Question

Explain working of AC circuit breaker using SCR with circuit diagram.

Answer 1

AC Circuit Breaker: 

The circuit configuration of static AC circuit breaker using SCR is shown in the figure. When switch ‘S’ is closed, the SCRs T1 and T2 are fired in positive and negative half-cycles respectively. During positive half-cycle, T1 receives gate current through D2 || R2, switch S and R3 and it conducts. At the end of positive half-cycle, T1 is turned off due to natural current zero. In the negative half-cycle, T2 receives gate current through D1 || R1, R3andswitch S and it conducts. It is turned off at the end of this negative half cycle due to natural current zero value. When the load current is required to be interrupted, the switch S is opened. It results in blocking of gate currents of both SCRs and hence both SCRs are maintained off. When switch S is opened at any instant in a particular half-cycle, the load current continue to flow through conducting SCR till the end of this half-cycle, however in the next half-cycle the other SCR will not be fired due to non-availability of gate current. Thus the maximum time delay for breaking the circuit is one half-cycle.