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Protection Riddle No.12 - Inability of Earth fault protection system
As you know, for earth fault protection of electrical system, the residual current measurement is often performed using the three line CTs.
But in this case the protective relay must be slightly time-delayed to avoid spurious trips caused by circulation of a false zero sequence current following a brief period of saturation in the current transformers (magnetizing current, switching surge or down stream faulty). This limitation leads to too high a setting compared to the maximum available earth fault current, and consequently to a loss of sensitivity.
This difficulty can be largely overcome by using a ring current transformer in neutral point of power transformer in order to achieve zero-sequence current measurement. In this case the earth fault relay can simultaneously provide both rapid and sensitive earth fault protection.

This technical recommendation is applied to a new island electrical system with power auto transformer by a design group. They used the earth fault relay and related NCT in neutral point of auto transformer. But they had not suitable response from earth fault protection against the occurrence of ground faults.

How can you explain the reason of protection system inability in this case?
Author : Hamid - From: IRAN
Sat, April 12, 2008 - 18:57
In the case of auto- transformer such arrangement will not work 
Author : V.K. Nambiathiri - From: indian
Sun, April 13, 2008 - 22:34
Author : Hamid - From: IRAN
Mon, April 14, 2008 - 09:47
In auto-transformer there is single winding only. Neutral is common. Hence any E/F current in load side shall not  return to neutral of aoto- transformer, but flow to neutral of transformer feeding the auto - transformer. Hence NCT placed in neutral of auto- transformer does not sense this fault current  
Author : V.K. Nambiathiri - From: Indian
Mon, April 14, 2008 - 09:55
Yes, please explain clearly.whether the earth fault current doesn't return to neutral point of auto transformer? Are you sure? 
Author : Hamid - From: IRAN
Mon, April 14, 2008 - 13:13
Author : V.K.Nambiathiri - From: India
Mon, April 14, 2008 - 19:23
Even a part of it? 
Author : Hamid - From: IRAN
Thu, April 24, 2008 - 11:23
Because of electrical and magnetically coupling between primary and secondary windings of auto transformers, the primary and secondary earth fault currents are mixed in neutral point of transformer. The difference phase angle between secondary current and induced primary current is approximately 180 degree, therefore the amount of through earth fault current is the difference of  Ip and Is , where the Ip and Is are the primary and secondary earth fault current respectively. So the earth fault protection system in neutral point can not detect actual primary or secondary earth fault currents.
Author : Hamid - From: IRAN
Mon, April 28, 2008 - 09:12
I think the reply of my friend Mr. Nambiathiri is correct  . the reply from M r. Hamid isn't accurate. In case we don't have any load in connection with secondary winding the zero sequence CT will work . In case the primary winding to be energized and secondary winding to be unloaded , then in case of earth fault this scheme will work.  
Author : khashayar - From: IRAN
Sun, May 18, 2008 - 09:42
Mr.Hamid, you are correct 
Author : V.K.Nambiathiri - From: India
Wed, May 21, 2008 - 07:31
In case  secondary winding to be unloaded , and auto transformer to be energized from primary  side actually it is like one wye winding . Now in case of earth fault  , the CT installed in neutral of wye winding will detect the earth fault current and will work . I told the reply from Mr. Hamid isn't completely accurate but it is correct .   
Fri, March 12th, 2010 - 08:21
Dear All,

Mr.Hamid's post on 24th April 2008 is correct.But it is not the whole truth.I would like add as  as follows:

1. The neutral  current, as already pointed out, will be  the difference between the primary and secondary currents.

2.  From the diagram  given in the riddle, it is noted that there is an unloaded tertiary winding. This will tend to make the  zero sequence currents flowing in the primary and secondary for a Ph-G fault more than than the case without the tertiary winding.
3.  Further, the tertiary winding will make the distribution of zero sequence currents between the primary and secondary different.It will be no more inversely proportional to the primary and secondary voltages. The primary contribution will be  less, depending upon the tertiary leakage reactance.
4.Since the current in the neutral will be 3 times the the difference between the primary and secondary zero sequence currents,the presence of tertiary winding will make the actual fault current in the neutral  more than that  expected without it. By properly designing the leakge reactance of the tertiary the neutral current can be made higher and the relay can be made sufficiently sensitive without increasing the setting.

Your comments please.
Author : K B Santharam - From: India
Sat, March 13th, 2010 - 18:04
Congratulation dear K B Santharam, your approaches are deep and conceptually, I enjoy it very much. 
Author : Hamid - From: Iran
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