Power conversion device, control method of power conversion device, and program

Abstract

To provide a power conversion device in which two DC lines for transmitting DC power are not grounded at the closest end, and which can lower and interrupt a fault current passing through an AC breaker, and to provide a control method of the power conversion device and a program.SOLUTION: In a power conversion device of an embodiment, a low-voltage terminal having a small absolute value of a difference from a ground potential is connected to a ground potential via a line impedance of a DC power transmission line, and the power conversion device includes a leg, a first short-circuit switch, and a second short-circuit switch. The leg corresponds to each phase of the three phase AC terminal, and a first arm connected between the low-voltage terminal and the three phase AC terminal and a second arm connected between the high-voltage terminal having a large absolute value of a difference from a ground potential and the three phase AC terminal are connected in series on the three phase AC terminal side. The first short-circuit switch short-circuits a potential between the low-voltage terminal and the first arm to a ground potential. The second short-circuit switch short-circuits a potential between the high-voltage terminal and the second arm to a ground potential.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] An embodiment of the present invention relates to a power conversion device, a control method for a power conversion device, and a program. [Background technology]

[0002] DC power transmission systems using power converters have been in operation for some time. In DC power transmission systems, power converters are connected between AC and DC systems and convert the power of each system into the other. In recent years, modular multilevel converters (MMCs) have been put into practical use as power conversion devices. MMCs are equipped with arm units containing multiple unit converters (hereinafter referred to as "cells") connected in series, and are capable of handling high voltages and large capacity by adding up the voltages output by each cell.

[0003] In a power conversion device, if a ground fault or short circuit occurs while the three phases of an AC system are out of balance, an excessive DC fault current may flow to the AC system. In this case, for example, an AC circuit breaker provided on the AC system side of the power conversion device electrically separates the AC system from the power conversion device. In this case, in order to enable the AC circuit breaker to cut off the power, the power conversion device needs to reduce the fault current passing through the AC circuit breaker to a level where an arc discharge occurring between the contacts is extinguished.

[0004] In this regard, for example, Patent Document 1 discloses a technology for reducing a fault current passing through an AC circuit breaker in a power conversion device configured such that one of two DC terminals connected to a DC system side is grounded without passing through the line impedance of a DC transmission line (hereinafter, grounding without passing through the impedance of a DC transmission line is also simply referred to as "near-end grounding"). In the conventional technology, a short-circuit switch is provided to connect the ungrounded DC terminal of the two DC terminals to the ground or directly to the ground side, and in the event of a fault, the short-circuit switch connects the ungrounded DC terminal to the ground side, thereby reducing the fault current passing through the AC circuit breaker.

[0005] However, power converters used in DC power transmission systems are not necessarily configured such that one of the DC terminals is near-end grounded without passing through the line impedance of the DC transmission line. That is, some power converters used in DC power transmission systems are configured such that neither of the two DC terminals is near-end grounded. Therefore, even if conventional technology is applied to a power converter configured such that neither of the two DC lines is near-end grounded, the power converter may not be able to sufficiently reduce the fault current passing through the AC circuit breaker due to a fault. This causes a problem in that the fault current that cannot be sufficiently reduced cannot be interrupted by the AC circuit breaker. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2018-148685 Summary of the Invention [Problem to be solved by the invention]

[0007] The problem to be solved by the present invention is to provide a power conversion device, a control method for a power conversion device, and a program that can reduce a fault current passing through an AC circuit breaker and interrupt the fault current in a power conversion device configured such that neither of two DC lines for transmitting DC power is grounded at the nearest end. [Means for solving the problem]

[0008] A power conversion device according to an embodiment converts power between a DC terminal and a three-phase AC terminal, the DC terminal having a high-voltage terminal with a large absolute value of difference from ground potential and a low-voltage terminal with a smaller absolute value of difference from ground potential than the high-voltage terminal. The low-voltage terminal is connected to ground potential via a line impedance of a DC transmission line that transmits converted DC power. The power conversion device includes legs, a first short-circuiting switch, and a second short-circuiting switch. Each leg corresponds to each phase of the three-phase AC terminal and includes a plurality of unit converters connected in series. A first arm is connected between the low-voltage terminal and the three-phase AC terminal, and a second arm is connected between the high-voltage terminal and the three-phase AC terminal, and the first arm is connected in series on the three-phase AC terminal side. The first short-circuiting switch shorts the potential between the low-voltage terminal and the first arm to the ground potential. The second short-circuiting switch shorts the potential between the high-voltage terminal and the second arm to the ground potential. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a diagram showing a configuration of a power conversion device according to a first embodiment and an example of a DC power transmission system to which the power conversion device is applied. [Figure 2] FIG. 2 is a diagram showing an example of the configuration of a unit converter in a leg included in a power conversion device. [Figure 3] 4 is a diagram showing an example of a current waveform of an AC current that is cut off by an AC circuit breaker provided in the power conversion device when a fault occurs. FIG. [Figure 4] FIG. 4 is a sequence diagram showing an example of a control flow of a first operation for interrupting a fault in a control device provided in the power conversion device. [Figure 5] FIG. 4 is a sequence diagram showing an example of a control flow of a second operation for interrupting a fault in a control device provided in the power conversion device. [Figure 6] FIG. 10 is a diagram showing an example of a configuration of a power conversion device according to a second embodiment and a DC power transmission system to which the power conversion device is applied. [Figure 7] FIG. 10 is a diagram showing an example of a configuration of a power conversion device according to a third embodiment and a DC power transmission system to which the power conversion device is applied. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, a power conversion device, a control method for a power conversion device, and a program according to an embodiment will be described with reference to the drawings.

[0011] (First embodiment) [Configuration of DC transmission system] Fig. 1 is a diagram showing a configuration of a power conversion device according to a first embodiment and an example of a DC power transmission system to which the power conversion device is applied. Fig. 1 shows an example in which a power conversion device 10 is provided at an interconnection point on the AC power source AC side in a DC power transmission system 1 that transmits power between an AC power source AC and a DC power source DC. The AC power source AC may be, for example, an AC system or an AC load. The DC power source DC may be, for example, a DC system or a DC load.

[0012] In the DC power transmission system 1, AC transmission lines of corresponding phases in a three-phase AC power source AC that supplies AC power are connected to three-phase AC terminals (AC terminal R, AC terminal S, AC terminal T) of the first phase (R phase), second phase (S phase), and third phase (T phase) of the power conversion device 10. Furthermore, in the DC power transmission system 1, two DC terminals (DC terminal A, DC terminal B) on the opposite side to the three-phase AC terminals of the power conversion device 10 are connected to corresponding DC transmission lines (DC transmission line LA, DC transmission line LB) of the DC power source DC that supplies DC power.

[0013] Here, in the DC power transmission system 1, the DC transmission line LA connected to the DC terminal A and the DC transmission line LB connected to the DC terminal B each have a line impedance (line impedance LIA, line impedance LIB). That is, in the DC power transmission system 1, DC power is supplied to the DC power source DC via the line impedance of each DC transmission line (hereinafter, when there is no need to distinguish between the line impedance LIA and the line impedance LIB, they will be referred to as "line impedance LI").

[0014] Furthermore, in the DC power transmission system 1, the DC transmission line LA between the line impedance LIA and the DC power source DC is grounded. In other words, in the DC power transmission system 1, the DC terminal A of the power conversion device 10 is connected to the ground potential via the line impedance LIA of the DC transmission line LA.

[0015] The power conversion device 10 converts AC power supplied by an AC power source AC into DC power supplied by a DC power source DC, and vice versa, between three-phase AC terminals (AC terminal R, AC terminal S, AC terminal T) and DC terminals (DC terminal A, DC terminal B). The DC terminal B of the power conversion device 10 is a high-voltage terminal having a large absolute value of difference with respect to ground potential, and the DC terminal A of the power conversion device 10 is a low-voltage terminal having a smaller absolute value of difference with respect to ground potential than the DC terminal B. Here, the low-voltage terminal is a terminal on the so-called return line side of a DC transmission line. In the following description, the DC terminal B is also referred to as the "high-voltage terminal B," and the DC terminal A is also referred to as the "low-voltage terminal A."

[0016] [Configuration of power conversion device] The power conversion device 10 is an example of a power conversion device equipped with a double-star-connected modular multilevel converter (MMC) that converts AC power and DC power mutually. The power conversion device 10 is configured such that a DC transmission line LA, which is grounded between a line impedance LIA and a DC power source DC, is connected to a low-voltage terminal A, and neither DC terminal is near-end grounded. If the DC power source DC were a power conversion device, the DC power source DC would be equivalent to a conventional power conversion device in which one of two DC transmission lines (here, the DC transmission line LA) is near-end grounded. The power conversion device 10 includes AC circuit breakers 11, transformers 12, and legs 13 corresponding to the three-phase AC terminals, a first short-circuit switch 14A corresponding to the low-voltage terminal A, a second short-circuit switch 14B corresponding to the high-voltage terminal B, and a control device 50.

[0017] The AC circuit breaker 11 is a circuit breaker that interrupts the AC line between the three-phase AC terminals and the transformer 12. The AC circuit breaker 11 is a circuit breaker in which a mechanical contact type switch is provided for each phase of the three-phase AC terminal. The interrupting operation of the AC circuit breaker 11 with respect to the AC line is controlled by the control device 50. More specifically, the AC circuit breaker 11 is controlled by the control device 50 to be in either a conductive state in which the AC line is electrically connected by closing each of the mechanical contact type switches, or a cut-off state in which the AC line is electrically cut-off by opening each of the mechanical contact type switches.

[0018] The transformer 12 is a transformer that electrically insulates the AC side of the AC power supply from the power conversion device 10 and adjusts the voltage level difference between the AC side of the AC power supply and each leg 13.

[0019] The number of legs 13 included in the power conversion device 10 corresponds to the number of phases of AC power supplied by (and supplied to) the AC power source AC. Fig. 1 shows a case in which the AC power source AC supplies three-phase AC power, namely, a first phase (R phase), a second phase (S phase), and a third phase (T phase), and an AC transmission line for the AC power of each phase is connected to a corresponding AC terminal. For this reason, Fig. 1 shows the configuration of the power conversion device 10 including three legs 13 corresponding to the respective AC terminals. The legs 13 corresponding to the respective three-phase AC terminals, i.e., corresponding to the respective phases of the AC power, have the same configuration.

[0020] In each leg 13, the connection point CP is connected to an AC line connected to an AC terminal of a corresponding phase of AC power supplied by the AC power source AC. More specifically, in the leg 13 corresponding to the R phase, the connection point CP is connected to an AC line connected to an AC terminal R corresponding to the R phase of the AC power source AC, in the leg 13 corresponding to the S phase, the connection point CP is connected to an AC line connected to an AC terminal S corresponding to the S phase of the AC power source AC, and in the leg 13 corresponding to the T phase, the connection point CP is connected to an AC line connected to an AC terminal T corresponding to the T phase of the AC power source AC. Fig. 1 shows a case in which the connection point CP of each leg 13 is connected to a corresponding AC line provided with an AC circuit breaker 11 and a transformer 12, that is, connected to an AC terminal of a corresponding phase of the AC power source AC via the AC circuit breaker 11 and the transformer 12.

[0021] In each leg 13, a terminal on the opposite side to the connection point CP is connected to a corresponding DC terminal. More specifically, in each leg 13, a terminal having the same potential as a low-voltage DC voltage output by the power conversion device 10 and having a small absolute value of difference from the ground potential is connected to a low-voltage terminal A, and a terminal having the same potential as a high-voltage DC voltage output by the power conversion device 10 and having a large absolute value of difference from the ground potential is connected to a high-voltage terminal B. In the following description, the terminal of the leg 13 connected to the low-voltage terminal A is also referred to as a DC terminal CA of the leg 13, and the terminal of the leg 13 connected to the high-voltage terminal B is also referred to as a DC terminal CB of the leg 13.

[0022] Each leg 13 includes, for example, two buffer reactors 15 (buffer reactor 15A and buffer reactor 15B) and two arms 16. Each arm 16 includes, for example, n (n is a natural number) unit converters (hereinafter also referred to as "cells") 162 connected in series. FIG. 1 shows a configuration in which each arm 16 includes two unit converters 162. The unit converter 162 is, for example, a half-bridge circuit such as a chopper cell. The unit converter 162 may also be, for example, a full-bridge circuit.

[0023] In each leg 13, a low-voltage side buffer reactor 15A and a low-voltage side arm 16 (hereinafter also referred to as a "first arm 16A") are connected in series, and a high-voltage side buffer reactor 15B and a high-voltage side arm 16 (hereinafter also referred to as a "second arm 16B") are connected in series. In each leg 13, a connection point between the buffer reactor 15A and the buffer reactor 15B is a connection point CP. In each leg 13, a DC terminal CA on the first arm 16A opposite to the buffer reactor 15A is a low-voltage terminal A of the power conversion device 10, and a DC terminal CB on the second arm 16B opposite to the buffer reactor 15B is a high-voltage terminal B of the power conversion device 10. In other words, in each leg 13, the first arm 16A and the buffer reactor 15A are connected in series in this order from the low-voltage terminal A side toward the connection point CP side, and are connected to the connection point CP. Furthermore, in each leg 13, second arm 16B and buffer reactor 15B are connected in series in this order from the high voltage terminal B side toward connection point CP side, and are then connected to connection point CP. As a result, in leg 13, first arm 16A and second arm 16B are connected in series between low voltage terminal A and high voltage terminal B.

[0024] The buffer reactor 15 prevents excessive short-circuit current from flowing between the three legs 13 corresponding to each phase due to differences in the voltage values ​​of the three-phase AC voltages. While FIG. 1 shows a case where the buffer reactor 15 is disposed on the connection point CP side of the arm 16 in each leg 13, the buffer reactor 15 may also be disposed on the opposite side of the arm 16 from the connection point CP side (i.e., on the DC terminal CA side or the DC terminal CB side) or at any position within the arm 16 (i.e., between any two unit converters 162 connected in series in the arm 16). The buffer reactor 15 may be replaced with a transformer with a special winding structure that has a leakage reactance sufficient to replace the function of a reactor. In this case, the buffer reactor 15 may be integrated with the transformer 12.

[0025] First arm 16A (which may include buffer reactor 15A) is an example of a “first arm,” and second arm 16B (which may include buffer reactor 15B) is an example of a “second arm.” Unit converter 162 is an example of a “unit converter.”

[0026] Each arm 16 generates a voltage of any voltage waveform, such as a multi-level waveform that is a stepped sine wave representing the AC waveform to be supplied to the corresponding phase of the AC power source AC, in response to control from the control device 50 for each unit converter 162 connected in series.

[0027] Here, an example of the configuration of the unit converter 162 included in the arm 16 will be described. Fig. 2 is a diagram showing an example of the configuration of the unit converter 162 in the leg 13 included in the power conversion device 10. As described above, the unit converter 162 may be, for example, a chopper cell (half-bridge circuit) or a full-bridge circuit. Fig. 2(a) shows an example of a unit converter 162 configured as a chopper cell (half-bridge circuit) (hereinafter referred to as "unit converter 162a"), and Fig. 2(b) shows an example of a unit converter 162 configured as a full-bridge circuit (hereinafter referred to as "unit converter 162b").

[0028] The unit converter 162a includes two switching elements Q (switching element Q1 and switching element Q2), two diodes D (diode D1 and diode D2), and a capacitor C. The switching element Q is, for example, a power semiconductor switching element such as an insulated gate bipolar transistor (IGBT). The switching element Q is not limited to an IGBT. The switching element Q may be any element as long as it is a self-arc-suppressing power semiconductor switching element that can realize a converter or inverter.

[0029] In the unit converter 162a, the switching element Q1 and the switching element Q2 are connected in series to each other. In the unit converter 162a, the series circuit of the switching element Q1 and the switching element Q2 and the capacitor C are connected in parallel to each other. In the unit converter 162a, each switching element Q and its corresponding diode D are connected in parallel to each other. In the unit converter 162a, the connection point between the emitter of the switching element Q1 and the collector of the switching element Q2 is the high-voltage side terminal TH(+) connected to the high-voltage terminal B side (DC terminal CB side) of the leg 13, and the connection point between the emitter of the switching element Q2 and the capacitor C is the low-voltage side terminal TL(-) connected to the low-voltage terminal A side (DC terminal CA side) of the leg 13.

[0030] Control signals from the control device 50 are input to the gates of the switching element Q1 and the switching element Q2 included in the unit converter 162a (a control voltage is applied or a control current is supplied). A gate signal gtp is input to the gate of the switching element Q1 as a control signal from the control device 50, and a gate signal gtn is input to the gate of the switching element Q2 as a control signal from the control device 50. As a result, each of the switching elements Q1 and Q2 is switched to either an ON state or an OFF state by the control device 50. For example, when a control signal (gate signal gtp, gate signal gtn) of "1 (High level)" is input, the respective switching elements Q are turned ON, and when a control signal (gate signal gtp, gate signal gtn) of "0 (Low level)" is input, the respective switching elements Q are turned OFF.

[0031] The capacitor C is charged or discharged according to the state of each switching element Q. In the unit converter 162a, the terminal voltage of the capacitor C (hereinafter referred to as "capacitor voltage Vc") is output as the terminal voltage between the high-voltage side terminal TH and the low-voltage side terminal TL of the unit converter 162a (hereinafter referred to as "cell voltage Vo"). More specifically, when the control device 50 sets the control signals (gtp, gtn) = (1, 0), for example, current flows in the order of the low-voltage side terminal TL, capacitor C, switching element Q1, and high-voltage side terminal TH, and the cell voltage Vo of the unit converter 162a matches the capacitor voltage Vc. In other words, when the control device 50 sets the control signals (gtp, gtn) = (1, 0), the capacitor C is inserted between the high-voltage side terminal TH and low-voltage side terminal TL of the unit converter 162a, and the capacitor voltage Vc is output as the cell voltage Vo. On the other hand, when the control device 50 sets the control signals (gtp, gtn) = (0, 1), for example, current flows in the order of the high-voltage side terminal TH, switching element Q2, and low-voltage side terminal TL, and the cell voltage Vo of the unit converter 162a becomes 0 [V]. In other words, when the control device 50 sets the control signals (gtp, gtn) = (0, 1), the high-voltage side terminal TH and low-voltage side terminal TL of the unit converter 162a are short-circuited, and current flows without passing through the capacitor C, so that the cell voltage Vo of the unit converter 162a becomes 0 [V].

[0032] The unit converter 162b includes four switching elements Q (switching element Q1 to switching element Q4), four diodes D (diodes D1 to D4), and a capacitor C. The functions and operations of the switching elements Q, diodes D, and capacitors C included in the unit converter 162b are similar to the functions and operations of the switching elements Q, diodes D, and capacitors C included in the unit converter 162a.

[0033] In the unit converter 162b, switching element Q1 and switching element Q2 are connected in series, and switching element Q3 and switching element Q4 are connected in series. In the unit converter 162b, the series circuit of switching element Q1 and switching element Q2, the series circuit of switching element Q3 and switching element Q4, and a capacitor C are connected in parallel. In the unit converter 162b, each switching element Q and its corresponding diode D are connected in parallel. In the unit converter 162b, the connection point between the emitter of switching element Q1 and the collector of switching element Q2 is the high-voltage side terminal TH(+) connected to the high-voltage terminal B side (DC terminal CB side) of leg 13, and the connection point between the emitter of switching element Q3 and the collector of switching element Q4 is the low-voltage side terminal TL(-) connected to the low-voltage terminal A side (DC terminal CA side) of leg 13.

[0034] A control signal from the control device 50 is input to the gates of each of switching elements Q1 to Q4 included in the unit converter 162b (a control voltage is applied or a control current is supplied). A gate signal gta is input to the gate of switching element Q1 as a control signal from the control device 50, a gate signal gtb is input to the gate of switching element Q2 as a control signal from the control device 50, a gate signal gtc is input to the gate of switching element Q3 as a control signal from the control device 50, and a gate signal gtd is input to the gate of switching element Q4 as a control signal from the control device 50. As a result, each of switching elements Q1 to Q4 is switched to either an ON state or an OFF state by the control device 50. In the unit converter 162b as well, the inter-terminal voltage of capacitor C (capacitor voltage Vc) is output as an inter-terminal voltage (cell voltage Vo) between the high-voltage side terminal TH and the low-voltage side terminal TL of the unit converter 162b. More specifically, when the control device 50 sets the control signals (gta, gtb, gtc, gtd) = (1, 0, 0, 1), for example, current flows in the order of low-voltage side terminal TL, switching element Q4, capacitor C, switching element Q1, and high-voltage side terminal TH, and the cell voltage Vo of the unit converter 162b matches the capacitor voltage Vc. In other words, when the control device 50 sets the control signals (gta, gtb, gtc, gtd) = (1, 0, 0, 1), the capacitor C is inserted between the high-voltage side terminal TH and low-voltage side terminal TL of the unit converter 162b, and the capacitor voltage Vc is output as the cell voltage Vo. On the other hand, when the control device 50 sets the control signals (gta, gtb, gtc, gtd) = (0, 1, 1, 0), for example, current flows in the order of high-voltage side terminal TH, switching element Q2, capacitor C, switching element Q3, and low-voltage side terminal TL, and the cell voltage Vo of the unit converter 162b coincides with the inverse of the capacitor voltage Vc. In other words, when the control device 50 sets the control signals (gta, gtb, gtc, gtd) = (0, 1, 1, 0), the capacitor C is inserted in the reverse direction between the high-voltage side terminal TH and low-voltage side terminal TL of the unit converter 162b, and the negative capacitor voltage Vc is output as the cell voltage Vo.Furthermore, when the control device 50 sets the control signals (gta, gtb, gtc, gtd) = (1, 0, 1, 0) or (0, 1, 0, 1), current flows, for example, in the order of the high-voltage side terminal TH, diode D1, switching element Q3, and low-voltage side terminal TL, or in the order of the high-voltage side terminal TH, switching element Q2, diode D4, and low-voltage side terminal TL, and the cell voltage Vo of the unit converter 162b becomes 0 [V]. In other words, when the control device 50 sets the control signals (gta, gtb, gtc, gtd) = (1, 0, 1, 0) or (0, 1, 0, 1), the high-voltage side terminal TH and low-voltage side terminal TL of the unit converter 162b are short-circuited, and current flows without passing through the capacitor C, and the cell voltage Vo of the unit converter 162b becomes 0 [V].

[0035] The unit converter 162 is not limited to the configuration of the unit converter 162a shown in FIG. 2(a) or the unit converter 162b shown in FIG. 2(b), and may have any configuration as long as it achieves the same functions as the unit converter 162a or the unit converter 162b.

[0036] By connecting a plurality of unit converters 162 (unit converter 162a or unit converter 162b) configured in this way in series, the arm 16 outputs a voltage obtained by adding together the cell voltages Vo of each unit converter 162 in which a capacitor C is inserted between the high-voltage side terminal TH and the low-voltage side terminal TL. Therefore, the arm 16 outputs a voltage corresponding to the number of unit converters 162 controlled by the control device 50 so that a capacitor C is inserted between the high-voltage side terminal TH and the low-voltage side terminal TL. In this way, the arm 16 generates a multilevel waveform in accordance with the control from the control device 50.

[0037] Returning to Fig. 1, the first short-circuit switch 14A is a switch for short-circuiting the DC line between the DC terminal CA and the low-voltage terminal A to the ground potential. The second short-circuit switch 14B is a switch for short-circuiting the DC line between the DC terminal CB and the high-voltage terminal B to the ground potential. Each of the first short-circuit switch 14A and the second short-circuit switch 14B is a switch having a withstand voltage rating for short-circuiting the DC line to the ground potential, and is a switch having a rapid response such that it becomes closed (ON) in, for example, the order of several tens of milliseconds.

[0038] The first short-circuit switch 14A and the second short-circuit switch 14B are, for example, mechanical switches or power semiconductor switching elements. When the first short-circuit switch 14A and the second short-circuit switch 14B are configured as mechanical switches, the cost of realizing the power conversion device 10 can be reduced.

[0039] When the first short-circuit switch 14A and the second short-circuit switch 14B are configured with power semiconductor switching elements, for example, they may be configured in the same way as the converter unit 162, where a switching element such as an insulated gate bipolar transistor (IGBT) and a diode are connected in parallel with each other (hereinafter referred to as a "semiconductor switch unit"), or a plurality of such semiconductor switch units may be connected in series. The semiconductor switch unit may be, for example, a thyristor. When the first short-circuit switch 14A and the second short-circuit switch 14B are configured with power semiconductor switching elements, it is possible to realize a power conversion device 10 that enables the AC circuit breaker 11 to more quickly interrupt the AC line.

[0040] The first short-circuit switch 14A and the second short-circuit switch 14B may be any switches that have a withstand voltage rating for short-circuiting the DC line to ground potential and have a fast response. For example, the first short-circuit switch 14A may be a metallic return transfer breaker (MRTB(S): Switch). That is, in the power conversion device 10, the first short-circuit switch 14A may be a return line forced arc-extinguishing device that is also arranged in a general DC transmission system as a component for eliminating a ground fault that occurs in the return line. In this case, the power conversion device 10 can be realized without newly adding the first short-circuit switch 14A.

[0041] The short-circuiting operation of the first short-circuit switch 14A and the second short-circuit switch 14B with respect to the corresponding DC lines is controlled by the control device 50. More specifically, the first short-circuit switch 14A and the second short-circuit switch 14B are controlled by the control device 50 to either an open state (off state) in which the DC lines are not short-circuited to the ground potential, or a closed state (on state) in which the DC lines are short-circuited to the ground potential.

[0042] The first short-circuit switch 14A is an example of a "first short-circuit switch," and the second short-circuit switch 14B is an example of a "second short-circuit switch."

[0043] The control device 50 controls the operating state of the power conversion device 10. More specifically, the control device 50 controls the voltage value of the voltage output from each arm 16 of the leg 13, that is, controls the power conversion operation in the power conversion device 10, controls the AC line interruption operation by the AC circuit breaker 11, and controls the short-circuit operation of the first short-circuit switch 14A and the second short-circuit switch 14B on the corresponding DC line. In the following description, when there is no need to distinguish between the first short-circuit switch 14A and the second short-circuit switch 14B, they will be referred to as the "short-circuit switch 14."

[0044] The control device 50 controls the operating state of the power conversion device 10 by, for example, executing a program (software) using a hardware processor such as a CPU (Central Processing Unit). The control device 50 may be realized by hardware (including circuitry) such as an LSI (Large Scale Integration), an ASIC (Application Specific Integrated Circuit), an FPGA (Field-Programmable Gate Array), or a GPU (Graphics Processing Unit), or may be realized by a combination of software and hardware. The control device 50 may be realized by a dedicated LSI. The program may be stored in advance in a storage device (storage device having a non-transitory storage medium) such as an HDD (Hard Disk Drive) or flash memory provided in the control device 50 or the power conversion device 10, or may be stored in a removable storage medium (non-transitory storage medium) such as a DVD or CD-ROM, and installed in the HDD or flash memory provided in the control device 50 or the power conversion device 10 by attaching the storage medium to a drive device provided in the control device 50 or the power conversion device 10.

[0045] The control device 50 includes, for example, a cell control unit 52, a circuit breaker control unit 54, and a short-circuit switch control unit 56.

[0046] The cell control unit 52 controls the voltage value of the voltage output from each arm 16 included in the leg 13. More specifically, as described above, the cell control unit 52 controls the switching element Q included in each unit converter 162 to the on state or the off state, thereby causing the voltage to be output from the arm 16, the voltage being the sum of the cell voltages Vo of each unit converter 162.

[0047] The circuit breaker control unit 54 controls the AC line interruption operation by the AC circuit breaker 11. When the circuit breaker control unit 54 detects a fault, for example, between DC terminal CA and DC terminal CB of the R phase based on the detection value of the detector, the circuit breaker control unit 54 controls the AC circuit breaker 11 to an interrupted state (open state), thereby interrupting the AC line.

[0048] The short-circuit switch control unit 56 controls the short-circuit operation of the first short-circuit switch 14A and the second short-circuit switch 14B on the corresponding DC line. When the circuit breaker control unit 54 detects a fault occurring between the DC terminal CA and the DC terminal CB in the leg 13 of each phase based on the detection value of the detector, the circuit breaker control unit 54 controls each of the first short-circuit switch 14A and the second short-circuit switch 14B to a closed state (on state), thereby short-circuiting the corresponding DC line to the ground potential.

[0049] With this configuration, the control device 50 controls the operating state of the power conversion device 10. Furthermore, when the control device 50 detects a fault occurring between DC terminal CA and DC terminal CB in leg 13 of each phase, it controls the AC circuit breaker 11 to interrupt the AC line and controls the first short-circuit switch 14A and the second short-circuit switch 14B to short-circuit the corresponding DC line, thereby disconnecting the AC transmission line of each phase connected to the AC power source AC from the DC transmission lines LA and LB connected to the DC power source DC.

[0050] In controlling the operating state of the power conversion device 10 by the control device 50, when the cell control unit 52 is performing a power conversion operation (outputting power from the arm 16), the control device 50 detects whether an accident (ground fault or short-circuit accident) has occurred between the DC terminal CA and the DC terminal CB in the leg 13 of each phase. More specifically, the control device 50 monitors the detection values ​​of a detector (not shown) provided at an arbitrary position in the power conversion device 10, and detects an accident that has occurred between the DC terminal CA and the DC terminal CB in the leg 13 of each phase based on the detection values ​​of the detector. The detector is, for example, a current detector that detects a current value and a current polarity, or a voltage detector that detects a voltage value. The detector is, for example, disposed at a position where it can detect the current value, current polarity, and voltage value of the DC terminal CA, the DC terminal CB, and the secondary side (leg 13 side) of the transformer 12. Based on the detection values ​​of these detectors, the control device 50 detects an accident that has occurred between the DC terminal CA and the DC terminal CB in the leg 13 of each phase.

[0051] The method of detecting a fault in the control device 50 is not limited to the method using the detector described above and the values ​​detected by this detector, and any method or configuration may be used as long as it is capable of detecting a fault occurring between the DC terminal CA and the DC terminal CB in the leg 13 of each phase. For example, in the power conversion device 10, the detectors may be disposed at positions where they can detect the current values, current polarity, and voltage values ​​of the DC terminal CA, the DC terminal CB, and the secondary side (leg 13 side) of the transformer 12, and the control device 50 may detect a fault occurring between the DC terminal CA and the DC terminal CB in the leg 13 of each phase by determining the difference between the detected values ​​of these detectors, deviation from the normal operating range, an increase in the amount of change, etc.

[0052] When the control device 50 detects an accident, the circuit breaker control unit 54 controls the AC circuit breaker 11 to cut off the AC line, and the short-circuit switch control unit 56 controls the first short-circuit switch 14A and the second short-circuit switch 14B to short-circuit the corresponding DC line.

[0053] Here, an example of AC current flowing into the AC line by the AC circuit breaker 11 when a fault is detected will be described. Fig. 3 is a diagram showing an example of a current waveform of AC current interrupted by the AC circuit breaker 11 provided in the power conversion device 10 when a fault occurs. Fig. 3 shows a current waveform of one phase of the AC power source AC, which is a three-phase AC system, i.e., a current waveform of one phase of the AC terminals of the three-phase AC terminals. Fig. 3(a) shows an example of AC system current flowing to the AC power source AC side when a fault occurs in the power conversion device 10 having a configuration not including the first short-circuit switch 14A and the second short-circuit switch 14B, i.e., a conventional power conversion device configuration (hereinafter referred to as "conventional power conversion device"). Fig. 3(b) shows an example of AC system current flowing to the AC power source AC side when a fault occurs in the power conversion device 10.

[0054] As shown in FIG. 3A, for example, if a fault is detected between the R-phase DC terminal CA and the DC terminal CB, the DC current flowing between the R-phase DC terminal CA and the DC terminal CB is superimposed on the AC system current as a fault current, causing an overall increase in the AC system current. As this AC system current increases, the points where the current periodically crosses zero (current zero points) disappear. Generally, when an AC circuit breaker 11 is used to interrupt an AC line, it is preferable to open the mechanical contact switch at the current zero point. However, in conventional power conversion devices, the interruption operation of the AC circuit breaker 11 is controlled when the current zero point has disappeared. This control of the interruption operation of the AC circuit breaker 11 therefore fails to interrupt a line where the AC system current has increased overall due to a fault. This is because the AC circuit breaker 11 cannot interrupt when the AC system current is increased overall, as shown in FIG. 3A.

[0055] In contrast, in the power conversion device 10, when an accident occurs, the control device 50 controls the short-circuit operation of bringing each of the first short-circuit switch 14A and the second short-circuit switch 14B into a closed state (on state) at an appropriate timing, and controls the interruption operation by the AC circuit breaker 11 based on the closed states (on states) of the first short-circuit switch 14A and the second short-circuit switch 14B. For example, as shown in (b) of FIG. 3, after the accident occurs, the control device 50 (more specifically, the short-circuit switch control unit 56) controls the short-circuit operation of bringing each of the short-circuit switches 14A and 14B into a closed state (on state) in that order, starting with the first short-circuit switch 14A and then the second short-circuit switch 14B. As a result, the DC transmission lines LA and LB are short-circuited to the ground potential, and a current that acts to lower the AC system current that rises due to the superposition of the DC current (fault current) and cause a current zero point where the current value crosses zero (i.e., a current that assists the drop in the AC system current (hereinafter referred to as a "zero-point auxiliary current")) flows, thereby intentionally causing a current zero point to exist. Then, the control device 50 (more specifically, the circuit breaker control unit 54) controls the interruption operation to cause the AC circuit breaker 11 to interrupt the AC line at the timing of the current zero point. As a result, the power conversion device 10 can safely interrupt the AC line in which the AC system current has risen overall due to the fault by controlling the interruption operation of the AC circuit breaker 11.

[0056] Therefore, in the control device 50, the AC circuit breaker 11 controls the AC line interruption operation and the first and second short-circuit switches 14A and 14B control the corresponding DC line short-circuit operation. After the short-circuit switch control unit 56 controls the first and second short-circuit switches 14A and 14B to a closed state (on state), the control device 50 checks whether the first and second short-circuit switches 14A and 14B are actually in a closed state (on state). This check can be performed, for example, based on the detection value of a detector used to detect whether a fault has occurred between the R-phase DC terminal CA and the DC terminal CB. More specifically, the control device 50 can check whether the first short-circuit switch 14A is actually in a closed state (on state) by, for example, calculating the difference between the detection values ​​of the detectors disposed before and after the connection point to which the first short-circuit switch 14A is connected. The control device 50 can, for example, determine the difference between the detection values ​​of the detectors located before and after the connection point to which the second short-circuit switch 14B is connected, thereby confirming whether the second short-circuit switch 14B is actually in a closed state (on state).

[0057] The method of checking whether each of the first short-circuit switch 14A and the second short-circuit switch 14B is actually in the closed state (on state) in the control device 50 is not limited to the method using the detector described above and the detection value by this detector. For example, if each of the first short-circuit switch 14A and the second short-circuit switch 14B is provided with a sensor that detects its own open state or closed state, it may be checked whether each of the first short-circuit switch 14A and the second short-circuit switch 14B is actually in the closed state (on state) based on the detection result of this sensor.

[0058] [First Operation of Power Conversion Device] Next, an example of an operation for breaking a fault when an accident occurs in the power conversion device 10, that is, control of the breaking operation by the circuit breaker control unit 54 provided in the control device 50 and control of the short-circuit operation by the short-circuit switch control unit 56 will be described. In the following explanation, for ease of explanation, it will be described that the control device 50 controls the breaking operation and the short-circuit operation.

[0059] 4 is a sequence diagram showing an example of the control flow of a first operation for interrupting a fault in the control device 50 provided in the power conversion device 10. In the following description, it is assumed that an fault (e.g., a ground fault) occurs between the DC terminal CA and the DC terminal CB of the R phase at time t0, for example. In this case, in the power conversion device 10, a fault current (ground fault current) flows from the AC power supply AC toward the location where the ground fault occurred (ground fault point), causing an imbalance in the currents of the three phases (R phase, S phase, and T phase). In the following description, it is assumed that the control device 50 detects the fault at time t1, for example.

[0060] In this case, the control device 50 turns off the switching element Q of the unit converter 162 provided in each arm 16 (control signal (gtp, gtn) = (0, 0)), causing each unit converter 162 to enter a state in which it does not control the cell voltage Vo (so-called gate block state).

[0061] Thereafter, for example, at time t12, the control device 50 outputs a control signal (hereinafter referred to as a "closing control signal SCA") for bringing the first short-circuit switch 14A into a closed state (on state). As a result, the first short-circuit switch 14A is brought into a closed state (on state), and the DC line between the DC terminal CA and the low-voltage terminal A (i.e., the DC transmission line LA) is short-circuited to the ground potential via the first short-circuit switch 14A. Furthermore, for example, at time t13, the control device 50 outputs a control signal (hereinafter referred to as a "closing control signal SCB") for bringing the second short-circuit switch 14B into a closed state (on state). As a result, the second short-circuit switch 14B is brought into a closed state (on state), and the DC line between the DC terminal CB and the high-voltage terminal B (i.e., the DC transmission line LA) is short-circuited to the ground potential via the first short-circuit switch 14A. Here, the control device 50 may output the closing control signal SCA at time t12 and the closing control signal SCB at time t13 in the reverse order or simultaneously. As a result, a zero point auxiliary current that assists the decrease of the R-phase AC system current flows, and a current zero point is present in the R-phase AC system current (see (b) of FIG. 3).

[0062] Thereafter, the control device 50 confirms that the first short-circuit switch 14A is actually in a closed state (on state) at time t14, for example. Furthermore, the control device 50 confirms that the second short-circuit switch 14B is actually in a closed state (on state) at time t15, for example. Here, the control device 50 may confirm the closed state (on state) of the first short-circuit switch 14A at time t14 and the closed state (on state) of the second short-circuit switch 14B at time t15 in the reverse order, or may perform these confirmations simultaneously. Here, it is assumed that the control device 50 has confirmed that the first short-circuit switch 14A and the second short-circuit switch 14B are actually in a closed state (on state).

[0063] In this case, for example, at time t16, the control device 50 outputs a control signal (hereinafter referred to as "open-circuit control signal SO") for putting the AC circuit breaker 11 into a cutoff state (open state). As a result, in the AC circuit breaker 11, each of the mechanical contact type switches corresponding to each phase of the three-phase AC terminals is put into an open state, and each AC line of the three-phase AC terminal is cut off. In other words, the AC transmission lines of each phase connected to the AC power source AC are disconnected from the DC transmission lines LA and LB connected to the DC power source DC.

[0064] Thereafter, for example, at time t7, the control device 50 confirms that the AC lines of all phases of the three-phase AC terminals have been interrupted by the AC circuit breaker 11. Here, it is assumed that the control device 50 has confirmed that the AC lines of all phases have been interrupted. In this case, the control device 50 ends the control of the first operation sequence (the sequence diagram shown in FIG. 4) for the fault that has occurred this time.

[0065] In the power conversion apparatus 10, the control device 50 controls the first operation sequence as follows: first, the short-circuit switch control unit 56 controls the first short-circuit switch 14A and the second short-circuit switch 14B to a closed state (ON state), and a zero-point auxiliary current that assists the reduction of a fault current superimposed on the AC system current due to the occurrence of a fault is passed through the AC line. In other words, in the power conversion apparatus 10, the short-circuit operation control by the short-circuit switch control unit 56 intentionally causes a current zero point to exist in the AC system current. Then, the circuit breaker control unit 54 causes the AC circuit breaker 11 to interrupt the AC line at the timing when the AC system current in the AC line reaches the current zero point. As a result, in the power conversion apparatus 10, even if a DC current (fault current) is superimposed on the AC system current due to the occurrence of a fault, causing an overall increase in the AC system current, the AC system current can be intentionally reduced to safely interrupt the AC line.

[0066] [Second Operation of Power Conversion Device] Next, a description will be given of another example of an operation for disconnecting a fault when an accident occurs in the power conversion device 10. In the following description, for ease of explanation, the description will be given assuming that the control device 50 controls the disconnection operation and the short-circuit operation.

[0067] Fig. 5 is a sequence diagram showing an example of the control flow of the second operation of interrupting a fault in the control device 50 included in the power conversion device 10. The operation from time t0 to time t7 shown in Fig. 5 includes the same control as the first operation in the sequence diagram shown in Fig. 4. Therefore, in the following explanation, a repeated explanation of the control similar to the control of the first operation in the sequence diagram shown in Fig. 4 will be omitted.

[0068] In the following description, it is assumed that an accident (e.g., a ground fault) occurs between DC terminal CA and DC terminal CB of the R phase at time t0. In the following description, it is assumed that the control device 50 detects the accident at time t1. In this case, too, the control device 50 puts each unit converter 162 into a gate-blocked state.

[0069] Thereafter, for example, at time t2, the control device 50 simultaneously outputs a closing control signal SCA to the first short-circuit switch 14A, a closing control signal SCB to the second short-circuit switch 14B, and an opening control signal SO to the AC circuit breaker 11. In other words, the control device 50 outputs the opening control signal SO to put the AC circuit breaker 11 into a blocking state (open state) before confirming that each of the first short-circuit switch 14A and the second short-circuit switch 14B is actually in a closing state (on state).

[0070] However, it is preferable that the timing at which the AC lines of the three-phase AC terminals are interrupted by the AC circuit breaker 11 is after the first short-circuit switch 14A and the second short-circuit switch 14B each actually enter a closed state (on state) and the DC line between the DC terminal CA and the low-voltage terminal A (i.e., the DC transmission line LA) and the DC line between the DC terminal CB and the high-voltage terminal B (i.e., the DC transmission line LA) are short-circuited to ground potential. That is, it is preferable that the control device 50 outputs the open-circuit control signal SO to the AC circuit breaker 11 in accordance with the timing at which the first short-circuit switch 14A and the second short-circuit switch 14B each actually enter a closed state (on state). Therefore, although the closing control signal SCA, the closing control signal SCB, and the opening control signal SO are output at the same timing in the sequence diagram shown in FIG. 5, the timing at which these control signals are output is not necessarily limited to the same timing. That is, the control device 50 may adjust the timing of outputting the open-circuit control signal SO so that each of the mechanical contact type switches provided in the AC circuit breaker 11 is in an open state after each of the first short-circuit switch 14A and the second short-circuit switch 14B is actually in a closed state (on state). More specifically, the control device 50 may output the closing control signal SCA and the closing control signal SCB to determine in advance the timing at which each of the first short-circuit switch 14A and the second short-circuit switch 14B is actually in a closed state (on state), and may output the open-circuit control signal SO in accordance with this timing.

[0071] Thereafter, for example, at time t7, the control device 50 confirms that the AC lines of all phases of the three-phase AC terminal have been cut off by the AC circuit breaker 11, in the same way as in the control of the first operation in the sequence diagram shown in FIG. 4, and when it is confirmed that the AC lines of all phases have been cut off, ends the control of this second operation sequence (sequence diagram shown in FIG. 5) for the accident that has occurred.

[0072] Even with such control of the sequence of the second operation by the control device 50, in the power conversion device 10, first, a zero-point auxiliary current that intentionally causes a current zero point to exist in the AC system current is passed through the AC line by controlling the short-circuit operation by the short-circuit switch control unit 56, and then, by the interruption operation by the circuit breaker control unit 54, the AC circuit breaker 11 interrupts the AC line at the timing when the AC system current in the AC line reaches the current zero point. As a result, in the second operation of the power conversion device 10, as in the first operation, the AC system current that increases overall due to a DC current (fault current) being superimposed on the AC system current due to the occurrence of a fault can be intentionally reduced, thereby safely interrupting the AC line.

[0073] Furthermore, in the second operation, by determining in advance the timing at which the first short-circuit switch 14A and the second short-circuit switch 14B actually enter a closed state (ON state) in response to control of the short-circuit operation by the short-circuit switch control unit 56, the open-circuit control signal SO can be output in accordance with this timing, i.e., the timing at which the AC system current in the AC line reaches a zero current point. Therefore, in the second operation, the process of confirming that the first short-circuit switch 14A and the second short-circuit switch 14B actually enter a closed state (ON state) can be omitted, reducing the processing load on the control device 50 and enabling the AC line to be interrupted more quickly and safely than in the first operation. Furthermore, in the second operation, the timing at which the AC system current in the AC line reaches a zero current point and the timing at which the AC circuit breaker 11 interrupts the AC line can be brought closer together. This shortens the time during which a fault current flows through the AC circuit breaker 11, thereby reducing the risk of the AC circuit breaker 11 breaking down due to factors such as heat generated in the AC circuit breaker 11 by a fault current. This allows the power conversion device 10 to achieve high reliability.

[0074] With such a configuration and operation, the power conversion device 10 includes a first short-circuit switch 14A for short-circuiting the DC line between the DC terminal CA and the low-voltage terminal A to ground potential, and a second short-circuit switch 14B for short-circuiting the DC line between the DC terminal CB and the high-voltage terminal B to ground potential. In the power conversion device 10, the control device 50 (more specifically, the short-circuit switch control unit 56) performs a short-circuit operation on the first short-circuit switch 14A and the second short-circuit switch 14B in the event of an accident, thereby intentionally causing a zero current point to exist in the AC system current connected to the AC circuit breaker 11. Thereafter, the control device 50 (more specifically, the circuit breaker control unit 54) performs a breaking operation on each of the mechanical contact type switches provided in the AC circuit breaker 11 so that they are in an open state until the AC system current of the AC line reaches the zero current point. As a result, in the power conversion device 10, even if both of the DC terminals to which the two DC lines for transmitting DC power are connected are not near-end grounded, it is possible to safely shut off the AC lines in which the AC system current has risen overall due to an accident that has occurred.

[0075] As described above, the power conversion device 10 of the first embodiment includes the first short-circuit switch 14A for short-circuiting the DC line between the DC terminal CA and the low-voltage terminal A to ground potential, and the second short-circuit switch 14B for short-circuiting the DC line between the DC terminal CB and the high-voltage terminal B to ground potential. In the power conversion device 10 of the first embodiment, the control device 50 (more specifically, the short-circuit switch control unit 56) performs a short-circuit operation on the first short-circuit switch 14A and the second short-circuit switch 14B in the event of an accident, thereby intentionally causing a zero current point to exist in the AC system current to which the AC circuit breaker 11 is connected. Thereafter, in the power conversion device 10 of the first embodiment, the control device 50 (more specifically, the circuit breaker control unit 54) performs a breaking operation on each of the mechanical contact type switches provided in the AC circuit breaker 11 so that they are in an open state until the AC system current of the AC line reaches the zero current point. As a result, in the power conversion device 10 of the first embodiment, even if both of the DC terminals to which the two DC lines for transmitting DC power are connected are not near-end grounded, it is possible to safely shut off the AC lines in which the AC system current has increased overall due to an accident that has occurred.

[0076] In the first embodiment described above, a case where the fault that occurs in the power conversion device 10 is a ground fault has been described. However, even when the fault that occurs in the power conversion device 10 is a short-circuit fault, the power conversion device 10 can safely shut off the AC line in the same way. The operation and control in this case can be easily considered based on the operation and control in the control device 50 described with reference to Figs. 4 and 5. Therefore, a detailed description of the operation and control when the fault that occurs in the power conversion device 10 is a short-circuit fault will be omitted.

[0077] The power conversion device 10 shown in FIG. 1 has been described as a unipolar power conversion device having one high-voltage terminal B. For this reason, a positive voltage DC transmission line is generally connected to the high-voltage terminal B in the power conversion device 10. However, there are also bipolar power conversion devices. In this case, the power conversion device 10 has a configuration similar to that on the high-voltage terminal B side as a configuration on the side to which a negative voltage DC transmission line is connected. However, this configuration and the operation and control of the control device 50 need only be equivalent to the configuration of the power conversion device 10 and the operation and control of the control device 50 described with reference to FIGS. 1 to 5, and can be easily considered based on the above-described configuration, operation, and control. Therefore, detailed description of the configuration when the power conversion device 10 is a bipolar power conversion device and the operation and control of the control device 50 will be omitted.

[0078] (Second embodiment) [Configuration of DC transmission system] A second embodiment will be described below. Fig. 6 is a diagram showing an example of a configuration of a power converter according to the second embodiment and a DC power transmission system to which the power converter is applied. The configuration of a power converter 20 according to the second embodiment shown in Fig. 6 and a DC power transmission system 2 to which the power converter 20 is applied is a configuration in which the power converter 10 of the first embodiment shown in Fig. 1 and the DC power transmission system 1 to which the power converter 10 is applied are replaced with the power converter 20. The components of the DC power transmission system 2 and the power converter 20 include components having the same functions as the components of the DC power transmission system 1 and the power converter 10. In the following description, the components of the DC power transmission system 2 and the power converter 20 that have the same functions as the components of the DC power transmission system 1 and the power converter 10 are denoted by the same reference numerals, and detailed description thereof will be omitted.

[0079] Similar to the power conversion device 10, the power conversion device 20 also converts AC power supplied by an AC power source AC into DC power supplied by a DC power source DC, and vice versa, between three-phase AC terminals (AC terminal R, AC terminal S, AC terminal T) and DC terminals (DC terminal A, DC terminal B). In the power conversion device 20 as well, the DC terminal B is a high-voltage terminal (high-voltage terminal B) having a large absolute value of difference with respect to ground potential, and the DC terminal A is a low-voltage terminal (low-voltage terminal A) having a smaller absolute value of difference with respect to ground potential than the DC terminal B.

[0080] [Configuration of power conversion device] Like the power converter 10, the power converter 20 is also an example of a power converter equipped with a double-star-connected modular multilevel converter that converts AC power and DC power mutually. Like the power converter 10, the power converter 20 is also configured such that a DC transmission line LA, which is grounded between a line impedance LIA and a DC power source DC, is connected to a low-voltage terminal A. However, when the power converter 20 is considered alone, none of the DC terminals are near-end grounded. The power converter 20 includes AC circuit breakers 11 and legs 23 corresponding to the three-phase AC terminals, a first short-circuit switch 14A corresponding to the low-voltage terminal A, a second short-circuit switch 14B corresponding to the high-voltage terminal B, and a control device 50.

[0081] Power conversion device 20 has a configuration in which legs 13 included in power conversion device 10 are replaced with legs 23. Therefore, in power conversion device 20, transformer 12 and buffer reactors 15A and 15B included in each leg 13 are replaced with multi-winding transformers 27. Also in power conversion device 20, each of legs 23 corresponding to each of the three-phase AC terminals, that is, corresponding to each phase of AC power, has the same configuration.

[0082] The multi-winding transformer 27 included in each leg 23 is a star-connected transformer. The multi-winding transformer 27 has at least three or more windings and performs the same function as the transformer 12 and buffer reactor 15 included in the power conversion device 10. In other words, the multi-winding transformer 27 also provides electrical insulation between the AC power source AC and the power conversion device 10, and adjusts the voltage level difference between the AC power source AC and the arms 16 included in each leg 23.

[0083] The multi-winding transformer 27 includes an AC system-side winding 272 connected to a three-phase AC terminal of a corresponding phase of the AC power supply AC, a first DC system-side winding 274 connected to a three-phase AC terminal of the first arm 16A, a second DC system-side winding 276 connected to a three-phase AC terminal of the second arm 16B, and an iron core (not shown). The multi-winding transformer 27 may include a stabilizing winding for suppressing harmonics flowing into each system. The AC system-side winding 272, the first DC system-side winding 274, and the second DC system-side winding 276 are each wound around an iron core (not shown). The first DC system-side winding 274 and the second DC system-side winding 276 have the same number of turns. The first DC system side winding 274 and the second DC system side winding 276 are connected in series on the three-phase AC terminal sides of the first arm 16A and the second arm 16B. That is, the first DC system side winding 274 and the second DC system side winding 276 are connected in series between the first arm 16A and the second arm 16B. The first DC system side winding 274 and the second DC system side winding 276 have opposite polarities to each other by having their negative polarities connected to each other.

[0084] In the multi-winding transformer 27 provided in each leg 23, one end of the AC system side winding 272 opposite to the AC power source AC is grounded. Furthermore, in the multi-winding transformer 27 provided in each leg 23, the connection point (neutral point) where the first DC system side winding 274 and the second DC system side winding 276 are connected is connected to each other.

[0085] With this configuration, in the power conversion device 20, a DC current flows from the second arm 16B side to the first arm 16A side via the second DC system-side winding 276 and the first DC system-side winding 274. At this time, because the second DC system-side winding 276 and the first DC system-side winding 274 are connected in series with opposite polarities as described above, the DC magnetomotive forces caused by the DC currents flowing through them have opposite polarities and cancel each other out, so no DC magnetic flux is generated in the iron cores (not shown). Furthermore, because the DC magnetomotive forces caused by the DC currents cancel out in each leg 23, even if the current balance is disrupted due to an accident, the iron cores (not shown) of each multi-winding transformer 27 can operate without biased magnetism or saturation.

[0086] Furthermore, in the power conversion device 10, the transformer 12 and the buffer reactors 15A and 15B provided in each leg 13 were factors that increased the size and cost of the power conversion device 10, but in the power conversion device 20, by replacing them with the multi-winding transformer 27, it is possible to reduce the size and cost of the power conversion device 20.

[0087] In the power conversion device 20, the operation and control of the control device 50 are similar to the operation and control of the control device 50 provided in the power conversion device 10. Therefore, a detailed description of the operation and control in the event of an accident in the power conversion device 20 will be omitted.

[0088] As described above, the power conversion device 20 of the second embodiment, like the power conversion device 10 of the first embodiment, includes a first short-circuit switch 14A for short-circuiting the DC line between the DC terminal CA and the low-voltage terminal A to ground potential and a second short-circuit switch 14B for short-circuiting the DC line between the DC terminal CB and the high-voltage terminal B to ground potential. Similarly to the power conversion device 10 of the first embodiment, the power conversion device 20 of the second embodiment also includes a control device 50 (more specifically, a short-circuit switch control unit 56) that, in the event of an accident, performs a short-circuit operation on the first short-circuit switch 14A and the second short-circuit switch 14B to intentionally create a zero current point in the AC system current connected to the AC circuit breaker 11. Then, similar to the power conversion device 10 of the first embodiment, the control device 50 (more specifically, a circuit breaker control unit 54) also performs a circuit break operation on the mechanical contact type switches of the AC circuit breaker 11 so that each switch is in an open state until the AC system current of the AC line reaches the zero current point. As a result, in the power conversion device 20 of the second embodiment, as in the power conversion device 10 of the first embodiment, even if both of the DC terminals to which the two DC lines for transmitting DC power are connected are not near-end grounded, it is possible to safely shut off an AC line in which the AC system current has increased overall due to an accident that has occurred.

[0089] (Third embodiment) [Configuration of DC transmission system] A third embodiment will be described below. Fig. 7 is a diagram showing an example of a configuration of a power converter according to the third embodiment and a DC power transmission system to which the power converter is applied. The configuration of a power converter 30 according to the third embodiment shown in Fig. 7 and a DC power transmission system 3 to which the power converter 30 is applied is a configuration in which the power converter 10 of the first embodiment shown in Fig. 1 and the DC power transmission system 1 to which the power converter 10 is applied are replaced with the power converter 30. The components of the DC power transmission system 3 and the power converter 30 include components having the same functions as the components of the DC power transmission system 1 and the power converter 10. In the following description, the components of the DC power transmission system 3 and the power converter 30 that have the same functions as the components of the DC power transmission system 1 and the power converter 10 are denoted by the same reference numerals, and detailed description thereof will be omitted.

[0090] Similar to the power conversion device 10, the power conversion device 30 also converts AC power supplied by an AC power source AC into DC power supplied by a DC power source DC, and vice versa, between three-phase AC terminals (AC terminal R, AC terminal S, AC terminal T) and DC terminals (DC terminal A, DC terminal B). In the power conversion device 30 as well, the DC terminal B is a high-voltage terminal (high-voltage terminal B) having a large absolute value of difference with respect to ground potential, and the DC terminal A is a low-voltage terminal (low-voltage terminal A) having a smaller absolute value of difference with respect to ground potential than the DC terminal B.

[0091] [Configuration of power conversion device] Like the power converter 10, the power converter 30 is also an example of a power converter including a double-star-connected modular multilevel converter that converts AC power and DC power mutually. Like the power converter 10, the power converter 30 is also configured such that a DC transmission line LA, which is grounded between a line impedance LIA and a DC power source DC, is connected to a low-voltage terminal A. However, when the power converter 30 is considered alone, none of the DC terminals are near-end grounded. The power converter 30 includes AC circuit breakers 11, transformers 12, and legs 13 corresponding to the three-phase AC terminals, a first short-circuit switch 14A corresponding to the low-voltage terminal A, a first impedance 38A, a second short-circuit switch 14B corresponding to the high-voltage terminal B, a second impedance 38B, and a control device 50.

[0092] The power conversion device 30 is configured such that a first impedance 38A is added between the DC line between the DC terminal CA and the low-voltage terminal A and the first short-circuit switch 14A, and a second impedance 38B is added between the DC line between the DC terminal CB and the high-voltage terminal B and the second short-circuit switch 14B. In other words, the power conversion device 30 is configured such that the first short-circuit switch 14A short-circuits the DC line between the DC terminal CA and the low-voltage terminal A to ground potential via the first impedance 38A, and the second short-circuit switch 14B short-circuits the DC line between the DC terminal CB and the high-voltage terminal B to ground potential via the second impedance 38B.

[0093] Each of the first impedance 38A and the second impedance 38B is, for example, a resistor, a reactor, or a combination of these components connected in series. In the power conversion device 30, the magnitude (current value) of the zero-point auxiliary current that flows when the first short-circuit switch 14A and the second short-circuit switch 14B are closed (ON) can be changed by each of the first impedance 38A and the second impedance 38B. For example, decreasing the impedance components of the first impedance 38A and the second impedance 38B increases the zero-point auxiliary current, whereas increasing the impedance components of the first impedance 38A and the second impedance 38B decreases the zero-point auxiliary current. Minimizing the impedance components of the first impedance 38A and the second impedance 38B maximizes the zero-point auxiliary current, thereby minimizing the fault current flowing through the AC circuit breaker 11.

[0094] The configuration of the first short-circuit switch 14A and the first impedance 38A is an example of a "first short-circuit switch," and the configuration of the second short-circuit switch 14B and the second impedance 38B is an example of a "second short-circuit switch."

[0095] With this configuration, the power conversion device 30 can reduce the fault current flowing in the AC circuit breaker 11 due to the occurrence of a fault, and can also reduce the zero point auxiliary current. As a result, the power conversion device 30 can minimize the effect on the AC circuit breaker 11 caused by the flow of the fault current and the zero point auxiliary current. As a result, the power conversion device 30 can reduce the risk of the AC circuit breaker 11 breaking down, and a highly reliable power conversion device can be realized.

[0096] In the power conversion device 30, the operation and control of the control device 50 are similar to the operation and control of the control device 50 provided in the power conversion device 10. Therefore, a detailed description of the operation and control when a fault occurs in the power conversion device 30 will be omitted.

[0097] As described above, the power converter 30 of the third embodiment, like the power converter 10 of the first embodiment, includes a first short-circuit switch 14A for short-circuiting the DC line between the DC terminal CA and the low-voltage terminal A to ground potential and a second short-circuit switch 14B for short-circuiting the DC line between the DC terminal CB and the high-voltage terminal B to ground potential. Similarly to the power converter 10 of the first embodiment, the power converter 30 of the third embodiment also includes a control device 50 (more specifically, a short-circuit switch control unit 56) that, in the event of an accident, performs a short-circuit operation on the first short-circuit switch 14A and the second short-circuit switch 14B to intentionally create a zero current point in the AC system current connected to the AC circuit breaker 11. Then, similar to the power converter 10 of the first embodiment, the power converter 30 of the third embodiment also includes a control device 50 (more specifically, a circuit breaker control unit 54) that performs a circuit break operation on each of the mechanical contact type switches of the AC circuit breaker 11 to an open state until the AC system current of the AC line reaches the zero current point. As a result, in the power conversion device 30 of the third embodiment, as in the power conversion device 10 of the first embodiment, even if both of the DC terminals to which the two DC lines for transmitting DC power are connected are not near-end grounded, it is possible to safely shut off an AC line in which the AC system current has increased overall due to an accident that has occurred.

[0098] Moreover, in the power conversion device 30 of the third embodiment, the magnitude (current value) of the zero-point auxiliary current can be changed by adjusting the impedance components of a first impedance 38A connected in series between the DC line between the DC terminal CA and the low-voltage terminal A and the first short-circuit switch 14A, and a second impedance 38B connected in series between the DC line between the DC terminal CB and the high-voltage terminal B and the second short-circuit switch 14B. As a result, in the power conversion device 30 of the third embodiment, the fault current flowing through the AC circuit breaker 11 due to the occurrence of a fault can be reduced to a zero-point auxiliary current of sufficient magnitude to reduce the current to the point where the AC circuit breaker 11 can be interrupted, thereby reducing the risk of the AC circuit breaker 11 breaking down and realizing a highly reliable power conversion device.

[0099] In the third embodiment described above, a power conversion device 30 having a configuration in which a first impedance 38A and a second impedance 38B are added to the power conversion device 10 of the first embodiment has been described. However, the first impedance 38A and the second impedance 38B may be added to the power conversion device 20 of the second embodiment. In this case, the configuration of the power conversion device 30 and the operation and control of the control device 50 need only be equivalent to the configurations of the power conversion device 20 and the power conversion device 30 of the second embodiment described above and the operation and control of the control device 50, and can be easily considered based on the configuration, operation, and control described above. Therefore, a detailed description of the configuration of the power conversion device 30 and the operation and control of the control device 50 in this case will be omitted.

[0100] As described above, the power conversion device of each embodiment includes a first short-circuit switch for short-circuiting the DC line between the low-voltage terminal and the terminal of the leg connected to the low-voltage terminal to ground potential, and a second short-circuit switch for short-circuiting the DC line between the high-voltage terminal and the terminal of the leg connected to the high-voltage terminal to ground potential. In the power conversion device of each embodiment, a control device (more specifically, a short-circuit switch control unit) performs a short-circuit operation on the first short-circuit switch and the second short-circuit switch in the event of an accident, thereby intentionally causing a zero current point to exist in the AC system current to which the AC circuit breaker is connected. Thereafter, in the power conversion device of each embodiment, a control device (more specifically, a circuit breaker control unit) performs a breaking operation so that each of the mechanical contact type switches provided in the AC circuit breaker is in an open state until the AC system current of the AC line reaches the zero current point. As a result, in the power conversion device of each embodiment, even if both of the DC terminals to which two DC lines for transmitting DC power are connected are not near-end grounded, it is possible to safely shut off an AC line in which the AC system current has risen overall due to an accident that has occurred.

[0101] In the power conversion apparatus of each of the above-described embodiments, the control device 50 included in the power conversion apparatus controls the conversion operation of the double-star-connected modular multilevel converter, the AC line interruption operation by the AC circuit breaker 11, and the short-circuiting operation of the first short-circuit switch 14A and the second short-circuit switch 14B. However, the control device 50 is not limited to being included in the power conversion apparatus. For example, the control device 50 may be included in a separate device disposed outside the power conversion apparatus, or the control device 50 itself may be disposed outside the power conversion apparatus as a separate control device. Even in this case, the configuration of the power conversion apparatus and the operation and control of the control device 50 may be equivalent to the configuration of the power conversion apparatus and the operation and control of the control device 50 of each of the above-described embodiments.

[0102] According to at least one embodiment described above, there is provided a power conversion device (10) for converting power between a DC terminal having a high-voltage terminal (B) with a large absolute value of difference with respect to ground potential and a low-voltage terminal (A) with a smaller absolute value of difference with respect to ground potential than the high-voltage terminal, and three-phase AC terminals (R, S, T), wherein the low-voltage terminal is connected to a ground potential via a line impedance (LIA) of a DC transmission line (LA) for transmitting converted DC power, and the arms (16) correspond to each phase of the three-phase AC terminal and include a plurality of unit converters (162) connected in series, the first arm (16A) being an arm connected between the low-voltage terminal and the three-phase AC terminal, and the low-voltage terminal is connected to a ground potential via a line impedance (LIA) of a DC transmission line (LA) for transmitting converted DC power. The present invention provides a power converter having a leg (13) in which a first arm (16B) connected between the low-voltage terminal and the first arm is connected in series on the three-phase AC terminal side, a first short-circuit switch (14A) that shorts the potential between the low-voltage terminal and the first arm to ground potential, and a second short-circuit switch (14B) that shorts the potential between the high-voltage terminal and the second arm to ground potential, thereby realizing a power converter in which, in the event of a fault, a zero-point auxiliary current that intentionally causes a current zero point to exist in the AC system current can be passed through the AC line, thereby reducing the DC fault current flowing on the AC system side, in a configuration in which neither of the two DC lines for transmitting DC power is grounded. In other words, a power converter in which, in the configuration in which neither of the two DC lines for transmitting DC power is grounded, can reduce the fault current passing through the AC circuit breaker and interrupt the fault current.

[0103] Although several embodiments of the present invention have been described, these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their modifications are included within the scope and spirit of the invention, as well as within the scope of the invention described in the claims and their equivalents. [Explanation of symbols]

[0104] 1, 2, 3... DC power transmission system, 10, 20, 30... power conversion device, 11... AC circuit breaker, 12... transformer, 13... leg, 14A... first short-circuit switch, 14B... second short-circuit switch, 15, 15A, 15B... buffer reactor, 16... arm, 16A... first arm (low-voltage side arm), 16B... second arm (high-voltage side arm), 162, 162a, 162b... unit converter (cell), 23... leg, 27... multi-winding transformer, 272... AC system side winding, 274... first DC system side winding, 276... second DC system side winding, 38A... first impedance, 38B ···Second impedance, 50··Control device, 52···Cell control unit, 54···Breaker control unit, 56···Short-circuit switch control unit, R, S, T···AC terminal, A···Low-voltage terminal (DC terminal), B···High-voltage terminal (DC terminal), LA, LB···DC transmission line, LI, LIA, LIB···Line impedance, Q, Q1, Q2, Q3, Q4···Switching element, D, D1, D2, D3, D4···Diode, C···Capacitor, TH···High-voltage side terminal, TL···Low-voltage side terminal, AC···AC power supply, DC···DC power supply, CA···DC terminal (high-voltage side), CB···DC terminal (low-voltage side), CP···Connection point

Claims

1. A power conversion device that converts power between a DC terminal and a three-phase AC terminal, the DC terminal having a high-voltage terminal having a large absolute value of a difference between the high-voltage terminal and a ground potential and a low-voltage terminal having a smaller absolute value of a difference between the high-voltage terminal and the ground potential, the low-voltage terminal is connected to a ground potential via a line impedance of a DC transmission line that transmits the converted DC power; a leg including a plurality of unit converters connected in series, the leg corresponding to each phase of the three-phase AC terminal, the first arm being the arm connected between the low voltage terminal and the three-phase AC terminal, and the second arm being the arm connected between the high voltage terminal and the three-phase AC terminal, the first arm being the arm connected in series on the three-phase AC terminal side; a first short-circuit switch that short-circuits the potential between the low voltage terminal and the first arm to the ground potential; a second short-circuit switch that short-circuits the potential between the high-voltage terminal and the second arm to the ground potential; A power conversion device comprising:

2. a control device that controls interruption operations of AC circuit breakers corresponding to each phase of the three-phase AC terminals, the AC circuit breakers being provided in an AC line between the three-phase AC terminals and a connection point between the three-phase AC terminal side of the first arm and the three-phase AC terminal side of the second arm of the leg, and that controls open and closed states of the first short-circuit switch and the second short-circuit switch; Furthermore, the control device controls the first short-circuit switch and the second short-circuit switch to the closed state, and controls the interruption operation of the AC circuit breaker to interrupt the AC line based on the closed states of the first short-circuit switch and the second short-circuit switch. The power conversion device according to claim 1 .

3. the control device controls the first short-circuit switch and the second short-circuit switch to the closed state, and controls the interruption operation of the AC circuit breaker to interrupt the AC line after the first short-circuit switch and the second short-circuit switch are in the closed state. The power conversion device according to claim 2 .

4. the control device controls the first short-circuit switch and the second short-circuit switch to the closed state, and controls the breaking operation of the AC circuit breaker so as to break the AC line before the first short-circuit switch and the second short-circuit switch are brought into the closed state. The power conversion device according to claim 2 .

5. a resistor is connected between the first short-circuit switch and / or the second short-circuit switch and the ground potential; The power conversion device according to claim 1 .

6. a reactor is connected between the first short-circuit switch and / or the second short-circuit switch and the ground potential; The power conversion device according to claim 1 .

7. the first short-circuit switch and / or the second short-circuit switch are adjusted so that an impedance component between the first short-circuit switch and the ground potential is minimized; The power conversion device according to claim 1 .

8. the first short-circuit switch and the second short-circuit switch are mechanical switches. The power conversion device according to claim 1 .

9. the first short-circuit switch and the second short-circuit switch are semiconductor switches. The power conversion device according to claim 1 .

10. The first short-circuit switch is a return line forced arc extinguishing device. The power conversion device according to claim 1 .

11. the legs further include a multi-winding transformer corresponding to each phase of the three-phase AC terminals and connected between the three-phase AC terminal side of the first arm and the three-phase AC terminal side of the second arm, Each of the multi-winding transformers comprises: an AC system side winding connected to the three-phase AC terminal of the corresponding phase; a first DC system side winding connected to the first arm side; a second DC system side winding connected to the second arm side and connected in series with the first DC system side winding in an opposite polarity; The power conversion device according to claim 1 , comprising:

12. A power conversion device that converts power between a DC terminal and a three-phase AC terminal, the DC terminal having a high-voltage terminal having a large absolute value of a difference between the high-voltage terminal and a ground potential and a low-voltage terminal having a smaller absolute value of a difference between the high-voltage terminal and the ground potential, the low-voltage terminal is connected to a ground potential via a line impedance of a DC transmission line that transmits the converted DC power; a leg including a plurality of unit converters connected in series, the leg corresponding to each phase of the three-phase AC terminal, the first arm being the arm connected between the low voltage terminal and the three-phase AC terminal, and the second arm being the arm connected between the high voltage terminal and the three-phase AC terminal, the first arm being the arm connected in series on the three-phase AC terminal side; a first short-circuit switch that short-circuits the potential between the low voltage terminal and the first arm to the ground potential; a second short-circuit switch that short-circuits the potential between the high-voltage terminal and the second arm to the ground potential; a control device that controls a breaking operation of an AC circuit breaker corresponding to each phase of the three-phase AC terminal, the AC circuit breaker being provided in an AC line between the three-phase AC terminal and a connection point between the three-phase AC terminal side of the first arm and the three-phase AC terminal side of the second arm of the leg, and that controls an open state and a closed state of the first short-circuit switch and the second short-circuit switch; A control method for a power conversion device comprising: The computer of the control device controlling the first short-circuit switch and the second short-circuit switch to the closed state; controlling the interruption operation of the AC circuit breaker to interrupt the AC line based on the closed states of the first short-circuit switch and the second short-circuit switch; A method for controlling a power conversion device.

13. A power conversion device that converts power between a DC terminal and a three-phase AC terminal, the DC terminal having a high-voltage terminal having a large absolute value of a difference between the high-voltage terminal and a ground potential and a low-voltage terminal having a smaller absolute value of a difference between the high-voltage terminal and the ground potential, the low-voltage terminal is connected to a ground potential via a line impedance of a DC transmission line that transmits the converted DC power; a leg including a plurality of unit converters connected in series, the leg corresponding to each phase of the three-phase AC terminal, the first arm being the arm connected between the low voltage terminal and the three-phase AC terminal, and the second arm being the arm connected between the high voltage terminal and the three-phase AC terminal, the first arm being the arm connected in series on the three-phase AC terminal side; a first short-circuit switch that short-circuits the potential between the low voltage terminal and the first arm to the ground potential; a second short-circuit switch that short-circuits the potential between the high-voltage terminal and the second arm to the ground potential; a control device that controls a breaking operation of an AC circuit breaker corresponding to each phase of the three-phase AC terminal, the AC circuit breaker being provided in an AC line between the three-phase AC terminal and a connection point between the three-phase AC terminal side of the first arm and the three-phase AC terminal side of the second arm of the leg, and that controls an open state and a closed state of the first short-circuit switch and the second short-circuit switch; A program for controlling a power conversion device comprising: The computer of the control device controlling the first short-circuit switch and the second short-circuit switch to the closed state; controlling the interruption operation of the AC circuit breaker to interrupt the AC line based on the closed states of the first short-circuit switch and the second short-circuit switch; program.

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