Abstract
The mechanism of arc extinction in circuit-breakers is discussed. Records of voltage, current, and arc resistance near current zero for an air-blast circuit-breaker are given to illustrate the process and to show how the current may be brought to zero prematurely.Sudden changes of arc resistance are shown to occur near current zero, when the r.m. s. current is low, as for unloaded transformers or shunt reactors, and the way in which these can give rise to current oscillations and sudden interruption of current (current chopping) is illustrated. It is shown that this process is affected by the circuit capacitance and by the circuit-breaker characteristics.The process of current chopping may give rise to high voltages across the shunt reactor or transformer, and the way in which such voltages can be limited by restriking of the circuit-breaker gap is illustrated for both air-blast and oil-break circuit-breakers. Ways of estimating the limiting effect of these gaps from the electric strength of circuit-breaker contact-gaps are given.Comparison of three-phase site conditions and test conditions is made, and limitations of the latter are pointed out.The relation between switching voltages and r.m. s. magnetizing current is studied, and it is concluded that the switching of unloaded transformers gives lower voltages than the switching of shunt reactors. Suitable ways in which the voltage across a shunt reactor may be reduced are described.
Published Version
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