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Reply To: Protection riddle no.5 – The protection system false operation

#1190
Hamid

    Is existence of CT secondary current possible without any its primary current? Yes it is possible in transient duty. That current named decay or subsidence current. The subsidence current is the current that flows through a CT burden after the line breaker opens. As you know, the circuit breaker failure over current relay (50BF) should be fast pickup and dropout relay. The subsidence current may affect the dropout time of breaker failure over current element. If the 50BF element is picked up the breaker failure time delay, other breakers must trip to isolate the failed breaker. CT subsidence current keep the 50BF element asserted longer than necessary and may contribute to a false breaker failure operation in tightly coordinated systems. Nowadays some digital base protective relays are able to detect CT subsidence current.