Abstract:
A switching circuit protection technique includes detecting the self-firing of at least one switching element due to an over-voltage in a switching circuit having multiple parallel circuits, applying a firing pulse to all switching elements and the switching circuit in response to the detected self-firing and firing all of the switching elements at the same time in response to a firing pulse.

Description:
This application is a continuation of application Ser. No. 07/899,296, filed Jun. 16, 1992, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to an apparatus for protecting switching circuits having parallel multiple switching elements with the characteristic of self-firing due to an over-voltage and a method of protecting the switching circuits. 
     2. Description of the Prior Art 
     Over-voltages are generated in the field windings of generators and motors according to asynchronous inputs, short circuits, and the like. Over-voltage protection circuits, in which switching elements, for example thyristors, are applied to the protection of these devices from over-voltages. FIGS. 3 and 4 show prior art over-voltage protection circuits having thyristors as switching elements. 
     In FIG. 3,  41  is a generator,  42  is a field winding of generator  41 ,  43  is a thyristor rectifier which supplies direct current (DC) power to the field winding  42  and  44  is an over-voltage protection circuit in which thyristors  45  are used for protecting from over-voltages generated in the field winding  42 . 
     The thyristors  45  in over-voltage protection circuit  44  are caused to fire by over-voltages generated in the field coil  42  and form a short circuit, thus protecting the generator  41  and the thyristor rectifier  43 . In order to improve the reliability of this over-voltage protection circuit, thyristors  45  are used which have the characteristic of self-firing without a gate signal when a certain level of voltage is impressed (hereafter, these thyristors  45  are referred to as VBO free thyristors). 
     FIG. 4 is a detailed diagram of a prior art over-voltage protection circuit which uses VBO free thyristors. Although a suitable number of VBO free thyristors may be connected in series or parallel according to the voltage and current used in the over-voltage protection circuit, FIG. 4 shows the case of  3  parallels. Elements  10 ,  20  and  30  are reactors which equalize the current shares between the parallel circuits. Elements  11 ,  12 ,  21 ,  22 ,  31  and  32  are VBO free thyristors. Element  55  is a gate circuit which supplies gate pulses to VBO free thyristors  11 ,  12 ,  21  , 22  , 31  and  32  when an ON command is supplied to the over-voltage protection circuit. 
     In the circuit in FIG. 4, VBO free thyristors  11 ,  12 ,  21 ,  22 ,  31  and  32  can be switched ON by detecting faults such as circuit over-voltage or generator short-circuits. However, even when there is no ON command, VBO free thyristors  11 ,  12 ,  21 ,  22 ,  31  and  32  can fire by themselves and prevent over-voltage. 
     In the prior art circuit in FIG. 4, when VBO free thyristors  11 ,  12 ,  21 ,  22 ,  31  and  32  self-fire due to over-voltage, they do not fire simultaneously due to randomness of firing voltage levels between individual thyristors and temperature variations. If it is assumed that the firing level of VBO free thyristor  11  is the lowest, VBO free thyristor  11  will fire first. The voltage impressed on VBO free thyristor  12  will increase due to the firing of VBO free thyristor  11 . Therefore, VBO free thyristor  12  will also fire almost simultaneously. In this way, the series circuit which includes VBO free thyristors  11  and  12  will fire. 
     On the other hand, when the self-firing levels of the series circuits which include VBO free thyristors  21 ,  22 ,  31  and  32  are high, over-voltage is inhibited due to the firing of the series circuit which includes VBO free thyristors  11  and  12 . Therefore, the series circuits which include VBO free thyristors  21 ,  22 ,  31  and  32  will not fire. For this reason, the circuit current concentrates in one of the three parallel circuits, so that three times the current flows through VBO free thyristors  11  and  12 , and VBO free thyristors  11  and  12  will break down. 
     As described above, due to randomness between the self-firing voltages of individual VBO free thyristors and temperature variations, the flow of current becomes concentrated in one of the parallel circuits. In order to prevent this, devices with completely identical characteristics were selected from a suitable number of VBO free thyristors and these were connected in parallel. In addition, temperature control for the unit was required in order to prevent the temperatures from becoming uneven. Thus, there was the problem of the unit becoming bulky and also very costly. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide an apparatus for protecting switching circuits and an effective method of protecting the switching circuits which prevents current concentration in one circuit of the switching elements which are connected in parallel as described above. 
     In order to achieve the above object, an apparatus of this invention is comprised of a switching circuit composed by connecting in parallel multiple circuits in which at least one switching element having the characteristic of self-firing according to an over-voltage are connected in series; detect means for detecting the self-firing of at least one switching element in the switching circuit; fire means for applying an firing pulse to all the switching elements in response to an output signal from the detect means. And a method of this invention entails detecting the self-firing of at least one switching element in the switching circuit and applying a firing pulse to all the switching elements in response to the detected self-firing. 
     In the invention constructed as above, a firing pulse is supplied to the other&#39;switching elements which are connected with a switching element which has fired according to over-voltage. Therefore, current concentration in one of the parallel connected multiple circuits can be prevented. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram showing a first embodiment of this invention. 
     FIG. 2 is a schematic diagram showing a second embodiment of this invention. 
     FIG. 3 is a schematic diagram showing a system used for an over-voltage protection circuit. 
     FIG. 4 is a schematic diagram showing a prior art example. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The following is a description of an embodiment of this invention with reference to the drawings. 
     FIG. 1 is a schematic diagram of a first embodiment of this invention, and the same symbols are given to devices which operate with the same functions as in FIGS. 3 and 4. And in this embodiment, VBO free thyristors are used as the switching elements. 
     In FIG. 1,  50  is a current sensor which detects the current flowing in the VBO free thyristor circuit;  51  is a level sensor which detects whether or not the output signal of current sensor  50  has exceeded a pre-set level;  52  is a NOT circuit which inserts signals;  53  is an AND circuit which takes a logical product and  54  is an OR circuit which takes a logical sum. 
     In the circuit in FIG. 1, if, for instance, VBO free thyristor  11  is assumed to arc due to over-voltage, the shared voltage of VBO free thyristor  12  will increase due to the firing of VBO free thyristor  11 . Thus, VBO free thyristor  12  will also fire almost simultaneously due to over-voltage. In this way, one of the three parallel circuits which includes VBO free thyristors  11  and  12  will fire. For this reason, a current will flow through VBO free thyristors  11  and  12 , and this current will be detected by current sensor  50 . If this current exceeds a pre-set level, the level sensor will detect this and generate an output. In order to detect that the firing is due to an over-voltage, the logical product with the inserted signal of the ON command is taken, by AND circuit  53 . By this means, the fact that some VBO free thyristors have fired and a current is flowing in a state where there is no ON command, that is to say that they have fired due to an over-voltage, can be detected. 
     The logic sum of the output of AND circuit  53  with the ON command is taken by OR circuit  54 , and supplied to gate circuit  55 . When a signal is inputted, gate circuit  55  impresses firing pulses on all VBO free thyristors  11 ,  12 ,  21 ,  22 ,  31  and  32  simultaneously. 
     In the above way, when VBO free thyristor  11  has fired due to over-voltage, the fact that a current is flowing is detected and firing pulses are also applied to the other VBO free thyristors  21 ,  22 ,  31  and  32 . Therefore, since VBO free thyristors  21 ,  22 ,  31  and  32  also arc, the VBO free thyristors of all three parallels fire. Thus, current concentration in one circuit can be prevented. 
     FIG. 2 is a schematic diagram showing a second embodiment of this invention. In FIG. 2,  56  are voltage sensors which detect the voltage between the anodes and cathodes of VBO free thyristors  11  and  12 ,  21  and  22 ,  31  and  32 . Also in FIG. 2, instead of detecting the firing of a VBO free thyristor using current as in FIG. 1, the over-voltage firing of a VBO free thyristor is detected by the fact that the voltage between the anode and the cathode of the VBO free thyristor caused by that voltage has become zero. Level sensors  51  in FIG. 2 detect that the voltages between the anodes and the cathodes have become almost zero. When a voltage has become almost zero in the state where there is no ON command, gate circuit  55  supplies firing pulses to all VBO free thyristors  11 ,  12 ,  21 ,  22 ,  31  and  32 . In FIG. 2, voltage sensors  56  are connected one to each multiple series devices. However, even if voltage sensors  56  are connected to every individual VBO free thyristor, the effect of this invention will not be altered. 
     Also, in FIG. 1, current sensor  50  detects the entire current. However, even if the current is detected by connecting current sensors  50  to every individual parallel circuit, the effect of this invention will not be altered. 
     Although the invention has been described in its preferred form with a certain degree of particularity, it is understood that the present disclosure of the preferred form may be changed in the details of construction and other combinations and arrangements of parts may be resorted to without departing from the spirit and the scope of the invention as hereinafter claimed. 
     When this invention is used as described above, current concentration in one of the parallel switching circuits can be prevented and the switching elements can be prevented from breaking down.