Power conversion device and its control device

Abstract

To provide a power conversion apparatus which can achieve both of reduction in time for initial charging and discharging of the power conversion apparatus and downsizing of an initial charging device, as well as a control apparatus for the same.SOLUTION: The power conversion apparatus includes: unit converters each configured by parallel-connecting a capacitor and a switching circuit formed of two sets of series-connected switching elements to which reflux diodes are parallel-connected; a power converter in which circulation current suppression reactors are series-connected to constitute an arm, two sets of the series-connected arms constitute a leg, a connection point of the two sets of arms is connected to a phase of a first AC power source so as to serve as an AC terminal, and both ends of the leg serve as DC terminals; and a charging and discharging device in which one end is connected to a second AC power source and the other end is connected to the phase of the first AC power source. The charging and discharging device includes a transformer with a tap capable of switching between a high-voltage tap position and a low-voltage tap position, and when starting and stopping the power converter, the charging and discharging device is connected to the power converter, in place of the first AC power source.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a power conversion device using a modular multilevel converter (hereinafter referred to as an MMC converter) and a control device thereof.

Background Art

[0002] The MMC converter is composed of a voltage source such as a capacitor which is an energy storage element, and an arm converter with two terminals formed by connecting a plurality of unit converters each composed of a half-bridge circuit in series. The unit converter generates a desired voltage by controlling the PWM modulation rate of the half-bridge circuit. The capacitor which is an energy storage element fluctuates in voltage by repeatedly charging and discharging in a cycle determined by the output AC frequency of the MMC converter.

[0003] The two-terminal arm converter connects the first terminal of the arm converter to each phase terminal of the AC power supply, and connects the second terminal connected in star to the terminal of the DC power supply. The arm converters connected to each phase in this way generate a desired AC voltage and control the AC current, and at the same time, superimpose a DC current to realize power conversion with the DC power supply.

[0004] The control of the MMC converter includes current control for adjusting the arm current to an external AC current command and DC current command, a function of keeping the average voltage of the capacitor balanced between the unit converters by mutually adjusting the PWM modulation rates of the half-bridge circuits provided in the unit converters within the arm (hereinafter referred to as inter-stage balance control), and a function of keeping the total stored energy of the capacitors in the arm converter balanced between the arm converters (hereinafter referred to as inter-phase balance control). To realize this inter-phase balance control, a circuit element for suppressing the circulating current between the arm converters is required.

[0005] Patent Document 1 discloses means for providing a circulating current suppression reactor between the first terminal of the arm converter and the AC power supply terminal in order to suppress the circulating current. This means disclosed in Patent Document 1 is a double-star-shaped MMC converter (hereinafter referred to as a DSMMC converter) which is a form of the MMC converter.

[0006] In addition, Non-Patent Document 1 discloses a means of connecting the DC terminals of two DSMMC converters in a back-to-back manner to form a power conversion device, connecting one AC power terminal to the power grid, and connecting the other AC power terminal to a rotating machine. According to the means disclosed in Non-Patent Document 1, since no DC current is superimposed on the rotating machine even when connected to the MMC converter, it is suitable for the case of variable-speed operation of a rotating machine directly connected to the power grid and operating at a constant frequency.

[0007] On the other hand, Patent Document 2 discloses a pre-charging device for charging the capacitor of a DSMMC converter. The DSMMC converter charges the capacitor from a pre-charging power source when starting up for the first time. Also, when the DSMMC converter is stopped due to maintenance of the power conversion device or the like, the capacitor remains charged at a high voltage. Therefore, when stopping, the capacitor charged to a high voltage is discharged in advance to make it in a safe state.

[0008] According to Patent Document 2, during the pre-charging operation, a means is disclosed for connecting a pre-charging resistor between the pre-charging power source and the DSMMC converter to pre-charge the capacitor while suppressing the inrush current. Also, during the discharging operation, a means is disclosed for discharging the energy stored in the capacitor by flowing the discharge current of the capacitor through the pre-charging resistor by combining the pre-charging resistor and a three-phase short-circuit switch and operating the DSMMC converter in that state.

Prior Art Documents

Patent Documents

[0009]

Patent Document 1

Patent Document 2

Non-Patent Documents

[0010]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0011] The present invention relates to a power conversion device composed of a DSMMC converter and its control device. In this case, the main problems to be considered and solved are as follows.

[0012] The first problem is that since it takes time for the capacitor to be initially charged and discharged, the time required for the power conversion device to start and stop becomes longer. In particular, it becomes an obstacle when applying to applications such as pumped-storage power plants and offshore wind power plants that often require start-up and stop in a short time. Patent Document 2 does not disclose a means for simultaneously initially charging and discharging two DSMMC converters. Therefore, it takes twice as long to initially charge and discharge two DSMMC converters as compared with one DSMMC converter.

[0013] The second problem is that the initial charging device becomes large-sized. The enlargement of the equipment becomes an obstacle when applying to applications with strict installation area and volume limitations, such as pumped-storage power plants and offshore wind power plants. The reason for the enlargement of the initial charging device is that the cooling device for the initial charging resistor has to be enlarged. According to Patent Document 2, when the DSMMC converter is initially charged and discharged, the capacitor is always charged and discharged through the initial charging resistor. Especially during the discharging operation, all the energy stored in the capacitor is consumed by the initial charging resistor. Therefore, when discharging in a short time, the cooling device has to be enlarged to improve the heat dissipation performance in order to avoid destruction due to the temperature rise of the resistor.

[0014] In order to reduce the energy consumption during resistance, for example, there is a means of regenerating the energy stored in the capacitor to the AC power supply to which the DSMMC converter is connected by the AC current control of the DSMMC converter. However, this means has the drawback that the regenerative operation range depends on the voltage VAC of the AC power supply to which the DSMMC converter is connected. In other words, the regenerative operation range by the AC current control is up to the capacitor voltage VC at which the modulation rate M of the DSMMC converter does not reach the overmodulation operation region.

[0015] Calculate the energy reduction amount of the initial charge resistance when the above regenerative operation by AC current control is applied. The modulation rate M of the DSMMC converter is defined by equation (1). Also, the total energy EC stored in all unit converter capacitors per DSMMC converter is defined by equation (2).

[0016]

Equation

[0017]

Equation

[0018] In equations (1) and (2), VAC is the effective value of the line-to-line voltage of the AC power supply to which the DSMMC converter is connected, k is the number of unit converters per arm converter, VC is the capacitor voltage in the unit converter, and C is the capacitance of the capacitor.

[0019] In general, since it is economical to design a power conversion circuit including a DSMMC converter with a high voltage utilization rate, the modulation rate during rated operation is designed to be about 0.8 to 0.9. Assuming that VAC, k, and C in equations (1) and (2) are constant values and the modulation rate during rated operation is 0.85, the overmodulation operation region (M ≧ 1.0) is reached when VC drops by about 17%. That is, from equation (2), the energy consumed by the initial charging resistance is about 70% by this means when the means of Patent Document 2 is 100% (all consumed by resistance). Since the reduction of energy by about 30% is the limit, the miniaturization of the initial charging device by this means is limited.

[0020] From the above, an object of the present invention is to solve the above problems and provide a power conversion device and its control device that can achieve both shortening of the initial charging and discharging times of the power conversion device and miniaturization of the initial charging device.

Means for Solving the Problems

[0021] Therefore, in the present invention, "a power conversion device including a unit converter configured by connecting in parallel a switching circuit in which two sets of switching elements connected in parallel with a circulating diode are connected in series and a capacitor, and a circulating current suppression reactor is connected in series to form an arm, two sets of arms connected in series form a leg, the connection point of the two sets of arms is connected to the phase of the first AC power supply as an AC terminal, and both ends of the leg are used as DC terminals, and a charge and discharge device having one end connected to the second AC power supply and the other end connected to the phase of the first AC power supply. The charge and discharge device includes a tapped transformer that can be switched between a high voltage tap position and a low voltage tap position, and is characterized in that it is connected to the power converter instead of the first AC power supply when the power converter starts and stops."

[0022] In the present invention, a unit converter is configured by connecting in parallel a switching circuit in which two sets of switching elements with parallel-connected circulation diodes are connected in series and a capacitor, and a circulating current suppression reactor is connected in series to form an arm. Two sets of series-connected arms form a leg. The connection points of the two sets of arms are connected to the phase of a first AC power supply, and both ends of the leg are used as DC terminals to form a power converter. A control device for the power conversion device includes a charge / discharge device with one end connected to a second AC power supply and the other end connected to the phase of the first AC power supply. The charge / discharge device includes a tapped transformer that can be switched between a high-voltage tap position and a low-voltage tap position. When the power converter starts and stops, it is connected to the power converter instead of the first AC power supply. When the power converter starts, the control device shifts from a resistance charging mode in which the second AC power supply is connected to the AC terminal of the power converter through a resistance and the high-voltage tap position of the tapped transformer, to a bypass charging mode in which the second AC power supply is connected to the AC terminal of the power converter through the tapped transformer, and then shifts to a switching charging mode in which the unit converter is arced. After that, the first AC power supply is connected to the power converter instead of the second AC power supply. The control device for the power conversion device is characterized in this way.

[0023] In the present invention, a unit converter is configured by connecting in parallel a switching circuit in which two sets of switching elements with a circulating diode connected in parallel are connected in series and a capacitor, and a circulating current suppression reactor is connected in series to form an arm. Two sets of series-connected arms form a leg. The connection point of the two sets of arms is connected to the phase of the first AC power supply, and both ends of the leg are used as DC terminals. A power converter control device includes a charge and discharge device having one end connected to the second AC power supply and the other end connected to the phase of the first AC power supply. The charge and discharge device includes a tapped transformer that can be switched between a high voltage tap position and a low voltage tap position. When the power converter starts and stops, it is connected to the power converter instead of the first AC power supply. The control device, when the power converter stops, connects to the second AC power supply via the low voltage tap position of the tapped transformer instead of the first AC power supply, and gives the gate pulse of the PWM modulation rate determined by the current control unit to the switching element to operate the power converter. It is characterized in that it shifts from the regenerative discharge mode of operating the power converter to the resistive discharge mode of operating by connecting to the second AC power supply through a resistor from the low voltage tap position of the tapped transformer.

Advantages of the Invention

[0024] According to the present invention, since all the capacitors of the power conversion device composed of the DSMMC converter can be charged and discharged simultaneously, the start-up and stop times of the power conversion device can be accelerated.

[0025] Also, according to the embodiment of the present invention, by providing a tap on the pre-charging transformer, it becomes possible to select different AC power supply voltage values during the pre-charging operation and the discharging operation. Especially during the discharging operation, by lowering the AC power supply voltage value, the regenerative operation range to the pre-charging power supply can be expanded, so the energy consumed by the resistor can be significantly reduced, and it is possible to configure a pre-charging device that can perform pre-charging and discharging at high speed without causing an increase in the size of the cooling device for dissipating heat from the resistor.

Brief Description of the Drawings

[0026]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Mode for Carrying Out the Invention

[0027] Hereinafter, a power conversion device and its control device according to an embodiment of the present invention will be described with reference to the drawings. Note that the present invention is not limited by this embodiment.

[0028] In the following description, in Embodiment 1, a main circuit configuration example of the power conversion device of the present invention will be described. In Embodiment 2, a starting control method of a power conversion device enabling high-speed initial charging will be described. In Embodiment 3, a stopping control method of a power conversion device enabling high-speed discharging will be described. In Embodiment 4, a main circuit configuration example of a power conversion device different from that of Embodiment 1 will be described.

Embodiment

[0029] FIG. 1 is a diagram showing a main circuit configuration example of a power conversion device according to Embodiment 1 of the present invention. The power conversion device includes a power converter 1 (DSMMC converters 1A and 1B in the illustrated example) and a pre-charging device 4.

[0030] In the main circuit configuration example of the power conversion device in FIG. 1, the DC terminals PA, PB and NA, NB of the two sets of DSMMC converters 1A, 1B are connected in series, and the AC terminals UA, VA, WA of the two sets of DSMMC converters 1A, 1B are connected to three-phase power supplies 2A, 2B via main circuit breakers 6A, 6B respectively, so that the power between the three-phase power supplies 2A, 2B can be interchanged, thereby showing a case where a frequency conversion system or a DC power transmission system is configured. However, the present invention is applicable to a power conversion device including at least one set of power converter 1 and pre-charging device 4. The voltage phase detection means 3 calculates the voltage phase θ from the three-phase AC voltage and outputs it to the DSMMC control device 22.

[0031] The pre-charging device 4, which is another main component of the power conversion device, functions to charge the internal capacitor of the DSMMC converter 1A when it is in a state before startup and thus the internal capacitor is uncharged. Also, when the DSMMC converter 1A is stopped, it functions to discharge the internal capacitor when it is in a charged state. In this sense, the pre-charging device 4 can be called a charge and discharge device. The terminals UP, VP, WP of the pre-charging device 4 in FIG. 1 are connected to the same lines as the AC terminals UA, VA, WA of the DSMMC converter 1A. Also, the terminals AA, BA, CA of the pre-charging device 4 are connected to the pre-charging power supply 5.

[0032] Next, the DSMMC converter, which is the power converter 1, will be described in detail taking 1A as an example. Since 1B has the same configuration as 1A, the description thereof will be omitted. The DSMMC converter 1A includes an A terminal, a B terminal, six arm converters 17UP, 17UN, 17VP, 17VN, 17WP, 17WN having two terminals, and six current detectors 18 for detecting the arm currents IUP, IUN, IVP, IVN, IWP, IWN of each phase, six circulating current suppression reactors 19, high resistors 20P, 20N, and current detectors 21P, 21N.

[0033] The A terminals of the arm converters 17UP, 17VP, 17WP and the B terminals of 17UN, 17VN, 17WN are connected to the AC terminals UA, VA, WA via the current detectors 18 and the circulating current suppression reactors 19. The B terminals of the arm converters 17UP, 17VP, 17WP and the A terminals of 17UN, 17VN, 17WN are connected to the DC terminals PA, NA and also connected to the DC terminals PB, NB of the other DSMMC converter 1B. Further, the arm converter 17 is provided with C terminals and D terminals connected to the DSMMC control device 22.

[0034] The above connection relationship in the DSMMC converter 1A, to express it very simply, is that in short, each arm is constituted by a series circuit of the arm converter 17 and the circulating current suppression reactor 19, each leg is constituted by a series circuit of two sets of arms, the connection points of two sets of arms in three sets of legs are connected to each phase of the AC power supply, and a Grez connection configuration is adopted in which both ends of the three sets of legs are used as DC terminals.

[0035] The DC terminals PA, NA of the DSMMC converter 1A are grounded through the high resistors 20P, 20N to fix the potential, and the DC voltage VDC is differentially measured by the current detectors 21P, 21N. The measured DC voltage VDC is sent to the DSMMC control device 22.

[0036] The DSMMC control device 22 calculates the gate pulse g to be applied to each unit converter using the arm currents IUP, IUN, IVP, IVN, IWP, IWN, the DC voltage VDC, the voltage phase θ, and the capacitor voltage VC of each unit converter obtained via the D terminal of the arm converter 17, and outputs it to the C terminal of each arm converter 17. The above description is about the internal structure of the DSMMC converter 1A. Since the structure of the other DSMMC converter 1B is the same as that of the DSMMC converter 1A, the description is omitted in this embodiment.

[0037] Next, the initial charging device 4 of the present invention will be described. The initial charging device 4 is composed of an initial charging device circuit breaker 7, a three-phase short-circuit circuit breaker 8, a discharge resistor 9, an initial charging resistor 10, a discharge circuit breaker 11, an initial charging circuit breaker 12, a resistor bypass circuit breaker 13, a tapped initial charging transformer 14, a transformer tap switching circuit breaker 15, and 16.

[0038] The initial charging device circuit breaker 7 is an AC circuit breaker for connecting the initial charging device 4 and the initial charging power supply 5. The three-phase short-circuit circuit breaker 8 is an AC circuit breaker for star-connecting the discharge resistors 9 of each phase during the discharge operation. The discharge resistor 9 is a resistor for consuming part of the energy stored in the capacitor during the discharge operation. The discharge circuit breaker 11 is an AC circuit breaker for connecting the discharge resistor 9 to the circuit during the discharge operation. The initial charging resistor 10 is a resistor for preventing the inrush overcurrent during the initial charging operation. The initial charging circuit breaker 12 is an AC circuit breaker for connecting the initial charging resistor 10 to the circuit during the initial charging operation. The resistor bypass circuit breaker 13 is an AC circuit breaker for connecting the circuit without passing through the discharge resistor 9 or the initial charging resistor 10.

[0039] The feature of the initial charging device 4 of the power conversion device according to Embodiment 1 of the present invention is that it is provided with a tapped initial charging transformer 14. And different tap positions are adopted when initially charging to start up the power converter 1 and when discharging to stop the power converter 1. During charging, it operates at the full voltage tap position (high voltage tap position), and during discharging, it operates at the half voltage tap position (low voltage tap position).

[0040] More specifically, the transformer 14 for initial charging with taps has first terminals AT, BT, and CT on the side of the initial charging power supply 5 (hereinafter referred to as the primary winding side), and second terminals UT1, VT1, WT1 and third terminals UT2, VT2, WT2 on the side of the DSMMC converter 1A (hereinafter referred to as the secondary winding side). The second terminals UT1, VT1, WT1 and the third terminals UT2, VT2, WT2 are configured such that the turns ratios with the primary winding are different. By configuring in this way, the AC voltage values of the respective terminals on the secondary winding side are different. Also, when the AC voltage value applied to the primary winding is the same, the terminals are configured such that the AC voltage of the secondary winding is higher at the second terminals UT1, VT1, WT1 than at the third terminals UT2, VT2, WT2. The second terminals UT1, VT1, WT1 are used during the initial charging operation, and the third terminals UT2, VT2, WT2 are used during the discharging operation.

[0041] The circuit breakers 15 and 16 for transformer tap switching are AC circuit breakers for switching the connection terminals on the secondary winding side of the transformer 14 for initial charging with taps. During the initial charging operation, the circuit breaker 15 for transformer tap switching is operated to be "closed" and 16 to be "open" so as to connect the second terminals UT1, VT1, WT1 to the AC terminals UA, VA, WA of the DSMMC converter 1A. These tap positions are so-called full tap positions (high voltage tap positions).

[0042] On the other hand, during the discharging operation, the circuit breaker 15 for transformer tap switching is operated to be "open" and 16 to be "closed" so as to connect the third terminals UT2, VT2, WT2 to the AC terminals UA, VA, WA of the DSMMC converter 1A. These tap positions are so-called half tap positions (low voltage tap positions).

[0043] As described above, in the present invention, the opening and closing conditions of each circuit breaker (circuit breaker 7 for initial charging device, three-phase short-circuit circuit breaker 8, discharging circuit breaker 11, initial charging circuit breaker 12, resistor bypass circuit breaker 13, circuit breakers 15 and 16 for transformer tap switching) in the initial charging device 4 are switched for each operation mode to form a circuit during the initial charging operation and a circuit during the discharging operation.

[0044] According to Embodiment 1, a power conversion device can be configured that satisfies circuit configurations and conditions suitable for charging and discharging of capacitors in the DSMMC converter 1, respectively.

[0045] Thus, by providing taps on the pre-charging transformer 14, it becomes possible to select different AC power supply voltage values during the pre-charging operation and the discharging operation. In particular, during the discharging operation, by lowering the AC power supply voltage value, the regeneration operation range to the pre-charging power supply can be expanded, so that the energy consumed by the resistor can be significantly reduced, and a pre-charging device that can perform pre-charging and discharging at high speed without causing an increase in the size of the cooling device for dissipating heat from the resistor can be configured.

Embodiment

[0046] In Embodiment 2, a startup control method for a power conversion device that enables high-speed pre-charging will be described.

[0047] FIG. 2 is a diagram showing a circuit configuration example of the arm converter 17. Hereinafter, the arm converter 17UP will be described. Since the other arm converters 17UN, 17VP, 17VN, 17WP, and 17WN have the same configuration, the description of the other arm converters will be omitted in this embodiment.

[0048] The arm converter 17UP is a circuit in which k unit converters 23 are connected in series. k is the number of unit converters 23 per arm converter. The unit converter 23 includes a switching circuit in which two switching elements 24H and 24L to which a circulating diode is connected in parallel are connected in series, a capacitor 25 that is an energy storage element, and a voltage detector 26 that detects the voltage VC across the capacitor 25, which are connected in parallel, respectively. In addition, it includes two gate drive units (GDUs) 28H and 28L for controlling the gates of the switching elements 24H and 24L. The signal converter (CONV) 29 outputs the voltage VC across the capacitor 25 detected by the voltage detector 26 to the DSMMC control device 22.

[0049] The A terminal to the D terminal in Fig. 2 are the input and output terminals of the arm converter 17UP, corresponding to the A terminal to the D terminal of the arm converter in Fig. 1. Among them, the C terminal is the input terminal for receiving 2×k gate pulses transmitted from the DSMMC control device 22 to the arm converter 17UP. The D terminal is the output terminal for transmitting the voltage VC across both ends of the k capacitors 25 transmitted from the arm converter 17UP to the DSMMC control device 22.

[0050] Next, the initial charging operation of the present invention will be described with reference to Figs. 3 and 4. Fig. 3 is a time chart showing the opening and closing conditions of each circuit breaker in the initial charging device 4 in Fig. 1 and the gate pulse output conditions of the DSMMC converters 1A and 1B during the initial charging operation. In Fig. 3, the three-phase short-circuit breaker 8 and the discharge breaker 11 are circuit breakers not related to the initial charging operation, and since they are always "open" during the initial charging operation period from time t0c to time t4c to be described later, their operation descriptions are omitted.

[0051] Fig. 4 shows the capacitor voltage waveforms of the DSMMC converters 1A and 1B and the AC current waveforms of each part in the initial charging device 4 during the initial charging operation of the present invention. Fig. 4 shows, from top to bottom, the capacitor voltage of the arm converter 17UP on the DSMMC converter 1A side, the capacitor voltage of the arm converter 17UN, the capacitor voltage of the arm converter 17UP on the DSMMC converter 1B side, the capacitor voltage of the arm converter 17UN, the currents IPA, IPB, IPC in the initial charging device 4, the currents IRDA, IRDB, IRDC flowing through the discharge resistor 9, the currents IRPA, IRPB, IRPC flowing through the initial charging resistor 10, and the currents IBA, IBB, IBC flowing through the resistor bypass breaker 13.

[0052] The horizontal axes of Figs. 3 and 4 are time t, and the times from t0c to t4c in Figs. 3 and 4 correspond to each other and represent the same time. Note that the capacitor voltage waveforms show only one capacitor voltage waveform of the representative unit converter in each arm converter 17UP or 17UN. Also, the respective AC currents are in the following relationships. IPA = IRDA + IRPA + IBA ···· (a) IPB = IRDB + IRPB + IBB ···· (b) IPC = IRDC + IRPC + IBC ··· (c) Using FIGS. 3 and 4, the process of the initial charging passing through each mode of the resistive charging mode, the bypass charging mode, and the DSMMC switching charging mode will be described.

[0053] First, at time t0c in FIG. 3, the main circuit breakers 6A and 6B are "open", and the three-phase power supplies 2A and 2B and the DSMMC converters 1A and 1B are electrically disconnected. Also, the initial charging device breaker 7 is "closed", the initial charging breaker 12 is "closed", the resistor bypass breaker 13 is "open", the transformer tap changer breaker 15 is "closed", and 16 is "open". The DSMMC converters 1A and 1B and the initial charging power supply 5 are connected via the initial charging resistor 10 and the tapped initial charging transformer 14 in the initial charging device 4. Also, the second terminals UT1, VT1, and WT1 on the secondary winding side of the tapped initial charging transformer 14 are connected to the AC terminals UA, VA, and WA of the DSMMC converter 1A. The gate pulse output flags FLG_GDB of the DSMMC converters 1A and 1B are "0", and the switching operations of the DSMMC converters 1A and 1B are stopped.

[0054] Next, the period from time t0c to time t1c is the operation period in the resistive charging mode. To charge the capacitors of the two DSMMC converters 1A and 1B via the initial charging resistor 10, a charging current flows through the initial charging resistor 10. As shown in FIG. 4, an excessive inrush current can be prevented by the initial charging resistor 10.

[0055] Next, at time t1c, the initial charging breaker 12 is switched from "closed → open", and the resistor bypass breaker 13 is switched from "open → closed". Thereby, the period from time t1c to time t2c hereafter is the operation period in the bypass charging mode. The initial charging resistor 10 is bypassed by the resistor bypass breaker 13 to charge the capacitors of the two DSMMC converters 1A and 1B. As shown in FIG. 4, even if the initial charging resistor 10 is bypassed, the inrush current flowing into the initial charging device 4 can be significantly limited. Also, the period from time t1c to time t2c can increase the initial charging speed of the capacitor compared to the period from time t0c to time t1c when charging via the initial charging resistor 10.

[0056] The reason why the inrush current can be significantly limited is that the capacitor is charged to a constant voltage during the period from time t0c to time t1c.

[0057] Next, at time t2c, the gate pulse output flag FLG_GDB of the DSMMC converter 1A is switched from "0→1", and the switching operation of the DSMMC converter 1A is started. At time t3c, the gate pulse output flag FLG_GDB of the DSMMC converter 1B is switched from "0→1", and the switching operation of the DSMMC converter 1B is started.

[0058] The period from time t2c to time t4c shown in FIG. 4 is the operation period in the DSMMC switching charging mode. The DSMMC converters 1A and 1B are switched, and the capacitors of the DSMMC converters 1A and 1B are charged to the target voltage value (1 p.u.). The initial charging operation is completed at time t4c when the target voltage value is reached.

[0059] One of the characteristic matters in the startup control in the second embodiment is that during the period from time t2c to time t4c, the switching operation is started only by the lower switching element 24L of the unit converter 23 in FIG. 2, and the upper switching element 24H of the unit converter 23 maintains the state of stopping the switching operation.

[0060] By controlling in this way, the capacitor can be charged efficiently. The reason will be explained with reference to FIG. 5. In FIG. 5 showing the current path of the unit converter for efficiently charging the capacitor, on the A side of the horizontal axis, the arm current IUP is positive, and on the B side of the horizontal axis, the arm current IUP is negative. Also, No. 1, No. 2, and No. 3 on the vertical axis represent the gate on and off states of the switching elements 24H and 24L. The definitions of positive and negative currents correspond to FIG. 1. The circuit configuration of the unit converter 23 and the current path flowing under this condition are shown in the matrix of the horizontal and vertical axes, indicating the charge and discharge state of the capacitor.

[0061] According to the summary of FIG. 5, the matrix of No. 1-A is in a discharge operation mode for discharging the capacitor, the matrices of No. 1-B and No. 3-B are in a charge operation mode for charging the capacitor, and the other matrices are in a bypass operation mode in which no current passes through the capacitor 25.

[0062] Among these, the operation mode of No. 1 is an operation mode for discharging the capacitor 25, which is inappropriate for use during the initial charging operation. This inappropriate mode is such that when the arm current IUP is positive, the upper switching element 24H of the unit converter 23 is gate-on and the lower switching element 24L is gate-off. Therefore, in the operation of Embodiment 2 of the present invention, the gate pulse output of the upper switching element 24H is actively stopped to make the switching element always in the off operation.

[0063] By controlling in this way, the two operation modes No. 1-A and No. 1-B of No. 1 are omitted from the operation conditions of FIG. 5, and the four operation modes of No. 2 and No. 3 are always selected. As a result, since the operation mode for discharging the capacitor 25 is not selected, the capacitor can be efficiently charged. The above is the reason for operating only the lower switching element 24L of the unit converter 23 during the period from time t2c to time t4c in the present invention.

[0064] FIG. 6 shows a control block diagram of the DSMMC control device 22 during the initial charging operation. Using the control block of FIG. 5, a specific control method during the period from time t2c to time t4c will be described. Note that FIG. 6 describes the DSMMC control device 22 of the DSMMC converter 1A. Since the control block configuration of the DSMMC control device 22 of the other DSMMC converter 1B is the same, the detailed description is omitted in this embodiment.

[0065] The DSMMC control device 22 in Fig. 6 is composed of a modulation rate determination means 30, a PWM calculation means 31, and a gate pulse output determination circuit 32. The gate pulses gHUP, gHVP, gHWP, gHUN, gHVN, gHWN which are the outputs in Fig. 6 are the gate pulses applied to the upper switching elements 24H of the unit converter 23 in Fig. 2, and the gate pulses gLUP, gLVP, gLWP, gLUN, gLVN, gLWN are the gate pulses applied to the lower switching elements 24L of the unit converter 23. The gate pulses output "0" or "1", and when it is "1", it is defined that the switching element is in the on operation, and when it is "0", the switching element is in the off operation. As described in Fig. 5, during the initial charging operation, since the gate pulses applied to the upper switching elements 24H are always in the off operation, the gate pulses gHUP, gHVP, gHWP, gHUN, gHVN, gHWN always output "0".

[0066] The second characteristic matter in the startup control in Embodiment 2 is to shorten the startup time by performing modulation rate control in the DSMMC switching charging mode. For this reason, the modulation rate determination means 30 illustrated in Fig. 7 determines and outputs the modulation wave Mref to be applied to each arm converter 17UP, 17UN, 17VP, 17VN, 17WP, 17WN of the DSMMC converter 1A during the period from time t2c to time t4c (DSMMC switching charging mode). Regarding the DSMMC converter 1B side, the modulation wave Mref to be applied to each arm converter of the DSMMC converter 1B during the period from time t3c to time t4c is determined and output.

[0067] Fig. 7 shows an example of the modulation wave Mref at each time output from the modulation rate determination means 30. At time t2c in Fig. 7, the modulation rate determination means 30 on the DSMMC converter 1A side outputs a modulation wave of modulation rate Mref_A, and compares it with the carrier wave Carry to calculate the gate pulses gLUP, gLVP, gLWP, gLUN, gLVN, gLWN. At time t4c, the modulation rate determination means 30 of the DSMMC converter 1A outputs a modulation wave of modulation rate Mref_B, and compares it with the carrier wave Carry to calculate the gate pulses gLUP, gLVP, gLWP, gLUN, gLVN, gLWN.

[0068] During the period from time t2c to time t4c, the modulation wave Mref is changed in a ramp shape, and the lower switching element 24L operates while changing the on / off ratio. By changing the modulation wave Mref with time as shown in FIG. 7, the ratio of the charging operation mode and the bypass operation mode in FIG. 5 can be changed, so that the charging speed of the capacitor voltage of the DSMMC converter 1A can be adjusted.

[0069] Regarding the operation of the DSMMC converter 1B, the other operations are the same except that the gate pulse output start time changes from t2c to t3c. In this embodiment, the reason for shifting the gate pulse output start times of the DSMMC converters 1A and 1B to t2c and t3c is to adjust the charging speeds of the respective capacitors so that the times to the target voltage value (1 p.u.) coincide.

[0070] The PWM calculation means 31 in FIG. 6 compares the modulation wave Mref calculated by the modulation rate determination means 30 with the carrier wave Carry and outputs a gate pulse.

[0071] The gate pulse output determination circuit 32 is a circuit that selects whether to start or stop the switching operation of the DSMMC converter 1A. When the gate pulse output flag FLG_GDB is "0", the gate pulse output is always turned off to stop the switching operation. When the gate pulse output flag FLG_GDB is "1", the gate pulse output from the PWM calculation means 31 is output.

[0072] The capacitor is charged to the target voltage value by the above control method. At the time t4c when the target voltage value is reached, the primary charging device breaker 7, the resistor bypass breaker 13, and the transformer tap changeover breaker 15 are switched from "closed → open", the gate pulse output flag FLG_GDB is switched from "1 → 0", and the primary charging operation is completed by electrically disconnecting the DSMMC 1A, 1B and the primary charging device 4.

[0073] According to the second embodiment, charging can be performed in a short time.

Embodiment

[0074] In Embodiment 3, a stop control method for a power conversion device enabling high-speed discharge will be described with reference to FIGS. 8 and 9.

[0075] FIG. 8 is a time chart showing the opening and closing of each circuit breaker in the primary charging device 4 in FIG. 1 and the gate pulse output operation of the DSMMC converters 1A and 1B during the discharge operation. Note that the primary charging circuit breaker 12 in FIG. 8 is a circuit breaker not related to the discharge operation, and since it is "open" during the discharge operation period from t0d to t3d described later, the operation explanation is omitted.

[0076] FIG. 9 shows the capacitor voltage waveforms of the DSMMC converters 1A and 1B and the AC current waveforms of each part in the primary charging device 4 during the discharge operation of the present invention. Note that the explanation of each waveform name in FIG. 9 is the same as that in FIG. 4. The time from time t0d to time t3d in FIGS. 8 and 9 corresponds to each other and represents the same time.

[0077] Using FIGS. 8 and 9, the discharge through each mode of the regenerative discharge mode, discharge mode switching, and resistive discharge mode will be described.

[0078] At time t0d in FIG. 9, the main circuit circuit breakers 6A and 6B switch from "closed → open", and in order to shift to the discharge operation mode of regenerating to the primary charging power supply 5 by the AC current control of the DSMMC converter, the three-phase power supplies 2A and 2B and the DSMMC converters 1A and 1B are electrically disconnected. Also, the primary charging device circuit breaker 7 switches from "open → closed", the three-phase short-circuit circuit breaker 8 is "open", the discharge circuit breaker 11 is "open", the resistance bypass circuit breaker 13 switches from "open → closed", the transformer tap switching circuit breaker 15 is "open", and 16 switches from "open → closed", and the DSMMC converters 1A and 1B are connected to the primary charging power supply 5 via the tapped primary charging transformer 14 in the primary charging device 4. Also, the third terminals UT2, VT2, and WT2 on the secondary winding side of the tapped primary charging transformer 14 are connected to the AC terminals UA, VA, and WA of the DSMMC converter 1A. The gate pulse output flag FLG_GDB of the DSMMC converters 1A and 1B switches from "0 → 1", and the DSMMC converters 1A and 1B start the switching operation.

[0079] The period from time t0d to time t1d is an operation period in the regenerative discharge mode, and is a discharge operation mode in which the energy stored in the capacitors of the DSMMC converters 1A and 1B is regenerated to the initial charging power supply 5 by the AC current control of the DSMMC converter without passing through the discharge resistor 9. In order to regenerate to the initial charging power supply 5 without passing through the discharge resistor 9, it can be seen in FIG. 9 that the discharge current is flowing through the resistor bypass breaker 13. Also, during this period, the voltage of the capacitors of the DSMMC converters 1A and 1B can be discharged to 50%.

[0080] Generally, since it is economical for a power conversion device to be designed with a high voltage utilization rate, when the modulation rate during rated operation is 0.85, overmodulation operation occurs due to a capacitor voltage drop of about 17%, and thereafter, regenerative operation to the power supply becomes impossible. Therefore, thereafter, the energy of the capacitor has to be consumed by the discharge resistor.

[0081] On the other hand, in this embodiment, the DSMMC converter can operate without overmodulation until the capacitor voltage reaches 45%. The reason is that, as explained in Embodiment 1, the connection terminals on the secondary winding side of the tapped initial charging transformer 14 are switched from the second terminals UT1, VT1, WT1 to the third terminals UT2, VT2, WT2 to lower the AC voltage value of the power supply.

[0082] Specifically, with the transformer tap-changing circuit breaker 15 in the "open" state and 16 in the "closed" state, the third terminals UT2, VT2, WT2 on the secondary winding side of the step-up transformer 14 with taps are connected to the AC terminals UA, VA, WA of the DSMMC converter 1A. The second terminals UT1, VT1, WT1 and the third terminals UT2, VT2, WT2 are configured such that the turns ratio with the primary winding is different. In this embodiment, when the AC voltage value applied to the primary winding is the same, the turns ratio is configured such that the AC voltage of the secondary winding has the terminal voltage of the second terminals UT1, VT1, WT1 twice as high as the terminal voltage of the third terminals UT2, VT2, WT2, and is connected to the third terminals UT2, VT2, WT2 during the discharge operation of this embodiment. Then, the AC voltage value of the power supply can be reduced to approximately half compared to connecting to the second terminals UT1, VT1, WT1. Since the modulation ratio has a margin due to the AC voltage of the secondary winding of the step-up transformer 14 with taps dropping to approximately half, the capacitor voltage can be discharged up to 45%. With the above configuration, most of the energy stored in the capacitor is regenerated to the power supply.

[0083] At time t1d, the gate pulse output flags FLG_GDB of the DSMMC converters 1A and 1B are switched from "1→0", the switching operations of the DSMMC converters 1A and 1B are stopped once, and preparations are made to shift to the discharge operation mode using the discharge resistor 9.

[0084] At time t2d, the breaker 7 for the pre-charging device is switched from "closed→open", the three-phase short-circuit breaker 8 is switched from "open→closed", the discharge breaker 11 is switched from "open→closed", and the resistor bypass breaker 13 is switched from "closed→open". Also, the gate pulse output flags FLG_GDB of the DSMMC converters 1A and 1B are switched from "0→1", the switching operations of the DSMMC converters 1A and 1B are restarted, and a shift is made to the discharge operation mode using the discharge resistor 9. The series of periods from time t1d to time t2d corresponds to the discharge mode switching.

[0085] The period from time t2d to time t3d is the operation period in the resistive discharge mode, which is a discharge operation mode that consumes the energy stored in the capacitors of the DSMMC converters 1A and 1B by passing current through the discharge resistor 9 through the AC current control of the DSMMC converter. During the period from time t0d to time t1d, the voltage of the capacitor can be discharged from 100% to 45%. That is, from Equation (2), if the energy consumed by the discharge resistor 9 is set to 100% (all consumed by the resistor) in Patent Document 2, this embodiment is about 20%, and the energy consumed by the discharge resistor 9 can be reduced by about 80%. Therefore, the enlargement of the cooling device can be avoided, and the initial charging device can be miniaturized.

[0086] Fig. 10 shows the control block diagram during the discharge operation of the DSMMC control device 22. Using the control block in Fig. 10, the specific control method of the DSMMC converter 1A during the period from time t0d to time t1d and the period from time t2d to time t3d will be described. Note that for the DSMMC converter 1B, since the modulation ratio is constant at 0.5 during the period from time t0d to time t1d and the period from time t2d to time t3d, the description is omitted.

[0087] The DSMMC control device 22 at the time of stop shown in Fig. 10 is composed of an AC current calculation means 33, a three-phase to two-phase conversion means 34, an AC current control means 35, a three-phase to two-phase inverse conversion means 36, a Q-axis current command calculation means 37, a PWM calculation means 31, a gate pulse output determination circuit 32, and a dead time calculation means 38. Among them, since the PWM calculation means 31 and the gate pulse output determination circuit 32 are the same as those during the initial charging operation, the description is omitted.

[0088] The AC current calculation means 33 calculates the AC currents IU, IV, and IW from the arm currents IUP, IUN, IVP, IVN, IWP, and IWN of each phase detected by the current detector 18 as follows. IU = IUP - IUN ··· (d) IV = IVP - IVN ··· (e) IW = IWP - IWN ··· (f) The three-phase to two-phase conversion means 34 converts the three-phase alternating current IU, IV, IW into the Q-axis current IQ and the D-axis current ID. The relationships among IU, IV, IW, IQ, ID, and the voltage phase θ are given by Equation (3).

[0089]

Number

[0090] The alternating current control means 35 generates the alternating voltage commands VQref and VDref so that the alternating currents IQ and ID follow the alternating current command values IQref and IDref. The alternating current control means 35 is composed of a proportional-integral controller.

[0091] The three-phase to two-phase inverse conversion means 36 converts VQref and VDref into the three-phase alternating voltage commands VUref, VVref, and VWref. The relationships among IQ, ID, IU, IV, IW, and the voltage phase θ are given by Equation (4).

[0092]

Number

[0093] The alternating voltage commands VUParef, VVParef, and VWParef given to the arm converters 17UP, 17VP, and 17WP and the alternating voltage commands VUNaref, VVNaref, and VWNaref given to the arm converters 17UN, 17VN, and 17WN are generated from VUref, VVref, and Vwref. Also, the direct voltage command VDCref / 2 is added to the alternating voltage command to calculate the arm voltage commands VUPref, VUNref, VVPref, VVNref, VWPref, and VWNref.

[0094] The dead time calculation means 38 has a function of adding a dead time to the gate pulse obtained from the PWM calculation means 31 to prevent simultaneous conduction of the upper switching element 24H and the lower switching element 24L.

[0095] During the period from time t2d to time t3d in FIG. 9, the currents IRDA, IRDB, and IRDC flowing through the discharge resistors 9 are decreasing because the Q-axis current command calculation means 37 in FIG. 10 calculates and changes the Q-axis current command IQref according to the DC voltage VDC. The reason for changing the Q-axis current command IQref according to the DC voltage VDC during the period from time t2d to time t3d will be explained.

[0096] During the period from time t2d to time t3d, if the discharge operation is continued with IQref as a constant current command, when the capacitor voltage decreases, the DC voltage VDC decreases and enters the overmodulation operation region, and IQ cannot follow IQref. To avoid this, the Q-axis current command calculation means 37 calculates the Q-axis current command IQref according to the DC voltage VDC. Specifically, IQref is set to satisfy equation (5). R is the resistance value of the discharge resistor 9, and X is the leakage reactance of the pre-charging transformer 14 with taps.

[0097]

Equation

[0098] After discharging the capacitor to the target voltage by the above means, at time t3d, all circuit breakers are turned "on", the gate pulse output flags FLG_GDB of the DSMMC converters 1A and 1B are switched from "1→0", and the DSMMC 1A, 1B and the pre-charging device 4 are electrically disconnected to complete the discharge operation, and the power conversion device can be safely stopped.

[0099] By the pre-charging operation described above, the capacitor of the power conversion device composed of two DSMMC converters can be charged simultaneously while preventing the inrush overcurrent during the pre-charging operation, so the start-up and stop times of the power conversion device can be speeded up. Also, during the discharge operation, a tap for voltage adjustment is provided on the pre-charging transformer 14, and a function of adjusting the power supply voltage value by switching the tap of the transformer is provided, so the regeneration operation range during discharge is widened, and the energy consumed by the discharge resistor can be reduced by about 80% compared with the means in Patent Document 2.

Embodiment

[0100] FIG. 11 is a diagram showing a main circuit configuration example of the power conversion device according to Embodiment 4 of the present invention. FIG. 11 is different from FIG. 1 of Embodiment 1 in that the discharge resistor 9 and the discharge breaker 11 in the initial charging device 4 are removed. The initial charging resistor 10 functions as the removed discharge resistor, and the initial charging breaker 12 functions as the discharge breaker.

[0101] By configuring as shown in FIG. 11, it becomes possible to further reduce the size of the initial charging device.

Description of Signs

[0102] 1A, 1B: DSMMC converter 2A, 2B: Three-phase power source 3: Voltage phase detection means 4: Initial charging device 5: Initial charging power source 6A, 6B: Main circuit breaker 7: Initial charging device breaker 8: Three-phase short-circuit breaker 9: Discharge resistor 10: Initial charging resistor 11: Discharge breaker 12: Charging breaker 13: Resistor bypass breaker 14: Initial charging transformer with taps 15, 16: Transformer tap changeover breaker 17UP, 17UN, 17VP, 17VN, 17WP, 17WN: Arm converter 18, 21P, 21N: Current detector 19: Circulating current suppression reactor 20P, 20N: High resistance 23: Unit converter 24H, 24L: Switching element 25: Capacitor, 26: Voltage detector 28H, 28L: Gate drive unit (GDU) 29: Signal converter 30: Modulation rate determination means 31: PWM calculation means 32: Gate Pulse Output Judgment Circuit 33: Alternating Current Calculation Means 34: Three-Phase to Two-Phase Conversion Means 35: Alternating Current Control Means 36: Two-Phase to Three-Phase Inverse Conversion Means 37: Q-Axis Current Command Calculation Means 38: Dead Time Calculation Means

Claims

1. A power conversion device including: a unit converter (23) configured by connecting in parallel a switching circuit in which two sets of switching elements (24H, 24L) with antiparallel diodes are connected in series and a capacitor (25), and connecting a plurality of the unit converters in series; an arm configured by connecting a current detector (18) and a circulating current suppressing reactor (19) in series; a leg configured by connecting two sets of the arms in series; connecting connection points of the two sets of the arms of each phase leg to a phase of a first AC power supply (2A) via a main circuit breaker (6A) as AC terminals (UA, VA, WA), and making both ends of each phase leg DC terminals (PA, NA); a power converter (1A); and a charge / discharge device (4) having one end (AA, BA, CA) connected to a second AC power supply (5) and the other end (UP, VP, WP) connected to the AC terminals (UA, VA, WA). The charge / discharge device (4) of the power conversion device includes a tapped transformer (14) that can be switched between all tap positions (UT1, VT1, WT1) and half tap positions (UT2, VT2, WT2). When starting up the power converter, it is connected to the all tap positions (UT1, VT1, WT1) of the tapped transformer (14), and when stopping the power converter (1A), it is connected to the half tap positions (UT2, VT2, WT2) of the tapped transformer (14), so that the charge / discharge device (4) is connected to the power converter (1A) instead of the first AC power supply (2A). A power conversion device characterized by this.

2. A control device for a power conversion device including: a unit converter (23) configured by connecting in parallel a switching circuit in which two sets of switching elements (24H, 24L) with antiparallel diodes are connected in series and a capacitor (25), and connecting a plurality of the unit converters in series; an arm configured by connecting a current detector (18) and a circulating current suppressing reactor (19) in series; a leg configured by connecting two sets of the arms in series; connecting connection points of the two sets of the arms of each phase leg to a phase of a first AC power supply (2A) via a main circuit breaker (6A) as AC terminals (UA, VA, WA), and making both ends of each phase leg DC terminals (PA, NA); a power converter (1A); and a charge / discharge device (4) having one end (AA, BA, CA) connected to a second AC power supply (5) and the other end (UP, VP, WP) connected to the AC terminals (UA, VA, WA). The charging and discharging device (4) includes a tapped transformer (14) that can be switched between full tap positions (UT1, VT1, WT1) and half tap positions (UT2, VT2, WT2). When the power converter (1A) is started, the charging and discharging device (4) is connected to the power converter (1A) instead of the first AC power supply (2A). The control device (22), when starting the power converter (1A), connects the second AC power supply (5) to the AC terminals (UA, VA, WA) of the power converter (1A) through the initial charging resistor (10), the initial charging circuit breaker (12), and the full tap positions (UT1, VT1, WT1) of the tapped transformer (14) of the charging and discharging device (4), and charges the capacitor (25) connected in parallel with the switching circuit. It shifts from the resistance charging mode to the bypass charging mode where the second AC power supply (5) is connected to the AC terminals (UA, VA, WA) of the power converter (1A) through the resistance bypass circuit breaker (13) of the charging and discharging device (4) and the full tap positions (UT1, VT1, WT1) of the tapped transformer (14) to charge the capacitor (25) connected in parallel with the switching circuit. After that, it operates to shift to the switching charging mode where the switching elements (24H, 24L) of the unit converter (23) are arced by the gate pulses (gHUP, gLUP, gHUN, gLUN, gHVP, gLVP, gHVN, gLVN, gHWP, gLWP, gHWN, gLWN) from the control device (22) to charge the capacitor (25) connected in parallel with the switching circuit. After the charging of the capacitor (25) connected in parallel with the switching circuit is completed, it controls to connect the first AC power supply (2A) to the power converter (1A) instead of the charging and discharging device (4). A control device for a power conversion device characterized by this.

3. A control device for a power conversion device according to claim 2, In the switching charging mode where the control device (22) arcs the switching elements (24H, 24L) of the unit converter (23) with the gate pulses (gHUP, gLUP, gHUN, gLUN, gHVP, gLVP, gHVN, gLVN, gHWP, gLWP, gHWN, gLWN) from the control device (22), it is characterized in that it controls to increase the PWM modulation rate of the gate pulses applied to the switching circuit over time. A control device for a power conversion device.

4. A control device for a power conversion device according to claim 2, in the switching charging mode in which the control device (22) arcs the switching elements (24H, 24L) of the unit converter (23) with gate pulses (gHUP, gLUP, gHUN, gLUN, gHVP, gLVP, gHVN, gLVN, gHWP, gLWP, gHWN, gLWN) from the control device (22), the gate pulses (gHUP, gHUN, gHVP, gHVN, gHWP, gHWN) applied to the upper switching element (24H) of the switching circuit in which two sets of the switching elements (24H, 24L) are connected in series are always set to OFF signals during the switching charging mode, so as to control to prevent the capacitor (25) connected in parallel with the switching circuit from entering a discharging state. A control device for a power conversion device characterized by this.

5. A control device for a power conversion device according to claim 2, when the power converter (1A) stops, the charge / discharge device (4) is connected to the power converter (1A) instead of the first AC power supply (2A). When the power converter (1A) stops, the control device (22) connects to the second AC power supply (5) through the half-tap positions (UT2, VT2, WT2) of the tapped transformer (14) and the resistor bypass breaker (13), and applies gate pulses (gHUP, gLUP, gHUN, gLUN, gHVP, gLVP, gHVN, gLVN, gHWP, gLWP, gHWN, gLWN) that are the PWM modulation rates determined by the current control means (35) of the control device (22) to the switching elements (24H, 24L) to operate the power converter (1A) and discharge the capacitor (25) connected in parallel to the switching circuit in a regenerative discharge mode. Then, connect the half-tap positions (UT2, VT2, WT2) of the tapped transformer, the discharge breaker (11), the discharge resistor (9), and the three-phase short-circuit breaker (8), disconnect the second AC power supply (5) with the primary charging device breaker (7), and apply gate pulses (gHUP, gLUP, gHUN, gLUN, gHVP, gLVP, gHVN, gLVN, gWHP, gLWP, gHWN, gLWN) that are the PWM modulation rates determined by the current control means (35) of the control device (22) to the switching elements (24H, 24L) to operate the power converter (1A) and control to shift to a resistive discharge mode for discharging the capacitor (25) connected in parallel to the switching circuit. A control device for a power conversion device, characterized by the above.

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