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Associate

Join Date: Oct 2013
Posts: 41

CT Secondary Earthing

03/04/2017 3:04 PM

Dear all

In 3 phase current transformer star connection what is the difference between

connected S1 or S2 to earth , and are in both cases the primary and secondary

current in phase or there 180 degree between them

thanks for all

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Guru

Join Date: Apr 2010
Posts: 4837
Good Answers: 458
#1

Re: CT secondary earthing

03/04/2017 3:31 PM

I'm assuming "Relay" is a circuit to detect faults.

If all three CTs are reversed, nothing changes except you are 1/2 cycle ahead or behind in time.

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Power-User

Join Date: Jul 2016
Posts: 217
Good Answers: 7
#3
In reply to #1

Re: CT secondary earthing

03/06/2017 8:44 AM

If no other elements are connected, then it makes no difference. However, in some cases to detect power direction flow, voltage elements will connected to the same relay. Then the relative polarity of the CTs is important, because the relay designer assumes a certain connection convention. You can reverse the documented voltage coil connections to make it work as designed.

Also, CTs used to detect faults in conjunction with other CTs all need to be connected in a like manner, for the protection to work as documented, or you need to adjust the relay connection to suit.

Lastly, you are missing one wire from your relay back to the neutral of the CTs. Relays are most typically single phase sensing.

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Guru
Engineering Fields - Power Engineering - New Member

Join Date: May 2007
Location: NYC metropolitan area.
Posts: 2870
Good Answers: 398
#2

Re: CT Secondary Earthing

03/05/2017 10:52 AM

Trick question. In order to answer it correctly you have to know the polarity marks on both the primary and secondary of each CT.

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Guru

Join Date: Dec 2010
Posts: 1108
Good Answers: 76
#4

Re: CT Secondary Earthing

03/06/2017 3:40 PM

What is meaning of drawing symbols below generator symbol??

If that earth symbol means common point of star generator winding is earthed, then I would draw as on left of your sketch, so secondary mirrors primary. If you have CTs involved, best to draw generator windings as "inductors" within circle with Y, Δ interconnection or whatever.

As already suggested, you should mark polarity on primary bars and secondary - that is at left of your sketch P1 is at top of each primary conductor with S1 at top of secondary, P2 and S2 at bottom.

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