Power conversion device

Abstract

This power conversion device (100) comprises: a power converter (6) that is connected to an AC system (2) via a reactor element (3) and performs power conversion between the AC system (2) and a DC circuit (4); and a control device (5) that controls the voltage output from the power converter. The control device (5) includes: an AC voltage control unit (48) that generates a reference voltage command value for the output AC voltage of the power converter (6); a phase generation unit (45) that generates a reference phase of the output AC voltage of the power converter (6); a correction voltage generation unit (50) that generates a correction voltage command value by correcting the reference voltage command value using a correction value for suppressing overcurrent due to the biased magnetization of the reactor element (3); and an AC voltage command generation unit (51) that generates a command value for the output AC voltage of the power converter (6) on the basis of the correction voltage command value and the reference phase.

Description

Power Conversion Device

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

[0002] In recent years, many distributed power sources using renewable energy such as solar power generation facilities have been introduced into power systems. Distributed power sources are often connected to the power system via power converters. As the number of distributed power sources connected to the power system increases, the proportion of synchronous generators connected to the power system decreases, and the inertial energy within the power system decreases, raising concerns about an increase in the frequency change rate, an increase in frequency fluctuation, and a decrease in stability. Therefore, voltage-controlled (Grid-Forming) control, which controls the output voltage of a power converter, has been attracting attention in recent years, rather than current-controlled (Grid-Following) control, which controls the output current of a power converter.

[0003] Japanese Patent Laid-Open Publication No. 2017-143621 (Patent Document 1) discloses a power conversion device provided between a DC circuit and an AC circuit. The power conversion device includes a unit converter, a positive arm and a negative arm each having a unit converter, an AC current path extending from a connection point between the positive arm and the negative arm, an AC current sensor that detects the current in the AC current path, and a management unit that controls the unit converter so as to suppress the DC component of the AC current path based on a measurement value of the AC current sensor.

[0004] Japanese Patent Application Laid-Open No. 2017-143621

[0005] In Patent Document 1, suppression of the DC component on the AC system side (for example, the transformer side) is considered because when a DC component flows into a transformer, it causes biased magnetism that causes magnetization in one direction in the iron core, which may lead to magnetic saturation.

[0006] The power conversion device disclosed in Patent Document 1 employs a current-controlled control system that detects the current flowing through an AC terminal, calculates the DC component of the detected current, and calculates a correction voltage to make the DC component zero. This correction voltage is added to an AC control voltage calculated using the AC current value as the output value and the AC current command value as the target value. However, Patent Document 1 does not disclose or suggest any technology for suppressing the DC component on the AC system side in a voltage-controlled control system.

[0007] An object in one aspect of the present disclosure is to continue stable operation of a voltage-controlled power conversion device by suppressing a DC component in an AC system.

[0008] A power conversion device according to an embodiment includes a power converter connected to an AC system via a reactor element and performing power conversion between the AC system and a DC circuit, and a control device that controls a voltage output from the power converter. The control device includes an AC voltage control unit that generates a reference voltage command value for an output AC voltage of the power converter, a phase generation unit that generates a reference phase for the output AC voltage of the power converter, a correction voltage generation unit that generates a correction voltage command value by correcting the reference voltage command value using a correction value that suppresses overcurrent due to biased magnetization of the reactor element, and an AC voltage command generation unit that generates a command value for the output AC voltage of the power converter based on the correction voltage command value and the reference phase.

[0009] According to the present disclosure, in a voltage-controlled power conversion device, it is possible to continue stable operation by suppressing the DC component of an AC system.

[0010] FIG. 1 is a diagram illustrating an example of the configuration of a power conversion device. FIG. 2 is a circuit diagram illustrating an example of a converter cell. FIG. 3 is a block diagram illustrating an example of the hardware configuration of a control device. FIG. 4 is a diagram illustrating an example of the functional configuration of a control device according to a first embodiment. FIG. 5 is a diagram illustrating a time change in magnetic flux density in a magnetic body of a transformer. FIG. 6 is a diagram illustrating an example of the configuration of a correction voltage generation unit according to a first embodiment. FIG. 7 is a diagram illustrating an example of the configuration of a correction voltage generation unit according to a second embodiment. FIG. 8 is a diagram illustrating an example of the functional configuration of a control device according to a third embodiment. FIG. 9 is a diagram illustrating an example of the configuration of a correction voltage generation unit according to a third embodiment.

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

[0012] [Configuration underlying each embodiment] <Overall configuration> Fig. 1 is a diagram showing an example configuration of a power conversion device 100. Referring to Fig. 1, the power conversion device 100 is connected between an AC system 2 and a DC circuit 4. The DC circuit 4 includes an energy storage element connected to a DC terminal of a power converter 6. The energy storage element is, for example, an electric double layer capacitor or an energy storage device including a storage battery such as a lithium ion battery. Alternatively, the DC circuit 4 includes a DC terminal of another power converter connected to the DC terminal of the power converter 6. In this case, by linking the two power converters, a back-to-back (BTB) system is configured for connecting AC power systems with different rated frequencies, etc.

[0013] The power conversion device 100 includes a self-excited power converter 6 and a control device 5 that controls the voltage output from the power converter 6 (i.e., performs voltage-controlled control). Typically, the power converter 6 is configured by a modular multilevel converter (MMC) that includes a plurality of converter cells 1 (corresponding to the "cells" in FIG. 1 ) connected in series with each other. Note that a "converter cell" is also called a "sub module" or a "unit converter."

[0014] The power converter 6 is connected to the DC circuit 4 and performs power conversion between the DC circuit 4 and the AC system 2. Specifically, the power converter 6 converts DC power output from the DC circuit 4 into AC power and outputs the AC power to the AC system 2 via the transformer 3. Connecting the power converter 6 to the AC system 2 via the transformer 3 makes it possible to boost the voltage of the power converter 6, thereby enabling the power converter 6 to be made more compact. The power converter 6 also converts AC power from the AC system 2 into DC power and outputs the DC power to the DC circuit 4. The power converter 6 is controlled by the control device 5 as a voltage source capable of outputting an AC voltage with a voltage phase and voltage amplitude different from those of the system voltage.

[0015] 1 , the power converter 6 includes a plurality of arms for each phase of the AC system 2. Specifically, the power converter 6 includes a plurality of leg circuits 8u, 8v, and 8w (hereinafter, collectively referred to as "leg circuits 8") connected in parallel with each other between a positive DC terminal (i.e., a high-potential side DC terminal) Np and a negative DC terminal (i.e., a low-potential side DC terminal) Nn.

[0016] A leg circuit 8 is provided for each of the multiple phases constituting the AC. The leg circuit 8 is connected between the AC system 2 and the DC circuit 4 and performs power conversion between the two circuits. FIG. 1 shows an example in which the AC system 2 is a three-phase AC system, and three leg circuits 8u, 8v, and 8w are provided corresponding to the U phase, V phase, and W phase, respectively. Note that if the AC system 2 is a single-phase AC system, two leg circuits are provided.

[0017] The AC terminals Nu, Nv, and Nw provided in the leg circuits 8u, 8v, and 8w, respectively, are connected to the AC system 2 via a transformer 3. The AC system 2 is, for example, an AC power system including an AC power source. For ease of illustration, FIG. 1 does not show the connection between the AC terminals Nv and Nw and the transformer 3. The DC terminals provided in common to each leg circuit 8 (i.e., a positive DC terminal Np and a negative DC terminal Nn) are connected to the DC circuit 4.

[0018] In the example of FIG. 1 , the power converter 6 is connected to the AC system 2 via the transformer 3 serving as a reactor element. However, the power converter 6 may also be connected to the AC system 2 via an interconnection reactor serving as a reactor element. Specifically, instead of using the transformer 3 of FIG. 1 , the leg circuits 8u, 8v, 8w may be configured to be connected to the AC system 2 via an interconnection reactor. Furthermore, instead of the AC terminals Nu, Nv, Nw, primary windings may be provided in the leg circuits 8u, 8v, 8w, respectively, and the leg circuits 8u, 8v, 8w may be AC-connected to the transformer 3 or the interconnection reactor via secondary windings magnetically coupled to the primary windings. In this case, the primary windings may be reactors 7a, 7b described below. That is, the leg circuit 8 is electrically (i.e., DC- or AC-connected) to the AC system 2 via connection parts provided in each of the leg circuits 8u, 8v, 8w, such as the AC terminals Nu, Nv, Nw or the above-described primary windings.

[0019] The leg circuit 8u is divided into a positive arm 13u extending from the positive DC terminal Np to the AC terminal Nu, and a negative arm 14u extending from the negative DC terminal Nn to the AC terminal Nu. The connection point between the positive arm 13u and the negative arm 14u is connected to the transformer 3 as the AC terminal Nu. The positive DC terminal Np and the negative DC terminal Nn are connected to the DC circuit 4. The leg circuit 8v includes a positive arm 13v and a negative arm 14v, and the leg circuit 8w includes a positive arm 13w and a negative arm 14w. Since the leg circuits 8v and 8w have the same configuration as the leg circuit 8u, the leg circuit 8u will be described below as a representative example.

[0020] In leg circuit 8u, positive arm 13u includes a plurality of converter cells 1 cascaded together and reactor 7a. The plurality of converter cells 1 and reactor 7a are connected in series. Negative arm 14u includes a plurality of converter cells 1 cascaded together and reactor 7b. The plurality of converter cells 1 and reactor 7b are connected in series.

[0021] The reactor 7a may be inserted at any position in the positive arm 13u, and the reactor 7b may be inserted at any position in the negative arm 14u. There may be a plurality of reactors 7a and a plurality of reactors 7b. The inductance values ​​of the reactors may be different from each other. Furthermore, only the reactor 7a in the positive arm 13u or only the reactor 7b in the negative arm 14u may be provided.

[0022] The power conversion apparatus 100 further includes an AC voltage detector 10, an AC current detector 15, DC voltage detectors 11a and 11b, and arm current detectors 9a and 9b provided in each leg circuit 8. These detectors measure electrical quantities (i.e., current and voltage) used to control the power conversion apparatus 100. Signals detected by these detectors are input to the control device 5.

[0023] The AC voltage detector 10 detects three-phase AC voltages Vsysu, Vsysv, and Vsysw (hereinafter also collectively referred to as "AC voltages Vsys") of the AC system 2. The AC current detector 15 detects three-phase AC currents Isysu, Isysv, and Isysw (hereinafter also collectively referred to as "AC currents Isys") of the AC system 2. The DC voltage detector 11a detects a DC voltage Vdcp at a positive-side DC terminal Np connected to the DC circuit 4. The DC voltage detector 11b detects a DC voltage Vdcn at a negative-side DC terminal Nn connected to the DC circuit 4.

[0024] The arm current detectors 9a and 9b provided in the U-phase leg circuit 8u detect a positive arm current Iup flowing in the positive arm 13u and a negative arm current Iun flowing in the negative arm 14u, respectively. The arm current detectors 9a and 9b provided in the V-phase leg circuit 8v detect a positive arm current Ivp and a negative arm current Ivn, respectively. The arm current detectors 9a and 9b provided in the W-phase leg circuit 8w detect a positive arm current Iwp and a negative arm current Iwn, respectively.

[0025] In the following description, the positive arm currents Iup, Ivp, and Iwp are collectively referred to as the positive arm current Iarmp. The negative arm currents Iun, Ivn, and Iwn are collectively referred to as the negative arm current Iarmn. The positive arm current Iarmp and the negative arm current Iarmn are collectively referred to as the arm current Iarm.

[0026] The DC current Idc output from the power converter 6 is detected by a DC current detector (not shown). The DC current Idc corresponds to the value of a DC current flowing between the power converter 6 and the DC circuit 4. The DC current Idc may be calculated by the following equation (1) using the positive arm currents Iup, Ivp, Iwp and the negative arm currents Iun, Ivn, Iwn.

[0027] Idc=(Iup+Ivp+Iwp+Iun+Ivn+Iwn) / 2 (1) <Configuration Example of Converter Cell> Fig. 2 is a circuit diagram showing an example of a converter cell 1. The converter cell 1 shown in Fig. 2(a) has a circuit configuration called a half-bridge configuration. This converter cell 1 includes a series body formed by connecting two switching elements 31p and 31n in series, a capacitor 32 as a storage element, and a voltage detector 33. The series body and capacitor 32 are connected in parallel. The voltage detector 33 detects a capacitor voltage Vcap, which is the voltage across both ends of capacitor 32.

[0028] The converter cell 1 shown in Fig. 2(b) has a circuit configuration called a full-bridge configuration. This converter cell 1 includes a first series body formed by connecting two switching elements 31p1 and 31n1 in series, a second series body formed by connecting two switching elements 31p2 and 31n2 in series, a capacitor 32, and a voltage detector 33. The first series body, the second series body, and the capacitor 32 are connected in parallel. The voltage detector 33 detects a capacitor voltage Vcap.

[0029] The two switching elements 31p and 31n in Fig. 2(a) and the four switching elements 31p1, 31n1, 31p2, and 31n2 in Fig. 2(b) are configured by connecting a free wheel diode in antiparallel to a self-extinguishing semiconductor switching element such as an IGBT (Insulated Gate Bipolar Transistor) or a MOSFET (Metal Oxide Semiconductor Field-Effect Transistor). Also, in Fig. 2(a) and Fig. 2(b), a capacitor such as a film capacitor is mainly used as the capacitor 32.

[0030] In the following description, the switching elements 31p, 31n, 31p1, 31n1, 31p2, and 31n2 are also collectively referred to as switching elements 31. Furthermore, the on / off of the semiconductor switching elements in the switching elements 31 is simply referred to as "on / off of the switching elements 31."

[0031] 2A, both terminals of switching element 31n are defined as input / output terminals G1 and G2. The switching operations of switching elements 31p and 31n output the voltage across capacitor 32 and zero voltage. For example, when switching element 31p is on and switching element 31n is off, the voltage across capacitor 32 is output. When switching element 31p is off and switching element 31n is on, zero voltage is output.

[0032] Next, referring to Fig. 2(b), the midpoint between switching elements 31p1 and 31n1 and the midpoint between switching elements 31p2 and 31n2 are defined as input / output terminals G1 and G2 of converter cell 1, respectively. Converter cell 1 shown in Fig. 2(b) outputs a positive voltage or zero voltage by turning on switching element 31n2, turning off switching element 31p2, and alternately turning on switching elements 31p1 and 31n1. Converter cell 1 shown in Fig. 2(b) can also output a zero voltage or negative voltage by turning off switching element 31n2, turning on switching element 31p2, and alternately turning on switching elements 31p1 and 31n1.

[0033] In this embodiment, the converter cell 1 may have the half-bridge cell configuration shown in Fig. 2(a) or the full-bridge configuration shown in Fig. 2(b). Also, a converter cell having a configuration other than those shown above, for example, a converter cell using a circuit configuration also called a 1.5 half-bridge configuration in which the switching element 31p2 in Fig. 2(b) is replaced with only a diode, may be used.

[0034] <Example of Hardware Configuration of Control Device> Fig. 3 is a block diagram showing an example of the hardware configuration of the control device 5. The control device 5 in Fig. 3 is configured based on a computer. Referring to Fig. 3, the control device 5 includes one or more input converters 70, one or more sample-and-hold (S / H) circuits 71, a multiplexer (MUX) 72, and an A / D converter 73. Furthermore, the control device 5 includes one or more central processing units (CPUs) 74, random access memories (RAMs) 75, and read-only memories (ROMs) 76. Furthermore, the control device 5 includes one or more input / output interfaces 77, an auxiliary storage device 78, and a bus 79 that interconnects the above components.

[0035] The input converter 70 includes an auxiliary transformer for each input channel. Each auxiliary transformer converts the detection signal from each electrical quantity detector in FIG. 1 into a signal with a voltage level suitable for subsequent signal processing. 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 electrical quantity received from the corresponding input converter 70 at a specified sampling frequency.

[0036] The multiplexer 72 sequentially selects the signals held in the plurality of sample-and-hold circuits 71. The A / D converter 73 converts the signal selected by the multiplexer 72 into a digital value. Note that by providing a plurality of A / D converters 73, A / D conversion may be performed in parallel on detection signals of a plurality of input channels.

[0037] The CPU 74 controls the entire control device 5 and executes arithmetic processing according to a program. The RAM 75 as a volatile memory and the ROM 76 as a nonvolatile memory are used as the main memory of the CPU 74. The ROM 76 stores programs, setting values ​​for signal processing, and the like. The auxiliary storage device 78 is a nonvolatile memory with a larger capacity than the ROM 76, and stores programs, data on detected values ​​of electricity, and the like. The input / output interface 77 is an interface circuit for communication between the CPU 74 and external devices.

[0038] At least a part of the control device 5 may be configured using circuits such as a field programmable gate array (FPGA) and an application specific integrated circuit (ASIC), or at least a part of the control device 5 may be configured using analog circuits.

[0039] The first to fourth embodiments will be described in detail below. Embodiment 1. <Functional Configuration of Control Device> FIG. 4 illustrates an example of the functional configuration of a control device 5 according to the first embodiment. Referring to FIG. 4, the control device 5 includes coordinate conversion units 21 and 25, a frequency detection unit 22, an AC power calculation unit 23, and an AC current calculation unit 24. The control device 5 further includes a capacitor voltage control unit 41, a DC current control unit 42, a frequency control unit 43, an active power control unit 44, a phase generation unit 45, a reactive power control unit 47, an AC voltage control unit 48, a correction voltage generation unit 50, a coordinate conversion unit 51, a voltage command generation unit 52, a balance control unit 54, a circulating current control unit 55, a carrier generation unit 56, and a PWM control unit 57. Each of these functions is realized by a processing circuit. The processing circuit may be dedicated hardware or a CPU that executes a program stored in the internal memory of the control device 5. When the processing circuit is dedicated hardware, the processing circuit may be configured, for example, by an FPGA, an ASIC, or a combination thereof.

[0040] The coordinate converter 21 calculates the rotating coordinate axis components (i.e., d- and q-axis components) of the AC current Isys. Specifically, the coordinate converter 21 performs three-phase / two-phase conversion on the AC currents Isysu, Isysv, and Isysw using a reference phase θc of the output voltage of the power converter 6 to calculate the d-axis component (hereinafter also referred to as the "d-axis current Isysd") and the q-axis component (hereinafter also referred to as the "q-axis current Isysq") of the AC current Isys. The coordinate converter 21 also performs three-phase / two-phase conversion on the AC voltages Vsysu, Vsysv, and Vsysw using the reference phase θc to calculate the d-axis component (hereinafter also referred to as the "d-axis voltage Vd") and the q-axis component (hereinafter also referred to as the "q-axis voltage Vq") of the AC voltage Vsys.

[0041] The frequency detection unit 22 detects the system angular frequency ωsys of the AC voltages Vsysu, Vsysv, and Vsysw.

[0042] The AC current calculation unit 24 calculates the active power P and reactive power Q of the AC system 2 based on the d-axis current Isysd, the q-axis current Isysq, the d-axis voltage Vd, and the q-axis voltage Vq. The active power P and reactive power Q correspond to the detected values ​​of the active power and reactive power output from the power converter 6 to the AC system 2, respectively. Therefore, in the following description, the active power P and reactive power Q are also referred to as the active power detected value P and reactive power detected value Q of the AC system 2, respectively.

[0043] The AC current calculation unit 24 calculates the AC current output from the power converter 6 using the positive arm currents Iup, Ivp, and Iwp and the negative arm currents Iun, Ivn, and Iwn. For example, the AC current Iacu output from the AC terminal Nu to the transformer 3 is a current value obtained by subtracting the negative arm current Iun from the positive arm current Iup (i.e., Iacu = Iup - Iun). Similarly, the AC current Iacv output from the AC terminal Nv is expressed as "Iacv = Ivp - Ivn," and the AC current Iacw output from the AC terminal Nw is expressed as "Iacw = Iwp - Iwn." Hereinafter, the AC currents Iacu, Iacv, and Iacw will also be collectively referred to as "AC current Iac."

[0044] The AC current Iac corresponds to the secondary side current of the transformer 3 (i.e., the current on the side of the power converter 6). The AC current Iac is calculated by a known method based on the AC current Isys and information about the transformer 3 (e.g., the transformation ratio). Note that when an interconnection reactor is used instead of the transformer 3, the AC current Iac and the AC current Isys coincide with each other.

[0045] The coordinate conversion unit 25 performs three-phase / two-phase conversion on the AC currents Iacu, Iacv, and Iacw using the reference phase θc, and calculates the d-axis component (hereinafter also referred to as the "d-axis current Id") and the q-axis component (hereinafter also referred to as the "q-axis current Iq") of the AC current Iac.

[0046] The capacitor voltage control unit 41 generates a DC current command value Idc* for the power converter 6 based on the capacitor voltage command value Vcap* and the capacitor voltage Vcap of the capacitor 32 included in the power converter 6. The capacitor voltage command value Vcap* is a command value given for the average voltage value of all capacitors included in the power converter 6. The capacitor voltage control unit 41 generates the DC current command value Idc* so that the average voltage value of all capacitors follows the capacitor voltage command value Vcap* (for example, so that the deviation between the capacitor voltage command value Vcap* and the average voltage value of all capacitors becomes zero).

[0047] The DC current control unit 42 generates a DC voltage command value Vdc* so that the DC current Idc follows the DC current command value Idc* (for example, so that the deviation between the DC current command value Idc* and the DC current Idc becomes zero).

[0048] The frequency control unit 43 generates an active power command value P* so that the system angular frequency ωsys follows the frequency command value ωsys* (for example, so that the deviation between the frequency command value ωsys* and the system angular frequency ωsys becomes 0). Note that the active power command value P* may be a command value in response to a request from a higher-level device or a command value set by a system operator.

[0049] The active power control unit 44 generates a reference angular frequency ωc of the AC voltage output from the power converter 6 (hereinafter also simply referred to as "output AC voltage") so that the active power detection value P follows the active power command value P* (for example, so that the deviation between the active power command value P* and the active power detection value P becomes zero).

[0050] The phase generator 45 generates a reference phase θc based on the system angular frequency ωsys and a reference angular frequency ωc that is generated based on the active power detection value P and the active power command value P*. The phase generator 45 generates the reference phase θc of the output AC voltage of the power converter 6 so that the system angular frequency ωsys follows the reference angular frequency ωc (for example, so that the deviation between the reference angular frequency ωc and the system angular frequency ωsys becomes zero).

[0051] The reactive power control unit 47 generates an amplitude command value Vmag* of the output AC voltage of the power converter 6 so that the reactive power detection value Q follows the reactive power command value Q* (for example, so that the deviation between the reactive power command value Q* and the reactive power detection value Q becomes zero).

[0052] The AC voltage control unit 48 calculates reference voltage command values ​​Vd0* and Vq0* of the output AC voltage of the power converter 6 based on the amplitude command value Vmag* generated based on the reactive power detected value Q and the reactive power command value Q*, and the d-axis voltage Vd and q-axis voltage Vq of the AC voltage Vsys, which are the AC voltage detection values. Vd0* corresponds to the d-axis component of the reference voltage command value, and Vq0* corresponds to the q-axis component of the reference voltage command value.

[0053] The correction voltage generation unit 50 generates the correction voltage command values ​​Vds* and Vqs* by correcting the reference voltage command values ​​Vd0* and Vq0* using a correction value for suppressing an overcurrent due to biased magnetism of a reactor element (for example, the transformer 3). Specifically, the correction voltage generation unit 50 calculates a correction value that removes the DC component (and low-frequency component) of the AC current Isys of the AC system 2, and generates the correction voltage command values ​​Vds* and Vqs* based on the reference voltage command values ​​Vd0* and Vq0* and the correction value.

[0054] Here, we will explain magnetic saturation in the magnetic body provided in the transformer, which can occur due to biased magnetization of the reactor element. Figure 5 is a diagram showing the time change in magnetic flux density in the magnetic body of the transformer. Specifically, Figure 5(a) shows the time change in magnetic flux density in the magnetic body of the transformer when a voltage including a DC component and a fundamental wave component is applied to the transformer. Figure 5(b) shows the time change in magnetic flux density in the magnetic body of the transformer when a voltage including a DC component, a low frequency component, and a fundamental wave component is applied to the transformer. In this embodiment, the DC component is the component corresponding to 0 Hz. The low frequency component is the component corresponding to a frequency higher than 0 Hz and lower than the fundamental wave frequency (e.g., 60 Hz).

[0055] When a DC component of voltage is applied to a transformer, the magnetic flux density becomes biased to either positive or negative (i.e., biased magnetism occurs). In the example of Fig. 5(a), the DC component of the voltage applied to the transformer biases the magnetic flux density in the positive direction (i.e., biased magnetism occurs in the positive direction), and the magnetic flux density of the fundamental wave component is superimposed, causing magnetic saturation (i.e., the magnetic flux density reaches saturation magnetic flux density). Note that magnetic saturation occurs after time T1 from the time of voltage application.

[0056] Furthermore, when a DC component and a low-frequency component of a voltage are applied to a transformer, the magnetic bias becomes large transiently, which makes it easier for magnetic saturation to occur. In the example of Figure 5(b), magnetic saturation occurs after time T2 from the time when the voltage is applied.

[0057] As shown in Figures 5(a) and 5(b), when a voltage (or current) containing DC and low-frequency components is applied to a transformer, magnetic bias occurs in the magnetic material of the transformer, making it prone to magnetic saturation and resulting in overcurrent. Therefore, it is necessary to prevent at least the outflow of DC components to the transformer, and it is preferable to also prevent the outflow of low-frequency components to the transformer. In a voltage-controlled power conversion device, the amplitude and frequency of the output AC voltage are controlled, but the output AC current is not directly controlled. Therefore, if the AC current contains DC and low-frequency components due to the influence of transient fluctuations in the output power, these components must be appropriately suppressed. Therefore, the correction voltage generation unit 50 according to the first embodiment generates correction voltage command values ​​Vds* and Vqs* that suppress overcurrent due to magnetic bias.

[0058] 6 is a diagram showing an example of the configuration of correction voltage generation unit 50 according to the first embodiment. Referring to FIG. 6, correction voltage generation unit 50 includes filters 151 and 152, subtractors 153 and 154, proportional units 155 and 156, and adders 157 and 158.

[0059] The filter 151 generates a d-axis current Iddc by filtering the d-axis current Id, and the filter 152 generates a q-axis current Iqdc by filtering the q-axis current Iq. For example, the filters 151 and 152 are band-pass filters.

[0060] As described above, DC components and low-frequency components of AC electrical quantities (e.g., AC current and AC voltage) can cause magnetization bias in the transformer 3. Here, in a coordinate system in which the dq coordinate axes are synchronized with the positive-phase fundamental component, the DC components and low-frequency components of three-phase AC in the stationary coordinate system appear as components near the fundamental component on the dq coordinate axes. Therefore, the filters 151 and 152 are configured to extract at least the fundamental component on the dq coordinate axes. Specifically, the filters 151 and 152 are configured to extract the fundamental component on the dq coordinate axes and components in a frequency band close to the fundamental component. Typically, the components in a frequency band close to the fundamental component are components in a frequency band lower than the fundamental frequency (e.g., 60 Hz) and equal to or greater than 90% of the fundamental frequency (e.g., 54 Hz). While the configuration for suppressing magnetization bias due to the positive-phase component has been described here, the filters may also be configured to suppress magnetization bias due to the negative-phase component. In this case, the components in the frequency band close to the fundamental wave component are components in a frequency band that is higher than the fundamental wave frequency and equal to or lower than 110% of the fundamental wave frequency (for example, 66 Hz).

[0061] The subtractor 153 calculates the deviation between the d-axis current command value Iddc* (e.g., 0) and the d-axis current Iddc (i.e., "Iddc*-Iddc"). The subtractor 154 calculates the deviation between the q-axis current command value Iqdc* (e.g., 0) and the q-axis current Iqdc (i.e., "Iqdc*-Iqdc").

[0062] Proportionalizer 155 generates a d-axis component Vdcmp of the correction value by multiplying the deviation output from subtractor 153 by gain K. Proportionalizer 156 generates a q-axis component Vqcmp of the correction value by multiplying the deviation output from subtractor 154 by gain K.

[0063] An adder 157 adds a d-axis component Vdcmp of the correction value to a d-axis component Vd0* of the reference voltage command value to generate a d-axis component Vds* of the corrected voltage command value. An adder 158 adds a q-axis component Vqcmp of the correction value to a q-axis component Vq0* of the reference voltage command value to generate a q-axis component Vqs* of the corrected voltage command value. By correcting the reference voltage command values ​​Vd0* and Vqs*0 using the correction values ​​Vdcmp and Vqcmp, corrected voltage command values ​​Vds* and Vqs* that cancel out (i.e., remove) the DC component and low-frequency component of the AC system 2 are generated.

[0064] In the example of Figure 6, the correction voltage generation unit 50 is configured to generate the correction values ​​Vdcmp and Vqcmp based on the d-axis current Id and the q-axis current Iq, but the correction values ​​Vdcmp and Vqcmp may also be generated based on the d-axis voltage Vd and the q-axis voltage Vq.

[0065] In this case, filter 151 generates d-axis voltage Vddc by filtering d-axis voltage Vd. Subtractor 153 calculates the deviation (i.e., "Vddc*-Vddc") between d-axis voltage command value Vddc* (e.g., 0) and d-axis voltage Vddc. Proportional adjuster 155 multiplies the deviation by gain K to generate d-axis component Vdcmp of the correction value. Similarly, q-axis component Vqcmp is generated using q-axis voltage Vqdc and q-axis voltage command value Vqdc* generated by filter 152.

[0066] In the example of FIG. 6, proportional control is employed in which the deviations output from the subtractors 153 and 154 are multiplied by a gain K, but proportional-integral control may also be employed.

[0067] Here, when the reactor element is the transformer 3, the AC current Iac can be expressed by a known method using the AC current Isys of the AC system 2 and information about the transformer 3 (for example, a transformation ratio, etc.). When the reactor element is an interconnection reactor, the AC current Iac coincides with the AC current Isys of the AC system 2. Therefore, the AC current Iac and the AC voltage Vsys can be regarded as AC electrical quantities of the AC system 2.

[0068] Therefore, the correction voltage generating unit 50 generates dq-axis components (e.g., dq-axis currents Iddc, Iqdc or dq-axis voltages Vddc, Vqdc) by performing filtering to extract fundamental wave components from the dq-axis components (e.g., dq-axis currents Id, Iq or dq-axis voltages Vd, Vq) of the AC electrical quantities (e.g., AC current Iac, AC voltage Vsys) of the AC system 2.

[0069] Next, the correction voltage generator 50 generates the dq-axis components Vdcmp, Vqcmp of the correction value by feedback control (e.g., proportional control) based on the dq-axis component command values ​​(e.g., dq-axis current command values ​​Iddc*, Iqdc* or dq-axis voltage command values ​​Vddc*, Vqdc*) and the filtered dq-axis components.The correction voltage generator 50 then adds the dq-axis components Vdcmp, Vqcmp of the correction value to the dq-axis components Vd0*, Vq0* of the reference voltage command value to generate the dq-axis components Vds*, Vqs* of the correction voltage command value.

[0070] 4 again, the coordinate conversion unit 51 generates an AC voltage command value Vac* indicating a command value for the output AC voltage of the power converter 6, based on the reference phase θc and the corrected voltage command values ​​Vds* and Vqs*. Specifically, the coordinate conversion unit 51 performs two-phase / three-phase conversion on the corrected voltage command values ​​Vds* and Vqs* on the d- and q-axes using the reference phase θc, thereby generating three-phase AC voltage command values ​​Vacu*, Vacv*, and Vacw* (i.e., the AC voltage command value Vac*). Thus, the coordinate conversion unit 51 functions as an "AC voltage command generation unit" that generates the AC voltage command value Vac*.

[0071] The AC voltage command value Vac* generated as described above reflects the correction voltage command values ​​Vds* and Vqs* generated by the correction voltage generating unit 50. The correction voltage command values ​​Vds* and Vqs* are obtained by correcting the reference voltage command values ​​Vd0* and Vq0* so as to suppress the DC component and low-frequency component in the three-phase AC of the AC system 2. Therefore, according to the AC voltage command value Vac*, the output AC voltage of the power converter 6 is controlled so as to suppress the DC component and low-frequency component.

[0072] The voltage command generating unit 52 generates a voltage command value Vcnv* based on the DC voltage command value Vdc* and the AC voltage command value Vac*. Typically, the voltage command generating unit 52 generates voltage command values ​​for the positive and negative arms of each phase based on the AC voltage command values ​​Vacu*, Vacv*, and Vacw* and the DC voltage command value Vdc*. Specifically, for the U phase, the voltage command value for the positive arm 13u of the U phase is a value obtained by subtracting the AC voltage command value Vacu* from the DC voltage command value Vdc*. The voltage command value for the negative arm 14u of the U phase is a value obtained by adding the AC voltage command value Vacu* to the DC voltage command value Vdc*. Voltage command values ​​for the positive and negative arms of the V and W phases are generated in a similar manner. The voltage command generating unit 52 generates voltage command values ​​Vucnv*, Vvcnv*, Vwcnv* (i.e., voltage command value Vcnv*) for the converter cells 1 of each phase based on the voltage command values ​​of the positive and negative arms of each phase.

[0073] The balance control unit 54 generates a circulating current command value Iz* for balancing the voltages of the capacitors 32 between the phases and between the arms. Specifically, the balance control unit 54 controls the balance of the voltages of the capacitors 32 between the phases by executing feedback control for making the voltage average value of each capacitor 32 included in the leg circuit 8 (i.e., the capacitor voltage average value) follow the inter-phase balance voltage command value.

[0074] The balance control unit 54 also controls the voltage balance between the positive and negative arms of the capacitors 32 by executing feedback control to make the negative-side capacitor voltage average value follow the positive-negative balance voltage command value (i.e., the positive-side capacitor voltage average value). The balance control unit 54 generates the circulating current command value Iz* by adding a feedback calculation value for controlling the voltage balance of the capacitors 32 between the phases and a feedback calculation value for controlling the voltage balance of the capacitors 32 between the positive and negative arms.

[0075] The circulating current control unit 55 calculates the circulating current Iz using each arm current Iarm, and generates an offset signal Caoff of the carrier signal so that the circulating current Iz follows the circulating current command value Iz* (for example, so that the deviation between the circulating current Iz and the circulating current command value Iz* becomes zero). Note that the U-phase circulating current Izu, V-phase circulating current Izv, and W-phase circulating current Izw flowing in the closed circuit of the power converter 6 that does not include the AC system 2 and the DC circuit 4 in its path are expressed by the following equations (2), (3), and (4), respectively.

[0076] Izu=(Iup+Iun) / 2-Idc / 3 (2) Izv=(Ivp+Ivn) / 2-Idc / 3 (3) Izw=(Iwp+Iwn) / 2-Idc / 3 (4) The carrier generating unit 56 generates a carrier signal Ca using the offset signal Caoff. The carrier signal Ca is composed of a periodic signal such as a triangular wave.

[0077] The PWM control unit 57 generates a control signal for the power converter 6 based on the voltage command value Vcnv*. Specifically, the PWM control unit 57 executes PWM (Pulse Width Modulation) control based on the voltage command values ​​Vucnv*, Vvcnv*, and Vwcnv* to generate a gate signal GP that controls the on / off driving of the switching element 31 of each converter cell 1 of each phase.

[0078] For example, a method for generating a voltage command value Vucnv* for each converter cell of the U phase will be described. The PWM control unit 57 generates a PWM modulation signal by comparing the voltage of the voltage command value Vucnv* with the voltage of the carrier signal Ca. When the voltage of the voltage command value Vucnv* is higher than the voltage of the carrier signal Ca, the PWM modulation signal is set to a high level. Conversely, when the voltage of the carrier signal Ca is higher than the voltage command value Vucnv*, the PWM modulation signal is set to a low level. The carrier signal Ca is generated so as to shift the timing of the PWM signals among the N converter cells 1 of each phase to achieve phase-shift PWM control.

[0079] The PWM control unit 57 also generates the voltage command value Vvcnv* for each V-phase converter cell 1 and the voltage command value Vwcnv* for each W-phase converter cell 1 in the same manner as above. As a result, based on the PWM modulation signal, the PWM control unit 57 generates a gate signal GP that controls on / off of each of the switching elements 31 included in the converter cell 1. Each switching element 31 of each converter cell 1 is driven on / off in accordance with the gate signal GP.

[0080] <Advantages> According to the first embodiment, the DC component and low-frequency component of the AC system 2, which cause biased magnetization of the reactor element, can be suppressed, and therefore the power converter 6, whose voltage is controlled by the control device 5, can continue to operate stably.

[0081] Embodiment 2 In the first embodiment, a configuration has been described in which filtering is performed on the dq-axis currents Id and Iq to extract fundamental wave components in order to generate the correction values ​​Vdcmp and Vqcmp, as shown in Fig. 6. In the second embodiment, a configuration will be described in which filtering is performed on the currents of the stationary coordinate axis components (i.e., the αβ-axis components) in order to generate the correction values ​​Vdcmp and Vqcmp.

[0082] 7 is a diagram showing an example of the configuration of a correction voltage generation unit according to the second embodiment. Referring to FIG. 7, correction voltage generation unit 50A includes coordinate conversion units 251 and 254, filters 252 and 253, subtractors 153 and 154, proportional units 155 and 156, and adders 157 and 158. Correction voltage generation unit 50A corresponds to correction voltage generation unit 50 in FIG. 4, but is denoted by the letter "A" for convenience in order to distinguish it from correction voltage generation unit 50 according to the first embodiment. This also applies to the third and fourth embodiments.

[0083] 7, the coordinate converter 251 performs dq / αβ conversion on the d-axis current Id and the q-axis current Iq using the reference phase θc to calculate the α-axis current Iα and the β-axis current Iβ. The α-axis current Iα corresponds to the α-axis component of the AC current Isys, and the β-axis current Iβ corresponds to the β-axis component of the AC current Isys.

[0084] The filter 252 generates an α-axis current Iαf by filtering the α-axis current Iα. The filter 253 generates a β-axis current Iβf by filtering the β-axis current Iβ. For example, the filters 252 and 253 are low-pass filters, moving average filters, etc.

[0085] Here, the DC component and low-frequency components of the three-phase AC appear as components near the fundamental wave on the dq coordinate axes and as components near DC on the αβ coordinate axes. Therefore, the filters 252 and 253 are configured to extract at least the DC component. Specifically, the filters 252 and 253 are configured to extract the DC component and components in a frequency band close to the DC component. Typically, the components in a frequency band close to the DC component are components in a frequency band higher than the DC component (i.e., 0 Hz) and equal to or lower than 10% (e.g., 6 Hz) of the fundamental wave frequency (e.g., 60 Hz). When the three-phase AC current includes a DC component, the α-axis current Iαf and the β-axis current Iβf become αβ-axis currents including a DC component.

[0086] The coordinate converter 254 performs αβ / dq transformation on the α-axis current Iαf and the β-axis current Iβf using the reference phase θc to calculate the d-axis current Iddc and the q-axis current Iqdc. The processes of the subtractors 153 and 154, the proportional converters 155 and 156, and the adders 157 and 158 are the same as those described in Fig. 6. Therefore, as in the case described in Fig. 6, the reference voltage command values ​​Vd0* and Vqs*0 are corrected using the correction values ​​Vdcmp and Vqcmp to generate corrected voltage command values ​​Vds* and Vqs* that cancel out the DC component and low-frequency component of the AC system 2.

[0087] The correction voltage generator 50A may generate the correction values ​​Vdcmp and Vqcmp based on the d-axis voltage Vd and the q-axis voltage Vq. In this case, the coordinate converter 251 uses the reference phase θc to perform dq / αβ transformation on the d-axis voltage Vd and the q-axis voltage Vq to calculate the α-axis voltage Vα and the β-axis voltage Vβ. The filter 252 performs filtering on the α-axis voltage Vα to generate the α-axis voltage Vαf. The filter 253 performs filtering on the β-axis voltage Vβ to generate the β-axis voltage Vβf. The coordinate converter 254 uses the reference phase θc to perform αβ / dq transformation on the α-axis voltage Vαf and the β-axis voltage Vβf to calculate the d-axis voltage Vddc and the q-axis voltage Vqdc. The processing details of the subtractors 153 and 154 and the proportionalizers 155 and 156 are as described above.

[0088] Therefore, the correction voltage generator 50A generates αβ-axis components (e.g., αβ-axis currents Iαf, Iβf or αβ-axis voltages Vαf, Vβf) by filtering the αβ-axis components (e.g., αβ-axis currents Iα, Iβ or αβ-axis voltages Vα, Vβ) of the AC electricity quantities (e.g., AC current Iac, AC voltage Vsys) of the AC system 2 to extract DC components. Next, the correction voltage generator 50A performs coordinate transformation on the filtered αβ-axis components to generate dq-axis components (e.g., dq-axis currents Iddc, Iqdc or dq-axis voltages Vddc, Vqdc).

[0089] Next, the correction voltage generator 50A generates dq-axis components Vdcmp, Vqcmp of the correction value by feedback control based on the dq-axis component command values ​​(for example, dq-axis current command values ​​Iddc*, Iqdc* or dq-axis voltage command values ​​Vddc*, Vqdc*) and the dq-axis components obtained by coordinate transformation of the filtered αβ-axis components.The correction voltage generator 50 then adds the dq-axis components Vdcmp, Vqcmp of the correction value to the dq-axis components Vd0*, Vq0* of the reference voltage command value to generate the dq-axis components Vds*, Vqs* of the correction voltage command value.

[0090] As shown in Fig. 6, the correction voltage generator 50 according to the first embodiment is configured to extract components near the fundamental wave using a filter. Therefore, there is a possibility that the filter will have a large effect on the phase (for example, the phase of the output signal will be shifted relative to the input signal of the filter). In this case, the phase of the correction value will also be shifted, and the effect of suppressing the DC component and low-frequency component of the three-phase AC may be reduced.

[0091] On the other hand, the correction voltage generating unit 50A according to the second embodiment is configured to extract components near DC using a filter, as shown in Fig. 7, so that the influence of the filter on the phase can be reduced, and therefore the DC component and low-frequency component of the three-phase AC can be more effectively suppressed.

[0092] <Advantages> According to the second embodiment, the DC components and low-frequency components of the AC system 2 that cause biased magnetization of the reactor elements can be more effectively suppressed, and therefore the power converter 6 can continue to operate more stably.

[0093] Third Embodiment In the first and second embodiments, the configuration has been described in which the DC component and low-frequency component of the AC system 2 are suppressed by constantly correcting the reference voltage command values ​​Vd0* and Vq0* using the correction values ​​Vdcmp and Vqcmp. According to this configuration, control is executed to suppress a DC component that is not actually present and that is generated due to a detection error or the like by each detector (e.g., valve branch current detectors 9a and 9b) shown in FIG. 1 (e.g., correcting the reference voltage command values ​​Vd0* and Vq0* using the correction values ​​Vdcmp and Vqcmp). In this case, the AC voltage command value Vac* may be unnecessarily varied, potentially increasing ripples in the output AC voltage and the output AC current.

[0094] Therefore, in the third embodiment, a configuration will be described in which it is determined whether or not to correct the reference voltage command values ​​Vd0*, Vq0* using the correction values ​​Vdcmp, Vqcmp based on the magnitude of the DC component of the AC system 2 and the components in a frequency band close to the DC component.

[0095] Fig. 8 is a diagram showing an example of a functional configuration of a control device 5B according to the third embodiment. Referring to Fig. 8, control device 5B has a configuration in which correction voltage generation unit 50 in control device 5 of Fig. 4 is replaced with a correction voltage generation unit 50B. While correction voltage generation unit 50 of Fig. 4 receives as input reference voltage command values ​​Vd0*, Vq0* and dq-axis currents Id, Iq, correction voltage generation unit 50B of Fig. 8 receives as input reference voltage command values ​​Vd0*, Vq0* and AC current Iac.

[0096] Fig. 9 is a diagram showing an example of the configuration of a correction voltage generation unit 50B according to embodiment 3. Referring to Fig. 9, correction voltage generation unit 50B includes the components shown in Fig. 6 , a coordinate conversion unit 301, a calculation unit 303, a filter 304, an absolute value calculation unit 305 (corresponding to "ABS" in the diagram), a comparison unit 306, and output control units 311 and 312.

[0097] The coordinate converter 301 converts the AC currents Iacu, Iacv, and Iacw from three-phase to two-phase using the reference phase θc to calculate the d-axis current Id and the q-axis current Iq. The processes of the filters 151 and 152, the subtractors 153 and 154, and the proportional converters 155 and 156 are the same as those described in FIG. 6 .

[0098] The calculation unit 303 calculates a sum current Iac0, which is the sum of the AC currents Iacu, Iacv, and Iacw of the respective phases. The filter 304 generates a sum current Iac0f by filtering the sum current Iac0. For example, the filter 304 is a low-pass filter, a moving average filter, or the like. The filter 304 is configured to extract at least the DC component that causes bias magnetization of the transformer 3. Specifically, the filter 304 is configured to extract the DC component and components in a frequency band close to the DC component. In this case, the sum current Iac0f is a current that includes the DC component and components in a frequency band close to the DC component.

[0099] The absolute value calculation unit 305 calculates the current absolute value |Iac0f| of the sum current Iac0f. The comparison unit 306 compares the current absolute value |Iac0f| with a threshold value Th1. If the current absolute value |Iac0f| is equal to or greater than the threshold value Th1, the comparison unit 306 outputs a signal sw having a value of "1" to the output control units 311 and 312. If the current absolute value |Iac0f| is less than the threshold value Th1, the comparison unit 306 outputs a signal sw having a value of "0" to the output control units 311 and 312. The threshold value Th1 may be determined based on, for example, an error of the valve branch current detectors 9a and 9b, or may be determined based on the rated value of the AC current Iac.

[0100] When the value of the signal sw is "1", the output control unit 311 outputs the correction value Vdcmp input from the proportional unit 155 to the adder 157, and when the value of the signal sw is "0", the output control unit 311 outputs "0" to the adder 157. When the value of the signal sw is "1", the output control unit 312 outputs the correction value Vqcmp input from the proportional unit 156 to the adder 158, and when the value of the signal sw is "0", the output control unit 312 outputs "0" to the adder 158.

[0101] Therefore, when the current absolute value |Iac0f| is equal to or greater than the threshold value Th1, the correction values ​​Vdcmp, Vqcmp are added to the base voltage command values ​​Vd0*, Vq0* to generate the corrected voltage command values ​​Vds*, Vqs*. On the other hand, when the current absolute value |Iac0f| is less than the threshold value Th1, the correction is not performed, and the base voltage command values ​​Vd0*, Vq0* are generated as the corrected voltage command values ​​Vds*, Vqs*.

[0102] In the example of Fig. 9, a configuration in which filters are applied to the d-axis and q-axis currents Id and Iq as in the first embodiment has been described, but a configuration in which filters are applied to the αβ-axis currents Iα and Iβ as in the second embodiment may also be used. Furthermore, while the signal sw is output based on a value obtained by applying a filter to the sum current Iac0, the signal sw may also be output based on values ​​obtained by applying a filter to the AC currents Iacu, Iacv, and Iacw of each phase. As in the first and second embodiments, the correction values ​​Vdcmp and Vqcmp may be generated using the dq-axis voltages Vd and Vq instead of the dq-axis currents Id and Iq.

[0103] Furthermore, a signal sw may be output based on the AC voltages Vsysu, Vsysv, and Vsysw of each phase. In this case, a sum voltage Vsys0, which is the sum of the AC voltages Vsysu, Vsysv, and Vsysw, is calculated, and the voltage absolute value |Vsys0| is compared with a threshold value Th2. When the voltage absolute value |Vsys0f| is equal to or greater than the threshold value Th2, a signal sw having a value of "1" is output. When the current absolute value |Vsys0f| is less than the threshold value Th2, a signal sw having a value of "0" is output. The threshold value Th2 may be determined, for example, based on an error of the AC voltage detector 10 or based on the rated value of the AC voltage Vsys.

[0104] According to the above, the correction voltage generating unit 50B performs filtering to extract DC components from the AC electrical quantities of the AC system 2 (e.g., the sum current Iac0, the sum voltage Vac0, the AC current Iac of each phase, and the AC voltage Vsys of each phase), and calculates absolute values ​​of the electrical quantities after filtering (e.g., the current absolute value |Iac0f|, the voltage absolute value |Vsys0|).

[0105] When the absolute value is equal to or greater than the threshold, the correction voltage generator 50B generates the correction voltage command values ​​Vds* and Vqs* by adding the correction values ​​Vdcmp and Vqcmp to the reference voltage command values ​​Vd0* and Vq0*. On the other hand, when the absolute value is less than the threshold, the correction voltage generator 50B generates the reference voltage command values ​​Vd0* and Vq0* as the correction voltage command values ​​Vds* and Vqs*.

[0106] <Advantages> According to the third embodiment, when the DC component and the component in a frequency band close to the DC component of the AC system 2 are large, a correction is made using a correction value to suppress bias magnetization of the reactor element, but when the DC component and the component in a frequency band close to the DC component are small, the correction is not made using the correction value. Therefore, the DC component and the low-frequency component are appropriately suppressed while preventing an operation that unnecessarily fluctuates the AC voltage command value, and the power converter 6 can continue to operate stably.

[0107] Fourth Embodiment In a modular multilevel conversion power converter 6, the capacitor voltage fluctuations of the converter cells 1 are large, so the harmonic components output by each converter cell 1 are not completely eliminated, and DC components remain, which may appear in the AC current. When the AC current contains a fundamental component and a DC component, an imbalance in the capacitor voltage occurs between the positive arm and the negative arm in a double star MMC such as that shown in Figure 1.

[0108] Therefore, in the fourth embodiment, a configuration will be described in which it is determined whether or not to correct the reference voltage command values ​​Vd0*, Vq0* using the correction values ​​Vdcmp, Vqcmp according to the positive / negative imbalance of the capacitor voltage.

[0109] Fig. 10 is a diagram showing an example of a functional configuration of a control device 5C according to embodiment 4. Referring to Fig. 10, control device 5C differs from control device 5 of Fig. 4 in that correction voltage generation unit 50 is replaced with a correction voltage generation unit 50C and in that a capacitor voltage average value calculation unit 60 is added.

[0110] The capacitor voltage average value calculation unit 60 calculates the average value Vcapk_m (where k = p, n, and m = u, v, w) of the capacitor voltage of each converter cell 1 included in each arm (i.e., the positive arms 13u to 13w and the negative arms 14u to 14w).

[0111] The correction voltage generating unit 50C receives as input the reference voltage command values ​​Vd0*, Vq0*, the d-axis currents Id, Iq, and the average value Vcapk_m.

[0112] Fig. 11 is a diagram showing an example of the configuration of correction voltage generation unit 50C according to embodiment 4. Referring to Fig. 11, correction voltage generation unit 50C includes the components of correction voltage generation unit 50 in Fig. 6, output control units 311 and 312 described in Fig. 9, subtractors 401 to 403, filters 404 to 406, absolute value calculation units 407 to 409, a minimum value selection unit 410, and a comparison unit 420.

[0113] The subtractor 401 calculates the deviation ΔVcap_u (here, Vcapp_u-Vcapn_u) between the average value Vcapp_u of the capacitor voltage Vcap of each converter cell 1 included in the positive arm 13u and the average value Vcapn_u of the capacitor voltage Vcap of each converter cell 1 included in the negative arm 14u.

[0114] The filter 404 generates the deviation ΔVcap_uf by filtering the deviation ΔVcap_u. For example, the filter 404 is a low-pass filter, a moving average filter, or the like. The filter 404 is configured to extract at least the DC component that causes the bias magnetization of the transformer 3. Specifically, the filter 404 is configured to extract the DC component and components in a frequency band close to the DC component. In this case, the deviation ΔVcap_uf includes the DC component and components in a frequency band close to the DC component. The absolute value calculation unit 407 calculates the absolute value |ΔVcap_uf| of the deviation ΔVcap_uf.

[0115] Similarly, for the V phase, the absolute value |ΔVcap_vf| of the deviation ΔVcap_vf is calculated by the subtractor 402, the filter 405, and the absolute value calculation unit 408. For the W phase, the absolute value |ΔVcap_wf| of the deviation ΔVcap_wf is calculated by the subtractor 403, the filter 406, and the absolute value calculation unit 409.

[0116] The minimum value selection unit 410 selects the minimum value Vcapdif from the absolute values ​​|ΔVcap_uf|, |ΔVcap_vf|, and |ΔVcap_wf|. The comparison unit 420 compares the minimum value Vcapdif with a threshold value Th3. If the minimum value Vcapdif is equal to or greater than the threshold value Th3, the comparison unit 420 outputs a signal sw having a value of "1" to the output control units 311 and 312. If the minimum value Vcapdif is less than the threshold value Th3, the comparison unit 420 outputs a signal sw having a value of "0" to the output control units 311 and 312. The threshold value Th3 may be determined based on, for example, an error of the voltage detector 33 that detects the capacitor voltage, or may be determined based on the rated value of the capacitor voltage.

[0117] The processes of the filters 151 and 152, the subtractors 153 and 154, and the proportional converters 155 and 156 are the same as those described in Fig. 6. The processes of the output control units 311 and 312 are the same as those described in Fig. 9.

[0118] Therefore, when the minimum value Vcapdif is equal to or greater than the threshold value Th3, the correction values ​​Vdcmp and Vqcmp are added to the reference voltage command values ​​Vd0* and Vq0* to generate the corrected voltage command values ​​Vds* and Vqs*. On the other hand, when the minimum value Vcapdif is less than the threshold value Th3, the correction is not performed, and the reference voltage command values ​​Vd0* and Vq0* are generated as the corrected voltage command values ​​Vds* and Vqs*.

[0119] In the fourth embodiment as well, a filter may be applied to the αβ-axis currents Iα and Iβ, and the correction values ​​Vdcmp and Vqcmp may be generated using the dq-axis voltages Vd and Vq instead of the dq-axis currents Id and Iq.

[0120] According to the above, the correction voltage generating unit 50C performs filtering to extract the DC component from the deviation (e.g., deviation ΔVcap_u) between the voltage average value (e.g., average value Vcapp_u) of each storage element (e.g., capacitor 32) included in the positive arm (e.g., positive arm 13u of the U phase) and the voltage average value (e.g., average value Vcapn_u) of each storage element included in the negative arm (e.g., negative arm 14u of the U phase), and calculates the absolute value of the deviation after filtering (e.g., absolute value |ΔVcap_uf|).

[0121] If the absolute value is equal to or greater than the threshold value Th3 (i.e., if the imbalance in capacitor voltage between the positive and negative arms is large), the correction voltage generator 50C generates the correction voltage command values ​​Vds* and Vqs* by adding the correction values ​​Vdcmp and Vqcmp to the reference voltage command values ​​Vd0* and Vq0*. On the other hand, if the absolute value is less than the threshold value Th3 (i.e., if the imbalance is small), the correction voltage generator 50C generates the reference voltage command values ​​Vd0* and Vq0* as the correction voltage command values ​​Vds* and Vqs*.

[0122] Advantages According to the fourth embodiment, when the imbalance in the capacitor voltage between the positive arm and the negative arm is large, a correction is made using a correction value to suppress bias magnetization of the reactor element, but when the imbalance is small, no correction is made using a correction value. Therefore, the DC component and low-frequency component are appropriately suppressed while preventing operations that unnecessarily fluctuate the AC voltage command value, and the power converter 6 can continue to operate stably.

[0123] <Advantages> Other Embodiments (1) In the above-described embodiment, the control device 5 including the active power control unit 44 and the reactive power control unit 47 generates the reference voltage command values ​​Vd0*, Vq0* and the reference phase θc. However, the present invention is not limited to this configuration. For example, instead of providing these components, the control device 5 may generate the reference voltage command values ​​Vd0*, Vq0* and the reference phase θc by executing well-known virtual synchronous generator control.

[0124] (2) The configurations exemplified as the above-described embodiments are examples of the configurations of the present disclosure, and may be combined with other known technologies, or may be modified, such as by omitting some parts, within the scope of the gist of the present disclosure. Furthermore, the above-described embodiments may be implemented by appropriately adopting the processes and configurations described in other embodiments.

[0125] The embodiments disclosed herein should be considered to be illustrative in all respects and not restrictive. The scope of the present disclosure is defined by the claims, not by the above description, and is intended to include all modifications within the meaning and scope of the claims.

[0126] REFERENCE SIGNS LIST 1 Converter cell, 2 AC system, 3 Transformer, 4 DC circuit, 5, 5B, 5C Control device, 6 Power converter, 7a, 7b Reactor, 8u, 8v, 8w Leg circuit, 9a, 9b Arm current detector, 10 AC voltage detector, 11a, 11b DC voltage detector, 13u to 13w Positive side arm, 14u to 14w Negative side arm, 15 AC current detector, 22 Frequency detection unit, 23 AC power calculation unit, 24 AC current calculation unit, 31n1, 31n2, 31n, 31p2, 31p1, 31p Switching element, 32 Capacitor, 33 Voltage detector, 41 Capacitor voltage control unit, 42 DC current control unit, 43 Frequency control unit, 44 Active power control unit, 45 Phase generation unit, 47 Reactive power control unit, 48 AC voltage control unit, 50, 50A, 50B, 50C correction voltage generation unit, 52 voltage command generation unit, 54 balance control unit, 55 circulating current control unit, 56 carrier generation unit, 57 PWM control unit, 60 capacitor voltage average value calculation unit, 70 input converter, 71 sample hold circuit, 72 multiplexer, 73 A / D converter, 74 CPU, 75 RAM, 76 ROM, 77 input / output interface, 78 auxiliary storage device, 79 bus, 100 power conversion device, 306, 420 comparison unit, 311, 312 output control unit, 410 minimum value selection unit.

Claims

1. A power conversion device comprising: a power converter connected to an AC system via a reactor element and performing power conversion between the AC system and a DC circuit; and a control device that controls a voltage output from the power converter, wherein the control device includes: an AC voltage control unit that generates a reference voltage command value for an output AC voltage of the power converter; a phase generation unit that generates a reference phase for the output AC voltage of the power converter; a correction voltage generation unit that generates a correction voltage command value by correcting the reference voltage command value using a correction value for suppressing overcurrent due to biased magnetism of the reactor element; and an AC voltage command generation unit that generates a command value for the output AC voltage of the power converter based on the correction voltage command value and the reference phase.

2. The power conversion device according to claim 1, wherein the correction voltage generation unit calculates the correction value so as to remove a DC component from the AC electricity quantity of the AC system, and generates the correction voltage command value based on the reference voltage command value and the correction value.

3. The power conversion device according to claim 2, wherein the correction voltage generation unit generates second dq-axis components by performing a first filter process to extract fundamental wave components from first dq-axis components, which are rotational coordinate axis components of the AC electrical quantity of the AC system; generates the dq-axis components of the correction value by feedback control based on a dq-axis component command value and the second dq-axis components; and generates the dq-axis components of the correction value by adding the dq-axis components of the correction value to the dq-axis components of the reference voltage command value.

4. The power conversion device according to claim 2, wherein the correction voltage generation unit generates a second αβ-axis component by performing a second filter process for extracting a DC component from a first αβ-axis component, which is a stationary coordinate axis component of the AC electricity quantity of the AC system; generates a second dq-axis component by coordinate transforming the second αβ-axis component; generates the dq-axis component of the correction value by feedback control based on a dq-axis component command value and the second dq-axis component; and generates the dq-axis component of the correction value by adding the dq-axis component of the correction value to the dq-axis component of the reference voltage command value.

5. A power conversion device according to any one of claims 2 to 4, wherein the correction voltage generation unit performs filtering on the AC electrical quantity to extract a DC component, calculates a first absolute value of the electrical quantity after the filtering process, and, if the first absolute value is equal to or greater than a first threshold value, generates the correction voltage command value by adding the correction value to the reference voltage command value.

6. The power conversion device according to claim 5, wherein when the first absolute value is less than the first threshold value, the correction voltage generation unit generates the reference voltage command value as the correction voltage command value.

7. The power conversion device according to any one of claims 2 to 4, wherein the power converter includes a plurality of arms for each phase of the AC system, each of the plurality of arms having a plurality of converter cells cascaded to one another, each of the plurality of converter cells having a plurality of switching elements and a storage element connected to the plurality of switching elements, the plurality of arms being constituted by a positive side arm and a negative side arm, and the correction voltage generation unit performs filtering to extract a DC component on a deviation between an average voltage value of each of the storage elements included in the positive side arm and an average voltage value of each of the storage elements included in the negative side arm, and calculates a second absolute value of the deviation after the filtering, and generates the correction voltage command value by adding the correction value to the reference voltage command value when the second absolute value is equal to or greater than a second threshold.

8. The power conversion device according to claim 7, wherein, when the second absolute value is less than the second threshold value, the correction voltage generation unit generates the reference voltage command value as the correction voltage command value.

9. A power conversion device according to any one of claims 1 to 8, wherein the AC voltage control unit generates the reference voltage command value based on a reactive power detection value of the AC system, a reactive power command value, and an AC voltage detection value of the AC system, and the phase generation unit generates the reference phase based on an active power detection value of the AC system, an active power command value, and a system angular frequency of the AC system.

10. The power conversion device according to any one of claims 1 to 9, wherein the power converter is a modular multilevel conversion type power converter.

11. The power conversion device according to any one of claims 1 to 10, wherein the reactor element is a transformer.

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