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Protection riddle No.2 – Stability of transformer differential protection
Imagine a hypothetical substation with a standard differential protection scheme that has been used to protect one of transmission power transformer with DYn connection.
The selected current ratio of CTs in each side of power transformer is proportional to the rated current of the same side; Also CTs secondary interconnections are in opposite of power transformer interconnections. (E.g. star in primary and delta in secondary side of power transformer). The differential protective relay is static base and low impedance type.
According to the technical recommendations, all secondary protection circuits (containing: CTs, star point of relay current elements,…) shall be grounded in one point.

But here there is a problem. Studies and observations has shown that protection system sometimes is unstable in the sever earth faults which are out of zone.
A young and clever electrical engineer who works in the consultant technical office has studied this problem and believes that the way of secondary circuits grounding (grounding of star point of relay current elements) is the reason of that mentioned instability. What do you think?

How can you explain the reason of that mentioned differential protection instability?
Author : Hamid - From: Iran
Tue, June 9th, 2009 - 16:38
yes he is right because in case of earth fault the fault current may insert from ground which causes malfunction ...
why they ground the secondary winding to the ground point  i think we need to analyse it using a suitable software .

Author : iqbal - From: Pakistan
Thu, March 11th, 2010 - 09:25
Dear All,

I know this is a riddle posted about a year back. Sorry , I came across this only now.

All CT neutrals ( both main CT and auxiliary CTs shoulb earthed at only one point, preferably at the protection panel. Further there is no need to earth neutral of the operating winding of the differential relay in this case since zero sequence currents are filtered out from both sides of the differential relay.

If the above precaution is not taken, during system earth faults, different CT neutral points would be at different potentials causing circulating currents in the CT secondaries and cause maloperation of the relay. Your comments please! 
Author : K B Santharam - From: India
Thu, March 11th, 2010 - 16:39
Yes I agree you, according to IEEE C37.91-1985 PAGE 38, the modern differential relays are of even low burden than the usual auxiliary current transformers. Thus, the common point of the relay connections should not be connected to the common point of the Y-connected transformers  ( a connection which is necessary without the zero-sequence filter). Only the filter neutral should be connected to the current transformer common point.

Unfortunately some of reference documents ,such as ABB special publications, have not considered this technical point. Please note to diagarm below:

Author : Hamid - From: Iran
Wed, March 2nd, 2016 - 08:07
I don't believe that the grounding is the issue. It is true that severe out of zone faults will cause the differential relay to operate. The reason is due to the saturation of cts that causing such trips and should be covered in setting slope of differential relays. 
Author : Russell - From: Filipin
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