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Protection Riddle No.77 - Restricted Earth Fault Trip due to inrush current
I need some help. In our 50MVA, 132/33 kV star/star transformer, at the time of charging from HV side, the transformer trips on REF (restricted earth fault) due to magnetizing inrush current.

Rated current: 218.7 Amp
Line CT ratio: 800/400/200/1 Amp
tap selected 400/1 Amp
setting on MJT 36 Numerical relay.
Earth fault = 0.2 In
TMF = 0.15 In
Inrush current in three phases is:
Ia = 0.42
Ib = 0.05
Ic = 0.51

EF current in neutral = 0.58 In
Relay operating time = 0.94 s

Since, inrush dyeing time > 2.5 s, the relay trips.

At TMS 0.3, the relay operating time = 1.89s
At TMS 0.4, the relay operating time = 2.52s

Customer advised to change TMS to 0.4 (2.52s), he does not agree since,

i) It will affect his normal earth fault protection time of 0.94s which is not desirable.
ii) Other similar transformer in the same substation does not give the problem.

The transformer is made with step lap core construction using Hi-B grade material using 1.6 tesla flux density.

Can anyone help me analyze and suggest what to do?
Author : Sreelatha - From: India
Thu, October 14th, 2010 - 13:23
It seems you face to Current Transformers saturation due to DC offset earth fault current and its bad effects. For more information you can refer to Protection Riddle No.38 and Mr. K B Santharam excellent reply.
Advising with numerical relay manufacturer and using the higher tap of current transformer or increasing of relay time setting can be some solutions for mentioned mal-operation.
Author : Hamid - From: Iran- Firouzabad in Fars
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