GB2221794A - Circuit breaker - Google Patents

Abstract

A circuit breaker arrangement comprising a first mechanical switch 1 in a circuit to be protected, a second mechanical switch 2 and a semiconductor switching unit 3 (e.g. thyristor) in parallel with each other and in series with the first switch, and means for operating the switches and switching unit in a predetermined sequence, such that breaking or making said circuit is effected by the switching unit. <IMAGE>

Description

Circuit Breaker This invention relates to circuit breaker arrangements, and an object of the invention is to provide a particularly effective form of circuit breaker as will be apparent from the following description.

According to the invention a circuit breaker arrangement comprises a first mechanical switch in a circuit to be protected, a second mechanical switch and a semiconductor switching unit in parallel with each other and in series with the first switch, and means for operating the switches and switching unit in a predetermined sequence, such that breaking or making said circuit is effected by the switching unit.

As the mechanical switches do not make or break a circuit carrying a substantial current, but are only arranged to carry such current when already closed or to support a recovery voltage, contact damage due to arcing is effectively avoided and the lives of the switches accordingly prolonged.

The semiconductor switching unit incorporates one or more thyristors and is preferably unidirectional, switching of the unit preferably being arranged to take place at an instant of zero current, with the whole switching operation being effected within a half cycle of the a.c. supply.

Any convenient form of control means may be employed for generating control signals for effecting the operation of the switches and switching unit in the required sequence.

The sequence of operations from the circuit breaker "closed" to the circuit breaker "open" condition will now be described by way of example with reference to the accompanying drawing, in which there is shown a first mechanical switch 1 in series with a parallel combination of a second mechanical switch 2 and a thyristor switching unit 3.

In the initial condition the switches 1 and 2 are closed and the unit 3 is off.

1. The thyristor of the unit 3 is fired so as to cause the unit to conduct.

2. The switch 2 is opened.

3. The thyristor drive is turned off.

4. The switch 1 is opened when the thyristor has cleared.

5. The switch 2 is then opened.

The sequence of operations from the circuit breaker "open" to the circuit breaker "closed" condition starts with the switch 1 open, the switch 2 closed and the thyristor unit 3 off.

1. The switch 2 is first opened.

2. The switch 1 is closed.

3. The thyristor of the unit 3 is fired.

4. The switch 2 is closed.

A control circuit (not shown) of any convenient form is arranged to apply a control signal to the thyristor unit 3 to fire the latter, and further control signals to effect the opening/closure of the switches 1,2 as appropriate in the required sequence.

As mentioned above, the mechanical switches 1, 2 do not need to make or break substantial currents, but are limited to switching relatively small leakage and capacitive currents, the thyristor unit being responsible for making and clearing the normal load and fault current. The switch 1 is used to act as the "isolation gap" which is normally required to be provided in circuit breaker arrangements.

The current rating of the thyristor switching unit 3 will be mainly dominated by its I2t capabilities, but it will be observed that it only serves to carry current for a very short time in both the closing and opening operations of the arrangement.

Further it will be seen that the voltage rating of the thyristor unit is limited to its power frequency overvoltage blocking capabilities and not the normal impulse withstand normally associated with power circuit breakers. During the thyristor switching operation the thyristor will be protected from impulse type overvoltages.

The circuit breaker arrangement in accordance with the invention thus: 1. Minimises the number of semiconductor switches required.

2. Makes possible the use of simple fast acting mechanical switches operating in air or a dielectric fluid medium.

3. With appropriate control gear it can be arranged to effect current interruption within a designated half cycle of the a.c. supply.

Claims (6)

1. A circuit breaker arrangement comprising a first mechanical switch in a circuit to be protected, a second mechanical switch and a semiconductor switching unit in parallel with each other and in series with the first switch, and means for operating the switches and switching unit in a predetermined sequence, such that breaking or making said circuit is effected by the switching unit.

2. A circuit breaker arrangement according to Claim 1, in which the semiconductor switching unit incorporates one or more thyristors.

3. A circuit breaker arrangement according to Claim 1 or 2, in which the semiconductor switching unit is unidirectional.

4. A circuit breaker arrangement according to Claim 1, 2, or 3, in which the operating means is arranged to switch the semiconductor unit at an instant of zero current.

5. A circuit breaker arrangement according to 1, 2, 3 or 4, in which the whole switching operation is effected within a half cycle of the a.c. supply.

6. A circuit breaker arrangement substantially as described herein with reference to the accompanying drawing.