Switching circuit for over-voltage protection

Abstract: A SWITCHING CIRCUIT FOR OVER-VOLTAGE PROTECTION is intended to protect a load, such as a generator winding, from sudden, unpredictable power surges. Such circuits commonly consist of a self firing, voltage responsive switch, e.g., a thyristor, deployed in parallel with the load (4,2). Several thyristors may be connected in parallel to improve current carrying capacity. In such a circuit one thyristor may fire in advance of the others leading to a current overload through it. To avoid this the present invention provides means responsive to any one of the thyristors (11, 12, 21, 22, 31, 32) firing, to apply a firing pulse to fire all the other thyristors.

Description: This invention relates to a switching circuit for over-voltage protection of a load having a plurality of parallel switching elements with the characteristic of self-firing in response to an over-voltage and a method of protecting the switching elements. 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 load provided by the field winding of the generator 41, 43 is a thyristors 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 protection from over-voltage 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 parallel (sub-circuits). 10, 20 and 30, are reactances which equalise the currents shared between the parallel circuits. 11, 12, 21, 22, 31 and 32 are VBO fee thyristors. 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-firing 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 became 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 becoming unequal. Thus, there is a problem with the unit becoming bulky and very costly. 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 parallel sub-circuit of the switching elements. In order to achieve the above object, there is provided a switching circuit for over-voltage protection comprising a plurality of parallel sub-circuits for connection in parallel with a load, each sub-circuit including a self-firing switching element responsive to an over-voltage to change the sub-circuit from an open to a closed, conductive, state, characterised in that the circuit includes means responsive to any one switching element firing to apply a firing pulse to all the switching elements. In a method according to the present invention the self-firing of any of the self-firing switching elements in any one of a plurality of parallel sub-circuits, each connected in parallel with a load, is detected, and used to stimulate the firing of all of the switching elements to switch all of the sub-circuits from an open circuit, non-conductive, state to a conductive, closed circuit state. In the invention constructed as above, a firing pulse is supplied to the other 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 sub-circuits can be prevented. 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. 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 element. 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 reverses 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 21 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 sub-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 reverse 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 parallells fire. Thus, current concentration in one sub-circuit can be prevented. Fig. 2 is a schematic diagram showing a second embodiment of this invention. In Figure 2, 56 are voltage sensors which detect the voltages 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 each to 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. 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.

Claims: A switching circuit comprising a plurality of parallel sub-circuits each including at least one switching element (11, 12, 21, 22, 31, 32), each switching element having a self-firing characteristic when subjected to an over-voltage characterised in that the circuit includes a detecting means (50, 51, 52, 53, 54, 56) for detecting the self-firing of any of the switching elements (11, 12, 21, 22, 31, 32) in the switching circuit: and a firing means (55) for applying a firing pulse to all the switching elements (11, 12, 21, 22, 31, 32) in response to an output signal from the detecting means (50, 51, 52, 53, 54, 56). A switching circuit as claimed in claim 1, characterised in that each of the sub-circuits consists of a plurality of switching elements (11, 12, 21, 22, 31, 32) connected in series. A switching circuit as claimed in claim 1 or claim 2, characterised in that each switching element (11, 12, 21, 22, 31, 32) is a thyristor. A switching circuit as claimed in any preceding claim, wherein the detecting means is a current meter (50). A switching circuit as claimed in any of claims 1 to 3, wherein the detecting means is a voltage meter (56). A method of protecting a switching circuit comprising multiple parallel sub-circuits each having at least one switching element (11, 12, 21, 22, 31, 32) with the characteristic of self-firing when subjected to an over-voltage characterised by the steps of detecting the self-firing of at least one switching element (11, 12, 21, 22, 31, 32) in response to an over-voltage applying a firing pulse to all the switching elements (11, 12, 21, 22, 31, 32) in the switching circuit in response to the detected self-firing; in order to fire all the other switching elements (11, 12, 21, 22, 31, 32) in response to the firing pulse.

Inventors: KANAI TAKEO C O INTELLECTUAL P, KANAI, TAKEO, C/O INTELLECTUAL PROPERTY DIV.
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1992
Main classification: H02H7
IPC codes: H02M1, H02H7, H02H9, G05F1