GB2182812A

GB2182812A - Current supply apparatus

Info

Publication number: GB2182812A
Application number: GB08525378A
Authority: GB
Inventors: Professor Barry Williams; Richard Fletcher
Original assignee: MK Electric Ltd
Current assignee: Novar ED&S Ltd
Priority date: 1985-10-15
Filing date: 1985-10-15
Publication date: 1987-05-20
Also published as: GB8525378D0; GB2182812B

Abstract

A circuit for supplying electric current contains a semiconductor switch 5 for use as a rapidly acting circuit-breaker arranged to respond to a main detector 7. The circuit may supply current to a number of subcircuits 3 having their own switches 4 and fault detectors 8 so that in the event of a fault the semiconductor switch 5 may disconnect all the subcircuits. The faulty subcircuit may then be disconnected under zero current conditions and power restored to the healthy subcircuits immediately. The semiconductor switch 5 may be controlled by a microprocessor. <IMAGE>

Description

SPECIFICATION Current supply apparatus This invention relates to apparatus for supplying electric current to single or multiphase circuits provided with means for protecting the circuit and/or devices or items of current using equipment connected to the circuit from damage which may be caused by a shortcircuit or other fault.

Power supply circuits for household, commercial and industrial use are normally protected by fuses or circuit-breakers intended to break the circuit if an excessive current, which may be caused by a short-circuit for example, passes through the conductors of the circuit.

A fuse is responsive to the heating effect of an excessive current and breaks the circuit by melting. An ordinary circuit-breaker responds to either the heating effect or the magnetic field generated by excessive current and breaks the circuit mechanically. However, such protection devices take a significant period of time to operate when an excessive current occurs, thereby allowing a considerable amount of power to be dissipated at the fault, and in the supply and protection apparatus.

The present invention is intended to provide means by which an electric current supply circuit may be broken very rapidly in the event of a circuit fault or intentionally by other external control means. In any circuit, excessive current caused by a fault takes a finite time to build up to the prospective excess current and the means for breaking the circuit provided by the invention preferably operates sufficiently fast to break the circuit before the excessive current caused by the fault can reach a high value, thereby reducing the amount of power dissipated at the fault, and in the supply and protection apparatus.

The invention is also intended to provide automatic circuit protection means applicable to a main supply circuit feeding electric current to subcircuits in which a fault in one of the sub-circuits causes, successively, the main current supply circuit to be broken, the faulty sub-circuit to be broken, and the main current supply to be re-established so that power is restored to the remaining healthy sub-circuits.

According to one aspect of the invention, there is provided apparatus for supplying electric current to a circuit for supplying current to devices or items of current using equipment, the apparatus comprising a semiconductor switch through which the current passes, at least one detector capable of detecting an electrical fault in the circuit or in the devices or appliances connected thereto, and means responsive to the detector or detectors for emitting a voltage to turn off the semiconductor switch to disconnect or interrupt supply of current to the circuit.

Semiconductor switches in general are capable of much faster action than fuses or circuit-breakers using mechanical contacts to make and break the circuit. The semiconductor switch, for use with alternating current, may comprise a pair of thyristors arranged antiparallel in the circuit, a pair of bipolar or field effect transistors, a pair of field controlled thyristors, a diode bridge provided with a single semiconductor switch, or a pair of antiparallel gate turn-off thyristors. A pair of gate turn-off thyristors is preferred as the current through the switch may be turned off, in response to an applied reverse gate voltage, at any point on the alternating current cycle whereas in an ordinary thyristor, when the gate voltage is withdrawn, the current continues until the next zero-current point in the cycle.A gate turn-off thyristor is therefore capable of reducing the current to zero in a very short time, which may be 3 micro-seconds or less, on application of a reverse gate voltage.

The fault detector may be arranged to generate a signal when a fault is detected and the signal may be received by a control unit which emits a voltage to turn the semiconductor switch on or off according to the signal received from the detector. The control unit may comprise an analogue or digital logic control unit but in a preferred embodiment the control unit is supported by a secondary control unit which may then comprise a computer such as a programmed microprocessor. This control unit may generate the appropriate voltages in response to the signals or combinations of signals received from the detector or detectors.When the semiconductor switch comprises gate turn-off thyristors the control unit may be programmed to emit a gate turn- off voltage, in order to switch off the semiconductor switch immediately, in response to one signal or signal combination received from the detector or detectors, and simply to withdraw the safe turn-on voltage in response to another signal or signal combination to allow the semiconductor switch to turn-off at a point on the alternating current cycle when the current is zero. With this arrangement the current may be turned off immediately when required but in appropriate circumstances it may be turned off at the zero point of the current cycle in order to minimise stresses on the component parts of the circuit.The invention may be applied to an installation in which a main supply circuit, provided with the main semiconductor switch, feeds one or more subcircuits which in turn supply current to devices or items of current using equipment and each sub-circuit is provided with is own switch to disconnect that sub-circuit from the main supply circuit. With this arrangement, switching off the main semiconductor switch reduces the current in all the sub-circuits to zero and the switches of the sub-circuits may be operated to open the sub-circuits under zero current conditions avoiding stresses in the sub circuits and their switching devices. The subcircuit switches may be actuable by voltages emitted by the secondary control unit.The sub-circuits may be provided with their own detectors to emit a signal in the event of a fault in the respective sub-circuit and this signal may be fed to the secondary control unit, which can process the various signals received and generate voltages to actuate the appropriate switches.

When several detectors are arranged to feed signals to the secondary control unit, each signal representing a condition of the respective circuit with which each detector is associated, the secondary control unit may be arranged to generate various combinations of voltages to operate the switches in a manner appropriate to the condition of the circuits. In one manner of operation, when a fault is detected in one of the sub-circuits the main semiconductor switch is opened by the primary control unit to break the current supply to all the circuits, the switch of the faulty subcircuit is opened by the secondary control unit to isolate that sub-circuit and the main semiconductor switch is then closed to restore power to the remaining healthy sub-circuits.

Because of the fast operation of the primary control unit and semiconductor switch this sequence of operations may be carried out very rapidly so that the power supplies to the subcircuits which are free from faults are interrupted only momentarily and the period of interruption may be sufficiently short not to apparently affect the operation of devices and current using equipment powered by the subcircuits.

When the main circuit is broken at a nonzero point on the alternating current cycle the line inductance of the circuit may generate a high transient voltage across the switch, derived from the energy stored in the circuit, and this voltage may be sufficient to damage the switch. It is therefore preferred to provide, in parallel with the switch or with the inductive part of the circuit, an energy absorbing device to limit the transient voltage generated.

This device may be a voltage dependant resistor.

The main circuit detector is preferably capable of reacting to an increase in current above the normal maximum very rapidly and a suitable device for this purpose may be a Hall effect detector. The sub-circuits may also be provided with similar or identical detectors and when the different sub-circuits are intended to carry different maximum currents the detectors or their associated electronic circuitry may be adjusted, for example by means of potentiometers, to the appropriate maximum currents required. In addition the main circuit and the sub-circuits may be provided with residual current detectors, arranged to emit signals to the control units in response to earth leakage currents.Other types of detector which may be used for measuring currents in the circuits include magneto resistor devices using the Gauss effect applied to semiconductors, optical fibre devices in which the plain of polarisation of polarised light is rotated by a magnetic field, and temperature sensitive devices such as thermistors. Other types of detector which may be used include devices for measuring the power delivered or power factor, or deviations of the alternating current from its normal sinusoidal or non-sinusoidal form. The control units may be sensitive to signals representing these parameters and programmed to respond to them by appropriate switching action.

Apparatus according to embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 shows schematically a circuit arrangement for supplying alternating electric current, Figure 2 shows schematically a semiconductor switch used in the arrangement of Fig. 1.

Figure 3 shows a circuit including protection for the semiconductor switch of Fig. 1.

Fig. 1 shows schematically apparatus for supplying electric current to an installation, such as a household, commercial or industrial application, where a number of individual circuits feed current to items of current using equipment or other devices.

Alternating current is fed from a source, indicated at 1, through a main circuit 2 to individual sub-circuits 3 which may be intended to feed currents of different magnitudes to different items of equipment. In the drawing five individual sub-circuits 3 are shown, to deliver different maximum currents such as from 6 to 40A, but it will be understood that any number of circuits 3, intended to supply any magnitude of current, may be fed from main circuit 2. The maximum current to be fed through circuit 2 may, for example, be 200A at 240V and 50 Hz. Although Fig. 1 shows a schematic diagram for a single phase installation, the same principle can be applied to multiphase installations.

Each of circuits 3 is provided with a switch 4, which may be a conventional relay, to make and break its respective circuit 3.

Circuit 2 is also provided with a semiconductor switch 5 to make or break the main feed circuit. In the embodiment shown switch 5 is controlled by the primary control unit 13 or the secondary control unit 6 which receive data from main detector 7 associated with circuit 2 and sub-circuit detectors 8 associated with the individual sub-circuits 3. The secondary control unit is also arranged to emit signals for opening and closing the switches 4.

Detector 7 may be located as shown in Fig. 1 or at any other relevant position in the mains supply circuit.

The purpose of detectors 7 and 8 is to monitor the current flow through circuits 2 and 3 and provide the control units with information representing these current flows. The control units are arranged to analyse this information and give appropriate instructions to switches 4 and 5 to make the break the appropriate circuits 2 and 3.

Fig. 2 shows switch 5 diagrammatically: it comprises a pair of antiparallel gate turn-off thyristors 9. Thyristors 9 are transferred from the blocking state to the conducting state on application of a voltage between the gate and the cathode; also application of a reverse gate voltage transfers the thyristor from the conducting state to the blocking state. Two thyristors are required, as shown, to control the alternating current through the switch. The turn-on and turn-off voltages are supplied by either the primary control unit 13, or the secondary control unit 6 as appropriate to the circumstance. In this embodiment each of the gate turn-off thyristors has a maximum rating of 600A controllable anode current and a 1300V forward and 650V reverse blocking capability.The thyristors have a typical storage time of 10,us and a typical current fall time of 3s at an anode current of 600A: thus application to the thyristors of the appropriate voltage from the control units can turn switch 5 on and off extremely rapidly, in a time of the order of a few microseconds. This type of semiconductor switch has low onstate power loss of less than 1 watt per amp.

With reference to Fig. 1, the function of detector 7 is to detect current in circuit 2 and feed the appropriate signal to control units 6 and 13. Detector 7 comprises a Hall effect electromagnetic current measurement module capable of measuring currents have a frequency from 0 (direct current) to 100kHz with a response time less than 1 s. This module is capable of measuring currents in circuit 2 of up to 200A with an accuracy of 1% and is galvanically isolated from circuit 2.

The primary control unit 13 is arranged such that, on receipt of a signal from detector 7 indicating a current in circuit 2 in excess of a predetermined value, such as 100A, semiconductor 5 is rendered non-conducting so that the current supply to circuit 2 is interrupted.

Because of the very fast response time of elements 5, 13 and 7 the current is interrupted very quickly, a few microseconds after detection of a current in excess of the predetermined value, and in the event of a shortcircuit fault the current is interrupted long before it reaches its maximum fault value, which could be very large. The response time of elements 5, 13 and 7 may be such that in the event of a short-circuit fault the maximum current before interruption may be only about 20A above the predetermined value. Thus, even with prospective short-circuit fault currents of several thousands of amps, the probability of damage caused by the fault current before interruption takes place is very small.

Detectors 8 associated with the sub-circuits 3 may be devices sensitive to currents in respective circuits 3, and/or to residual currents which may be caused by earth leakage affecting these circuits. Devices 8 are arranged to supply a signal to the secondary control unit.

In the event of excessive currents or earth leakage the secondary control unit is arranged to emit a signal to open the appropriate switch 4 after the main switch 5 has been opened. The primary control unit is arranged such that, on detection of excessive current in circuit 2 by detector 7, and excessive current or earth leakage in a sub-circuit 3 by a detector, 8 the current through circuit 2 is first reduced to zero by operation of switch 5 and the appropriate switch 4 is then opened by the secondary control unit to isolate the faulty sub-circuit from main circuit 2. Switches 4 are thus opened under zero-current conditions, eliminating arcing and other stresses upon them and their associated sub-circuits.The secondary control unit is further arranged so that, after operation of the appropriate sub circuit switch 4 to isolate a fault, the circuit switch 5 may be turned on to restore power to the remaining healthy sub-circuits 3. When a fault occurs in a sub-circuit the current feed to the remaining sub-circuits can then be restored a very short time after interruption caused by the fault. This time may be so short that the temporary interruption of power has little significant effect on the operation of items of current using equipment powered by the subcircuits 3 which are fault-free.

When current is flowing through circuit 2 and sub-circuits 3 energy is stored in the line inductance and other elements of these circuits and this energy is released, in the form of a very high voltage generated across switch 5, when the switch is opened. This energy may be sufficient to cause breakdown of the semiconductor switch or other parts. In order to avoid breakdown the switch may be protected by means of an energy absorbing device in the manner shown schematically in Fig.

3, which shows circuit 2 connected to one of the sub-circuits 3. The energy absorbing device 10 comprises a metal oxide varistor or equivalent device, connected across the source 1 as shown, which has the effect of absorbing the energy released on operation of switch 5 and clamping the voltage generated at a safe level.

Similarly, operation of switch 5 will release the energy stored by the inductance of the load, indicated schematically by 11, powered by sub-circuit 3 and to prevent generation of an excessively high voltage across the load another varistor 12 is connected across the load. A similar device is provided on the other sub-circuits 3 powered by circuit 2.

Varistors 10 and 12 may be replaced by semiconductor switches which remain open when switch 5 is closed but are closed, to clamp the voltages across source 1 and load 11 to zero, when switch 5 is opened. In an alternative arrangement the varistor or equivalent device 10 may be connected in parallel with switch 5 instead of source 1. Device 10 protects the switch from voltage surges in the current supply, in addition to absorbing energy from the line inductance.

The mode of operation of secondary control unit 6 may be controlled by means of display and key board 14 which comprises operating keys for emitting command signals to the secondary control unit and also indicators, which may be neon lamps or other electro luminescent devices, to indicate the state of circuits 2 and 3 controlled from the secondary control unit. The indicators may receive information from the secondary control unit to indicate the on/off condition of individual switches 4 and 5, and also to indicate the presence of a fault in any of the circuits perceived by the secondary control unit. Board 14 may comprise keys for instructing the secondary control unit to turn switches 4 and 5 off or on, allowing circuits 2 and 3 to be controlled manually under normal fault-free conditions.

In the embodiment described above the main detector 7 and primary control unit are primarily intended to detect an increase or rate of change of current in circuit 2 above a predetermined value or speed and to turn switch 5 off rapidly if such a condition, generally symptomatic of a short-circuit fault, occurs. However, detectors 7 and 8 may further comprise means sensitive to other magnitudes indicating the state of current flow in the circuits 2 and 3. For example, they may comprise thermistors to indicate an abnormal temperature rise in a circuit and the control units may be arranged to switch off the appropriate circuit if such a temperature rise occurs.Other parameters which may be measured by suitable means associated with circuits 2 and 3 and connected to the control units may be the voltage at a given point in the circuit, the power delivered, the power factor, the frequency of the current, energy reversal, and distortions of the waveform of the current.

Signals which are functions of these parameters may be fed to the control units and the control units arranged to instruct appropriate action. For example, either control unit may turn switch 5 off immediately, by applying a reverse gate signal to the switch 5 and either leave it off, or open one or more of switches 4 to isolate a particular sub-circuit and then turn switch 5 on again. Alternatively, either control unit may turn switch 5 off by reducing the on-state voltage but without applying reverse gate current, so that the switch is turned off when the alternating current passing through the switch falls to zero. This mode of switching off the current has the advantage that generation of transient currents and stresses on circuit elements are avoided or at least reduced. Switch 5 may be turned on again be restoration of the gate turn-on voltage to the switch.

The secondary control unit may be arranged to introduce a time delay between reception of signals from detectors 7 and/or 8 and switching off at switch 5 under appropriate circumstances. The secondary control unit may then be arranged to cause a warning indication, for example, a visual warning on board 14, to appear before switching off at switch 5 to give warning of impending disconnection or interruption of current through circuit 2.

In the embodiment described above semiconductor switch 5 comprises a gate turn-off thyristor switch. Other types of semiconductor switch, such as switches using a pair of ordinary thyristors, a triac, bipolar transistors, field effect transistors, field controlled transistors or a diode bridge using only one semiconductor switching device could be used. However, a gate turn-off thyristor switch is preferred as it is robust and is capable of very rapid turn-off.

Suitable gate turn-off thyristor switches are available commercially.

Claims

1. Apparatus for supplying electric current to a single or multiphase circuit for supplying current to devices or items of current using equipment, the apparatus comprising a semiconductor switch through which the current passes, at least one detector capable of detecting an electrical fault in the circuit or in the devices or items of current using equipment connected thereto, and means responsive to the detector or detectors for emitting a voltage to turn off or on and/or control the semiconductor switch to disconnect, connect or control the supply of current to the circuit.

2. Apparatus according to claim 1, in which the semiconductor switch is capable of being turned off immediately in response to said signal at any point on the cycle of alternating current passing through the circuit.

3. Apparatus according to claim 2, in which the semiconductor switch comprises a pair of antiparallel gate turn-off thyristors.

4. Apparatus according to claims 1, 2 or 3 in which the means responsive to the detector or detectors comprises a control unit or units arranged to receive signals from the detector or detectors and apply voltage to the semiconductor switch in response to the signals.

5. Apparatus according to claim 4, in which the control unit incorporates a computer.

6. Apparatus according to claim 4 or 5, in which the circuit comprises a main supply circuit in which the semiconductor switch is located and at least one sub-circuit fed with current by the main supply circuit, each subcircuit comprising a switch for disconnecting that sub-circuit.

7. Apparatus according to claim 6, in which the main supply circuit is provided with a main detector and each sub-circuit is provided with a respective sub-circuit detector, the detectors being arranged to detect electrical faults in their respective circuits and the control unit being responsive to both the main detector and each sub-circuit detector.

8. Apparatus according to claim 6, in which each sub-circuit is provided with a detector arranged to detect electrical faults in it's respective circuit and the control unit being responsive to each sub-circuit detector.

9. Apparatus according to claim 7, in which a control unit or units are capable of applying voltage to both the main semiconductor switch and the sub-circuit switches to turn the switches on and off in response to signals received from the detectors.

10. Apparatus according to claim 8, in which a control unit or units are capable of applying voltages to both the main semiconductor switch and the sub-circuit switches to turn the switches on and off in response to signals received from the detectors.

11. Apparatus according to claim 9, in which the control unit is arranged, on detection of a fault in a sub-circuit, to successively turn off the main semiconductor switch, turn off the switch of the sub-circuit in which the fault is detected, and turn the main semiconductor on to restore current to the remaining sub-circuits.

12. Apparatus according to claim 10, in which the control unit is arranged on detection of a fault in a sub-circuit, to successively turn off the main semiconductor switch, turn off the switch of the sub-circuit in which the fault is detected, and turn the main semiconductor on to restore current to the remaining subcircuits.

13. Apparatus according to any one of claims 6 to 12, in which the switches of the sub-circuits are semiconductor switches.

14. Apparatus according to any one of claims 7 to 12, in which a means is provided to control the main semiconductor and/or the sub-circuit switches manually by a remote keyboard or control devices.

15. Apparatus according to any one of claims 7 to 12, in which a means is provided to control the main semiconductor and/or the sub-circuit switches by external control signals.

16. Apparatus according to any one of claims 7 to 12, in which a means is provided for control or disconnection of any sub-circuit at any point on wave of the sinusoidal supply.

17. Apparatus according to any preceding claim, in which at least one circuit detector comprises a Hall effect detector adapted to emit a signal indicating a circuit fault when the current in that circuit exceeds a predetermined magnitude.

18. Apparatus according to any preceding claim, comprising at lease one device capable of limiting the voltage generated in the circuit across the semiconductor switch when the switch is turned off at a point on the alternating current cycle when the current is non-zero.

19. Apparatus according to claim 18, in which the device capable of limiting the voltage comprises a varistor.