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Electronaut
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Join Date: Dec 2009
Location: Mangalore, India
Posts: 65
Good Answers: 4

Generator Neutral Earthing

02/08/2010 7:52 AM

We have a CPP of 48MW capacity (8X6.0MW). The neutral of the generator is grounded using a NGR of 318ohms, 20A. There is an isolation switch between the neutral point of the generator and the NGR and the normal practice is to run the generator with NGR switch closed. In this context, which of the following options will be better and why?

a) Run the generators with NGR switch closed.

b) Close the NGR switch of one of the generator and keep the switches of the other NGRs open.

We have not faced any problem whatsoever by running the generators with the NGR switch closed. The alternator rating is 11kV, 8145kVA, 428A, 750rpm. All the generators are connected to a single 11kV bus with no bus-coupler. The power plant is a base load type with a constant load of about 36.0MW with 7 DG sets running in parallel.
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avid0g
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Join Date: Feb 2010
Posts: 127
Good Answers: 3
#1

Re: Generator Neutral Earthing

02/08/2010 8:27 AM

Please let me paraphrase you to be sure we-all have this understood.
Each NGR has 318ohms, 20A rating.
All generator neutrals are bonded together.
All generator chassis and frames are bonded together.
mode a) Close all NGR switches on all generators.
mode b) Close only one NGR switch.
And one last question, please.
List all abnormal conditions that would automatically open the switch(es)?

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Electronaut
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Location: Mangalore, India
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#2
In reply to #1

Re: Generator Neutral Earthing

02/08/2010 8:52 AM

You have got it right. The NGR switch is a mechanical switch with no provision for automatic opening. No operation whatsoever in any case will open the NGR switch.
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ovide brudo
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Join Date: Oct 2007
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#3

Re: Generator Neutral Earthing

02/08/2010 4:46 PM

Hello: If I may venture an opinion :(a) if you operate all 7 gens with all 7 ngrs closed, then the max Ig ( line-to-ground fault) in each ngr would be 20A or a possible total of 140A for all 7 which are in parallel. This means that 1 gen could have an Ig as high as 140A, because this current would split up into 7 x 20A pieces. (b) If you operate with all 7 gens but with only 1 ngr closed, then the Ig for any gen is limited to 20A.

So you would have to check with the gen manufacturer to see if an Ig of 20A or 140A can be tolerated; on the other hand, if the gens can tolerate more than 20A of Ig, then by having more than 1 ngr in the circuit, you increase the reliability( by avoiding operation with no ngr, since the ngr switches are manual).
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msamad
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Join Date: Feb 2009
Location: Houston, USA
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#4
In reply to #3

Re: Generator Neutral Earthing

02/08/2010 5:53 PM

For further explanation, see the previous posting: http://cr4.globalspec.com/comment/376202 and especially read the comment # 6. There is mathematical expressions to clarify this.

- MS

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avid0g
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#6
In reply to #3

Re: Generator Neutral Earthing

02/08/2010 10:44 PM

A generator does not have to withstand all the fault current of its own NGR, as long as all NGR equipment grounds are bonded together. It is the bonding jumpers that have to withstand the full current. Beyond the NGR, the fault current travels on the shared neutral conductors.

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avid0g
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#5

Re: Generator Neutral Earthing

02/08/2010 10:14 PM

If there is a fault at the generation yard, the fault current could be up to 34.6 Amperes per NGR, which exceeds the component rating by 72%.

Let us assume that an NGR will fail gracefully, but 20 Amperes can be sustained continually. So any fault mechanism with an impedance greater than 232 Ohms would last indefinitely. Such a fault would be generating up to 95 Kilowatts of heat.

If all eight NGR switches are closed, the fault current could be up to 276 Amps. Any fault mechanism with an impedance greater than 29 Ohms would also be sustained indefinitely, at least by the NGR set. Such a fault would generate up to 761 KW of heat.

I humbly suggest that you don't have to concern yourself with the generator specifications, as a fault current can occur from anywhere, not just in a generator. You do need alarms on the NGR current monitor(s), and you do need to consider the current capacity of all existing equipment grounding conductors.

The whole point of using an NGR is to allow continued power distribution during a fault, while it is being isolated. To this end, you have to consider how much fault current is required to guarantee the shutdown of a circuit with the fault. This is generally achieved with Ground Fault Circuit Interrupters. So the spec. for the primary GFCIs and the number of paralleled NGR elements must work together.

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avid0g
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#7
In reply to #5

Re: Generator Neutral Earthing

02/09/2010 1:38 AM

Let us assume that one NGR is used, the fault current could be up to 20 Amps. A fault with an resistance of 318 Ohms would generate the maximum amount of heat, 31.7 Kilowatts at 10 Amps. A 1 Ohm short would dissipate 396 Watts at 20 Amps.

If eight NGR switches are closed, the fault current could be up to 160 Amps. A fault with a resistance of 40 Ohms would generate the maximum amount of heat, 252 KW at 80 Amps. A 1 Ohm short would dissipate 24 KW at 159 Amps.

The whole point of using an NGR is to allow continued power distribution during a fault, to limit the current to what circuit breakers can interrupt and to limit damage until either a GFCI (Ground Fault Circuit Interrupter) trips or the circuit is isolated manually.

So if possible, limit excitement in you life and use just one NGR.

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