Differential protection and auto transfer system

[m.s.j]

In a low voltage substation, including two incomings and a coupling circuit breakers, incoming C.Bs are connected to two individual power transformers which are fed from upstream medium voltage system. The two low voltage bus bars are equipped by a fast auto transfer system which is considered for coupling C.B auto closing when one of two bus bars is dead.
Also each transformer is protected by standard differential relay that must trip medium and low voltage C.Bs via fast interposing trip relays.
The problem observed when a short circuit occurred in one of two transformer windings. The differential relay operated and only relevant medium voltage circuit breaker tripped. The fault clearing stage was done successfully and related low voltage bus bar was dead. In this case the auto transfer system closed the low voltage coupling circuit breaker, and the short circuit point energized again.
Indeed, the relevant low voltage C.B of faulty transformer couldn't open on time. Last investigation didn't show any problem in low voltage C.B mechanism or related circuits.
How can you explain the reason of low voltage C.B inactivity?


BEST REGARDS
MSJ


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[anorlunda]

Perhaps miscalibrated because of abnormal high side impedance.

The solution should be an interlock preventing auto transfer.