Patent Publication Number: US-2023154708-A1

Title: Switchgear and power converter

Description:
TECHNICAL FIELD 
     The present disclosure relates to a switchgear for a power device and to a power converter using the switchgear. 
     BACKGROUND ART 
     In recent years, in a self-commutated converter used for high voltage direct current (HVDC) or static synchronous compensator (STATCOM) as a DC power transmission system, a modular multilevel converter (MMC) has been introduced. The MMC converter is configured by connecting submodules, which are small converters, in series in multiple stages. Redundancy can be achieved in units of submodules, and submodules can be easily replaced even at a time of failure. 
     In the MMC converter, each submodule is provided with a switchgear that short-circuits an output terminal of the failed submodule in order to continuously operate even when some submodules are failed. The switchgear needs to operate at a high speed in order to suppress an influence of the failure accompanied by an arc generation in the submodule. There is a switchgear including a drive unit that operates using a blasting technique in an event of failure (see, for example, PTL 1). 
     CITATION LIST 
     Patent Literature 
     PTL 1: National Patent Publication No. 2011-510440 
     SUMMARY OF INVENTION 
     Technical Problem 
     However, since such a conventional blasting technique uses a drive unit utilizing gunpowder, the drive unit provided in the switchgear needs to be periodically replaced in order to ensure reliability, and there is a possibility that cost is generated at the time of replacement. 
     The present disclosure has been made to solve the above problem, and an object of the present disclosure is to provide a switchgear that reduces a cost of periodic replacement of a drive unit and ensures reliability. 
     Solution to Problem 
     A switchgear of the present disclosure includes a fixed electrode, a movable electrode provided to face the fixed electrode, the movable electrode being contactable with and separable from the fixed electrode, a piston provided opposite to the fixed electrode to drive the movable electrode via a movable shaft connected to the movable electrode, a container to accommodate the piston, and an arc generation mechanism including metal and provided in the container at a position opposite to the fixed electrode, the arc generation mechanism bridging a terminal unit connected to an external circuit, the arc generation mechanism opening a circuit with the terminal unit When energized to generate an arc at the opened portion. 
     Advantageous Effects of Invention 
     The switchgear according to the present disclosure does not require periodic replacement of the drive unit, and can ensure reliability. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is an example of a sectional view of a switchgear according to a first embodiment. 
         FIG.  2    is a diagram illustrating a flow of a closing operation of the switchgear according to the first embodiment. 
         FIG.  3    is an example of a sectional view of a switchgear according to a second embodiment. 
         FIG.  4    is a diagram illustrating a flow of a closing operation of the switchgear according to the second embodiment. 
         FIG.  5    is an example of a sectional view of a switchgear according to a third embodiment. 
         FIG.  6    is a diagram illustrating a flow of a closing operation of the switchgear according to the third embodiment. 
         FIG.  7    is an example of a sectional view of a switchgear according to a fourth embodiment. 
         FIG.  8    is a diagram illustrating a flow of a closing operation of the switchgear according to the fourth embodiment. 
         FIG.  9    is an example of a sectional view of a switchgear according to a fifth embodiment. 
         FIG.  10    is a diagram illustrating a flow of a closing operation of the switchgear according to the fifth embodiment. 
         FIG.  11    is a configuration diagram of a power converter according to a sixth embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Embodiment 
       FIG.  1    is an example of a sectional view of a switchgear  1 A according to the present embodiment. Switchgear  1 A will be described with reference to  FIG.  1   . 
     As illustrated in  FIG.  1   , switchgear  1 A includes a fixed electrode  2 , a movable electrode  3 , a vacuum container  5 , an upper terminal  6 , a lower terminal  7 , a drive unit  10 A, an arc generation mechanism  20 A, and a terminal unit  22 . Terminal unit  22  is connected to a drive power supply  30 , and is, for example, a power supply terminal. 
     Drive unit  10 A includes a movable shaft  4  and a container  12 , and container  12  accommodates a piston  11 , an opening spring  13 , and metal arc generation mechanism  20 A. Container  12  is desirably insulating. 
     Movable electrode  3  is provided so as to face fixed electrode  2 , and is contactable with and separable from fixed electrode  2 . 
     Movable electrode  3  is connected to movable shaft  4 . Movable shaft  4  is desirably insulating. 
     A contact point at which fixed electrode  2  and movable electrode  3  contact with each other is accommodated in vacuum container  5 . 
     Piston  11  is provided opposite to fixed electrode  2 , and drives movable electrode  3  via movable shalt  4 . Piston  11  slides along container  12 . 
     Arc generation mechanism  20 A is provided in container  12  opposite to fixed electrode  2 , and bridges terminal unit  22 . Terminal unit  22  is connected to drive power supply  30  which is an external circuit, and arc generation mechanism  20 A, terminal unit  22 , and drive power supply  30  constitute a circuit. 
     When energized, arc generation mechanism  20 A opens a circuit with terminal unit  22  to generate an arc at an opened portion. Due to a pressure of the generated arc, piston  11  operates in a direction to close movable electrode  3 . 
     A movable part operated by the pressure of the arc is movable electrode  3 , movable shaft  4 , and piston  11 . A fixed part of switchgear  1 A is other than the movable part. 
     In the present embodiment, arc generation mechanism  20 A is a fuse. 
       FIG.  2    is a diagram illustrating a flow of a closing operation of switchgear  1 A according to the present embodiment.  FIG.  2   ( 1 ) illustrates an open state which is a position in an initial state,  FIG.  2   ( 2 ) illustrates a driving state in a midway of operation of arc generation mechanism  20 A, and  FIG.  2   ( 3 ) illustrates a closed state after operation of arc generation mechanism  20 A. 
     In the open state, which is the position in the initial state in  FIG.  2   ( 1 ), piston  11  is positioned in a space on an open side in container  12 . Arc generation mechanism  20 A bridges contact terminal unit  22 . In the open state, opening spring  13 , which is an elastic member, is connected to and held by the movable part. As another example of the holding in the open state, the movable part may be sucked and held on the open side by a weight of the movable part, a permanent magnet, or an electromagnet. Otherwise, in a state of an initial position, an outer wall accommodating switchgear  1 A or a component connecting the fixed part and the movable part may be provided, and the component may be released or destroyed during the operation of arc generation mechanism  20 A. 
     An arrow P in  FIG.  2   ( 2 ) indicates an energization path in a case where drive power supply  30  energizes arc generation mechanism  20 A. An arrow Q indicates a direction in which the movable part moves by a pressure in container  12 . 
     When energization is performed, as indicated by arrow P, a current flows from drive power supply  30  to arc generation mechanism  20 A via terminal unit  22 , and thus, arc generation mechanism  20 A is immediately burned out, the circuit with terminal unit  22  is opened, and an arc  23  is generated at the opened portion. When arc  23  is generated in container  12 , a temperature of air in container  12  rapidly rises and the pressure in container  12  rises. As a result, piston  11  receives the pressure, operates to rise in a direction of Q, which is a closing direction of movable electrode  3 , and drives movable electrode  3  in the closing direction via movable shaft  4 . 
     In the closed state in  FIG.  2   ( 3 ), an energization path is formed by upper terminal  6 , fixed electrode  2 , movable electrode  3 , and lower terminal  7  after closing. 
     When energized, an electromagnetic repulsive force is generated between fixed electrode  2  and movable electrode  3 , and it is therefore necessary to pressurize the contact point between fixed electrode  2  and movable electrode  3 . In a pressure contact method, for example, an elastic member may be provided inside movable shaft  4 , and the elastic member may be connected to each of movable electrode  3  and movable shaft  4  to pressurize movable electrode  3  and movable shaft  4 , or a pressure that can adjust and hold a decreasing pressure in piston  11  may be used. 
     Similarly to the holding in the open state, the holding in the closed state is performed by connecting an elastic member that generates a load in the closing direction to the movable part. As an example of the elastic member, movable shaft  4  and the outer wall accommodating switchgear  1 A are connected and held by a disk spring. As another example of the holding in the closed state, the movable part may be sucked and held on the closed side by a weight of the movable part, a permanent magnet, or an electromagnet. Otherwise, a component that connects the movable part and the outer wall accommodating switchgear  1 A may be provided and held at a position of the closed state. 
     In the present disclosure, by receiving pressure generated as a result of a rapid rise in the temperature in container  12  due to arc  23  generated by burning out of arc generation mechanism  20 A provided in switchgear  1 A, movable electrode  3  can move in the closing direction to close switchgear  1 A. Thus, since an explosion technology is not used, the components do not need to be periodically replaced, and it is possible to obtain switchgear  1 A with long-term high reliability. 
     In addition, switchgear  1 A can be operated at a high speed by utilizing a rapid increase in pressure in container  12  due to arc  23 . 
     In a case where gunpowder or other media is used for driving piston  11 , the gunpowder or other media needs to be managed so as not to explode. However, in the present embodiment, it is sufficient to provide arc generation mechanism  20 A that bridges terminal unit  22 , and therefore, assembling work of switchgear  1 A is simple. 
     Drive power supply  30  to be energized to generate an arc is desirably a direct current that is unlikely to be naturally extinguished. 
     Since drive power supply  30  discharges a pulse current at once when the pulse current includes a capacitor charge and discharge circuit, switchgear  1 A can be operated at even higher speed. 
     In a case where drive power supply  30  is a capacitor discharge circuit, when electric charge of the capacitor disappears at the time of operation, arc  23  in arc generation mechanism  20 A is naturally extinguished. 
     By meandering in container  12 , the fuse used in arc generation mechanism  20 A according to the present embodiment can be expected to have an effect of increasing a length of generated arc  23  and increasing a pressure rise. 
     In the present embodiment, switchgear  1 A that performs the closing operation has been described with reference to  FIG.  2   , but an opposite driving direction may be used for a switchgear that requires a high-speed opening operation. 
     Second Embodiment 
     A difference between the first embodiment and a second embodiment is a difference in the arc generation mechanism. Although the fuse is used in the arc generation mechanism according to the first embodiment, a switch is used in the arc generation mechanism according to the second embodiment. 
     Hereinafter, only the difference between the first embodiment and the second embodiment will be described, and description of the same or corresponding parts will be omitted. As for the reference signs, the same or corresponding parts as those in the first embodiment are denoted by the same reference signs, and the description thereof will be omitted. 
       FIG.  3    is an example of a sectional view of a switchgear  1 B according to the present embodiment. Switchgear  1 B will be described with reference to  FIG.  3   . 
     A drive unit  10 B includes movable shaft  4  and container  12 , and container  12  accommodates piston  11 , opening spring  13 , and an arc generation mechanism  20 B. 
     In the present embodiment, arc generation mechanism  20 B is a switch. Arc generation mechanism  20 B includes a fixed conductor  20 B 1 , a movable conductor  20 B 2 , and a contact  20 B 3 . One end of movable conductor  20 B 2  is contactable with and separable from one fixed conductor  20 B 1  at a contact point. The other end of movable conductor  20 B 2  is connected to the other fixed conductor  20 B 1  by contact  20 B 3 , and can perform separation and contact operation with a contact point via contact  20 B 3 . Movable conductor  20 B 2  may be a plate instead of movable conductor  20 B 2  or a flexible conductor having flexibility may be used without using contact  20 B 3  as long as being connected to one fixed conductor  20 B 1  and being contactable with and separable from the other fixed conductor  20 B 1 . Fixed conductor  20 B 1  is provided by bridging terminal unit  22  in container  12 , and arc generation mechanism  20 B, terminal unit  22 , and drive power supply  30  constitute a circuit. 
     When energized, arc generation mechanism  20 B separates movable conductor  20 B 2  from the contact point with fixed conductor  20 B 1  by the electromagnetic repulsive force, and generates an arc at the opened portion by mechanically opening the circuit with terminal unit  22 . Due to a pressure of the generated arc, piston  11  operates in a direction to close movable electrode  3 . 
       FIG.  4    is a diagram illustrating a flow of a closing operation of switchgear  1 B according to the present embodiment.  FIG.  4   ( 1 ) illustrates an open state which is a position in an initial state,  FIG.  4   ( 2 ) illustrates a driving state in a midway of operation of arc generation mechanism  20 B, and  FIG.  4   ( 3 ) illustrates a closed state after operation of arc generation mechanism  20 B. 
     In the open state, which is the position in the initial state in  FIG.  4   ( 1 ), piston  11  is disposed in contact with container  12 . As in the first embodiment, in the open state in the present embodiment, opening spring  13 , which is an elastic member that generates a load in an opening direction, is connected to and held by the movable part. Fixed conductor  20 B 1  and movable conductor  20 B 2  of arc generation mechanism  20 B are in contact with each other at the contact point. 
     An arrow P in  FIG.  4   ( 2 ) indicates an energisation path in a case where drive power supply  30  energizes arc generation mechanism  20 B. An arrow Q indicates a direction in which the movable part moves by a pressure in container  12 . 
     When energized, as indicated by arrow P, the current flows from drive power supply  30  to arc generation mechanism  20 B via terminal unit  22 , and thus, in arc generation mechanism  20 B, an electromagnetic repulsive force is generated at the contact point between fixed conductor  20 B 1  and movable conductor  20 B 2 . Movable conductor  20 B 2  is separated from fixed conductor  20 B 1  by the generated electromagnetic repulsive force. A contact pressure load of arc generation mechanism  20 B is set in advance such that movable conductor  20 B 2  separates from fixed conductor  20 B 1 . When movable conductor  20 B 2  separates from fixed conductor  20 B 1 , a circuit with terminal unit  22  is opened, and arc  23  is generated at the contact point between fixed conductor  20 B 1  and movable conductor  20 B 2  bridging terminal unit  22 . 
     The pressure by gas heated and expanded by arc  23  increases an operating speed of piston  11  in a direction of arrow Q. 
     As a result, piston  11  receives the pressure, operates to rise in the closing direction of movable electrode  3 , and drives movable electrode  3  in the closing direction via movable shaft  4 . 
     In the closed state in  FIG.  4   ( 3 ), as in the first embodiment, an energisation path is formed by upper terminal  6 , fixed electrode  2 , movable electrode  3 , and lower terminal  7  after closing. When energized, an electromagnetic repulsive force is generated between fixed electrode  2  and movable electrode  3 , and thus as in the first embodiment, the contact point between fixed electrode  2  and movable electrode  3  is pressurized to maintain the closed state. 
     In the present disclosure, as in the first embodiment, by using the pressure rise of arc  23  generated by the electromagnetic repulsive force of arc generation mechanism  20 B provided in switchgear  1 B, movable electrode  3  can move in the closing direction to close switchgear  1 B. 
     Thus, since an explosion technology is not used, the components do not need to be periodically replaced, and it is possible to obtain switchgear  1 B with long-term high reliability. 
     In addition, switchgear  1 B can be operated at a high speed by utilizing a rapid increase in pressure in container  12  due to arc  23 . 
     Since arc generation mechanism  20 A is a fuse in the first embodiment, the fuse is blown during operation, and thus the fuse needs to be replaced once the operation is performed. 
     On the other hand, arc generation mechanism  20 B which is a switch in the present embodiment allows the operation to be performed a plurality of times. It is therefore possible to check the operation in advance or periodically, and switchgear  1 B is more reliable. 
     As in the first embodiment, in a case where gunpowder or other media is used for driving piston  11 , the gunpowder or other media needs to be managed so as not to explode, for example. However, in the present embodiment, it is sufficient to provide arc generation mechanism  20 B that bridges terminal unit  22 , and therefore, assembling work of switchgear  1 B is simple. 
     In the present embodiment, switchgear  1 B that performs the closing operation has been described with reference to  FIG.  4   , but an opposite driving direction may be used for a switchgear that requires a high-speed opening operation. 
     Third Embodiment 
     A difference between the first and second embodiments and a third embodiment is a difference in the arc generation mechanism. 
     The arc generation mechanism according to the third embodiment uses a conductor. 
     Hereinafter, only the difference from the first and second embodiments will be described, and description of the same or corresponding parts will be omitted. As for the reference signs, the same or corresponding parts as those in the first and second embodiments are denoted by the same reference signs, and the description thereof will be omitted. 
       FIG.  5    is an example of a sectional view of a switchgear  1 C according to the present embodiment. Switchgear  1 C will be described with reference to  FIG.  5   . 
     A drive unit  10 C includes movable shaft  4  and container  12 , and container  12  accommodates piston  11 , opening spring  13 , and an arc generation mechanism  20 C. 
     In the present embodiment, arc generation mechanism  20  C is the conductor described as movable conductor  20 B 2  according to the second embodiment. In the second embodiment, movable conductor  20 B 2  is connected to fixed conductor  20 B 1  to be a switch. Arc generation mechanism  20 C according to the present embodiment is provided inside piston  11 , and disposed on an inner side of a pressure receiving surface in piston  11  facing terminal unit  22  so as to bridge terminal unit  22 . Arc generation mechanism  20 C is contactable with and separable from terminal unit  22 . 
     Arc generation mechanism  20 C is connected to drive power supply  30  via terminal unit  22 , and arc generation mechanism  20 C, terminal unit  22 , and drive power supply  30  constitute a circuit. 
       FIG.  6    is a diagram illustrating a flow of a closing operation of switchgear  1 C according to the present embodiment.  FIG.  6   ( 1 ) illustrates an open state which is a position in an initial state,  FIG.  6   ( 2 ) illustrates a driving state in a midway of operation of arc generation mechanism  20 C, and  FIG.  6   ( 3 ) illustrates a closed state after operation of arc generation mechanism  20 C. 
     In the open state, which is the position in the initial state in  FIG.  6   ( 1 ), piston  11  is positioned to be contact with container  12 . Terminal unit  22  is in contact with and bridged with arc generation mechanism  20 C. As in the first and second embodiments, in the open state in the present embodiment, opening spring  13  which is an elastic member is connected to and held by the movable part. 
     An arrow P in  FIG.  6   ( 2 ) indicates an energization path in a case where drive power supply  30  energizes arc generation mechanism  20 C. An arrow Q indicates a direction in which the movable part moves by a pressure in container  12 . 
     When energized, as indicated by arrow P, a current flows from drive power supply  30  to arc generation mechanism  20 C via terminal unit  22 , and thus, in arc generation mechanism  20 C, an electromagnetic repulsive force is generated by an interaction with a current flowing through terminal unit  22 . The generated electromagnetic repulsive force is a force in the direction of arrow Q which is the closing direction. Due to the generated electromagnetic repulsive force, arc generation mechanism  20 C is separated from terminal unit  22  to open a circuit with terminal unit  22 , and arc  23  is generated between terminal unit  22  and arc generation mechanism  20 C. As illustrated in  FIG.  6   ( 2 ), arc  23  is generated at two opened portions where arc generation mechanism  20 C bridges terminal unit  22 . 
     The pressure of generated arc  23  and an electromagnetic force increase an operating speed of piston  11  in the direction of arrow Q. 
     As a result, piston  11  receives the pressure, operates to rise in the closing direction of movable electrode  3 , and drives movable electrode  3  in the closing direction via movable shaft  4 . 
     In the closed state in  FIG.  6   ( 3 ), as in the first and second embodiments, an energization path is formed by upper terminal  6 , fixed electrode  2 , movable electrode  3 , and lower terminal  7  after closing. When energized by the energization path, an electromagnetic repulsive force is generated between fixed electrode  2  and movable electrode  3 , and thus, the contact point between fixed electrode  2  and movable electrode  3  is pressurized to maintain the closed state. 
     In the present disclosure, as in the first and second embodiments, gas in a space near a bottom surface of container  12  in  FIGS.  5  and  6    is heated and expanded by arc  23  generated with terminal unit  22  by the electromagnetic repulsive force of arc generation mechanism  20 C provided in switchgear  1 C, and movable electrode  3  receives the pressure of the expanded gas and can move in the closing direction to close switchgear  1 C. Thus, since an explosion technology is not used, the components do not need to be periodically replaced, and it is possible to obtain switchgear  1 C with long-term high reliability. 
     In addition, switchgear  1 C can be operated at a high speed by utilizing a rapid increase in pressure in container  12  due to arc  23 . 
     The expanding gas does not need to be disposed at the position of the bottom surface of container  12  illustrated in  FIGS.  5  and  6    but only needs to be disposed at such a position that the air can be expanded by generated arc  23 . 
     In the present embodiment, since arcs  23  are generated at two places, the pressure of the gas heated and expanded by arcs  23  is increased more than in the first and second embodiments. Therefore, in the present embodiment, switchgear  1 C can be operated at a higher speed by the electromagnetic repulsive three in the closing direction generated in arc generation mechanism  20 C in addition to a driving force due to the pressure rise of arcs  23  generated at two places. 
     As in the first and second embodiments, in a case where gunpowder or other media is used for driving piston  11 , the gunpowder or other media needs to be managed so as not to explode, for example. However, in the present embodiment, it is sufficient to provide arc generation mechanism  20 C that bridges terminal unit  22 , and therefore, assembling work of switchgear  1 C is simple. 
     In the present embodiment, switchgear  1 C that performs the closing operation has been described with reference to  FIG.  6   , but an opposite driving direction may be used for a switchgear that requires a high-speed opening operation. 
     In the present exemplary embodiment, arc generation mechanism  20 C is provided inside piston  11 . However, arc generation mechanism  20 C only needs to be provided in container  12  in which piston  11  generating arc  23  can be driven. For example, arc generation mechanism C may be disposed on the pressure receiving surface facing an outside of piston  11  and a surface of piston  11  facing terminal unit  22 . However, since air leaks from a gap between piston  11  and container  12 , a volume of a space between terminal unit  22  and the pressure receiving surface is preferably large. 
     Fourth Embodiment 
     A difference between the first to third embodiments and a fourth embodiment is a difference in the arc generation mechanism. 
     The arc generation mechanism according to the fourth embodiment uses a plurality of conductors. 
     Hereinafter, only the difference from the first to third embodiments will be described, and description of the same or corresponding parts will be omitted. As for the reference signs, the same or corresponding parts as those in the first to third embodiments are denoted by the same reference signs, and the description thereof will be omitted. 
       FIG.  7    is an example of a sectional view of a switchgear  1 D according to the present embodiment. Switchgear  1 D will be described with reference to  FIG.  7   . 
     A drive unit  10 D includes movable shaft  4  and container  12 , and container  12  accommodates piston  11 , opening spring  13 , and an arc generation mechanism  20 D. 
     In the present embodiment, arc generation mechanism  20 D is a plurality of conductors. In the third embodiment, the conductor is provided in the piston  11 , but the conductor according to the present embodiment is also provided in container  12  in addition to piston  11 . That is, arc generation mechanism  20 D includes a conductor disposed on the inner side of the pressure receiving surface of piston  11  facing terminal unit  22  so as to bridge terminal unit  22 , and a conductor disposed in container  12  so as to face the pressure receiving surface of piston  11 . The conductor disposed in container  12  and the conductor disposed in piston  11  are provided in contact with each other so as to be connected in series. The conductor in piston  11  of arc generation mechanism  20 D is contactable with and separable from terminal unit  22 . 
     Arc generation mechanism  20 D is connected to drive power supply  30  via terminal unit  22 , and arc generation mechanism  20 D, terminal unit  22 , and drive power supply  30  constitute a circuit. 
       FIG.  8    is a diagram illustrating a flow of a closing operation of switchgear  1 D according to the present embodiment.  FIG.  8   ( 1 ) illustrates an open state which is a position in an initial state,  FIG.  8   ( 2 ) illustrates a driving state in a midway of operation of arc generation mechanism  20 D, and  FIG.  8   ( 3 ) illustrates a closed state after operation of arc generation mechanism  20 D. 
     In the open state, which is the position in the initial state in  FIG.  8   ( 1 ), piston  11  is positioned to be contact with container  12 . Terminal unit  22  is in contact with and bridged with arc generation mechanism  20 D. As in the first to third embodiments, in the open state in the present embodiment, opening spring  13 , which is an elastic member that generates a load in the opening direction, is connected to and held by the movable part. 
     An arrow P in  FIG.  8   ( 2 ) indicates an energization path in a case where drive power supply  30  energizes arc generation mechanism  20 D. An arrow Q indicates a direction in which the movable part moves by the pressure in container  12 . 
     When energized, as indicated by arrow P, a current flows from drive power supply  30  to arc generation mechanism  20 D via terminal unit  22 , and thus, in arc generation mechanism  20 D, an electromagnetic repulsive force is generated between the conductor in piston  11 , the conductor in container  12 , and terminal unit  22  by an interaction with the current flowing through terminal unit  22 . The generated. electromagnetic repulsive force is a force in the direction of arrow Q which is the closing direction. Due to the generated electromagnetic repulsive three, the conductor in container  12 , which is arc generation mechanism  20 D, is separated from terminal unit  22  to open a circuit with terminal unit  22 , and arc  23  is generated between terminal unit  22 , the conductor in piston  11 , and the conductor in container  12 . 
     As illustrated in  FIG.  8   ( 2 ), arc is generated at four opened portions where arc generation mechanism  20 D bridges terminal unit  22 . The pressure of generated arc  23  and an electromagnetic force increase an operating speed of piston  11  in the direction of arrow Q. As a result, piston  11  drives movable electrode  3  in the closing direction via movable shaft  4 . 
     In the closed state in  FIG.  8   ( 3 ), as in the first to third embodiments, an energization path is formed by upper terminal  6 , fixed electrode  2 , movable electrode  3 , and lower terminal  7  after closing. When energized by the energization path, an electromagnetic repulsive force is generated between fixed electrode  2  and movable electrode  3 , and thus, the contact point between fixed electrode  2  and movable electrode  3  is pressurized to maintain the closed state. 
     In the present disclosure, as in the first to third embodiments, the gas is heated and expanded by arc  23  generated between terminal unit  22  and arc generation mechanism  20 D by the electromagnetic repulsive force of arc generation mechanism  20 D provided in switchgear  1 D, and movable electrode  3  receives the pressure of the heated and expanded gas and can move in the closing direction to close switchgear  1 D. Thus, since an explosion technology does not need to be used, the components do not need to be periodically replaced, and it is possible to obtain switchgear  1 D with long-term high reliability. 
     In addition, switchgear  1 D can be operated at a high speed by utilizing a rapid increase in pressure in container  12  due to arc  23 . 
     In the present embodiment, since arcs  23  are generated at four places, the pressure by arcs  23  is increased more than in the first to third embodiments. Therefore, in the present embodiment, switchgear  1 D can be operated at a higher speed by the electromagnetic force in the closing direction generated in arc generation mechanism  20 D in addition to a driving force due to the pressure rise of arcs  23  generated at four places. 
     As in the first to third embodiments, in a case where gunpowder or other media is used for driving piston  11 , the gunpowder or other media needs to be managed so as not to explode, for example. However, in the present embodiment, it is sufficient to provide arc generation mechanism  20 D that bridges terminal unit  22 , and therefore, assembling work of switchgear  1 D is simple. 
     In the present embodiment, switchgear  1 D that performs the closing operation has been described with reference to  FIG.  8   , but an opposite driving direction may be used for a switchgear that requires a high-speed opening operation. 
     Fifth Embodiment 
     A difference between the first to fourth embodiments and the fifth embodiment is whether an ablation material is disposed in the switchgear. 
     Hereinafter, only the difference from the first and fourth embodiments will be described, and description of the same or corresponding parts will be omitted. As for the reference signs, the same or corresponding parts as those in the first to fourth embodiments are denoted by the same reference signs, and the description thereof will be omitted. 
       FIG.  9    is an example of a sectional view of a switchgear  1 E according to the present embodiment. Switchgear  1 E will be described with reference to  FIG.  9   . 
     A drive unit  10 E includes movable shaft  4  and container  12 , and container  12  accommodates piston  11 , opening spring  13 , and an arc generation mechanism  20 E. 
     In the present embodiment, as in the first embodiment, arc generation mechanism  20 E is a fuse and is connected to drive power supply  30  via terminal unit  22 , and arc generation mechanism  20 E, terminal unit  22 , and drive power supply  30  constitute a circuit. 
     Switchgear  1 E according to the present embodiment includes an ablation. material  50 . Ablation material  50  is a material, such as a thermoplastic resin, which is decomposed and vaporized by light and heat of an arc. In the present embodiment, as illustrated in  FIG.  9   , one ablation material  50  is provided inside piston  11 , and disposed on the inner side of the pressure receiving surface in piston  11  facing terminal unit  22 . The other ablation material  50  is disposed in container  12  so as to face the pressure receiving surface of piston  11 . 
       FIG.  10    is a diagram illustrating a flow of a closing operation of switchgear  1 E according to the present embodiment.  FIG.  10   ( 1 ) illustrates an open state which is a position in an initial state,  FIG.  10   ( 2 ) illustrates a driving state in a midway of operation of arc generation mechanism  20 E, and  FIG.  10   ( 3 ) illustrates a closed state after operation of arc generation mechanism  20 E. 
     In the open state, which is the position in the initial state in  FIG.  10   ( 1 ), piston  11  is positioned so as to be in contact with the space on the open side in container  12 . Terminal unit  22  is bridged with arc generation mechanism  20 E. As in the first to fourth embodiments, in the open state in the present embodiment, opening spring  13 , which is an elastic member that generates a load in the opening direction, is connected to and held by the movable part. 
     An arrow P in  FIG.  10   ( 2 ) indicates an energization path in a case where drive power supply  30  energizes arc generation mechanism  20 E. An arrow Q indicates a direction in which the movable part moves by the pressure in container  12 . 
     When energization is performed, as indicated by arrow P, a current flows from drive power supply  30  to arc generation mechanism  20 E via terminal unit  22 , and thus, as in the first embodiment, arc generation mechanism  20 E is immediately burned out, the circuit with terminal unit  22  is opened, and an arc  23  is generated at the opened portion. A surface of ablation material  50  is decomposed by arc  23  and vaporized. 
     In the present embodiment, in container  12 , in addition to the pressure due to the gas heated and expanded by generated arc  23 , the pressure due to the gas vaporized from ablation material  50  is generated. 
     Receiving the pressure of generated arc  23  and the pressure due to ablation material  50 , piston  11  gains an increased operating speed in the direction of arrow Q. As a result, piston  11  receives the pressure, operates to rise in the closing direction of movable electrode  3 , and drives movable electrode  3  in the closing direction via movable shaft  4 . 
     In the closed state in  FIG.  10   ( 3 ), as in the first to fourth embodiments, an energization path is formed by upper terminal  6 , fixed electrode  2 , movable electrode  3 , and lower terminal  7  after closing. When energized by the energization path, an electromagnetic repulsive force is generated between fixed electrode  2  and movable electrode  3 , and thus as in the first embodiment, the contact point between fixed electrode  2  and movable electrode  3  is pressurized to maintain the closed state. 
     In the present disclosure, as in the first to fourth embodiments, due to arc  23  generated by burning out of arc generation mechanism  20 E provided in switchgear  1 E, the temperature in container  12  is increased. The expansion of the gas and the vaporization of ablation material  50  rapidly increase the temperature in container  12  to expand a volume. The pressure of the volume can move movable electrode  3  in the closing direction to close switchgear  1 E. Thus, since an explosion technology is not used, the components do not need to be periodically replaced, and it is possible to obtain a switchgear with long-term high reliability. 
     In addition, the switchgear can be operated at a high speed by utilizing a rapid increase in pressure in container  12  due to arc  23 . 
     In the present embodiment, since the pressure due to the vaporization of ablation material  50  is applied, the switchgear can be operated at a higher speed than in the first embodiment. 
     In the present embodiment, an ablation material is added to the fuse which is the arc generation mechanism according to the first embodiment. However, in any of the second to fourth embodiments, the ablation material can be similarly added to increase the pressure rise, and the speed can be further increased. 
     In the present embodiment, the ablation material is disposed in piston  11  and in container  12  facing the pressure receiving surface of piston  11 , hut the present invention is not limited thereto. For example, the ablation material may be disposed on a side of the pressure receiving surface of piston  11  in container  12 . Ablation material  50  only needs to be disposed at such a position as to be vaporized by arc  23  generated by energization of arc generation mechanism  20 E. 
     As in the first to fourth embodiments, in a case where gunpowder or other media is used for driving piston  11 , the gunpowder or other media does not need to be managed so as not to explode, for example. However, in the present embodiment, it is sufficient to provide arc generation mechanism  20 E that bridges terminal unit  22 , and therefore, assembling work of switchgear  1 E is simple. 
     In the present embodiment, switchgear  1 E that performs the closing operation has been described with reference to  FIG.  10   , but an opposite driving direction may be used for a switchgear that requires a high-speed opening operation. 
     Sixth Embodiment 
       FIG.  11    is a configuration diagram of power converter  100  according to a sixth embodiment. Power converter  100  illustrated in  FIG.  11    is a power converter of a modular multilevel converter (MMC) type that converts AC power into DC power.  FIG.  11   ( 1 ) is an overall configuration diagram of power converter  100 , and  FIG.  11   ( 2 ) is an example of a configuration diagram of a power module  101 . 
     As illustrated in  FIG.  11   ( 1 ), in power converter  100  according to the present embodiment, a plurality of power modules  101  are connected in series between each phase and a DC output line for an input system of three-phase AC power. 
     As illustrated in  FIG.  11    ( 2 ), in each of the plurality of power modules  101 , two power semiconductor circuits  102  are connected in series, and a capacitor  103  is connected as an energy accumulator in parallel with two power semiconductor circuits  102  connected in series. Further, switchgear  1 A according to the first embodiment is connected in parallel between an input terminal and an output terminal of each power module  101 . 
     Since power converter  100  according to the present embodiment includes switchgear  1 A, when power module  101  fails, switchgear  1 A is closed to short-circuit input and output of failed power module  101  to prevent power converter  100  as a whole from stopping operating. 
     Even when the output of failed power module  101  is short-circuited, power converter  100  can be continuously operated by providing redundancy to the number of power modules connected in series. 
     When a short-circuit failure occurs in a semiconductor element in power module  101 , there is a possibility that the element is broken with an arc. Therefore, in some cases, an explosion-proof structure may be provided around a power semiconductor. Since by closing switchgear  1 A at a high speed when an arc is generated, switchgear  1 A can shorten generation time of the arc, the explosion-proof structure of power module  101  power converter  100  can be simplified. 
     The switchgear illustrated in the second to fifth embodiments may be applied to switchgear  1 A according to the present embodiment. 
     When the number of power modules  101  becomes large, the cost of component replacement increases. However, the switchgear described in the first to sixth embodiments eliminates the need for replacing the components, and an inexpensive power converter can be provided by a simple explosion-proof structure. 
     Since the switchgear described in the second to fifth embodiments can be operated a plurality of times, the operation can be checked, and a more reliable power converter can be provided. 
     REFERENCE SIGNS LIST 
       1 A,  1 B,  1 C,  1 D,  1 E: switchgear,  2 : fixed electrode,  3 : movable electrode,  4 : movable shaft,  5 : vacuum container,  6 : upper terminal,  7 : lower terminal,  10 A,  10 B,  10 C,  10 D,  10 E: drive unit,  11 : piston,  12 : container,  13 : opening spring,  20 A,  20 B,  20 C,  20 D,  20 E: arc generation mechanism,  20 B 1 : fixed conductor,  20 B 2 : movable conductor,  20 B 3 : contact,  22 : terminal unit,  23 : arc,  30 : drive power supply,  50 : ablation material,  100 : power converter,  101 : power module,  102 : power semiconductor circuit,  103 : capacitor