Power conversion device and control method thereof

Abstract

To reduce capacitor voltage fluctuations.SOLUTION: A power conversion apparatus includes a delta connection unit in which one bridge cell or a plurality of bridge cells connected in series are delta-connected, and a control unit controlling cyclic current flowing through the delta connection unit by the plurality of bridge cells. The bridge cells each have: a plurality of semiconductor switch groups each including a plurality of semiconductor switches connected in series; and a capacitor connected in parallel to the plurality of semiconductor switch groups. The control unit uses, to calculate the command value of the cyclic current, a positive phase dq-axis component (vd+, vq+) of power system voltage, the delta connection unit being connected to a power system, a negative phase dq-axis component (vd-, vq-) of the power system voltage, a positive phase dq-axis component (id+, iq+) of current flowing in each phase of the delta connection unit, and a negative phase dq-axis component (id-, iq-) of current flowing in each phase of the delta connection unit.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] Modular multilevel converters (MMCs) are next-generation transformerless power converters suitable for large-capacity, high-voltage applications. MMCs are applicable to, for example, static synchronous compensators (STATCOMs) and high-voltage direct current (HVDC) systems. Delta-connected MMCs, in particular, are attracting attention because they can output a negative-phase reactive current by passing a circulating current through the delta connection. Delta-connected MMCs have an internal delta connection, and one or multiple bridge cells connected in series are provided on each phase of the delta connection (see, for example, Patent Document 1).

[0003] The bridge cell has a plurality of semiconductor switch groups each including a plurality of semiconductor switches connected in series, and a DC capacitor connected in parallel to the plurality of semiconductor switch groups. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 5800154 Summary of the Invention [Problem to be solved by the invention]

[0005] In an MMC, for example, if an unbalanced fault occurs in the power system, non-zero active power will flow steadily into each phase, which may cause fluctuations in the voltage of the DC capacitor. As a countermeasure, a circulating current (zero-phase current) may be passed through the delta connection to maintain the orthogonal relationship between the AC voltage and AC current of each phase, thereby maintaining the voltage balance of the DC capacitor.

[0006] Conventional methods, including those described in Patent Document 1, calculate circulating current commands based on the error between the average voltage of all three-phase DC capacitors and the average voltage of each phase capacitor. However, because the calculation method is based on the DC capacitor voltage error, if an unbalanced fault occurs in the power system, the circulating current may respond slowly to the fault, causing the voltage of each phase DC capacitor to fluctuate significantly. If an overvoltage occurs due to the voltage fluctuation of the DC capacitor, the protection circuit may shut down the operation of the power conversion device.

[0007] The present disclosure provides a power conversion device and a control method thereof that can reduce voltage fluctuations in a capacitor. [Means for solving the problem]

[0008] In one aspect of the present disclosure, a delta connection section in which one or a plurality of series-connected bridge cells are delta-connected; a control device that controls a circulating current flowing in the delta connection by the plurality of bridge cells, The bridge cell includes a plurality of semiconductor switch groups each including a plurality of semiconductor switches connected in series, and a capacitor connected in parallel to the plurality of semiconductor switch groups, The control device calculates the command value of the circulating current by using the positive-phase dq-axis component (v d + ,v q + ), the negative-phase dq-axis component of the voltage of the power system (v d - ,v q - ), the positive-phase dq-axis component of the current flowing through each phase of the delta connection (i d + ,i q + ) and the negative-phase dq-axis components of the current flowing through each phase of the delta connection (i d - ,i q - ) is used.

[0009] In another aspect of the present disclosure, A control method for a power conversion device including a delta connection portion in which one or a plurality of series-connected bridge cells are delta-connected, the control method controlling a circulating current flowing through the delta connection portion by the plurality of bridge cells, The bridge cell includes a plurality of semiconductor switch groups each including a plurality of semiconductor switches connected in series, and a capacitor connected in parallel to the plurality of semiconductor switch groups, The calculation of the command value of the circulating current is performed using the positive-phase dq-axis component (v d + ,v q + ), the negative-phase dq-axis component of the voltage of the power system (v d - ,v q - ), the positive-phase dq-axis component of the current flowing through each phase of the delta connection (i d + ,i q + ) and the negative-phase dq-axis components of the current flowing through each phase of the delta connection (i d - ,i q - ) is used to provide a method for controlling a power converter. [Effects of the Invention]

[0010] According to the present disclosure, it is possible to reduce voltage fluctuations in a capacitor. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a diagram illustrating a configuration example of a power conversion device according to an embodiment; [Figure 2] FIG. 2 is a diagram illustrating an example of the configuration of a control device. [Figure 3] FIG. 2 is a diagram illustrating an example of the configuration of a circulating current command unit. [Figure 4] FIG. 10 is a diagram showing an example of a simulation waveform at the time of a system unbalance fault when a conventional method is used for circulating current command calculation. [Figure 5] FIG. 10 is a diagram showing an example of a simulation waveform at the time of a system unbalance fault when the method disclosed herein is used for circulating current command calculation. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings.

[0013] FIG. 1 is a diagram showing an example of the configuration of a power conversion device according to an embodiment, illustrating an example of the circuit configuration of a delta-connected MMC. The MMC is applicable to, for example, a static synchronous compensator (STATCOM) and a high-voltage direct current (HVDC) transmission system. The power conversion device 400 shown in FIG. 1 includes a delta-connected unit 402 and a control device 401.

[0014] The delta connection unit 402 is a circuit in which one or a plurality of series-connected bridge cells are delta-connected. The delta connection unit 402 includes a plurality of clusters 50 (50UV, 50VW, 50WU) and a plurality of reactors 51 (51UV, 51VW, 51WU).

[0015] The UV phase cluster 50UV includes a plurality of bridge cells 52UV1, 52UV2, . . . 52UV connected in series via a pair of AC output terminals a and b. n 1 illustrates three bridge cells 52UV1, 52UV2, and 52UV3. The VW-phase cluster 50VW includes a plurality of bridge cells 52VW1, 52VW2, . . . 52VW connected in series via a pair of AC output terminals a and b. n 1 illustrates three bridge cells 52VW1, 52VW2, and 52VW3. The WU-phase cluster 50WU includes a plurality of bridge cells 52WU1, 52WU2, . . . 52WU connected in series via a pair of AC output terminals a and b. nFIG. 1 illustrates three bridge cells 52WU1, 52WU2, and 52WU3. n represents the number of bridge cells connected in series in each cluster and is an integer equal to or greater than 1. In other words, the number of bridge cells in one cluster may be as small as one. The UV phase, VW phase, and WU phase may also be simply referred to as the U phase, V phase, and W phase, respectively.

[0016] Multiple bridge cells 52 (52UV1 to 52UV n ,52VW1~52VW n ,52WU1~52WU n ) each have a pair of AC output terminals a, b and are connected in series via the pair of AC output terminals a, b. Each of the plurality of bridge cells 52 has its first AC output terminal a connected to the second AC output terminal b of one of the bridge cells adjacent to it, and its second AC output terminal b connected to the first AC output terminal a of the other bridge cell adjacent to it.

[0017] Cluster 50UV, cluster 50VW, and cluster 50WU are delta-connected via reactors 51UV, 51VW, and 51WU, and are connected to the power grid 300. The connection to the power grid 300 may be via a transformer (not shown). A circulating current flows within the delta connection. The control device 401 can adjust the negative-phase reactive current by controlling the circulating current flowing in the delta connection unit 402 by switching multiple bridge cells 52.

[0018] Each of the bridge cells 52 includes a power conversion circuit having a plurality of switching elements and a drive circuit unit that operates the power conversion circuit. The bridge cells 52 have the same configuration. The switching elements are, for example, semiconductor switches including a transistor and a diode connected in antiparallel to the transistor. Specific examples of transistors include an IGBT (Insulated Gate Bipolar Transistor) and a MOSFET (Metal Oxide Semiconductor Field Effect Transistor).

[0019] Each bridge cell 52 has the function of converting DC power in the capacitor 54 into AC power and outputting it to a pair of AC output terminals a and b, and the function of converting AC power input from the pair of AC output terminals a and b into DC power and supplying it to the capacitor 54.

[0020] Each bridge cell 52 includes a pair of AC output terminals a and b, a capacitor 54, a power conversion circuit 53, and a drive circuit unit (not shown) (for example, a GDU (Gate Drive Unit) and a power supply circuit).

[0021] The capacitor 54 is a capacitance element connected to the pair of AC output terminals a and b via the power conversion circuit 53.

[0022] The power conversion circuit 53 is an inverter circuit connected between the capacitor 54 and a pair of AC output terminals a and b, and converts power bidirectionally between DC and AC. The power conversion circuit 53 is connected in parallel to the capacitor 54. FIG. 1 illustrates a full-bridge circuit having a plurality of switching elements 56.

[0023] The power conversion circuit 53 has a plurality of semiconductor switch groups 55, each including a plurality of semiconductor switches connected in series. The plurality of semiconductor switch groups 55 are connected in parallel to a capacitor 54. The power conversion circuit 53 shown in FIG. 1 has a full-bridge configuration in which a first semiconductor switch group including a plurality of switching elements 56 connected in series and a second semiconductor switch group including a plurality of switching elements 56 connected in series are connected in parallel. A first AC output terminal a is connected to a connection point between the switching element 56 of the first upper arm and the switching element 56 of the first lower arm. A second AC output terminal b is connected to a connection point between the switching element 56 of the second upper arm and the switching element 56 of the second lower arm.

[0024] The plurality of switching elements 56 illustrated in FIG. 1 are IGBTs with diodes connected in antiparallel, but may be switching elements with a switching function such as MOSFETs or thyristors.

[0025] At least one of the switching element and the anti-parallel diode is preferably an element containing a wide bandgap semiconductor such as SiC (silicon carbide), GaN (gallium nitride), Ga2O3 (gallium oxide), or diamond. By using a wide bandgap semiconductor as the switching element, the effect of reducing loss in the switching element is enhanced. The switching element may also be an element containing a semiconductor such as Si (silicon). Similarly, by using an element containing a wide bandgap semiconductor as the diode, the effect of reducing loss in the diode is enhanced. The diode may also be an element containing a semiconductor such as Si (silicon).

[0026] Each bridge cell 52 includes a drive circuit unit (not shown) such as a GDU and a power supply circuit.

[0027] The GDU is a drive circuit that drives the power conversion circuit 53, and more specifically, a gate drive circuit that drives the gates of the multiple switching elements 56 that are configured in the power conversion circuit 53. The GDU drives the multiple switching elements 56 that are configured in the power conversion circuit 53 based on power supplied from the capacitor 54 via the power supply circuit.

[0028] The GDU turns on or off a corresponding switching element among the multiple switching elements 56 by applying a voltage between the gate and emitter of the corresponding switching element in accordance with a control signal from the control device 401. This operation generates a square-wave voltage between a pair of AC output terminals a and b of the bridge cell 52.

[0029] The control device 401 is a controller that generates control signals (e.g., PWM signals (pulse width modulated signals)) that turn on or off the multiple switching elements 56 in accordance with a carrier period Tc (the reciprocal of the carrier frequency) common to the multiple bridge cells 52. The control device 401 has a memory and a processor (e.g., a CPU (Central Processing Unit)). Each function of the control device 401 is realized by the processor operating according to a program stored in the memory. The functions of the control device 401 may be realized by an FPGA (Field Programmable Gate Array) or an ASIC (Application Specific Integrated Circuit).

[0030] The power conversion device 400 can output a multilevel voltage waveform having a voltage equal to or higher than the withstand voltage of the switching elements and with reduced harmonics by causing the control device 401 to output voltage waveforms with mutually different phases from each of the multiple bridge cells 52. Therefore, the power conversion device 400 can be applied to, for example, a reactive power compensator or a DC power transmission system directly connected to an extra-high voltage grid.

[0031] Furthermore, in the MMC, if an unbalanced fault occurs in the power system 300, for example, a steadily non-zero active power may flow into each phase, causing fluctuations in the voltage of the capacitor 54. As a countermeasure, the control device 401 of the power conversion device 400 controls the circulating current (zero-phase current) flowing through the delta connection 402, thereby maintaining the orthogonal relationship between the AC voltage and AC current of each phase and maintaining the balance of the voltage of the capacitor 54.

[0032] Fig. 2 is a diagram showing an example of the configuration of a control device. The control device 401 shown in Fig. 2 includes a circulating current command unit 21, a circulating current control unit 22, a U-phase balance control unit 23-u, a V-phase balance control unit 23-v, a W-phase balance control unit 23-w, a power control unit 24, and a voltage command value generation unit 25. Each unit of the control device 401 is realized by an arithmetic processing device such as a DSP or FPGA. In this example, the number of bridge cells is set to three, for example, so n=1, 2, and 3 in Fig. 2.

[0033] The circulating current command unit 21 outputs a circulating current command value i0 as a command for the circulating current to be passed through the delta connection unit 402. * The circulating current control unit 22, the U-phase balance control unit 23-u, the V-phase balance control unit 23-v, the W-phase balance control unit 23-w, the power control unit 24, and the voltage command value generation unit 25 may have the same configuration as that disclosed in the above-mentioned Patent Document 1.

[0034] Here, in FIG. 1, the line voltage v of the power system 300 S (v Suv ,v Svw ,v Swu ) is the current i flowing from the MMC to the power system 300. u ,i v ,i w ) is the current (inverter current) flowing through each phase of the delta connection 402. uv ,i vw ,i wu ) The voltage of the capacitor 54 in each bridge cell 52 is (v Cnu ,v Cnv ,v Cnw ) In this example, n is 1, 2, and 3.

[0035] The current (i u ,i v ,i w ) and the current (i uv ,i vw ,i wu ) and the relationship shown in Equations 2 to 4 holds.

[0036]

number

[0037]

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[0038]

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[0039]

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[0040] The positive-phase dq-axis components of the voltage (system voltage) of the power system 300 to which the delta connection 402 is connected are expressed as v d + ,v q + , the negative-phase dq-axis components of the system voltage are v d - ,v q - , where j is the imaginary unit, the three-phase system voltage (v Suv ,v Svw ,v Swu )teeth,

[0041]

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[0042]

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[0043]

number

[0044] The positive-phase dq-axis components of the inverter current are expressed as i d + ,i q + , the negative-phase dq-axis components of the inverter current are i d - ,i q - In order to suppress DC voltage fluctuations of the MMC (voltage fluctuations of the capacitor 54), the circulating current that should flow through the delta connection 402 is set as follows:

[0045]

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[0046]

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[0047]

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[0048]

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[0049] By circulating the zero-phase current (circulating current i0) in the delta connection 402, the orthogonal relationship between the vector of the system voltage and the vector of the inverter current is maintained. The condition under which the vector of the system voltage and the vector of the inverter current are orthogonal is as follows:

[0050]

number

[0051]

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[0052]

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[0053] From Equation 13,

[0054]

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[0055] From Equation 14,

[0056]

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[0057] From Equation 15,

[0058]

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[0059] From equations 16 to 18, equations 19 and 20 can be obtained.

[0060]

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[0061]

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[0062]

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[0063] 3 is a diagram showing an example of the configuration of the circulating current command unit. The circulating current command unit 21 uses a 3-phase / 2-phase conversion block to convert the system voltage (v Suv ,v Svw ,v Swu ) is converted from three-phase to two-phase. Suv ,v Svw ,v Swu ) is converted into a positive-phase dq-axis voltage (v d + ,v q + Similarly, the circulating current command unit 21 generates the system voltage (v Suv ,v Svw ,v Swu ) is converted into a 3-phase / 2-phase voltage by the dq-axis voltage converter block, and then the moving average is calculated by the moving average block. d - ,v q - On the other hand, the circulating current command unit 21 generates the inverter current (i uv ,i vw ,i wu The circulating current command unit 21 performs three-phase / two-phase conversion of the inverter current (i uv ,i vw ,i wu ) is converted into a positive-phase dq-axis current (i d + ,i q + Similarly, the circulating current command unit 21 generates the inverter current (i uv ,i vw ,i wuThe result of the three-phase / two-phase transformation of the negative-phase dq axis current (i d - ,i q - )

[0064] The circulating current command unit 21 calculates the positive-phase dq-axis voltage (v d + ,v q + , ), negative-phase dq-axis voltage (v d - ,v q - ), positive-sequence dq-axis current (i d + ,i q + ) and negative-phase dq-axis current (i d - ,i q - ) and calculate the circulating current command value i0 based on Equation 21. * Calculate the following.

[0065] The circulating current i0 is calculated by Equation 5, and the circulating current control unit 22 shown in FIG. 2 calculates this circulating current i0 as a circulating current command value i0 * That is, the circulating current control unit 22 forms a feedback loop that makes the circulating current command value i0 * A first command value v common to each phase is used to control the switching operation of the switching element 56 so that the circulating current i0 follows the A * The configuration of the circulating current control unit 22 may be the same as that shown in FIG. 4 of the above-mentioned Patent Document 1.

[0066] The configurations of the U-phase balance control unit 23-u, V-phase balance control unit 23-v, and W-phase balance control unit 23-w shown in Fig. 2 may be the same as those shown in Fig. 5 of Patent Document 1. Balancing control is performed for each phase and for each bridge cell within that phase. In this example, the number of bridge cells is set to three, so n=1, 2, and 3 in Fig. 2.

[0067] The U-phase balance control unit 23-u, the V-phase balance control unit 23-v, and the W-phase balance control unit 23-w control the output voltage (v Cnu ,v Cnv ,v Cnw ) and the inverter current (i uv ,i vw ,i wu ) to achieve voltage balance by generating effective power between them.

[0068] That is, for each bridge cell 52, the balance control unit for each phase calculates a second command value v for controlling the switching operation of the switching element 56 in the bridge cell by using a value obtained by multiplying the deviation between the DC capacitor average value for that phase and the voltage value of the DC capacitor in that bridge cell by the value of the AC current flowing into that phase. Bnu * , v Bnv * , v Bnw * Generate.

[0069] Specifically, the U-phase balance control unit 23-u calculates the U-phase capacitor voltage average value v Cuave and the voltage v of the U-phase capacitor 54 Cnu and the inverter current i flowing into the U phase. uv The second command value v for the U phase is obtained by multiplying the value of Bnu * The average U-phase capacitor voltage v Cuave is a value obtained by averaging the voltage values ​​of the U-phase capacitor 54.

[0070] The V-phase balance control unit 23-v calculates the V-phase capacitor voltage average value v Cvave and the voltage V of the V-phase capacitor 54 Cnv and the inverter current i flowing into the V phase. vw The second command value v for the V phase is obtained by multiplying the value of Bnv * The V-phase capacitor voltage average value v Cvave is a value obtained by averaging the voltage values ​​of the V-phase capacitor 54.

[0071] The W-phase balance control unit 23-w calculates the W-phase capacitor voltage average value v Cwave and the voltage v of the W-phase capacitor 54 Cnw and the inverter current i flowing into the W phase. wu The second command value v for the W phase is obtained by multiplying the value of Bnw * The W-phase capacitor voltage average value v Cwave is a value obtained by averaging the voltage values ​​of the W-phase capacitor 54.

[0072] The configuration of the power control unit 24 shown in Fig. 2 may be the same as that shown in Fig. 6 of the above-mentioned Patent Document 1. The power control unit 24 controls a third command value v uv * , v vw * , v wu * The block diagram shown in FIG. 6 of the above-mentioned Patent Document 1 is a general control block for positive-sequence reactive power control, negative-sequence reactive power control, and active power control for a static static compensation (STATCOM). * is the instantaneous active power command value on the power source side, q * represents the instantaneous reactive power command value on the power source side. Cave is the value obtained by averaging the voltage values ​​of all three phase capacitors 54, v C *represents the voltage command value of the capacitor 54. i represents the current flowing from the delta connection 402 to the power grid 300, v S represents the line voltage of the power system 300.

[0073] The first command value v generated by the circulating current command unit 21 and the circulating current control unit 22 A * , the second command value v generated by the balance control unit of each phase Bnu * , v Bnv * and v Bnw * , and the third command value v generated by the power control unit 24 uv * , v vw * and v wu * is input to the voltage command value generating unit 25.

[0074] The configuration of the voltage command value generator 25 shown in FIG. 2 may be the same as that shown in FIG. 7 of the above-mentioned Patent Document 1. The voltage command value generator 25 functions as a switching command value generator for generating a switching command value that controls the switching operation of the switching element 56 in the bridge cell 52 of each phase. The voltage command value generator 25 generates a first command value v A * and the second command value v Bnu * and the third command value v uv * The voltage command value v of the bridge cell 52 of the U phase is calculated by dividing by the number of bridge cells (3 in this example) and adding it. nu * The voltage command value generator 25 generates the first command value v A * and the second command value v Bnv * and the third command value v vw * The voltage command value v of the V-phase bridge cell 52 is calculated by dividing by the number of bridge cells (3 in this example) and adding it to the voltage command value v nv * The voltage command value generator 25 generates the first command value vA * and the second command value v Bnw * and the third command value v wu * The voltage command value v of the W-phase bridge cell 52 is calculated by dividing by the number of bridge cells (3 in this example) and adding it to the nw * Generate.

[0075] The generated voltage command value v nu * , v nv * and v nw * is normalized by the voltage of the capacitor 54 of each phase and used as a switching command value. This switching command value is compared by a switching control means (not shown) with a triangular wave carrier signal (maximum value: 1, minimum value: -1) of carrier frequency fc to generate a PWM switching signal. This PWM switching signal is generated using a processing device such as a DSP or FPGA. The generated PWM switching signal is used to control the switching of the switching element 56 in the corresponding bridge cell.

[0076] FIG. 4 shows an example of a simulated waveform at the time of a grid unbalance fault when a conventional method (Patent Document 1) is used for calculating a circulating current command. The simulation conditions are as follows: System unbalance fault: U phase ground fault U phase residual voltage: 50% U phase ground fault period: 150ms MMC operation mode: Rated capacitor operation When a U-phase ground fault occurs at t=1 s, the voltage of capacitor 54 fluctuates by up to 30% in the conventional method.

[0077] Figure 5 shows an example of a simulated waveform during a grid unbalance fault when the disclosed method is used for circulating current command calculation. The simulation conditions are the same as those in Figure 4. When a U-phase ground fault occurs at t = 1 s, the disclosed method suppresses the voltage of capacitor 54 to a maximum of 9%.

[0078] In this way, the positive-phase dq-axis component (v d + ,v q + ), and the reverse-phase dq-axis components (v d - ,v q - ), positive phase dq axis component (i d + ,i q + ) and antiphase dq axis components (i d - ,i q - ) is used to calculate the command value of the circulating current, so that, for example, if an unbalanced fault occurs in the power grid, the circulating current responds quickly to the fault. As a result, compared to conventional methods, the voltage fluctuation of the capacitor can be reduced. In addition, for example, the increase in the capacitance of the capacitor can be suppressed, allowing for the miniaturization and cost reduction of the power conversion device.

[0079] Although the embodiments have been described above, the technology of the present disclosure is not limited to the above-described embodiments, and various modifications and improvements are possible, such as combinations with or substitutions for part or all of other embodiments. [Explanation of symbols]

[0080] 50, 50UV, 50VW, 50WU cluster 51, 51UV, 51VW, 51WU reactor 52UV1, 52UV2, 52UV n Bridge Cell 52VW1, 52VW2, 52VW n Bridge Cell 52WU1, 52WU2, 52WU n Bridge Cell 53 Power Conversion Circuit 54 Capacitor 55 Semiconductor switches 56 Switching element 300 Power system 400 Power Converter 401 Control device 402 Delta connection 410 Converter control section 420 Current command section a,b AC output terminal

Claims

1. a delta connection section in which one or a plurality of series-connected bridge cells are delta-connected; a control device that controls a circulating current flowing in the delta connection by the plurality of bridge cells, The bridge cell includes a plurality of semiconductor switch groups each including a plurality of semiconductor switches connected in series, and a capacitor connected in parallel to the plurality of semiconductor switch groups, The control device calculates the command value of the circulating current by using the positive-phase dq-axis component (v ｄ ＋ , v ｑ ＋ ), the negative-phase dq-axis component of the voltage of the power system (v ｄ － , v ｑ － ), the positive-phase dq-axis component of the current flowing through each phase of the delta connection (i ｄ ＋ , i ｑ ＋ ) and the negative-phase dq-axis components (i ｄ － , i ｑ － ) is used, The command value i 0 * of the circulating current is expressed as follows, where j is the imaginary unit. [Equation 1] A power conversion device represented by

2. A control method for a power conversion device including a delta connection portion in which one or a plurality of series-connected bridge cells are delta-connected, the control method controlling a circulating current flowing through the delta connection portion by the plurality of bridge cells, The bridge cell includes a plurality of semiconductor switch groups each including a plurality of semiconductor switches connected in series, and a capacitor connected in parallel to the plurality of semiconductor switch groups, The calculation of the command value of the circulating current is performed by using the positive-phase dq-axis component (v ｄ ＋ , v ｑ ＋ ), the negative-phase dq-axis component of the voltage of the power system (v ｄ － , v ｑ － ), the positive-phase dq-axis component of the current flowing through each phase of the delta connection (i ｄ ＋ , i ｑ ＋ ) and the negative-phase dq-axis components (i ｄ － , i ｑ － ) and The command value i 0 * of the circulating current is expressed as follows, where j is the imaginary unit. [Equation 2] A control method for a power conversion device represented by the formula:

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