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Protection Riddle No.1 – The saving fuses
In the operation condition suddenly one sever earth fault occurred between one of the three phases and transformer case in secondary side. Because of fault location, the secondary electrical protection couldn't detect any fault current. (Differential or REF protection were not used)Therefore it seemed that there was no electrical protection system to prevent damaging of transformer ; because the primary zero phase sequence current due to secondary short circuit was trapped in delta connection and couldn’t appear in primary feeding line of transformer. However the incoming fuses saved transformer in last-ditch successfully.

How can you explain the fuses function against fault condition?

Author : Hamid - From: Iran
Thu, September 13, 2007 - 11:01
Probably the phase to earth fault evaluated to a cross country fault and in this case the primary fuses identified the fault in two fass.

Can you please tell if two or three fusses operated? probably only two operated.
Author : guest
Thu, September 13, 2007 - 11:03
You may need to sketch out the vector grouping of the transformer to have a clearer picture of the fault current path.

The single phase fault to frame or earth at LV side will be seen as phase to phase current on the primary windings. The magnitude of the fault will determine the magnitude of current on the primary side. In this case the fault currents were large enough to blow the fuses.

Author : guest
Thu, September 13, 2007 - 11:04
Thank you very much sir,
Yes you are right; however the zero sequence of primary fault current is absence in that circuit, but existing negative and positive sequence current can be caused fuses blown in some conditions.

Author : Hamid - From: IRAN
Tue, June 9th, 2009 - 16:01
Dear all

I believe that thefault level  for MVA short circuit will decide the fuse setting as well as but few factors need to be consider

Z % of transformer
available short circuit at the primary side
Author : iqbal - From: Pakistan
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