Power Conversion Equipment

Abstract

The power conversion device includes a power converter including a plurality of converter cells (1) connected in series. Each converter cell (1) includes a semiconductor circuit (25) including a plurality of first switching elements (22A, 22B) connected in series, a first storage element (28) connected in parallel to the semiconductor circuit (25), and a discharge circuit (30A) connected in parallel to the first storage element (28). The discharge circuit (30A) includes a plurality of discharge units (50A to 50C) connected in series. The discharge unit (50A) includes a first discharge resistor (31A), a second discharge resistor (32A), and a second switching element (33A). The first discharge resistor (31A) is connected in parallel to a first series body including the second discharge resistor (32A) and the second switching element (33A) connected in series.

Description

[Technical field]

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

[0002] In recent years, modular multilevel converters (MMCs) have become known as high-voltage, large-capacity power conversion devices that are applied to high-voltage systems such as power grids. MMCs are composed of arms in which multiple unit converters called converter cells are cascaded. Due to advantages such as high scalability and low harmonics, MMCs are widely used in AC / DC converters and static synchronous compensators (STATCOMs) that are applied to high-voltage direct current (HVDC) transmission.

[0003] Each converter cell includes a storage element (e.g., a capacitor), and it is necessary to discharge the capacitor included in the converter cell after the converter is stopped. In this regard, Patent Document 1 (Japanese Patent No. 6869625) discloses a power conversion device. The discharge circuit of the power conversion device includes a switch that is connected in parallel to the capacitor and closes the circuit to discharge the capacitor when the power converter is stopped, a discharge resistor connected in series to the switch, and a drive circuit that opens the switch when the stopped state of the power converter is released. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 6869625 Summary of the Invention [Problem to be solved by the invention]

[0005] The power conversion device according to Patent Document 1 is designed to rapidly discharge the capacitor of the unit converter when the power converter is stopped by using the above-mentioned configuration. However, in the MMC used for HVDC and the like, the voltage of the capacitor of the converter cell is high (for example, several kV), and therefore the switches of semiconductor elements and the like used in the corresponding discharge circuit may become large.

[0006] An object in one aspect of the present disclosure is to provide a power conversion device that can quickly discharge a storage element of a converter cell when the power converter is stopped, without increasing the size of the converter cell. [Means for solving the problem]

[0007] A power converter according to an embodiment includes a power converter including a plurality of converter cells connected in series. Each converter cell includes a semiconductor circuit including a plurality of first switching elements connected in series, a first storage element connected in parallel to the semiconductor circuit, and a discharge circuit connected in parallel to the first storage element. The discharge circuit includes a plurality of discharge units connected in series. The discharge unit includes a first discharge resistor, a second discharge resistor, and a second switching element. The first discharge resistor is connected in parallel to a first series body including the second discharge resistor and the second switching element connected in series.

[0008] A power converter according to another embodiment includes a power converter including a plurality of converter cells connected in series. Each of the converter cells includes a semiconductor circuit including a plurality of first switching elements connected in series, a power storage unit including a plurality of power storage elements connected in parallel to the semiconductor circuit, and a discharge circuit connected in parallel to the power storage unit. The discharge circuit includes a plurality of discharge units connected in series. Each of the plurality of discharge units is connected in parallel to a corresponding one of the power storage elements. The discharge unit includes a first discharge resistor, a second discharge resistor, and a second switching element. The first discharge resistor is connected in parallel to a first series body including the second discharge resistor and the second switching element connected in series. Effect of the Invention

[0009] According to the present disclosure, it is possible to quickly discharge the storage element of the converter cell when the power converter is stopped, without increasing the size of the converter cell. [Brief description of the drawings]

[0010] [Figure 1] FIG. 1 is a diagram illustrating a configuration example of a power conversion device. [Diagram 2] FIG. 2 is a diagram showing a configuration example of a converter cell according to the first embodiment. [Diagram 3] FIG. 2 is a block diagram showing an example of a hardware configuration of a control device. [Figure 4] FIG. 2 is a diagram illustrating a first configuration example of a discharge circuit. [Diagram 5] FIG. 11 is a diagram illustrating a second configuration example of a discharge circuit. [Figure 6] FIG. 11 is a diagram illustrating a third configuration example of a discharge circuit. [Figure 7] FIG. 11 is a diagram showing a configuration example of a converter cell 1X according to a second embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

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

[0012] Embodiment 1 <Overall composition> Fig. 1 is a diagram showing a configuration example of a power conversion device 100. Referring to Fig. 1, the power conversion device 100 is connected between an AC circuit 2 and a DC circuit 4. The DC circuit 4 is, for example, a DC power system including a DC transmission network or the like, or a DC terminal of another power conversion device. The DC circuit 4 may be configured to include a power storage device connected to the DC terminal of a power converter 6. The power storage device includes, for example, an electric double layer capacitor or a storage battery such as a lithium ion battery.

[0013] The power conversion device 100 includes a self-excited power converter 6 and a control device 5 for controlling the power converter 6. Typically, the power converter 6 is configured by a modular multilevel converter including a plurality of converter cells (corresponding to "cells" in FIG. 1) 1 connected in series with each other. A "converter cell" is also called a "sub module" or a "unit converter."

[0014] The power converter 6 is connected to the DC circuit 4 and performs power conversion between the DC circuit 4 and the AC circuit 2. Specifically, the power converter 6 converts the DC power output from the DC circuit 4 into AC power, and outputs the AC power to the AC circuit 2 via the transformer 3. The power converter 6 also converts the AC power from the AC circuit 2 into DC power, and outputs the DC power to the DC circuit 4.

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

[0016] The leg circuit 8 is provided for each of the multiple phases constituting the AC. The leg circuit 8 is connected between the AC circuit 2 and the DC circuit 4, and performs power conversion between the two circuits. Fig. 1 shows a case where the AC circuit 2 is a three-phase AC system, and three leg circuits 8u, 8v, and 8w are provided corresponding to the U phase, V phase, and W phase, respectively.

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

[0018] Instead of using the transformer 3 in Fig. 1, the leg circuits 8u, 8v, 8w may be configured to be connected to the AC circuit 2 via an interconnection reactor. Furthermore, instead of the AC terminals Nu, Nv, Nw, the leg circuits 8u, 8v, 8w may each be provided with a primary winding, and the leg circuits 8u, 8v, 8w may be AC-connected to the transformer 3 or the interconnection reactor via a secondary winding magnetically coupled to the primary winding. In this case, the primary winding may be the reactors 7a, 7b described below. That is, the leg circuit 8 is electrically (i.e., DC- or AC-connected) connected to the AC circuit 2 via a connection part provided in each leg circuit 8u, 8v, 8w, such as the AC terminals Nu, Nv, Nw or the above-mentioned primary winding.

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

[0020] In the leg circuit 8u, the positive arm 13u includes a plurality of converter cells 1 connected in cascade to each other and a reactor 7a. The plurality of converter cells 1 and the reactor 7a are connected in series to each other. The negative arm 14u includes a plurality of converter cells 1 connected in cascade to each other and a reactor 7b. The plurality of converter cells 1 and the reactor 7b are connected in series to each other.

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

[0022] The power conversion device 100 further includes an AC voltage detector 10, an AC current detector 15, DC voltage detectors 11a and 11b, and arm current detectors 9a and 9b provided in each leg circuit 8. These detectors measure electrical quantities (i.e., current, voltage) used to control the power converter 6. Signals detected by these detectors are input to the control device 5.

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

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

[0025] The power converter 6 may be configured with a delta-connection MMC. A delta-connection cascade three-phase MMC has a configuration in which a plurality of cells are cascaded and arms in which reactors are connected in series are delta-connected.

[0026] <Converter cell configuration> 2A and 2B are diagrams showing a configuration example of the converter cell 1 according to the embodiment 1. The converter cell 1 shown in Fig. 2A has a circuit configuration called a half-bridge configuration.

[0027] 2(a), the converter cell 1 includes a semiconductor circuit 25, a capacitor 28 as a power storage element, and a discharge circuit 30. The semiconductor circuit 25, the capacitor 28, and the discharge circuit 30 are connected in parallel.

[0028] The semiconductor circuit 25 includes two switching elements 22A and 22B connected in series, and diodes 23A and 23B. The diodes 23A and 23B are connected in anti-parallel (i.e., in parallel and in a reverse bias direction) to the switching elements 22A and 22B, respectively. The capacitor 28 is connected in parallel to the semiconductor circuit 25 and holds a DC voltage. The voltage Vc across the capacitor 28 is detected by a voltage detector included in the converter cell 1. The detected voltage Vc is input to the control device 5.

[0029] A connection node between the switching elements 22A and 22B is connected to a high potential side input / output terminal G1. A connection node between the switching element 22B and the capacitor 28 is connected to a low potential side input / output terminal G2.

[0030] When switching element 22A is on and switching element 22B is off, the voltage Vc of capacitor 28 is output between input / output terminals G1 and G2. When switching element 22A is off and switching element 22B is on, zero voltage is output between input / output terminals G1 and G2.

[0031] The converter cell 1 shown in Fig. 2(b) has a circuit configuration called a full-bridge configuration. With reference to Fig. 2(b), the converter cell 1 includes semiconductor circuits 25 and 26, a capacitor 28, and a discharge circuit 30. The semiconductor circuit 25, the semiconductor circuit 26, the capacitor 28, and the discharge circuit 30 are connected in parallel.

[0032] The semiconductor circuit 26 includes two switching elements 22C and 22D connected in series, and diodes 23C and 23D. The diodes 23C and 23D are connected in anti-parallel to the switching elements 22C and 22D, respectively. A connection node between the switching elements 22A and 22B is connected to a high-potential side input / output terminal G1, and a connection node between the switching elements 22C and 22D is connected to a low-potential side input / output terminal G2.

[0033] By turning on switching element 22D, turning off switching element 22C, and alternately turning on switching elements 22A and 22B, a positive voltage or zero voltage is output between input / output terminals G1 and G2. By turning off switching element 22D, turning on switching element 22C, and alternately turning on switching elements 22A and 22B, a zero voltage or negative voltage is output between input / output terminals G1 and G2.

[0034] Each of the switching elements 22A to 22D in Fig. 2(a) and Fig. 2(b) is a self-extinguishing switching element capable of controlling both on and off operations. The switching elements 22A to 22D are self-extinguishing semiconductor switching elements such as an IGBT (Insulated Gate Bipolar Transistor) or a MOSFET (Metal Oxide Semiconductor Field-Effect Transistor). In Fig. 2(a) and Fig. 2(b), a film capacitor or the like is mainly used for the capacitor 28.

[0035] 2(a) and 2(b) is connected in parallel to the capacitor 28, and is provided to discharge the energy of the capacitor 28. The specific configuration of the discharge circuit 30 will be described later.

[0036] In the following, an example will be described in which the converter cell 1 is configured as a half-bridge cell as shown in Fig. 2(a). However, the converter cell 1 may also be configured as a full-bridge cell as shown in Fig. 2(b).

[0037] <Control device hardware configuration> FIG. 3 is a block diagram showing an example of a hardware configuration of the control device 5. The control device 5 in FIG. 3 is configured based on a computer. Referring to FIG. 3, the control device 5 includes one or more input converters 70, one or more sample-and-hold (S / H) circuits 71, a multiplexer (MUX) 72, and an A / D converter 73. Furthermore, the control device 5 includes one or more central processing units (CPUs) 74, a random access memory (RAM) 75, and a read only memory (ROM) 76. Furthermore, the control device 5 includes one or more input / output interfaces 77, an auxiliary storage device 78, and a bus 79 that connects the above components to each other.

[0038] The input converter 70 includes an auxiliary transformer for each input channel, which converts the detection signal from each detector in FIG. 1 into a signal with a voltage level suitable for subsequent signal processing.

[0039] A sample-and-hold circuit 71 is provided for each input converter 70. The sample-and-hold circuit 71 samples and holds a signal representing an electrical quantity received from the corresponding input converter 70 at a specified sampling frequency.

[0040] The multiplexer 72 sequentially selects the signals held in the multiple sample-and-hold circuits 71. The A / D converter 73 converts the signal selected by the multiplexer 72 into a digital value. Note that by providing multiple A / D converters 73, A / D conversion may be performed in parallel on detection signals of multiple input channels.

[0041] The CPU 74 controls the entire control device 5 and executes arithmetic processing according to a program. The RAM 75 as a volatile memory and the ROM 76 as a non-volatile memory are used as the main memory of the CPU 74. The ROM 76 stores programs and setting values ​​for signal processing. The auxiliary storage device 78 is a non-volatile memory with a larger capacity than the ROM 76, and stores programs, data on detected electric quantity values, and the like.

[0042] The input / output interface 77 is an interface circuit for communication between the CPU 74 and an external device.

[0043] At least a part of the control device 5 may be configured using circuits such as a field programmable gate array (FPGA) and an application specific integrated circuit (ASIC), etc. Alternatively, at least a part of the control device 5 may be configured using analog circuits.

[0044] <Discharge circuit configuration> (Configuration example 1) Fig. 4 is a diagram showing a configuration example 1 of a discharge circuit. The discharge circuit 30A shown in Fig. 4 corresponds to the discharge circuit 30 shown in Fig. 2(a), but for the sake of convenience, it is given the symbol "A" to distinguish it from other configuration examples of the discharge circuit. This also applies to the following configuration examples 2 and 3.

[0045] 4, the discharge circuit 30A includes a plurality of discharge units 50A to 50C (hereinafter also collectively referred to as "discharge units 50") connected in series. In the example of FIG. 4, the number of discharge units 50 is three, but it is sufficient that there are two or more discharge units.

[0046] The discharge unit 50A includes discharge resistors 31A and 32A, a switching element 33A, and a diode 34A. Similarly, the discharge unit 50B includes discharge resistors 31B and 32B, a switching element 33B, and a diode 34B. The discharge unit 50C includes discharge resistors 31C and 32C, a switching element 33C, and a diode 34C.

[0047] Hereinafter, discharge resistors 31A to 31C are also collectively referred to as "discharge resistors 31," discharge resistors 32A to 32C are also collectively referred to as "discharge resistors 32," switching elements 33A to 33C are also collectively referred to as "switching elements 33," and diodes 34A to 34C are also collectively referred to as "diodes 34."

[0048] The discharge resistors 31A to 31C are the same resistor, and the resistance value thereof is "R1". The discharge resistors 32A to 32C are the same resistor, and the resistance value thereof is "R2". Typically, the resistance value R2 is set to be equal to or less than the resistance value R1. The rated voltage of the switching element 33 is lower than the rated voltages of the switching elements 22A and 22B. The discharge circuit 30A may be configured not to include the diodes 34A to 34C connected in anti-parallel to the switching elements 33A to 33C.

[0049] In each discharge unit 50, the discharge resistor 31 is connected in parallel to a series circuit including a series-connected discharge resistor 32 and a switching element 33. Specifically, the discharge resistor 31 is connected between the positive terminal and the negative terminal of the discharge unit 50. Moreover, the series circuit including the discharge resistor 32 and the switching element 33 is connected between the positive terminal and the negative terminal of the discharge unit 50.

[0050] In the discharge circuit 30A, when each of the switching elements 33A-33C is in the OFF state, a current flows only through each of the discharge resistors 31A-31C. In this case, the voltages applied to the switching elements 33A-33C are equal to the voltages applied to the discharge resistors 31A-31C, respectively. That is, a voltage obtained by dividing the voltage Vc of the capacitor 28 by the discharge resistor 31 of each discharge unit 50 is applied to each switching element 33. The equivalent resistance value of the discharge unit 50 when the switching element 33 is in the OFF state is resistance value R1.

[0051] In the discharge circuit 30A, when each of the switching elements 33A to 33C is in the on state, a current is divided between the discharge resistor 31 and the discharge resistor 32. The current flowing through the switching element 33 is equal to the current flowing through the discharge resistor 32. When the switching element 33 is in the on state, the equivalent resistance value of the discharge unit 50 is the combined resistance value R12 of the resistance value R1 and the resistance value R2 (i.e., R12=R1×R2 / (R1+R2)). Note that the resistance value R1 is greater than the combined resistance value R12.

[0052] From the above, it can be seen that the equivalent resistance value of the discharge circuit 30 when the switching elements 33A to 33C are in the off state (i.e., the resistance value R1) is greater than the equivalent resistance value of the discharge circuit 30 when the switching elements 33A to 33C are in the on state (i.e., the combined resistance value R12).

[0053] When the power converter 6 is in a stopped state, the control device 5 controls each of the switching elements 33A to 33C of the discharge circuit 30A to an ON state. This reduces the equivalent resistance value of the discharge circuit 30A, thereby facilitating the discharge of the capacitor 28 by the discharge circuit 30A while the power converter 6 is in a stopped state. Furthermore, by setting the resistance value R2 of the discharge resistor 32A to be equal to or less than the resistance value R1 of the discharge resistor 31A, the equivalent resistance value of the discharge circuit 30A can be further reduced, thereby further facilitating the discharge of the capacitor 28.

[0054] On the other hand, when the power converter 6 is in operation, the control device 5 controls each of the switching elements 33A to 33C of the discharge circuit 30A to the off state. This increases the equivalent resistance value of the discharge circuit 30, thereby making it possible to reduce the loss occurring in the discharge circuit 30A while the power converter 6 is in operation.

[0055] The control device 5, for example, determines that the power converter 6 is in an operating state when it receives an operation command for the power converter 6 from a higher-level device, and determines that the power converter 6 is in a stopped state when it receives a stop command for the power converter 6 from the higher-level device.

[0056] According to the above configuration, by providing a plurality of discharge units 50 in the discharge circuit 30A, a voltage obtained by dividing the capacitor voltage Vc by each discharge resistor 31 is applied to each switching element 33. Therefore, even if the voltage of the capacitor 28 of the converter cell 1 is high as in the case of MMC, it is not necessary to use a large switching element having high voltage resistance performance in the discharge circuit 30A. In other words, a small, low-loss element with a low rated voltage can be used as the switching element 33 of the discharge circuit 30A. As a result, the converter cell 1 can be made smaller and lighter.

[0057] Furthermore, when the power converter 6 is in a stopped state, the capacitor 28 of the converter cell 1 can be quickly discharged by reducing the equivalent resistance value of the discharge circuit 30A. On the other hand, when the power converter 6 is in an operating state, the loss generated in the discharge circuit 30A can be reduced by increasing the equivalent resistance value of the discharge circuit 30A.

[0058] (Configuration example 2) Fig. 5 is a diagram showing a second configuration example of a discharge circuit. Referring to Fig. 5, a discharge circuit 30B includes a plurality of discharge units 51A to 51C (hereinafter also collectively referred to as "discharge units 51") connected in series. Note that, although the number of discharge units 51 is three in the example of Fig. 5, it is sufficient that the number is two or more.

[0059] Discharge unit 51A has a configuration in which capacitor 35A is added to discharge unit 50A of Fig. 4. Specifically, capacitor 35A is connected in parallel to a series body including discharge resistor 32A and switching element 33A, and discharge resistor 31A. Similarly, discharge units 51B and 51C include capacitors 35B and 35C, respectively. Capacitors 35A to 35C (hereinafter also collectively referred to as "capacitor 35") are the same capacitor. The capacitance of capacitor 35 is smaller than the capacitance of capacitor 28. In addition, the rated voltage of capacitor 35 is smaller than the rated voltage of capacitor 28.

[0060] Here, the advantage of providing the capacitor 35 in the discharge circuit 30B will be described. Generally, a switching element, which is a semiconductor element, includes parasitic capacitance, so there is a certain switching transition time (for example, the time from when the gate driver is turned on to when the switching element actually turns on).

[0061] 4, even in a configuration that does not include a capacitor 35, if the variation in the switching timing of the multiple switching elements 33 (for example, the time difference between the switching timing of one switching element 33 and the switching timing of another switching element 33) is sufficiently small, voltage is not applied unevenly to some of the switching elements 33. For this reason, in the configuration of the discharge circuit 30A, it is preferable that the variation in the switching timing of the multiple switching elements 33 is smaller than the switching transition time of each switching element 33.

[0062] However, there are cases where the variation in switching timing becomes large. Suppose that the variation becomes large and, for example, only one of the three switching elements 33 (e.g., switching element 33A) is in the OFF state and the remaining switching elements 33 (e.g., switching elements 33B and 33C) are in the ON state.

[0063] In this case, the equivalent resistance value of the discharge unit 50B including the switching element 33B and the discharge unit 50C including the switching element 33C is a combined resistance value R12, and the equivalent resistance value of the discharge unit 50A including the switching element 33A is a resistance value R1. Since the resistance value R1 is greater than the combined resistance value R12, the voltage of the discharge unit 50A is greater than the voltages of the discharge units 50B and 50C. If the voltage of the discharge unit 50A at this time exceeds the withstand voltage of the switching element 33, the switching element 33 may break down.

[0064] 5, by providing capacitors 35A-35C in the discharge units 51A-51C, respectively, it is possible to prevent transient voltage concentration due to variations in switching timing. Note that the capacitance and rated voltage of capacitor 35 may be smaller than the capacitance and rated voltage of capacitor 28, respectively. Therefore, a small and inexpensive capacitor can be used as capacitor 35.

[0065] 5, the configuration in which the capacitor 35 is provided in each of the discharge units 51 has been described, but the present invention is not limited to this configuration. For example, the capacitor 35 may be provided in at least one of the multiple discharge units 51.

[0066] (Configuration example 3) Fig. 6 is a diagram showing a third configuration example of a discharge circuit. Referring to Fig. 6, a discharge circuit 30C includes a plurality of discharge units 52A to 52C (hereinafter also collectively referred to as "discharge units 52") connected in series. Note that, although the number of discharge units 52 is three in the example of Fig. 6, it is sufficient that the number is two or more.

[0067] Discharge unit 52A has a configuration in which a series body including a resistor 36A and a capacitor 37A connected in series is added to discharge unit 50A of FIG. 4. Specifically, the series body is connected in parallel to switching element 33A. The series body corresponds to an RC snubber circuit. Similarly, discharge unit 52B has a series body including a resistor 36B and a capacitor 37B connected in series. Discharge unit 52C has a series body including a resistor 36C and a capacitor 37C connected in series.

[0068] Resistors 36A to 36C (hereinafter also collectively referred to as "resistors 36") are the same resistor. Capacitors 37A to 37C (hereinafter also collectively referred to as "capacitors 37") are the same capacitor. The capacitance of capacitor 37 is smaller than the capacitance of capacitor 28. In addition, the rated voltage of capacitor 37 is smaller than the rated voltage of capacitor 28.

[0069] The advantages of the discharge circuit 30C shown in Fig. 6 are the same as those of the discharge circuit 30B shown in Fig. 5. In the example of Fig. 6, the configuration in which an RC snubber circuit is provided in all the discharge units 52 has been described, but the present invention is not limited to this configuration. For example, the configuration may be such that an RC snubber circuit is provided in at least one of the multiple discharge units 52. Furthermore, in the example of Fig. 6, an RC snubber circuit in which a resistor 36 and a capacitor 37 are connected in series is connected in parallel to the switching element 33. However, the configuration may be such that a C snubber circuit formed only of the capacitor 37 is connected in parallel to the switching element 33.

[0070] <Modifications of the control method> In the above, a configuration has been described in which when the power converter 6 is in a stopped state, the switching elements 33A to 33C are controlled to the ON state, and when the power converter 6 is in an operating state, the switching elements 33A to 33C are controlled to the OFF state.

[0071] However, when the power converter 6 is in operation, the on / off state of each of the switching elements 33A to 33C may be controlled based on the following conditions.

[0072] When the power converter 6 is in operation and the voltage Vc of the capacitor 28 is equal to or greater than the threshold value Th1, the control device 5 controls each switching element 33 of the discharge circuit 30 to the ON state. On the other hand, when the power converter 6 is in operation and the voltage Vc is less than the threshold value Th2, the control device 5 controls each switching element 33 of the discharge circuit 30 to the OFF state. However, the threshold value Th1 is set to be greater than the threshold value Th2.

[0073] As a result, when the voltage Vc of the capacitor 28 becomes equal to or higher than the threshold Th1, the discharge of the capacitor 28 is promoted, thereby preventing an overvoltage of the capacitor 28. On the other hand, when the voltage Vc becomes less than the threshold Th2, the discharge of the capacitor 28 is suppressed, thereby reducing the loss occurring in the discharge circuit 30.

[0074] In another aspect, the control device 5 controls the on / off state of each switching element 33 based on the deviation between the average value of the voltages Vc of all capacitors 28 included in multiple converter cells 1 in an arm (for example, any of the positive side arms 13u to 13w or the negative side arms 14u to 14w) (hereinafter referred to as the capacitor voltage average value Vav) and the voltage Vc in each converter cell 1 in that arm.

[0075] In this regard, the control device 5 includes an average calculation unit that calculates a capacitor voltage average value Vav corresponding to each arm. The control device 5 includes a deviation calculation unit that calculates a deviation ΔV (for example, ΔV=Vc-Vav) obtained by subtracting the capacitor voltage average value Vav from the voltage Vc of each of the multiple converter cells 1 in the arm.

[0076] When the power converter 6 is in an operating state and the deviation ΔV corresponding to a certain converter cell 1 is equal to or greater than the threshold Th3, the control device 5 controls the switching element 33 corresponding to the converter cell 1 to be in an on state. When the power converter 6 is in an operating state and the deviation ΔV corresponding to a certain converter cell 1 is less than the threshold Th4, the control device 5 controls the switching element 33 corresponding to the converter cell 1 to be in an off state. However, the threshold Th3 is set to be greater than the threshold Th4.

[0077] As a result, when the voltage Vc of the converter cell 1 is greater than or equal to the assumed value with respect to the capacitor voltage average value Vav in the arm to which the converter cell 1 belongs (for example, when ΔV≧Th3), the discharge of the capacitor 28 is promoted, so that the voltage Vc can be decreased. On the other hand, when the difference between the voltage Vc and the capacitor voltage average value Vav becomes small (for example, when ΔV<Th4), the discharge of the capacitor 28 is suppressed. Thereby, the variation in the voltage Vc within the arm can be suppressed.

[0078] Embodiment 2. FIG. 7 is a diagram showing a configuration example of a converter cell 1X according to Embodiment 2. Referring to FIG. 7, the converter cell 1X has a configuration in which the capacitor 28 is replaced with a power storage unit 60 in the converter cell 1 of FIG. 4. The power storage unit 60 includes three capacitors 61A to 61C (hereinafter also collectively referred to as "capacitor 61") connected in series.

[0079] Each of the plurality of discharge units 50 is connected in parallel to the corresponding capacitor 61. Specifically, the discharge unit 50A is connected in parallel to the capacitor 61A, the discharge unit 50B is connected in parallel to the capacitor 61B, and the discharge unit 50C is connected in parallel to the capacitor 61C.

[0080] With the above configuration, the converter cell 1X according to the second embodiment has the same advantages as those of the second configuration example shown in FIG. 5 and the third configuration example shown in FIG. 6. Specifically, by providing the capacitors 61A-61C connected in parallel to the discharge units 50A-50C, respectively, it is possible to prevent transient voltage concentration due to variations in switching timing. Furthermore, the converter cell 1X can reduce the number of parts compared to the second and third configuration examples. The method of controlling the switching element 33 by the control device 5 is the same as that of the first embodiment.

[0081] Other embodiments. (1) In the above-described first embodiment, configuration example 2 and configuration example 3 may be combined. For example, the discharge circuit 30C shown in FIG 6 may be further provided with the capacitor 35 of the discharge circuit 30B shown in FIG 5.

[0082] (2) In the above-described embodiment, a configuration has been described in which a normally-off type semiconductor switching element such as an IGBT is used as the switching element 33, but the present invention is not limited to this configuration. For example, a normally-on type semiconductor switching element such as a JFET (Junction Field Effect Transistor) may be used as the switching element 33. With this configuration, the JFET can always pass a current without receiving a power supply, and therefore, even if the power supply is lost, the discharge of the capacitor 28 can be promoted.

[0083] (3) The configurations exemplified as the above-mentioned embodiments are examples of the configurations of the present disclosure, and may be combined with other known technologies, or may be modified, such as by omitting some parts, without departing from the scope of the present disclosure. In addition, the above-mentioned embodiments may be implemented by appropriately adopting the processes and configurations described in other embodiments.

[0084] The embodiments disclosed herein should be considered to be illustrative and not restrictive in all respects. The scope of the present disclosure is defined by the claims, not the above description, and is intended to include all modifications within the scope and meaning equivalent to the claims. [Explanation of symbols]

[0085] 1,1X converter cell, 2 AC circuit, 3 transformer, 4 DC circuit, 5 control device, 6 power converter, 7a,7b reactor, 8u,8v,8w leg circuit, 9a,9b arm current detector, 10 AC voltage detector, 11a,11b DC voltage detector, 13u~13w positive arm, 14u~14w negative arm, 15 AC current detector, 22A~22D,33A~33C switching element, 23A~23D,34A~34C diode, 25,26 semiconductor circuit, 28,35A~35C,37A~37C,61A~61C capacitor, 30,30A~30C discharge circuit, 31A~31C,32A~32C discharge resistor, 36A~36C Resistor, 50A~50C, 51A~51C, 52A~52C Discharge unit, 60 Storage unit, 70 Input converter, 71 Sample and hold circuit, 72 Multiplexer, 73 A / D converter, 74 CPU, 75 RAM, 76 ROM, 77 Input / output interface, 78 Auxiliary storage device, 79 Bus, 100 Power conversion device.

Claims

1. A power converter comprising a plurality of converter cells connected in series, Each of said converter cells, A semiconductor circuit including a plurality of first switching elements connected in series, A first energy storage element connected in parallel to said semiconductor circuit, And a discharge circuit connected in parallel to said first energy storage element, Said discharge circuit includes a plurality of discharge units connected in series, Said discharge unit includes a first discharge resistor, a second discharge resistor, and a second switching element, Said first discharge resistor is connected in parallel to a first series body including said second discharge resistor and said second switching element connected in series, a power conversion device.

2. The rated voltage of said second switching element is smaller than the rated voltage of said first switching element, the power conversion device according to claim 1.

3. The resistance value of said second discharge resistor is less than or equal to the resistance value of said first discharge resistor, the power conversion device according to claim 1 or claim 2.

4. At least one of said plurality of discharge units further includes a second energy storage element connected in parallel to said first discharge resistor, the power conversion device according to claim 1 or claim 2.

5. The capacitance of said second energy storage element is smaller than the capacitance of said first energy storage element, The rated voltage of said second energy storage element is smaller than the rated voltage of said first energy storage element, the power conversion device according to claim 4.

6. At least one of said plurality of discharge units further includes a third energy storage element connected in parallel to said second switching element, the power conversion device according to claim 1 or claim 2.

7. At least one of the plurality of discharge units further includes a second series body including a third energy storage element and a resistor connected in series, The second series body is connected in parallel to the second switching element, and the power conversion device according to claim 1 or claim 2.

8. The capacitance of the third energy storage element is smaller than the capacitance of the first energy storage element, The rated voltage of the third energy storage element is smaller than the rated voltage of the first energy storage element, and the power conversion device according to claim 6.

9. The capacitance of the third energy storage element is smaller than the capacitance of the first energy storage element, The rated voltage of the third energy storage element is smaller than the rated voltage of the first energy storage element, and the power conversion device according to claim 7.

10. The variation in the switching timing of the plurality of second switching elements included in the plurality of discharge units is smaller than the switching transition time of the second switching element in each of the plurality of discharge units, and the power conversion device according to claim 1 or claim 2.

11. When the power converter is in an operating state, the second switching element is controlled to be in an off state, When the power converter is in a stopped state, the second switching element is controlled to be in an on state, and the power conversion device according to claim 1 or claim 2.

12. When the power converter is in a stopped state, the second switching element is controlled to be in an on state, When the power converter is in an operating state and the voltage of the first energy storage element is equal to or higher than a first threshold value, the second switching element is controlled to be in an on state, When the power converter is in an operating state and the voltage of the first energy storage element is less than a second threshold value, the second switching element is controlled to be in an off state, The power conversion device according to claim 1 or claim 2, wherein the first threshold value is larger than the second threshold value.

13. The power converter includes a plurality of arms, each of the plurality of arms includes two or more of the converter cells, an average value calculation unit that calculates an average value of the voltages of all the first energy storage elements included in the two or more converter cells in the arm; and a deviation calculation unit that calculates a deviation between the voltage of the first energy storage element and the average value for each of the two or more converter cells in the arm. When the power converter is in a stopped state, the second switching element is controlled to be in an on state, when the power converter is in an operating state and the deviation corresponding to one converter cell is equal to or greater than a third threshold value, the second switching element corresponding to the one converter cell is controlled to be in an on state, when the power converter is in an operating state and the deviation corresponding to the one converter cell is less than a fourth threshold value, the second switching element corresponding to the one converter cell is controlled to be in an off state. The power conversion device according to claim 1 or claim 2, wherein the third threshold value is larger than the fourth threshold value.

14. The power conversion device according to claim 1 or claim 2, wherein the second switching element is a normally-on type semiconductor element.

15. A power converter including a plurality of converter cells connected in series is provided, each of the converter cells includes a semiconductor circuit including a plurality of first switching elements connected in series, a power storage unit including a plurality of energy storage elements connected in parallel to the semiconductor circuit, and a discharge circuit connected in parallel to the power storage unit. The discharge circuit includes a plurality of discharge units connected in series. Each of the plurality of discharge units is connected in parallel to the corresponding power storage element. The discharge unit includes a first discharge resistor, a second discharge resistor, and a second switching element. The first discharge resistor is a power conversion device that is connected in parallel to a first series body including the second discharge resistor and the second switching element connected in series.

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