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Participant

Join Date: Jun 2007
Posts: 3

transformer protection

07/22/2007 1:55 AM

For a transformer of delta-star connection (delta side 6.6KV, Star side 415V) with star side earthed through resistor, what is the method of providing protection for phase to earth fault on the star side?

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Guru

Join Date: Jun 2007
Location: chennai,India
Posts: 592
Good Answers: 19
#1

Re: transformer protection

07/22/2007 3:53 AM

Please consider Differential Protection for the transformer.

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Ramesh,Freelance Electrical/automation Consultant
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Active Contributor

Join Date: Jul 2007
Location: INDIA
Posts: 14
#2

Re: transformer protection

07/23/2007 4:44 AM

hi,

For delta side we should connect CTs in resudal mode & in star CTs should in restricated E/F mode ( One CT in Neutral).

REF will work only for Trafo. internal fault only

Hope it will help

tadagids

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Commentator

Join Date: Feb 2007
Posts: 60
#3

Re: transformer protection

07/23/2007 8:52 AM

Two different froms of protection shall be considered:

1)as already mentioned, to read the residual current in the neutral of the transformer

2)to connec a differential protection in a connection named earth differntial protection. In this kind of connection, the relay will be connected in the neutral connectionto earth of the phase current transformers and the other side to the current transformer connected to the wye of the power transformer.

If you only use the regular differential protection, this will not be able to detect a phase to ground fault.

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Anonymous Poster
#4

Re: transformer protection

07/24/2007 12:04 AM

Your neutral is grounded through resistor to limit ground to earth fault current. To protect your down stream network, you should use GFCI " Ground fault current protector". This is used in two different ways. (1) If your distribution is 3ph, 4 wire system, all 3 phases and neutral are passed through one CT. CT functions as resultant current detector and which is zero at all the time except ground fault. Normally, settings are in the range of 4ma to limit touch voltages to safe level. (2) The another method is using one CT on grounding conductor. Whenever there is a ground fault on down stream distribution, ground current will return path through grounding resistor and back to transformer. In this case, CT will detect fault and trip breaker.

I think this is enough for your question.

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Power-User

Join Date: Oct 2006
Location: Kolkata, India
Posts: 447
#5
In reply to #4

Re: transformer protection

07/25/2007 8:46 AM

Overall Differential protection normally used for >5MVA trafo. Use 51N on upstream line side, 51NT at neutral as standby to 51N and co-ordinate. 64R (REF) for trafo internal fault. This is a bit conservative design and used in many power stations in India.

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BNDas
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Anonymous Poster (1); biswanath.das (1); elbf2801 (1); ramvinod (1); tadagids (1)

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