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Protection riddle no. 10 – Differential protection and auto transfer system
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?
Author : Hamid - From: Iran
Sun, September 14, 2008 - 11:36
Transformer vector group is to be looked in to. If it is delta/star and the fault is in M.V. side winding l.v. side C.b. may not operate 
Author : Nambiathiri - From: INDIA
Sun, September 14, 2008 - 13:22
Author : Hamid
Tue, September 16, 2008 - 07:28
As M.V. side is in delta connection, E/f in this winding produces no fault current 
Author : V.K.Nambiathiri - From: INDIA
Wed, September 17, 2008 - 06:41
There shall be no fault current if E/F occures in M.V. side delta winding 
Author : V.K. Nambiathiri - From: INDIA
Thu, September 18, 2008 - 14:17
When the higher voltage circuit breaker is tripped after a heavy internal fault, by protection which cannot conveniently provide an auxiliary contact with a dwell time long enough to ensure correct tripping of the lower voltage circuit breaker.
Typically the relay is initiated by transformer protection, such as overall differential where both the HV and LV circuit breakers are to be tripped simultaneously.
The HV circuit breaker may be expected to trip more quickly than the LV breaker. So the tripping signal is maintained by the delayed resetting, to ensure complete clearance by the fault may have been cleared largely by the HV circuit breaker and the protection has started to reset.
Therefore a good solution is applying of trip relay with suitable drop off time in LV voltage side for remaining of protection system trip command on LV circuit breaker for enough time.
Author : Hamid - From: IRAN
Tue, November 25, 2008 - 14:20
I agree with the author that any fault between the zone protected by differential CTs ie Primary and Secondary of transformer will  give trip  command to both MV and LV breakers siomultaneously sothat transformer is isolated. The differential relay 87T will operate for the faults within the zone and not any fault occuring out side the zone.
If there is a fault after the protected zone of 87T occuring say at LV bus and if the protective relay  gives  trip command only to MV breaker then LV breaker will not operate and bus fault continues even after ATs closes.This may be the reason of the case described here. 
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