Automatic switches to protect transformers

AN AUTOMATIC grounding switch in combination with an automatic air-break switch can be used in place of a circuit breaker in the 110-kv supply to a transformer. Since internal faults in transformers do not occur frequently, these high-voltage circuit breakers can be put to better use at terminals of transmission lines where faults are caused by lightning. The substitute for a circuit breaker on the high-voltage side of a transformer must provide for interrupting load, magnetizing, and fault currents. The low-voltage circuit breakers are necessary in all except very small single-bank stations for all three interrupting duties. An air-break switch on the high-voltage side of a transformer should be able to interrupt the magnetizing current or to break parallel in a station having two or three banks of transformers. The remaining duty is to interrupt current flowing from the 110-kv network into a faulted transformer. This can be done indirectly by putting a fault on the 110-kv bus which will operate circuit breakers at distant ends of the 110-kv transmission line. This is a drastic method of avoiding interruption of 110-kv fault current at the station flowing to transformer with an internal fault; but as this kind of fault occurs very seldom, such a general interruption can be endured occasionally. When two or more transmission lines are terminated at such stations, high-voltage circuit breakers are required at terminals of lines only, because the automatic air-break switch can be used to disconnect a faulted transformer after all sources of fault current have been removed.