Generator High Res. Grounding

[sobeys81]

Hi,

IEEE C62.92.2 part 2 for neutral grounding talks about using a disttribution transformer in the neutral for high resistance grounding of generator.

Recently, I saw an application of a 3 MVA 4.16 kV Generator where there was a NGR directly connected to the generator neutral rated at 15 A. I was wondering, what are the pros and cons of either methods. I would appreciate a word.

Do we ever use a Low resistance grounding on a medium voltage generator ?

Thanks

 

[trosepe]

A single phase distribution transformer is used to allow low resistance resistor to create a high resistance ground on the generator neutral. The transformer primary is connected in series between the generator neutral and ground. A resistor is connected to the secondary. The resistance on the secondary is reflected through to the primary by the square of the transformer turns ratio. This method also helps harmonic mitigation. As for the pros and cons; IEEE green book addresses these very well.

 

[ScottyUK]

Low impedance earthing is frequently found when the generator is connected directly to an industrial network (as distinct from a public utility) when the generator voltage is the same as the distribution voltage. It saves a transformer, at the risk of writing off the stator core in the event of a stator earth fault. The accountants think this is a good idea because it saves on first cost. Most rotating machines engineers would prefer a high impedance earth and a generator transformer to protect the machine.


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If we learn from our mistakes I'm getting a great education!

 

[sobeys81]

Thanks all. while I am writing this post, I have probably all papers ever published in IEEE on low and high resistance grounding on my table. here is the dilemna,
I have a 69 kV/4.16 kV transformer rated at 10 MVA and grounded with 25 A NGR.

If I ask you this question, Is it a low resistance or High resistance grounded system. IEEE says all the systems with NGR rated between 100-400 A are LRG and between 2-10 A are HRG.

What do I say about a 25 A ? I have made sure that it is higher than charging current.
I would appreciate a word. Thanks

 

[trosepe]

Do you alarm or trip on ground fault? You typically alarm in HRG systems and trip in LRG systems. If you alarm, you need to be certain all your system phase to ground insulation levels, especially underground cable, are rated for the system line to line voltage.

 

[sobeys81]

we will trip. Cables at 4.16 kV are by default rated for line to line voltage. What other issue i should be concerned about.

So you would call it a LRG system or a HRG system ?

Thanks

 

[rbulsara]

what trosepe said.

Afterall high and low are relative terms. Key is to trip when GF fault current is not sustainable and alarm only if the system is designed to withstand permitted GF fault current indefinitely.

Rafiq Bulsara

http://www.srengineersct.com

 

[trosepe]

I don't understand what you mean by "by default the cables are rated for line to line voltage". Cables rated 5KV, 100% insulation should only be used on solidly grounded systems. Cables with 133%insulation or 8KV rated are needed for resistance grounded systems. Your system (15 amp ground current)is more in the range of HRG systems. What is the max time rating of your resistor and how fast do you trip on ground fault?

 

[sobeys81]

Thanks trospe, Cables at 4.16 kV are unshielded and rated for 100% insulation. Does 100% and 133 % apply to unshielded cables as well ?

 

[trosepe]

No it doesn't; only shielded cable. Sorry for making the assumption of shielded cables.

 

[odlanor]

1- By Westinghouse - transmission and distribution handbook, a system not effective grounding should have grounding by above 50 ohms at neutral generator primary. So, 15A or 25A are high impedance grounding systems.
2- We usually use medium -resistance(100-600A) for generator less then 40MA. It is good for protection sensibility that should trip fast.
3-We use high impedance for generator more then 40MVA.It is good to protect against damage to stator core. There is no security for fast protection , so neutral current should be more then 30A , to avoid damage in stator core.
In test results showing effects of arc burning on stator core and laminations during earth faults in generator :
Less then 15A - negligible arc burning
15-30 A – slight burning
40A and above – severe burning.

 

[sobeys81]

Please dont mind, if my question is stupid.

I have heard this terminology stator core damage so many times. A fair size generator on 24 Kv carries 20,000 A full load current, so what damage 200 A ground fault current can do to a stator.

And if it does, I am sure I am missing some thing big time. I know that a ground fault would depending upon LRG or HRG system would bring over voltage during fault. So, if stator winding insulation is rated for certain 1.5 P.u., so we are ok. So why all this fuss is all about.

what is about this small small ground fault current that can do so damage to a stator which is capable of handling
so high full load current.

Thanks

 

[davidbeach]

Ground faults start off doing damage to the winding insulation. If the current is kept low enough, that's all that will happen. If the ground fault current is high enough, and lasts long enough, you start burning both copper and iron. Once you start burning iron you have a partial to complete restack to perform. If you can limit the damage to the insulation you can often minimize the necessary repair work. Burning copper pretty much guarantees at least a partial rewind if not a complete rewind. Rewinds are vastly less expensive than restacks.

 

[sobeys81]

Thanks davidbeach

"Ground faults start off doing damage to the winding insulation. If the current is kept low enough, that's all that will happen."

So this will happen because of overvolatge ? not because small 50 A ground fault current ?
Thanks

 

[davidbeach]

No ground fault current unless there is an insulation failure. Insulation failures can come from many causes; overvoltage is one of those causes.

 

[odlanor]

see :

http://library.abb.com/global/scot/...K60-202E_en_Generator_Protective_Relaying.pdf

page 4 and 5

http://books.google.com.br/books?id...ErNJ2&dq=generator ground protection&pg=PA120

 

[odlanor]

ERRORS:
3-We use high impedance for generator more then 40MVA.It is good to protect against damage to stator core. There is no security for fast protection , so neutral current should be LESS then 30A , to avoid damage in stator core

http://books.google.com.br/books?id...ErNJ2&dq=generator ground protection&pg=PA120

ITEM 5.2.3 CORE DAMAGE CAUSE BY GROUND FAULT

 

[sobeys81]

Thanks Odlanor.

Is it also a standard practice to shut down the generator excitation during a ground fault as well, if it is a stator grounding fault ?

Why I thought so because if gen. excitation is still on it will keep feeding the fault. Is this correct.

 

[odlanor]

Yes , this is the recomendation of IEEE-Std C37.102-1995 see fig.6-1a (59GN-50/51GN)

 

[slavag]

Hi sobeys81.
You must shut down exicitation in case of stator ground fault. For my pinion, isn't recommendation, it's MUST.

Best Regards.
Slava