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Protection Riddle No.63 - Over Design Restricted Earth Fault Protection
In an executed REF protection, the neutral CT No.1 after many years of system operation is fronted to burning due to unknown accident (see below figure). Operator group decide to replace of failed CT. They chose four more expensive CTs with two times rated power for prevention of same accident; they think the current transformer with higher volt-ampere capacity is more stable against thermal stresses due to secondary over loading. But in practice they face to mysteries case, after an sever earth fault on power transformer not only new neutral current transformer but also related protective relay was burned due to that sever earth fault current.
How can you explain the reason of that protection system failure? They think, they can always have a more reliable and safe protection system with an over design equipments choosing!! What is your idea?


Author : Hamid - From: Iran
 
#1
Wed, January 20th, 2010 - 14:20
when "VA" of a protective current transformer connected with a protection relay is properly sized , it is guaranteed that the current injected to the relay does not exceed e. g. 20 times of  the relay rated current . But , when this CT is replaced by a more expensive CT of higher burden , it is not guaranteed that the current injected to the relay is 20 times . it is higher than 20 times and will damage the relay .(In fact it goes to saturation region later and higher secondary current is generated)   
Author : amir khashayar - From: iran
 
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