminal P1 or the AC terminal Nu of the adjacent converter cell 7. Similarly, in the converter cell 7 located on the lower arm 6, the input / output terminal P1 is connected to the input / output terminal P2 or the AC terminal Nu of the adjacent converter cell 7, and the input / output terminal P2 is connected to the input / output terminal P1 or the low-potential DC terminal Nn of the adjacent converter cell 7.

[0193] Furthermore, the converter cell 7 constituting the power converter 110X may also be a converter cell other than the configurations illustrated in Figures 22 and 23, for example, a converter cell with a circuit configuration called a clamped double cell, and the switching element and energy storage element are not limited to the examples given above.

[0194] The power converter 110 shown in Figure 21 can also be controlled in the same way as described in Embodiments 1 to 3, by controlling the output AC current Iac (Iacu, Iacv, Iacw) according to the current command values ​​Idref, Iqref, or Iacref. In the power converter 110 of Figure 21, the switching elements 31p, 31n, 31p1, 31n1, 31p2, and 31n2 included in the converter cells 7 of the upper and lower arms of each phase can be controlled by PWM control (PWM control unit 270) according to the three-phase voltage command values ​​Vu*, Vv*, Vw*.

[0195] Furthermore, if the power converter 110 has an operating mode in which it operates as a voltage source, in addition to the modified configuration shown in Figure 21, then self-commutated power converters with other circuit configurations can also be applied to the power conversion system according to this embodiment.

[0196] The configuration described above as an example of an embodiment is merely one example of the configuration of this disclosure, and it is possible to combine it with other known technologies. In addition, without departing from the gist of this disclosure, it is also possible to omit parts of it or otherwise appropriately adopt processes and configurations described in other embodiments.

[0197] 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 the foregoing description, and all modifications within the meaning and scope of equivalents of the claims are intended. [Explanation of symbols]

[0198] 1 AC system, 3 DC system, 4, 4u, 4v, 4w leg circuit, 5 upper arm, 6 lower arm, 7 converter cell, 8A, 8B reactor, 9, 16 AC current detector, 9A arm current detector, 10 AC voltage detector, 11A, 11B DC voltage detector, 12, 32 energy storage element, 13 transformer, 17 DC current detector, 20 input converter, 21 sample-and-hold circuit, 22 multiplexer, 23 converter, 25 RAM, 26 ROM, 27 input / output interface, 28 auxiliary storage device, 29 bus, 31, 31n, 31n1, 31n2, 31p, 31p1, 31p2 switching element, 33 voltage detector, 100 power conversion system, 110, 110X power converter, 200 control device, 205 AC current control unit, 210, 220, 235, 237, 252, 255, 301 Coordinate transformation unit, 230, 230X, 230Y, 230Z Current command value setting unit, 231~233, 238 LPF (limiter function), 233X Current upper limit value adjustment unit, 234 LPF, 236 Limiter, 240, 240Y AC voltage command value calculation unit, 242 Subtractor, 245 Control calculation unit, 250, 250X, 250Y Voltage feedforward control unit, 253, 254, 256 Selector, 260 Three-phase voltage command generation unit, 270 PWM control unit, 311~314, 321~329 Ramp plate limiter (RRL), 400 Bandpass filter, 410 Gain setting unit, Iacmax, Idmax, Idqmax, Iqmax Current limit, Iac, Iacu, Iacv, Iacw; Output AC current, Idref, Iqref; Current command value, Idref0, Iqref0; Current command value (grid-connected mode), Isys, Isysu, Isysv, Isysw; Grid AC current, Kp; Gain, Nn; Low-potential DC terminal, Np; High-potential DC terminal, Nu, Nv, Nw; AC terminal, P1, P2; Input / Output terminal, SMD; Mode selection signal, Sul, Suu, Svl, Svu, Swl, Swu; On / Off control signal (switching element), Vacff, Vdff, Vqff; Voltage FF term, Vacref; AC voltage command value, Vac, Vacu, Vacv, Vacw; AC output voltage, Vdref, Vqref, Vu*, Vv*.Vw* is the voltage command value, Vsys, Vsysd, Vsysq, Vsysu, Vsysv, Vsysw are the system AC voltages, and fsys is the fundamental frequency.

Claims

1. a power converter including at least one self-extinguishing semiconductor switching element connected between an AC circuit and a DC circuit; an AC current detector for detecting an output AC current from the power converter to the AC circuit; a control device that generates an on / off command for the semiconductor switching element for controlling the power converter, The control device includes: a current command value setting unit that sets an AC current command value; an AC current control unit for generating the on / off command in accordance with an AC voltage command value calculated so that the output AC current detected by the AC current detector follows the AC current command value, the current command value setting unit sets the AC current command value based on a fundamental wave component of the output AC current detected by the AC current detector in a first operation mode in which the power converter operates as a voltage source; the AC current control unit, in the first operation mode, calculates the AC voltage command value according to a sum of a feedback component for compensating for a current deviation between the AC current command value and the output AC current detected by the AC current detector, and an AC component according to a frequency of the fundamental wave component.

2. The power conversion system of claim 1 , wherein the first operation mode is applied at least during a black start in which the power converter operates in a state where there is no power source in the AC circuit.

3. The power conversion system according to claim 1 , wherein the current command value setting unit sets the AC current command value so that an amplitude of the AC current following the AC current command value is equal to or smaller than a predetermined current upper limit value.

4. 2. The power conversion system according to claim 1, wherein the current command value setting unit sets the AC current command values ​​of the d-axis and q-axis based on a d-axis AC current and a q-axis AC current obtained by performing a rotational coordinate transformation of the output AC current detected by the AC current detector using an angular velocity according to a frequency of the fundamental wave component.

5. 5. The power conversion system according to claim 4, wherein the current command value setting unit sets the d-axis and q-axis AC current command values ​​according to d-axis current values ​​and q-axis current values ​​obtained by limiting the d-axis AC current and the q-axis AC current obtained by the rotational coordinate transformation so that their absolute values ​​fall within a predetermined range.

6. the current command value setting unit sets the d-axis AC current command value in accordance with a value obtained by limiting the d-axis AC current obtained by the rotational coordinate transformation so that the absolute value of the d-axis AC current is equal to or less than a predetermined first upper limit value, setting the AC current command value of the q-axis in accordance with a value obtained by limiting the q-axis AC current obtained by the rotational coordinate transformation so that the absolute value of the q-axis AC current is equal to or less than a second upper limit value; 5. The power conversion system according to claim 4, wherein the second upper limit value is calculated by multiplying a value obtained by subtracting a square of the set d-axis AC current command value from a square of a predetermined current upper limit value by (1 / 2).

7. the current command value setting unit sets the q-axis AC current command value in accordance with a value obtained by limiting the q-axis AC current obtained by the rotational coordinate transformation so that the absolute value of the q-axis AC current is equal to or less than a predetermined first upper limit value, setting the AC current command value of the d axis according to a value obtained by limiting the d axis AC current obtained by the rotational coordinate transformation so that the absolute value of the d axis AC current is equal to or less than a second upper limit value; 5. The power conversion system according to claim 4, wherein the second upper limit value is calculated by multiplying a value obtained by subtracting a square of the set q-axis AC current command value from a square of a predetermined current upper limit value by (1 / 2).

8. The power conversion system according to any one of claims 5 to 7, wherein the d-axis AC current and the q-axis AC current obtained by the rotational coordinate transformation are subjected to the limiter processing after passing through a low-pass filter.

9. 5. The power conversion system according to claim 4, wherein the current command value setting unit converts the d-axis AC current and the q-axis AC current obtained by the rotational coordinate transformation into amplitudes and phases of AC currents, performs limiter processing on the amplitudes so that they are equal to or less than a predetermined current upper limit value, and inversely converts the amplitude and the phase after the limiter processing into d-axis AC current and q-axis AC current, thereby setting the AC current command values ​​for the d-axis and q-axis.

10. The power conversion system according to claim 9 , wherein the d-axis AC current and the q-axis AC current obtained by the rotating coordinate transformation are converted into the amplitude and the phase after passing through a low-pass filter.

11. The power conversion system of claim 9 , wherein the amplitude is limited after passing through a low pass filter.

12. The current command value setting unit is a band pass filter that outputs the fundamental wave component of the output AC current detected by the AC current detector; The power conversion system according to claim 1 , wherein the current command value setting unit sets the AC current command value in accordance with an output from the band pass filter.

13. the band pass filter is configured to multiply the fundamental wave component by Kp and output the multiplied fundamental wave component, The current command value setting unit is 13. The power conversion system according to claim 12, further comprising a gain setting unit that variably sets the Kp within a range of 0 to 1.0 in accordance with an amplitude of the output AC current detected by the AC current detector so that the amplitude is equal to or less than a predetermined upper current limit value.

14. 14. The power conversion system according to claim 13, wherein the gain setting unit sets the Kp to 1.0 when the amplitude is equal to or less than the current upper limit value, and sets the Kp according to a value obtained by dividing the current upper limit value by the amplitude when the amplitude is greater than the current upper limit value.

15. The current command value setting unit is The power conversion system according to any one of claims 1 to 7 and 9 to 14, wherein, when switching from the first operation mode to a second operation mode in which the power conversion system operates in connection with the AC circuit, the AC current command value is changed by limiting a change amount of the AC current command value per unit time to be within a predetermined range.

16. an AC voltage detector for detecting an AC voltage of the AC circuit, the current command value setting unit sets the AC current command value in accordance with a command from outside the power conversion system in the second operation mode; The power conversion system according to claim 15 , wherein in the second operation mode, the AC current control unit calculates the AC voltage command value according to a sum of the feedback component and an AC component according to the AC voltage detected by the feedback component.

17. 16. The power conversion system according to claim 15, wherein the AC voltage command value is calculated under a condition that a change amount of the AC component per unit time is limited to be within a predetermined range when switching from the first operation mode to the second operation mode.

18. A power conversion system described in any one of claims 3 to 7 and 9 to 14, wherein the first operating mode is applied at least during a black start in which the power converter operates in a state where there is no power source in the AC circuit.

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