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Anonymous Poster

what is the application of NGR in fault level calculation of transformer and gee

04/06/2008 7:00 AM

dear all

what is the application of NGR in fault level calculation of transformer and generator .how can we calculate the resistance of NGR .what formulae is use for calculation for resistance of NGR .

regards

suresh paliwal

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Guru
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#1

Re: what is the application of NGR in fault level calculation of transformer and gee

04/06/2008 5:01 PM

http://acronyms.thefreedictionary.com/ngr

pick one, maybe we can help.

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Anonymous Poster
#3
In reply to #1

Re: what is the application of NGR in fault level calculation of transformer and gee

04/07/2008 4:02 PM

BrickTop,

NGR = Neutral Grounding Resistor

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#2

Re: what is the application of NGR in fault level calculation of transformer and gee

04/07/2008 3:25 PM

A) If low resistance configuration is chosen, then, ground fault current is greater than 25% of 3-phase value.

B) If high resistance configuration is chosen, then, ground fault current is between 1 to 5% of 3-phase value.

C) Ground fault protection scheme is important because affects resistor thermal rating, hence size.

D) For B) above, ground fault current must not be less than system charging current produced by phase to ground capacitance in 11 kV system.

E) For D) above, total capacitance calculation must include line to ground capacitance of cables, motors, transformers, and (often forgotten) surge protection capacitors, as well as lightning arrestors.

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Power-User

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#4

Re: what is the application of NGR in fault level calculation of transformer and gee

04/08/2008 2:30 AM

The ground system for your purpose has been dealt in length and breath in IEEE 32, 142 and 242. The types of system grounding normally used in industrial and commercial power systems are 1) Solid grounding 2) Low-resistance grounding 3) High-resistance grounding 4) Ungrounded. I am not discussion here the pros and cons of all these system as you have not asked it.

The NGR types of grounding, the resistor is connected between the neutral of the source (transformer or generator) secondary and the earth ground.

The line-to-ground capacitance associated with system components determines the magnitude of zero-sequence charging current. The resistor must be sized to ensure that the ground fault current limit is greater than the system's total capacitance-to-ground charging current. If not, then transient over-voltages can occur. The charging current of a system can be calculated by summing the zero-sequence capacitance or determining capacitive reactants of all the cable and equipment connected to the system.

So 1) calculate the capacitive charging current of the system from data sheet of all the equipment associated or calculate it from basic formula for all those equipment as mentioned by Reply-2 section (E).

2) Decide your limiting fault current as describe above.

3) Now you can apply the ohms law to calculate the R of NGR as R (NGR) = E /(sqrt3*I)

R(NGR) ≤X (Co)/3 ohms, I ≥ 3I (Co), W (NGR) = squired I * R(NGR) Watts. where E= L-N voltage of system, I= limiting fault current you have decided, I (Co)= capacitive charging current.

If any thing is missing above please point out.

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manirul
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#5
In reply to #4

Re: what is the application of NGR in fault level calculation of transformer and gee

12/31/2008 12:55 AM

Dear Manirul,

Will you explain why a transformer i.e. 22/3.3 kv 10 MVA ( Delta , Star) with 8% of impedence connected with NGR when feeding exclusively to 3.3 kV motors and 22/.4 kV, 3.15 MVA ( Delta , star) solidly grounded when connected to the other LV distribution.

Is it compalasory or having additional banifit to connect with NGR ( Motors feed transformer) on solid grounded system.

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Power-User

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#7
In reply to #5

Re: what is the application of NGR in fault level calculation of transformer and gee

01/18/2009 9:39 PM

Dear dwarish & vguru

I am irregular to cr4 due to project close out work, pls leave you email id so that i can send reply.

Thanks.

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manirul
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#6
In reply to #4

Re: what is the application of NGR in fault level calculation of transformer and gee

01/16/2009 7:26 AM

Hi, Is it require a NGR for a 1250 KVA, 11kv / 415v, 5% Impeadence Transformer....? if so then wht shld be the fault limiting current, Resistance, insulation grade, temp. rise....

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