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SR79
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Join Date: Jul 2011
Posts: 5

How to Calculate NGR?

07/18/2011 7:19 AM

We have 50kA, 1 sec S/C fault current level for transformer of 11/3.3 kV having Dyn11 vector group. The value of NGR is 6.92A. How is this NGR value calculated?
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wareagle
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Join Date: Jun 2008
Location: Alabama
Posts: 1604
Good Answers: 63
#1

Re: How to Calculate NGR?

07/18/2011 12:51 PM

I do not believe "The value of NGR is 6.92A". The value of NGR is in ohms. So you can use Ohm's law to calculate the value of the resistor.

I = E/R or R = E/I. Don't forget the calculate the required wattage.
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msamad
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Join Date: Feb 2009
Location: Houston, USA
Posts: 946
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#2

Re: How to Calculate NGR?

07/19/2011 9:00 AM

This thread may also help: http://cr4.globalspec.com/thread/61442/Earthing-Resistor-Calculation

- MS

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electricalexpert65
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Join Date: Oct 2009
Posts: 2061
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#3

Re: How to Calculate NGR?

07/26/2011 11:28 AM

NGR Value (in Ohms) = Phase Voltage of the system / desired earth fault current.

In your case, the desired earth fault current (on the 3.3kV star side, I presume), is decided as 6.92A. Hence, the Ohmic value of the NGR is: 275 Ohms.
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