GB2058496A - High speed switching system - Google Patents

Abstract

A high speed switching system for disconnecting high electrical currents in a current-carrying circuit between a power source 12 and a load circuit 14 includes overload sensing means (84) which triggers a solid-state switch (76) to trip a high- speed switching device 78, 80. An overcurrent responsive circuit interrupter means e.g. fuses 70-74, connected across the contacts of the switching device thereupon interrupts the current-carrying circuit. <IMAGE>

Description

SPECIFICATION High speed switching system This invention relates generally to overload or fault protection circuits, and more particularly, it relates to a new and improved high speed switching system for disconnecting high electrical currents from a load in response to an overload condition.

In U.S. Patent No. 3,215,896 issued to D. P.

Shattuck et al on November 2, 1965, there is disclosed a fast response overload protection circuit which includes a silicon controlled rectifier which is gated on in response to an error signal thereby causing the actuation of a series fuse associated with the load.

In U. S. Patent No. 3,493,815 issued to R. L.

Hurtle on February 3, 1 970, there is disclosed an electrical protection system which includes a fuse that is actuated by a silicon controlled rectifier responsive to fault current.

In U. S. Patent No. 3,525,019 to J. Lansch issued on August 1 8, 1 970, there is disclosed a power protection circuit which includes a relay responsive to an error voltage sensing means for turning off triacs to effectively open the load lines.

In U. S. Patent No. 3,868,552 issued on February 25, 1975, to A. Wickson, there is described an electrical circuit interrupter wherein an error signal gates on a silicon controlled rectifier which causes increased current to flow through a transformer secondary and across fuses thereby effectively shortening the period of time required for opening a circuit in response to electrical disturbances.

In U. S. Patent No. 3,957,329 to L. D.

McConnell issued on May 1 8, 1976, there is described a fault-current limiter which includes a fuse connected in parallel with the normally open contact of a vacuum switch.

In U. S. Patent No. 3,987,340 to F. W. Kussy issued on October 1 9, 1 976, there is shown a motor controller consisting of overload relay heaters, electromagnetic contactors, circuit breakers, and fusible elements paralleled by resistors.

Industry is increasingly interested in the use of inverters to vary the speed of induction motors.

However, a major problem has been encountered in utilizing such inverters when some of the solidstate switching devices in the inverterfail to commutate or switch off. This condition causes a short circuit to be applied to the motor terminals, thereby producing a short circuit current flow and a severe negative torque or braking effect. On applications of machines such as centrifugal turbocompressors in which the coupling between the motor and the drive train is maintained by a positive torque on a threaded shaft, the braking effect may cause damage due to the unscrewing of the motor from the drive train. Thus, it is desirable to provide a switching system for interrupting currents very quickly so as to disconnect the motor from the inverter before sufficient negative torque is developed to cause unscrewing.

Accordingly, it is a general object of the present invention to provide a new and improved high speed switching system for disconnecting high electrical currents from a load in response to an overload condition.

It is another object of the present invention to provide a high speed switching system which includes a vacuum interrupter switch having a plurality of contacts for carrying the full load current and a plurality of fuses connected in parallel with the contacts for carrying only the fault current.

It is another object of the present invention to provide a high speed switching system which includes a plurality of low current fuses responsive to overload conditions for interrupting extremely fast a load from a polyphase line.

It is still another object of the present invention to provide a high speed switching system which includes a fault detector, a solid-state semiconductor, a repulsion coil, a vacuum interrupter switch and rupturable fuses all operatively connected so as to disconnect very fast a load from a power source.

It is yet still another object of the present invention to provide a high speed switching system which is relatively simple in construction and easy to manufacture.

In accordance with these aims and objectives, there is provided in accordance with the instant invention a high speed switching system for disconnecting high electrical currents in a current carrying circuit between a power source and a load circuit which includes sensing means, solidstate switching means, a high-speed switching device, pick-up means, and an overcurrent responsive circuit interrupter means. The sensing means is operatively connected to a power source for detecting overload conditions to provide a gating signal. The solid-state switching means has a control terminal which is coupled to the sensing means for triggering on the same in response to the gating signal. Normally closed contacts of the high-speed switching device are connected in series with the current-carrying circuit.The pickup means is connected in series with the solidstate switching means for activating the switching device. The interrupter means is coupled in parallel with the switching device for interrupting the flow of current in the current-carrying circuit so as to disconnect the load from the power source when an overload condition is detected by the sensing means. Upon the occurrence of a fault-current from the sensing means which exceeds a predetermined value, the switching means activates the pick-up means to cause the normally closed contacts of the switching device to open thereby allowing the interrupter means to interrupt the current-carrying circuit.

These and other objects and advantages of the present inventions will become more fully apparent from the detailed description when read in conjunction with the accompanying drawing in which there is shown a functional diagram of a high speed switching system constructed in accordance with the present invention.

Referring now with particularity to the drawing, there is shown a functional diagram of a high speed switching system 10 constructed in accordance with the principles of the instant invention and connected between a three-phase power source such as inverter 12 and a load circuit consisting of a three-phase motor 14. The output terminals 18, 20 and 22 of the inverter 12 are joined to the respective polyphase lines 24, 26 and 28. The input terminals 30, 32 and 34 of the motor 14 are tied to respective polyphase lines 36, 38 and 40.

The high speed switching system 10 comprises an overcurrent responsive circuit interrupter means 42 and a switching network 44. The interrupter means 42 includes a plurality of respective fuse terminals 46, 48; 50, 52; and 54, 56 which are disposed in a spaced apart relationship. Conductors 58, 60 and 62 are connected between the respective terminals 46, 50, 54 and the respective polyphase lines 28, 26, 24. Conductors 64, 66 and 68 are connected between the respective terminals 48, 52, 56 and the respective polyphase lines 40, 38, 36.

Rupturable conductors or fuses 70, 72 and 74 are affixed to and coupled between the terminals 46, 48; 50, 52; and 54, 56, respectively.

The switching network 44 includes solid-state semi-conductor switching means 76 such as a silicon-controlled rectifier, pick-up means 78, a high-speed switching device 80 and power supply means 82. There is provided a sensing means such as fault detector 84 operatively connected to the inverter 12 by conductors 86 and 88 for measuring of the current drawn by the motor 14 or excessive current internal to the inverter 12.

The silicon-controlled rectifier 76 is arranged for triggering by the output of the fault detector 84 coupled to the gate and cathode terminals 90, 92 of the silicon-controlled rectifier via conductors 94, 96.

The power supply means 82 consist of an alternating current voltage source 98 connected to the primary winding 100 of transformer 102.

The alternating current voltage on the secondary winding 104 of the transformer 102 is half-wave rectified by diode 106 and resistor 108 for charging capacitor 110 to a desired d.c. level. This d.c. voltage on the capacitor 110 is coupled to one side of the pick-up means 78 formed as a repulsion coil. A diode 112 is provided across the capacitor 110 to prevent reverse polarization thereof. The other side of the repulsion coil 78 is connected to the anode terminal of the silicon controlled rectifier 76. The coil is utilized to create a magnetic field to control the operation of the switching device 80, which in practice is a conventional vacuum interrupter switch having a plurality of contacts a, b, and c. Each of the contacts a, b, and c is normally closed with each one being connected in series between the current-carrying lines.It should be noted that the respective contacts a, b, and c are disposed in parallel relationship with the respective fuses 70, 72, 74.

The fault detector 84 is preferably a noncontact d.c. and a.c. current sensor of the type manufactured and sold by F. W. Bell Inc. under their designation of IF--5000M. Series. For a detailed discussion of the theory, operation and characteristics of the repulsion coil 78, reference is made to the article entitled "Analysis of a Fast Acting Circuit Breaker Mechanism" by S. Basu andes K. D. Srivastava presented at the IEEE Summer Meeting and International Symposium on High Power Testing, Portland, Oregon, July 18-23, 1971. The vacuum interrupter switch can be of the type manufactured by ITT Jennings and designated with their part no. RP 1 57-1 which has a typical response time of period 0.3 ms.The fuses 70, 72 and 74 may be of any conventional commercially available type, the specific current rating on the fuses being selected to be of suitable value for the desired application.

In operation, upon the occurrence of an overload condition fault current above a predetermined value will be sensed by the fault detector 84 to produce an output gating signal to be applied to the gate or control terminal 90 of the silicon-controlled rectifier 76. S.uch a gate signal will trigger or turn on the silicon-controlled rectifier, thereby causing it to conduct. In this conducting state, current will flow through the anode of the silicon-controlled rectifier which energizes the repulsion coil 78. The actuation of the coil will in turn create a magnetic field to force the normally closed contacts a, b, and c of the vacuum interrupter switch 80 to open which diverts the main load current from the inverter 12 to flow through the fuses 70, 72, and 74.The load current will continue to flow to the motor 14 for a relatively short period of time required for the fuses to blow and open the circuit, thereby removing the motor load 14 from across the power source 12.

In one particular application of the invention, the peak fault current at the terminals 30, 32 and 34 of the motor 14 has been measured to be 4000 Amps or 2828 Amps rms. In the usual operation, the full motor load current is typically 450 Amps. It has also been determined by tests that the clearing time (a total of melting time and arc extinguishing time) of the switching system used must be less than 2 milliseconds so as to prevent unscrewing or uncoupling of the motor shaft from the drive tain. A 500 Amp fuse would have a melting time of just under one second for 60 hz operation. Thus, any attempt of normal fusing with a 500 Amp fuse would not suitably protect against the unscrewing because opening of the circuit is too slow allowing too much current to the motor load.

However, a much shorter period of time for disconnecting the power source 12 from the load 14 is obtained in the present invention due to the fact that the full load current of 450 Amps is being carried through the contacts a, b, and c ofthe vacuum interrupter switch 80 under normal conditions and does not flow through the fuses 70, 72, and 74. In other words, the fuses are shunted by the contacts under normal operation and are utilized only to carry the high overload currents. Therefore, much smaller fuses or relatively low current fuses can be selected for use in the circuit to render a much faster clearing time.

If a 20 Amp fuse having a 200,000 Amp interrupting capacity is used, the clearing time in the event of a fault current of 4,000 Amps would be approximately .89 milliseconds after the contacts of the vacuum interrupter switch 80 open.

From the foregoing detailed discussion, it can be thus seen that the present invention provides a new and improved high speed switching system which includes low current fuses for disconnecting the motor load from the inverter in a relatively short amount of time. Further, the high speed switching system includes a silicon-controlled rectifier, a repulsion coil and a vacuum interrupter switch for diverting the fault current to the fuses.

While there has been illustrated and described what is at present to be a preferred embodiment of the present invention, it will be understood by those skilled in the art that various changes and modifications may be made and equivalents may be substituted for elements thereof without departing from the true scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the central scope thereof. Therefore, it is intended that this invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention but that the invention will include all embodiments falling within the scope of the appended claims.

Claims (7)

1. A high speed switching system for disconnecting high electrical currents in a currentcarrying circuit between a power source and a load circuit comprising: sensing means operatively connected to a power source for detecting overload conditions to provide a gating signal; solid-state switching means having a control terminal coupled to said sensing means for triggering on said switching means in response to said gating signal; a high-speed switching device having normally closed contacts being connected in series with the current-carrying circuit; pick-up means connected in series with said solid-state switching means for activating said switching device; and overcurrent responsive circuit interrupter means coupled in parallel with said switching device for interrupting the flow of current in said current-carrying circuit so as to disconnect the load circuit from the power source when an overload condition is detected by said sensing means, said switching means activating said pickup means when fault current from said sensing means exceeds a pre-determined value to cause the normally closed contacts of said switching device to open thereby allowing said interrupter means to interrupt said current-carrying circuit.

2. A high speed switching system as claimed in Claim 1, wherein said sensing means comprises a fault detector.

3. A high speed switching system as claimed in Claim 1, wherein said solid-state switching means comprises a silicon-controlled rectifier.

4. A high speed switching system as claimed in Claim 1, wherein said switching device comprises a vacuum interrupter switch.

5. A high speed switching system as claimed in Claim 1, wherein said pick-up means comprises a repulsion coil.

6. A high speed switching system as claimed in Claim 1, wherein said interrupter means comprises a plurality of low current fuses fixedly supported at spaced apart terminals.

7. A high speed switching system constructed and arranged to operate substantially as hereinbefore described with reference to and as illustrated in the accompanying drawing.