The process of interrupting the current in an ac system is aided by the fact that ac current goes through zero every half-cycle, or approximately every 8 ms in a 60 Hz system. The absence of a natural current zero in dc makes it difficult to develop a dc circuit breaker. There are three principal problems that must be addressed:
• Forcing current zero in the interrupting element
• Controlling the over voltages caused by large changes in current as a function of time (di/dt) in a highly inductive circuit
• Dissipating large amounts of energy (tens of mega joules is not uncommon)
The second and third problems are solved by the application of zinc oxide varistors connected line to ground and across the breaking element. The first is the major problem, and several different solutions have been adopted by different manufacturers. Basically, current zero is achieved by inserting a counter voltage into the circuit.
In the circuit shown in Figure, opening CB (an air-blast circuit breaker) causes current to be commutated to the parallel LC tank. The commutating circuit will be oscillatory, which creates current zero in the circuit breaker. The opening of CB increases the voltage across the commutating circuit, which will be limited by the zinc oxide varistor ZnO1 by entering into conduction. The resistance R is the closing resistor in series with switch S.
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| Figure Block diagram of a dc circuit breaker (one module). |