Generator Neutral Grounding Method

[kibz]

Hi Guys,
We have a 3mw, 10.5 kv generator connected directly to 3.75MVA Step-up Transformer without a generator circuit breaker in between (Unit Connection). Relaying Books & IEEE Stds always recommend high resistance grounding method using distribution transformer with a low ohmic resistor in the transformer secondary for unit connected generating system.
We have a spare 100 amps, 80 ohm, 8kv Neutral Grounding Resistor in our storeroom.
Can we use this 80 ohm Low Resistance NGR for this type of installation? Is there any effect to the generator using low
resistance grounding instead of high resistance grounding aside from high ground fault current?
Please share your knowledge on this.
Thanks & More Power...Kibz

 

[waross]

Ask instead:
"What is the effect of high ground fault current?"
The higher the ground fault current, the higher the chance of arcing destroying the generator core in he event of an arcing ground fault.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[aruskuat]

Kibz, here is my answer :
1. Is 10.5 kV line-line or line-ground voltage? The rated voltage of NGR is line-ground voltage of your generator
2. The purpose of using high-resistance grounding, according to IEEE 142 and ANSI C37.101 is to limit the level of over voltage and SC when there is fault thus the continuity of supply form the generator to the load will be sustained. The low resistance grounding will be used for protection coordination of the 50/51 G

 

[kibz]

Guys,
The 3mw, 3phase generator is rated 10.5 kv line-line. The NGR is 8kv, 80 ohms, 100 amps. From my calculation the single-line to grd fault will be 76 amps. So we can use this NGR on a unit connected generator-step up transformer configuration, without generator circuit breaker. But the problem is, it will damage the generator core in case of fault. Am I correct?
Thanks..Kibz

 

[ppedUK]

Kibz:-
Your ground fault current calculation is correct at 76A for 80ohm NER, however 76A may still be too high to avoid major stator burning. Typical machines of this voltage and rating usually have ground fault current limited to <25A, but check with the manufacturer of the alternator.

 

[aruskuat]

Agree with that, high resistance grounding mostly used in generators <25A

 

[odlanor]

kibz,
your generator with 100A -resistor do not applied core stator damage.
see attached

 

[kibz]

Guys,
If I use high resistance grounding using distribution transformer & secondary resistor, I have to compute for the total capacitance
of the generator, transformer, arrester. I have no data for these capacitance. Is it safe to calculate HRG equipment using 10 Amps
of ground fault current?
Thanks..Kibz

 

[odlanor]

kibz,
Regarding the experience, the ground(100-400A) has designed an industrial tradition of over 75 years. Only recently (about 10-15 years) was questioned his performance on larger (20-60MVA) generator, due to the high-side neutral current associated with slow excitation, causing damage to the machine even after the opening of the breaker generator.
Perhaps your difficulty of information follows this view.
I have found difficulty in setting ground stator - 100% relay for low power generator with high impedance grounding.
I think you will regret in the future, as happened to me.