Patent Publication Number: US-7911303-B2

Title: Circuit breaker and opening and closing method thereof

Description:
CLAIM OF PRIORITY 
     The present application claims priority from Japanese application serial No. 2006-353644, filed on Dec. 28, 2006, the content of which is hereby incorporated by reference into this application. 
     FIELD OF THE INVENTION 
     The present invention relates to a circuit breaker and more particularly to a circuit breaker provided with an opening means which quickly drives an operating shaft of a vacuum valve toward an opening direction by electromagnetic repulsion, and an opening and closing method thereof. 
     BACKGROUND OF THE INVENTION 
     An opening means which quickly drives an operating shaft of a vacuum valve toward an opening direction by electromagnetic repulsion comprises an electromagnetic repulsion coil and a ring copper plate located opposite to it where the vacuum valve is opened by quickly exciting the electromagnetic repulsion coil of the opening means by a capacitor discharge or the like and using an electromagnetic repulsive force such as eddy current which occurs in the coil current and copper plate. 
     Circuit breakers with an electromagnetic repulsion mechanism are classified into direct-current circuit breakers and high-speed circuit breakers. In the former type, charge in a previously charged capacitor is injected in a direction reverse to the direction of line current to make a zero current point forcedly to interrupt the current. If an accidental short circuit occurs in the DC line, an overcurrent as determined by resistance and inductance as circuit constants, like fast-rising short-circuit current, flows, necessitating the breaker to operate quickly. 
     On the other hand, a high speed breaker is used in a private power generation system or the like and introduced in order to prevent electrical leakage from the private power generation equipment in a power failure, to prevent power supply systems from going down due to an overload, or assure continuous operation of a critical load by quickly switching from a defective power system to a normal one. This type of breaker also uses an electromagnetic repulsion mechanism because a response of the breaker must be within several milliseconds after receipt of an opening command. 
     One known example of such a breaker with an electromagnetic repulsion driving mechanism is the one disclosed in JP-A No. 2000-299041 which includes a vacuum valve, an operating mechanism provided in the opening and closing of the vacuum valve, and an electromagnetic repulsion driving mechanism provided midway in the operating mechanism and further includes a mechanism for reducing rebound of the movable electrode shaft in the course of current interruption. 
     SUMMARY OF THE INVENTION 
     However, the above conventional circuit breakers are compelled to provide a large electromagnetic repulsion driving mechanism and a larger power supply capacity because they not only have to obtain a prescribed opening speed but also require an electromagnetic repulsive force exceeding the attractive force of a permanent magnet for holding the closed state. Besides, since the electromagnetic repulsion driving mechanism and the mechanism for reducing rebound of the movable electrode shaft during action of the electromagnetic repulsion driving mechanism are vertically disposed in series between the vacuum valve and its operating mechanism, the electromagnetic repulsion driving mechanism and the mechanism for reducing rebound of the movable electrode shaft must be both moved when the vacuum valve is opened or closed. 
     Therefore, if the vacuum valve operating mechanism is of the electromagnetically driven type, its components such as the permanent magnet and exciting coil must have a large capacity, which means that the vacuum valve operating mechanism should be large enough. In addition, the vacuum valve operating mechanism can become less maneuverable due to its size. 
     The present invention has been made in view of the above circumstances and an object thereof is to provide a circuit breaker which enables an electromagnetic repulsion driving mechanism to easily cancel the closed state and reduces the rebound of a movable electrode shaft in the course of current interruption in a simple manner and provides high maneuverability, and an opening and closing method thereof. 
     In order to achieve the above objects, a circuit breaker according to the present invention that comprises a first coil in an electromagnet which opens and closes a vacuum valve, a second coil provided in the electromagnet together with the first coil, and an electromagnetic repulsion coil connected in series with the second coil, wherein the second coil and the electromagnetic repulsion coil are excited simultaneously in quick opening operation by electromagnetic repulsion. In another aspect of the present invention, the circuit breaker comprises a vacuum valve, an electromagnet including a coil for driving an operating shaft of the vacuum valve toward an opening direction by electromagnetic repulsion, a movable core and a permanent magnet, and an operating mechanism which excites the coil to close the vacuum valve, holds the vacuum valve closed by an attractive force of the permanent magnet and excites the coil in a direction reverse to an excitation direction in closing operation to open the vacuum valve, wherein, together with the coil, a second coil which is excited simultaneously with an electromagnetic repulsion coil for electromagnetic repulsion in quick opening operation by electromagnetic repulsion is provided in the electromagnet. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a left side view of a commutation type DC circuit breaker according to an embodiment of the present invention. 
         FIG. 2  is a back view of the commutation type DC circuit breaker shown in  FIG. 1  according to the invention. 
         FIG. 3  is a right side view of the commutation type DC circuit breaker shown in  FIG. 1  according to the invention. 
         FIG. 4  is a front view of the commutation type DC circuit breaker shown in  FIG. 1  according to the invention. 
         FIG. 5  is a system circuit diagram for the commutation type DC circuit breaker shown in  FIG. 1  according to the invention. 
         FIG. 6  is a time chart showing operation in case of an accident for the commutation type DC circuit breaker shown in  FIG. 1  according to the invention. 
         FIG. 7  is a time chart showing normal operation for the commutation type DC circuit breaker shown in  FIG. 1  according to the invention. 
         FIG. 8  is a diagram illustrating a coil excitation method in closing operation for the commutation type DC circuit breaker shown in  FIG. 1  according to the invention. 
         FIG. 9  is a diagram illustrating a coil excitation method in opening operation for the commutation type DC circuit breaker shown in  FIG. 1  according to the invention. 
         FIG. 10  is a diagram illustrating a coil excitation method in quick interruption for the commutation type DC circuit breaker shown in  FIG. 1  according to the invention. 
         FIG. 11  is a right side sectional view of a three-phase high speed circuit breaker as a circuit breaker according to the invention. 
         FIG. 12  is a back view of the three-phase high speed circuit breaker shown in  FIG. 11  according to the invention. 
         FIG. 13  is a front view of the three-phase high speed circuit breaker shown in  FIG. 11  according to the invention. 
         FIG. 14  is a left side sectional view of a switchgear incorporating the commutation type DC circuit breaker shown in  FIG. 1  according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     According to an embodiment of the present invention, a circuit breaker opening and closing method wherein a vacuum valve is opened or closed by excitation of a first coil, and a second coil provided in an electromagnet together with the first coil and an electromagnetic repulsion coil connected serially are simultaneously excited in quick opening operation by electromagnetic repulsion. The circuit breaker opening and closing method in which a coil of an electromagnet for driving an operating shaft of a vacuum valve toward an opening direction by electromagnetic repulsion is excited to close the vacuum valve, the vacuum valve is held closed by an attractive force of a permanent magnet of the electromagnet and the coil is excited in a direction reverse to an excitation direction in closing operation to open the vacuum valve is characterized in that a second coil provided in the electromagnet together with the coil and an electromagnetic repulsion coil for electromagnetic repulsion are simultaneously excited in quick opening operation by electromagnetic repulsion. 
     According to another embodiment of the present invention, an electromagnet coil is excited in a direction reverse to an excitation direction in closing operation simultaneously with opening operation by an electromagnetic repulsion driving mechanism, so that the attractive force of a permanent magnet to hold the closed state decreases and cancellation of the closed state becomes easy and rebound of a movable electrode shaft in the course of current interruption can be reduced in a simple manner and a highly maneuverable circuit breaker and an opening and closing method thereof can be obtained. 
     The object of providing a circuit breaker which enables an electromagnetic repulsion driving mechanism to easily cancel the closed state and reduces rebound of a movable electrode shaft in the course of current interruption in a simple manner and provides high maneuverability as well as an opening and closing method thereof is achieved in a simple manner. 
     Next, embodiments of a circuit breaker according to the present invention will be described. 
       FIGS. 1 to 7  and  FIG. 14  show an embodiment of a commutation type DC circuit breaker as a circuit breaker according to the present invention, in which  FIG. 1  is a left side sectional view of an embodiment of a commutation type DC circuit breaker as a circuit breaker according to the invention,  FIG. 2  is a back view of an embodiment of the commutation type DC circuit breaker shown in  FIG. 1  as a circuit breaker according to the present invention,  FIG. 3  is a right side sectional view of an embodiment of the commutation type DC circuit breaker shown in  FIG. 1  as a circuit breaker according to the present invention,  FIG. 4  is a front view of an embodiment of the commutation type DC circuit breaker shown in  FIG. 1  as a circuit breaker according to the present invention,  FIG. 5  is a system circuit diagram of an embodiment of the commutation type DC circuit breaker shown in  FIG. 1  as a circuit breaker according to the present invention,  FIG. 6  is a time chart showing operation in case of an accident in an embodiment of the commutation type DC circuit breaker shown in  FIG. 1  as a circuit breaker according to the present invention, and  FIG. 7  is a time chart showing normal operation in an embodiment of the commutation type DC circuit breaker shown in  FIG. 1  as a circuit breaker according to the present invention.  FIG. 14  is a left side sectional view of switchgear which uses an embodiment of a commutation type DC circuit breaker as a circuit breaker according to the present invention. 
     First Embodiment 
     First, the method of use and the method of operation for an embodiment of a commutation type DC circuit breaker as a circuit breaker according to the present invention will be described referring to  FIGS. 5 to 7 . 
     In  FIG. 5 , reference numeral  1  represents a DC power supply which supplies 1500 V through its positive pole in an ordinary DC feeding circuit. Reference numeral  2  represents a load such as a train. Reference numeral  3  represents a feeding line which supplies electricity to the load and reference numeral  4  represents a flyback line which connects the load  2  and the DC power supply  1 . The commutation type DC circuit breaker  5  as a circuit breaker according to the present invention is inserted midway in the feeding line  3  and switches electric power supplied from the DC power supply  1  to the load  2 . 
     The commutation type DC circuit breaker  5  is comprised of four switches, a first main switch  51 , a second main switch  52 , a first sub switch  53  and a second sub switch  54 , and a control unit  900 . The commutation type DC circuit breaker  5  is connected with a first capacitor  55 , a second capacitor  56  and a reactor  57 . The first main switch  51  and the second main switch  52  are inserted into the feeding line  3  serially and are located near the DC power supply  1  and near the load  2  respectively. The series circuit composed of the first sub switch  53 , first capacitor  55  and reactor  57  is connected in parallel with the main switch and the series circuit composed of the second sub switch  54  and second capacitor  56  is connected in parallel with the first capacitor  55 . 
     A current transformer  58  in the feeding line  3  detects the current of the feeding line  3  and sends the current value to an overcurrent tripping device  59 . The overcurrent tripping device  59  has a preset value for automatic interruption and outputs an opening command  11  when the current flowing through the feeding line  3  reaches the preset value. Upon receipt of an external command  10  or an opening command  11  from the overcurrent tripping device  59 , the control unit  900  gives an opening/closing command to the commutation type DC circuit breaker  5 . 
     The first sub switch  53 , which operates in conjunction with the first main switch  51 , once closes after opening of the first main switch  51  with time lag t 1  (for example, 2 ms) and then opens. On the other hand, the second sub switch  54 , which operates in conjunction with the second main switch  52 , opens time t 2  (for example, 2.5 ms) before opening of the second main switch  52 . 
     For operation of the load  2 , the first main switch  51  and second main switch  52  are closed to supply 1500 V DC to the load  2 . At this time, the first sub switch  53  is open and the second sub switch  54  is closed. The first capacitor  55  and second capacitor  56  are charged to +2000 V with reference to the DC power supply  1 . 
     If the load  2  is out of order or an earth fault occurs in the feeding line  3 , a very large fast-rising fault current, which depends on circuit constants, flows in the feeding line  3 . For example, if the circuit resistance is 15 mΩ and the circuit inductance is 150 pH, the maximum current attained is 100 kA and the maximum rush ratio is 10 kA/ms. If such a fault current occurs, the fault current must be interrupted quickly in order to minimize its influence on the equipment. First, the current transformer  58  detects the fault current value and sends it to the overcurrent tripping device  59 . If the overcurrent tripping device  59  is preset, for example, to 12000 A for automatic interruption, when the fault current reaches 12000 A, it sends an opening command  11  to the control unit  900 . According to a command from the control unit  900 , the first main switch  51  opens. As the first main switch  51  opens, the first sub switch  53  closes with time lag t 1 . Consequently, an LC resonance circuit, which consists of the first capacitor  55 , second capacitor  56 , reactor  57 , first main switch  51 , first sub switch  53  and second sub switch  54 , is established and the first capacitor  55  and second capacitor  56  previously charged by a charger  50  discharge electricity and a commutation current whose direction is reverse to the fault current direction is injected into the first main switch  51 . Assuming that the capacitance of the first capacitor  55  is 600 μF and the capacitance of the second capacitor  56  is 1200 μF, the maximum reverse commutation current value is 40 kA, which means that if the first sub switch  53  is closed before the fault current reaches 40 kA, the fault current is offset by the commutation current. At the time when the current passing through the first main switch  51  becomes zero, the main switch  51  finishes interruption. After the first main switch  51  opens, the second main switch  52  opens with time lag t 3 ; if time t 3  is set so as to satisfy the relation of t 3 &gt;t 1 +t 2 , the second sub switch  54  does not open before the first sub switch  53  closes and thus the first capacitor  55  and second capacitor  56  discharge electricity simultaneously, making it possible to deal with a large current as mentioned above. Even when the first main switch  51  has finished interruption, there is a period in which the first sub switch  53  and second sub switch  54  are both closed and thus the first capacitor  55  and second capacitor  56  are charged by the DC power supply  1 . This charge current is interrupted when the charge voltage rises and the circuit current becomes almost zero or below the vacuum valve chopping current. 
     On the other hand, interruption by the commutation type DC circuit breaker  5  in normal operation is done according to an external command  10 . Upon receipt of an opening command as an external command  10 , the first main switch  51  and second main switch  52  open simultaneously. At this time, since the second sub switch  54  opens time t 2  before the second main switch  52  opens, an LC resonance circuit, which consists of the first capacitor  55 , reactor  57 , first main switch  51  and first sub switch  53 , is established when the first sub switch  53  closes. 
     Out of the previously charged first capacitor  55  and second capacitor  56 , only the first capacitor  55  discharges electricity and a commutation current whose direction is reverse to the load current direction is injected into the first main switch  51 . Here, the maximum value of the load current is below the value preset on the overcurrent tripping device  59 , 12000 A. If the maximum commutation current is 14 kA when only the first capacitor  55  discharges electricity, the maximum load current of 12000 A is offset and when the current of the first main switch  51  becomes zero, the first main switch  51  finishes interruption. After the breaker opens, the second main switch  52  performs the function to disconnect the load  2  and the first capacitor  55 , and the load  2  and the second capacitor  56  to prevent an electric shock accident due to the capacitor charge voltage in the load circuit. 
     Next, an embodiment of the above commutation type DC circuit breaker  5  as a circuit breaker according to the present invention will be described referring to  FIGS. 1 to 4  and  FIG. 14 . 
     In an embodiment of the commutation type DC circuit breaker as a circuit breaker according to the present invention, the four switches are automatically activated at the above timings by two electromagnets and a mechanical link structure.  FIGS. 1 to 4  indicate that the circuit is in operation (the first main switch  51  and second switch  52  are closed). All the four switches are illustrated here as vacuum valves incorporating a pair of contacts but may be air switches or the like. 
     First, electrical connection in an embodiment of the commutation type DC circuit breaker  5  as a circuit breaker according to the present invention will be described. 
     A fixed feeder  100  of the first main switch  51  and a fixed feeder  114  of the second main switch  52  are connected to a bus bar  1000  ( FIG. 14 ) located outside the commutation type DC circuit breaker  5 . One end of the bus bar  1000  is connected with the reactor  57 . The other end of the reactor  57  is connected with the first capacitor  55  and second capacitor  56 . A movable conductor  62  of the first main switch  51  is electrically conductive to a movable feeder  120  through a power collector  101 . The movable feeder  120  is connected with the DC power supply  1 . The movable conductor  62  of the first main switch  51  and the movable conductor  69  of the first sub switch  53  are constantly connected electrically through conductors  102 ,  103 , a flexible conductor  104  and a conductor  105 . 
     A feeder  106  and a feeder  107  are fixed on a fixed conductor  108  of the first sub switch  53 . The feeder  107  is connected with a fixed conductor  109  of the second sub switch  54 . On the other hand, the feeder  106  is connected with the first capacitor  55  outside the commutation type DC circuit breaker  5 . A movable conductor  110  of the second sub switch  54  is connected with the second capacitor  56  through a conductor  111 , a flexible conductor  112  and a feeder  113 . A movable conductor  200  of the second main switch  52  is electrically conductive to a movable feeder  203  through a power collector  201 . The movable feeder  203  is connected with the load  2 . The system circuit shown in  FIG. 5  is implemented by the above electrical connections. 
     Next, the mechanical structure of an embodiment of the commutation type DC circuit breaker  5  as a circuit breaker according to the present invention will be described referring to  FIGS. 1 to 4 . 
     As shown in  FIG. 1 , the movable conductor  62  of the first main switch  51  is pin-connected with a member  64 . One end of an operating rod  65  is fixed on the member  64  and the other end is fixed on a hinge  66 . The movable conductor  69  of the first sub switch  53  is connected with the hinge  66  through a member  67  by a pin  534 . In other words, the movable conductor  62  of the first main switch  51  and the movable conductor  69  of the first sub switch  53  work in conjunction with each other. The operating rod  65 A penetrates a pin  150  whose top and bottom are flattened. A washer  153 , a contact pressure spring  151  and a washer  154  are held between the pin  150  and a nut  152  fixed on the operating rod  65 . 
     Also the operating rod  65  penetrates an electromagnetic repulsion coil  170  that constitutes an opening means, and a repulsion plate  171 . An eddy current is generated in the repulsion plate  171  by excitation of the electromagnetic repulsion coil  170 , and an electromagnetic repulsive force between the current of the electromagnetic repulsion coil  170  and the eddy current of the repulsion plate  171  is received by the member  64  through the repulsion plate  171  and the operating rod  65  moves upward in  FIG. 1  by the repulsive force. 
     As shown in  FIG. 3 , the movable conductor  200  of the second main switch  52  is pin-connected with a member  202 . One end of an operating rod  204  is fixed on the member  202 . The operating rod  204  penetrates a pin  206  with flattened abutment surfaces at the top and bottom. A washer  210 , a contact pressure spring  212  and a washer  214  are held between the pin  206  and a nut  208  fixed on the operating rod  202 . With the second main switch  52  open, the hexagonal part  216  at the top of the operating rod  202  is engaged with the pin  206  by a contact pressure spring  212 . On the other hand, in closing operation of the second main switch  52 , the pin  206  and hexagonal part  216  are disengaged at the moment the fixed contact  220  and movable contact  222  of the second main switch  52  contact each other, and as the contact pressure spring  212  is compressed, the load of the contact pressure spring  212  becomes a contact force of the contacts in the second main switch  52 . 
     As shown in  FIGS. 1 and 3 , the operating rod  65  of the first main switch  51  and the operating rod  202  of the second main switch  52  are driven by an electromagnet  301  located beside the first main switch  51  and second main switch  52  in an operating device case  300 . The shaft  302  of the electromagnet  301  is coupled with one lever  501  of a main shaft  500  through a member  303 . The other levers  503  and  499  of the main shaft  500  are coupled with an insulating rod  502  extending toward the first main switch  51  and an insulating rod  504  extending toward the second main switch  52  respectively. The insulating rod  502  is engaged with the pin  150  through a sub shaft  510  and the insulating rod  504  is engaged with the pin  206  through a sub shaft  512 . In other words, as shown in  FIGS. 1 and 3 , the attractive force of the electromagnet  301  is transmitted to the operating rod  65  of the first main switch  51  and the operating rod  204  of the second main switch  52  through the main shaft  500 , levers  501 ,  503 ,  499  provided on it and the sub shafts  510 ,  512  and levers  513 ,  514  provided on them. The first main switch  51  or the second main switch  52  is turned on (closed) by exciting a first coil  305   a  in the electromagnet  301  and moving a plunger  304  downward in the figure. 
     The first sub switch  53  is driven in conjunction with the first main switch  51  as mentioned above; however, in order to achieve operation timings as illustrated in  FIGS. 6 and 7 , a coupling member  530  and a lever  531  are provided as shown in  FIG. 1 . The coupling member  530  and lever  531  are connected with each other by a pin  533 . The other end of the coupling member  530  is connected with the lever  513  of the sub shaft  510 . On the other hand, the lever  531  freely rotates around a shaft  532  as a fulcrum. 
     When the first main switch  51  opens, the operating rod  65  moves upward in  FIG. 1  and the first sub switch  53  once closes and at the same time the lever  531  rotates counterclockwise, which causes the lever  531  to engage with the pin  534  in the hinge  66  and moves back the movable conductor  69  of the first sub switch  53  toward the opening direction (downward). The hole in the hinge  66  through which the pin  534  penetrates is made oval in order to enable the movable conductor  69  to move toward the opening direction (downward) regardless of the position of the operating rod  65 . Reference numeral  70  represents a spring which gives a contact force to the first sub switch  53 . 
     As shown in  FIG. 3 , a coupling member  540  and a lever  541  are also provided on the second sub switch  54 . When the second main switch  52  opens, the lever  541  rotates around the shaft  542  clockwise, which causes the lever  541  to engage with a pin  543  in a member  544  coupled with the movable conductor  110  of the second sub switch  54  and moves back the movable conductor  110  toward the opening direction (downward). Reference numeral  71  represents a spring which gives a contact force to the second sub switch  54 . The coupling members  530  and  540  are variable in length and used to adjust opening and closing timings for the main switches and sub switches. 
     In  FIG. 4 , reference numeral  555  represents a tripping spring which is provided in the operating device case  300  so as to work in conjunction with the main shaft  500 ; reference numeral  590  represents a capacitor which supplies exciting energy to the first coil  305   a ; and reference numeral  591  represents a control circuit for the electromagnet  301 . The area indicated by chain double-dashed line represents the control unit  900  for supplying exciting energy to the electromagnetic repulsion coil  170 , which is comprised of a capacitor  902  and a control board  903 . 
     Next, the structure of the electromagnet  301  will be described. The first coil  305   a  and second coil  305   b  are provided on a bobbin  901 . Fixed cores  903 ,  904 ,  905  are provided on the upper, outer circumferential and lower surfaces of the first coil  305   a  and second coil  305   b  and a permanent magnet  306  rests on the fixed core  903  on the upper surface. The movable core of the electromagnet  301  is composed of a movable circular plate  906  and the plunger  304  and held between the shaft  302  and a nut  907 . When the electromagnet  301  is turned on, the plunger  304  and a center leg  908  are in contact with each other. 
     Next, operation of an embodiment of the commutation type DC circuit breaker  5  as a circuit breaker according to the present invention will be described. 
     (Normal Closing Operation and Opening Operation) 
     In closing operation, the first coil  305   a  of the electromagnet  301  is excited to let the plunger generate an attractive force. This attractive force is transmitted to the operating rod  65  of the first main switch  51  and the operating rod  204  of the second main switch  52  through the main shaft  500  and sub shafts  510 ,  512 , so that the movable conductors  62  and  200  move downward and the first main switch  51  and second main switch  52  close. In closing operation, the contact pressure springs  151 ,  212  of the main switches  51 ,  52  and the tripping spring  555  in the operating device case  300  are elastically charged to prepare for opening of the first main switch  51  and second main switch  52 . 
     At this time, the first sub switch  53  becomes open as the pin  534  and hinge  66  are engaged, and the second sub switch  54  becomes closed as the lever  541  and pin  543  are disengaged. Upon completion of closing operation, the electromagnet  301  is de-excited. The reactive forces of the elastically charged contact pressure springs  151 ,  212  and tripping spring  555  are held by the attractive force of the permanent magnet  306  in the electromagnet  301 . At this time, a magnetic flux from the permanent magnet  306  is generated primarily in the route from the permanent magnet  306  through the movable plate  906 , plunger  304 , center leg  908 , fixed core  905 , fixed core  904  and fixed core  903  to the permanent magnet  306 . 
     In normal opening operation of the first main switch  51  and second main switch  52 , the first coil  305   a  is excited in a direction reverse to the excitation direction in the above closing operation. By reversely exciting the first coil  305   a , the magnetic flux between the plunger  304  and center leg  908  is cancelled and the attractive force of the electromagnet  301  is decreased. When the attractive force becomes smaller than the spring reactive force, the first main switch  51  and second main switch  52  start opening operation. In this electromagnetic operating mechanism, opening operation is basically done by the spring forces of the contact pressure springs  151 ,  212  and tripping spring  555 . In other words, reverse excitation of the first coil  305   a  only requires an energy enough to cancel the magnetic flux generated by the permanent magnet. 
     (Quick Interruption in Case of an Accident) 
     In quick interruption in case of an accident, the electromagnetic repulsion coil  170  is excited to generate an electromagnetic repulsive force from the repulsion plate  171 . The member  64  receives the electromagnetic repulsive force and the operating rod  65  coupled with the member  64  moves upward further flexing the contact pressure spring  151  until the first main switch  51  becomes open and the first sub switch  53  becomes closed. At this time, the main shaft  500  and sub shaft  510  are not activated and the second main switch  52  and second sub switch  54  remain unchanged. 
     The second coil  305   b  of the electromagnet  301  is connected in series with the electromagnetic repulsion coil  170  and excited simultaneously with the electromagnetic repulsion coil  170 . When the direction of excitation of the second coil  305   b  is set to be the same as the direction of excitation of the first coil  305   a  in opening operation (reverse excitation), the magnetic flux of the permanent magnet  306  is cancelled and the attractive force of the electromagnet  301  is thus decreased. When the attractive force of the electromagnet  301  becomes smaller than the reactive forces of the contact pressure springs  151 ,  212  and tripping spring  555 , the plunger  304  moves upward. In response, the second main switch  52  and second sub switch open as well. 
     Next, the control method of an embodiment of the commutation type DC circuit breaker as a circuit breaker according to the present invention will be described referring to  FIGS. 8 to 10 .  FIGS. 8 ,  9  and  10  are diagrams which explain operation of the control circuit in closing operation, normal opening operation and interruption in case of an accident, respectively. Normal closing operation and opening operation are controlled by the control board  591 . In closing operation, two contacts  934 ,  935  are turned ON to make a circuit and the switch  915  is turned ON to give the charge of the capacitor  590  to the first coil  305   a  in the electromagnet  301  ( FIG. 8 ). On the other hand, in normal opening operation, four contacts  930 ,  931 ,  932 ,  933  are activated to make a circuit and the switch  915  is turned ON to excite the first coil  305   a  in a direction reverse to the excitation direction in closing operation ( FIG. 9 ). The arrowed curves in  FIGS. 8 and 9  express the directions of exciting current flows. 
     In quick interruption in case of an accident, the switch  914  is turned ON to excite the electromagnetic repulsion coil  170  and the second coil  305   b  in the electromagnet  201  by the charge of the capacitor  902  located in the control unit  900  ( FIG. 10 ). 
     Since the first coil  305   a  and second coil b constitute a duplex winding structure, when one of them is excited, an induced voltage is generated in the other (electromagnetic induction). For the second coil  305   b , which is used for quick interruption, the inductance must be small because of high speed and the number of turns is 10 (turns) or so. On the other hand, for the first coil  305   a , which is used for normal opening or closing operation, the coil current must be decreased in order to reduce the burdens on the capacitor  590  and switch  915  and the number of turns is set to 200-400 turns. Since the induced voltage is proportional to the number of turns, the voltage induced in the second coil upon excitation of the first coil does not pose a problem (in normal closing or opening operation) but conversely, or in quick interruption, a voltage on the kilovolt-order is induced in the first coil  305   a.    
     In this embodiment, a surge voltage suppressor  954  is provided as a countermeasure against induced voltage in the first coil  305   a . The surge voltage suppressor  954  is located in parallel with the first coil  305   a  so that a circulating current flows between the first coil  305   a  and the surge voltage suppressor  954 . A zinc oxide varistor (ZNR) may be used for the surge voltage suppressor  954  but in consideration of durability to withstand frequent operation, it is desirable that it be comprised of a protective resistance  952  and a diode  950  as shown in  FIGS. 8 to 10 . As shown in  FIG. 10 , in quick interruption, induced current I flows through the surge voltage suppressor  954  and therefore the induced voltage in the first coil  305   a  is decreased. When the protective resistance  952  is decreased, the induced voltage is decreased but the electromagnet  301  is released for a longer time. In order to meet the circuit insulation requirement, the resistance should be as large as possible. 
     In this embodiment, more operating energy is required in closing operation in which the contact pressure springs  151 ,  212  and tripping spring  555  are elastically charged for driving, than in opening operation which basically uses the above spring forces. As explained earlier, in the case of the electromagnet  301  in this embodiment, for opening operation it is enough to cancel the magnetic flux of the permanent magnet  306  in the electromagnet  301 . For this reason, the diode  950  is disposed as shown in  FIGS. 8 to 10  so as not to affect closing operation, which requires a large energy. On the other hand, for normal opening operation, which uses a small operating energy, the current may be distributed to the surge voltage suppressor  954 . In other words, this surge voltage suppressor  954  may be said to be particularly effective for the electromagnet  301  in this embodiment, for which the opening operation energy is small. 
     Furthermore, this control method adopts the following ingenious approach. If an earth fault accident occurs just after the commutation type DC circuit breaker  5  is turned on, opening operation must be immediately started (trip-free operation). Unlike an AC system in which a current zero point always exists, in a DC system, if the fault current exceeds the commutation current due to delay in opening operation, interruption might fail. 
     In closing operation, if an earth fault accident occurs upon contact of the contacts of the first main switch  51  and second main switch  52 , the current transformer  58  in the feeding line  3  detects the fault current and sends an opening/closing command to the commutation type DC circuit breaker  5  through the overcurrent tripping device  59  and the control unit  900 . At this moment, the control circuit is as shown in  FIG. 8  because closing operation is under way in the commutation type DC circuit breaker  5 . If the electromagnetic repulsion coil  170  and second coil  305   b  are excited in this condition, an excessive induced current would flow in the low-impedance circuit (first coil  305   a -contact  935 -contact  933 -switch  915 -capacitor  590 -contact  930 -contact  934 -first coil  305   a ), which might cause a delay in opening time and damage to the switch  915 . Hence, in the control method in this embodiment, at the same time when the electromagnetic repulsion coil  170  is excited, the switch  915  is forced to turn OFF to break the low-impedance circuit. Therefore, the switch  914  and switch  915  must provide quick response and particularly the switch  915  must demonstrate a quick interruption performance. In order to meet the above requirement, a thyristor is used for the switch  914  and an FET or IGBT semiconductor switch is used for the switch  915 . 
     Next, the advantage of an embodiment of the commutation type DC circuit breaker  5  as a circuit breaker according to the present invention will be described. 
     The conventional circuit breaker includes a vacuum valve, an operating mechanism provided in the vacuum valve opening/closing direction, and an electromagnetic repulsion driving mechanism provided midway in the operating mechanism where a permanent magnet is installed in the operating mechanism and the closed state is held by the attractive force of the permanent magnet. In quick interruption, it is necessary to give the movable part of the electromagnet  301  an electromagnetic repulsive force which exceeds the result of subtraction of the reactive forces of the contact pressure springs  151 ,  212  and tripping spring  555  from the attractive force of the permanent magnet  306 , namely the surplus force to hold the closed state of the vacuum valve. In this type of circuit breaker, since for quick interruption it is necessary to not only achieve a prescribed opening speed but also provide an electromagnetic repulsive force in excess of the attractive force of the permanent magnet to hold the closed state, a large electromagnetic repulsion driving mechanism and a larger power supply capacity are needed. Also, a mechanism to reduce rebound of the movable electrode shaft due to the electromagnetic repulsion reactive force must be separately provided to prevent reclosing. 
     In this embodiment, the electromagnet  301  is reversely excited simultaneously with electromagnetic repulsion operation to release the attractive force of the permanent magnet  306  and facilitate quick interruption. As a consequence, a reliable circuit breaker which reduces rebound of the movable electrode shaft and prevents reclosing is provided. Apart from the first coil  305   a  intended for normal closing/opening operation, a second coil  305   b  with a small inductance which assures quick response is provided and connected in series with the electromagnetic repulsion coil  170  for simultaneous excitation. 
     In the case of the electromagnet  301  in this embodiment, opening operation, which basically relies on the accumulated elastic forces of the contact pressure springs  151 ,  212  and tripping spring  555 , is performed simply by canceling the magnetic flux of the permanent magnet  306  to give an attractive force, which is advantageous in assuring quick response. 
     Since the first coil  305   a  and second coil  305   b  constitute a duplex structure, when one of them is excited, an induced voltage is generated in the other coil. Although an induced voltage generated in the first coil  301   a , which has a larger number of turns, may be a problem for quick interruption in which magnetic flux variation is large, it is solved by the surge voltage suppressor  954  provided in parallel with the coil. The surge voltage suppressor  954 , composed of a protective resistance  952  and a diode  950 , assures durability to withstand frequent operation. When the surge voltage suppressor  954  is composed of a protective resistance  952  and a diode  950 , the diode  950  is arranged as follows. During opening operation which only requires a small operating energy, the exciting current is allowed to be distributed to the surge voltage suppressor  954 , and during closing operation which requires a larger operating energy, such current distribution is not allowed. This surge voltage suppressor  954  cannot be applied to an electromagnet which requires a large operating energy for both closing operation and opening operation. It is useful for a case that the accumulated elastic energies of the contact pressure springs  151 ,  212  and tripping spring  555  are used for opening operation as in this embodiment. 
     In order to achieve quick interruption (trip-free duty) after closing operation, at the same time when the switch  914  to excite the electromagnetic repulsion coil  170  is turned ON, the switch  915  to excite the first coil  302   a  is turned OFF. A thyristor is used for the switch  914  and an FET or IGBT semiconductor switch is used for the switch  915  to assure quick switch response and particularly assure quick interruption performance of the switch  915 . 
     Second Embodiment 
       FIGS. 11 to 13  show an embodiment of a three-phase high speed circuit breaker  600  as a circuit breaker according to the present invention, in which  FIG. 11  is a right side sectional view of the three-phase high speed circuit breaker  600  as a circuit breaker according to the present invention,  FIG. 12  is a back view thereof, and  FIG. 13  is a front view thereof, all indicating the closed state. In these figures, the parts designated by the same reference numerals as in  FIGS. 1 to 4  are the same parts. 
     In these figures, the three-phase high speed circuit breaker  600  includes a vacuum valve  601  incorporating a freely releasable contact. A fixed conductor  602  of a fixed electrode of the vacuum valve  601  is connected with a fixed feeder  603  located on the upper side. On the other hand, a movable conductor  604  of its movable electrode is electrically conductive to a movable feeder  606  through a power collector  605 . 
     The movable conductor  604  is coupled with one end of an insulating rod  607 . The other end of the insulating rod  607  is fixed on an operating rod  608 . The operating rod  608  penetrates a pin  609  with flattened abutment surfaces at the top and bottom. The pins  609  for three phases are all engaged with one lever  503  of a single main shaft  500 . A washer  611 , a contact pressure spring  612  and a washer  613  are held between the pin  609  and a nut  610  fixed on the operating rod  608 . With the vacuum valve  601  open, the hexagonal part  620  at the bottom of the operating rod  608  is engaged with the pin  609  by a contact pressure spring  612 . On the other hand, in closing operation of the vacuum valve  601 , the pin  609  and hexagonal part  620  are disengaged at the moment the fixed contact  621  and movable contact  622  of the vacuum valve  601  contact each other, and the load of the contact pressure spring  612  becomes a contact force of the contacts. 
     The operating rod  608  is communicated with an electromagnetic repulsion coil  170  and a repulsion plate  171  which constitute an opening means. As in the foregoing embodiment, an electromagnetic repulsive force generated in the repulsion plate  171  by excitation of the electromagnetic repulsion coil  170  is received by an insulating rod  607  and due to the repulsive force, an operating rod  608  moves downward in the figure. 
     The operating rod  608  is driven by an electromagnet  301  located beside the vacuum valve  601  in an operating device case  300 . The shaft  302  of the electromagnet  301  is coupled with the other lever  501  of the main shaft  500  through a member  303 . In other words, the attractive force of the electromagnet  301  is transmitted to the operating rod  608  through the main shaft  500 . The vacuum valve  601  is turned on by exciting a first coil  305   a  in the electromagnet  301  and moving a plunger  304  downward in the figure. 
     The structure of the electromagnet  301  is the same as that of the foregoing embodiment. The first coil  305   a  and second coil  305   b  are provided on a bobbin  901  and fixed cores  903 ,  904 ,  905  are provided on the upper, outer circumferential and lower surfaces of the first coil  305   a  and second coil  305   b  and a permanent magnet  306  rests on the fixed core  903  on the upper surface. The movable core of the electromagnet  301  is composed of a movable circular plate  906  and a plunger  304  and held between the shaft  302  and a nut  907 . When the electromagnet  301  is turned on, the plunger  304  and a center leg  908  are in contact with each other. 
     Next, operation of an embodiment of the above three-phase high speed circuit breaker  600  as a circuit breaker according to the present invention will be described. 
     In closing operation, the first coil  305   a  of the electromagnet  301  is excited by a precharged capacitor  590  as shown in  FIG. 13  to let the plunger generate an attractive force. This attractive force is transmitted to the operating rod  608  through the main shaft  500 , so that the movable conductor  604  moves upward and the vacuum valve  601  closes. Simultaneously with closing operation, the contact pressure spring  612  and the tripping spring  555  are elastically charged to prepare for opening operation. Upon completion of closing operation, the electromagnet  301  is de-excited. The reactive forces of the elastically charged contact pressure spring  612  and tripping spring  555  are held by the attractive force of the permanent magnet  306  in the electromagnet  301 . 
     In normal opening operation of the vacuum valve  601 , the first coil  305   a  is excited in a direction reverse to the excitation direction in closing operation. By reversely exciting the first coil  305   a , the magnetic flux generated by the permanent magnet  306  is cancelled and when the attractive force of the electromagnet  301  becomes smaller than the spring reactive force, the vacuum valve  601  start opening operation. 
     The second coil  305   b  of the electromagnet  301  is connected in series with an electromagnetic repulsion coil  170 . In quick interruption in case of an accident, the electromagnetic repulsion coil is excited by a control unit  900  ( FIG. 13 ) comprised of a capacitor  902  and a control board  903 . By the electromagnetic repulsive force generated in the repulsion plate  171 , the operating rod  608  moves downward further flexing the contract pressure spring  612  until the vacuum switch  601  becomes open. At this time, the main shaft  500  does not move yet. 
     In quick interruption, the electromagnetic repulsion coil  170  and the second coil  305   b  of the electromagnet  301  are simultaneously excited. When the direction of excitation of the second coil  305   b  is set to be the same as the direction of excitation of the first coil  305   a  in normal opening operation (reverse excitation), the attractive force of the permanent magnet  306  is decreased. When the sum of loads of the contact pressure spring  612  and tripping spring  555  exceeds the attractive force of the permanent magnet  306 , the plunger  304  begins to move upward. In response, the whole operating mechanism of the high speed circuit breaker  600  enters the open state. The control method of the high speed circuit breaker  600  is the same as in the foregoing embodiment and as illustrated in  FIGS. 8 to 10 . 
     In quick interruption by the vacuum valve  601 , it is necessary to give the movable part of the electromagnet  301  an electromagnetic repulsive force which exceeds the result of subtraction of the reactive forces of the contact pressure spring  612  and tripping spring  555  from the attractive force of the permanent magnet  306 , namely the surplus force to hold the closed state of the vacuum valve  601 . If the electromagnetic repulsive force is directly given to the electromagnet movable part, the reactive force incurs the risk of contact reclosing, which means that a mechanism to reduce rebound must be separately provided as in the prior art. In this embodiment, the electromagnet  301  is reversely excited simultaneously with electromagnetic repulsion operation to release the attractive force of the permanent magnet  306 , which suppresses rebound of the movable electrode shaft in the course of current interruption by the electromagnetic repulsion driving mechanism, making it possible to provide a reliable circuit breaker.