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Commentator

Join Date: Jun 2008
Posts: 65

# NGR Sizing

02/19/2011 2:17 AM

i shall design the NGR for Power transformer. the rating of power transfomer is 12/15 MVA,132/6.6KV. star-star winding. i have gone through the 2 to 3 documents.there no clear cut idea about restricted current.is there standard restricted current value is there.or we have to design ourself. i seen 200 A & Some where i seen 500A current. what is the value i have to design & why.kindly suggest me.

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Join Date: Jan 2011
Posts: 9
#1

### Re: NGR Sizing

02/19/2011 6:04 AM

hi

i don't know exactly what is the clear idea about this issue but i have seen many project in our company that designers have limited the ngr current to rated current of transformer (secondary).in your case 1312A.GOOD LUCK

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Join Date: Jun 2008
Posts: 65
#2

### Re: NGR Sizing

02/19/2011 6:29 AM

hi sir,

purpose of the NGR is to restrict the neutral current.if the transfomer full current is designed for NGR means.what is the use of NGR. Kindly advise

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#3

### Re: NGR Sizing

02/21/2011 9:33 AM

The NGR value is arrived at by dividing the phase to neutral voltage by the desired earth fault current. It is a general practice in the industry ti limit the earth fault current in cas of transformer to the rated full load current of the transformers.

Please note that if no NGR, that is, if solid earthing, then the single-line-to-earth fault currnt could be of the order of three phase suymmetricl fault current.

That's why one limits the earth fault curent by providing an NGR. Aren't all transformers designed to deliver their rated full load currents continuously? So even during an earth fault they would deliver the earth fault current - limited to their rated current by the NGR - continouslty.

As the transformer is the source, it will be high up in the discrimination scheme and hence it will have the maximum time delay in the discrimintion grading. So, even in such a case, the transformer will deliver the fault current continuously.

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Join Date: Jun 2008
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#4

### Re: NGR Sizing

02/21/2011 11:05 PM

Dear Sir,

thanks for your suggestion.Kindly make it clear what will be limiting current.is there any value?.

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#5

### Re: NGR Sizing

02/22/2011 5:03 PM

In my coal mining days, we had NGRs on all of our circuits feeding underground. It's embarrassing that I can't remember, but the NGR was sized, iirc, to limit the current to either 15 or 50 amps on the neutral.

I'm pretty sure we took steps to make sure all loads were connected as 3 phase, or, if single phase, phase to phase as opposed to phase to ground.

I'm not sure I ever *knew* exactly why we limited the current to that level, but, in retrospect, I have a suspicion. We (in the mining industry in general) used a lot of trailing cables, and occasionally we got something (a fault)--I can't remember the name that was used--but it was (again, iirc) a low current arcing fault.

Such a fault might start at the far end of a cable, and gradually arc all the way back to the source at fairly low current. This is very bad in a coal mine that has potentially explosive atmosphere of methane and coal dust (not to mention destroying an expensive cable).

I suspect the goal was to limit the current to a low enough value that would not support that kind of fault.

A similar limitation may be appropriate in other applications.

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#6

### Re: NGR Sizing

03/01/2011 6:49 AM

hi dear friend

ngr is used to reduce single phase to ground short circuit current of sec.side of transformer and its clear that we want to minimize this current to zero(isolated network).but for detecting the earth fault, relays need to see a acceptable magnitude of current,and that is why ,we don't minimize the earth fault current.the rated current of transformer is a good margine for detecting of earth fault by relay.

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Join Date: Jun 2008
Posts: 65
#7

### Re: NGR Sizing

03/01/2011 7:18 AM

i understood now. thanks for your(good & understandable) reply.

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