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Participant

Join Date: Mar 2018
Posts: 2

Neutral Distribution Bypassing 3p 3w MCC

03/24/2018 4:25 AM

Hi everyone,
I've been recently involved in a brown field project. It is a 25 years old plant which was powered by generator at first "stage 0" and later on a electrical overhead line was built so they included a distribution transformer.
As you can See under Stage 0, we have two generators connected to a 3Phase 3Wire MCC. So for having neutral (3P+N) for auxiliaries we have a isolation transformers, one of them 750KVA.
Under Stage 1, one distribution transformer 1.6MVA, Dyn has been added, so the neutral is available but not distributed. Transformer is protected through 87T upstream. We still maintain the Aux transformer 400/400V for having neutral downstream.
Apart of the advantages of the isolation transformer, I was thinking in remove it (it is 20 year old now) and make a Neutral distrubition, joining all the neutral point of the sources (generators and transformer).
The protection in the MCC is a Digitrip RMS510 Cutler - Hammer, with LTI and Ground protection in the incoming of the MCC.
What do you think about neutral distribution for avoiding Isolation transformers?
We should protect the Neutral with additional CT and/or relay? or just with the existing protections is enough.
Regards
Luis

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Guru
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#1

Re: Neutral Distribution Bypassing 3p 3w MCC

03/24/2018 9:57 AM

"As you can See under Stage 0"

I can't see anything, the drawings are too small.

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Participant

Join Date: Mar 2018
Posts: 2
#2

Re: Neutral Distribution Bypassing 3p 3w MCC

03/24/2018 10:30 AM

Yes you are righ,

let's see now.

STAGE 0.

STAGE 1.

PROPOSAL

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Power-User

Join Date: Jan 2010
Location: Hyderabad
Posts: 316
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#4
In reply to #2

Re: Neutral Distribution Bypassing 3p 3w MCC

03/30/2018 11:27 AM

zero sequence reactance of generators is much lower than that of positive sequence. For that reason it is best to avoid subjecting the generators to earth faults in the system. I would keep the isolation transformer.

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Raghunath
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Guru
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Join Date: Aug 2010
Location: Resting under the Major Oak
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#3

Re: Neutral Distribution Bypassing 3p 3w MCC

03/25/2018 4:54 AM

Will the neutral be switched at each ATS? If not use just the one N→E link which I would locate in the new transformer switchboard.

As for a 20 year old transformer, some of our transformers were installed in the 1930’s. I retired, they kept on working.

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