Generator Ground

[SilverArc]

Hi,
I have five 13.8 KV generators each rated 1.7 MW each. It is an emegency gen system for a mining application.
Here is my question: I have experience with 600 V emergency generator systems. Here is my policy for grounding:

"In case of emegency generator installation, If emergency generator has to operate in parallel with utility, then It needs to have a seprate ground and should be also connected to utility ground other wise it does not need to have a seprate ground. I am talking about 600 V level. "

Do I need to have a seprate Low resistance grounding for these 13.8 KV generators and also connect them to utility ground.

Actually the dilemna is that: How do I decide that this generator needs to have its own grounding or only utility ground will be fine.

I apologize, if I was not able to express my self clearly.

Thanks

 

[davidbeach]

If it is an emergency generation system, you have to look at the situation where you don't have the utility. Completely ungrounded would not be a good thing. If your 13.8kV system has any 8kV single phase loads, you need to solidly ground the generators; hopefully this is not the case.

If the generators feed a distribution system that can be generator, utility, or both in parallel, you need to look at how your ground fault detection works. Lots of issues to be considered, not nearly enough information to begin to work out the details.

 

[rbulsara]

There are more than one ways for generator grounding. If all of your loads are 3-wire (not requiring neutral), either solidly ground each generator as davidbeach sugguests or resistance ground them. Most installations we have designed with MV generators incorporate individual resistance grounding for each generator and the loads are 3-wire.

I would advise hiring competent and experienced electrical consultants for this type of project.

 

[SilverArc]

Thanks David beach. I am located in Canada and I have experience with mostly up to 600 V, 1.5 MW generators. I have used the utility neutral and generator neutral is connetcted to that with no parallel operation with utility.

Suppose, I have a situation where it is 600 V and 1.5 MW generator with parallel operation to utility. Then I will have to resort to an independent grounding of generator which as per new Arcing phenomena will be high resistance.

Please advise me, if it is a right proposition?


The question in regard to 13.8KV and 1.7 MW genertors is a study we are working on will be installed in Africa. The local utility is 230 KV and will be stepped down to 13.8 KV.
Now, my sr. engineer has not yet decided and the client is not sure, if he wants a parallel operation or not.

So, will my this rule with low voltage work. If the generators will operate in parallel with utility then I should go for low resistance grounding as it is a 13.8 KV system. If they will always operate during emergency operation then I can just hook to the utility neutral.

I understand this might be a lengthy question to answer. I have access to all IEEE standards online. Please advise, which standard can give me a better picture.

Thanks

 

[dpc]

I agree with David Beach that more info is needed. In **general terms**, it is usually preferred to ground a medium-voltage generator through some type of impedance to limit the machine damage in the event of a ground fault. Some generators are not braced to withstand a zero ohm line-to-ground fault, but in any event, core damage will be more severe if generator is solidly-grounded. Resistance-grounding is preferred in most cases.

 

[davidbeach]

Ideally, new installation, I'd seriously consider high resistance grounding each generator, paralleling them on a generator bus, and then connect that bus to the rest of the system through a delta-wye transformer. That way you can have the generators grounded and protected in the way that is best for the generators and depending on how you ground the wye point of the transformer you can have the rest of the system grounded in the manner that works best for that.

As rbulsara suggested, you really need to be working directly with someone familiar with these types of systems; you'll get much better results that way than you will here.

 

[SilverArc]

Thanks Everybody.

I completely agree with your suggestions. One last part here, which no body mentioned.

If I have to operate a 600 V gen. in parallel with utility, Do I have to for a resistance grounding or just connect generator neutral to utility neutral.

Thanks

 

[dpc]

No one mentioned it because it wasn't your question .

Again, the answer depends on your 600V system. If you have no line-to-neutral loads, you could resistance ground to limit ground fault current. However, 480V and 600V generators are commonly solidly-grounded.

 

[alehman]

IEEE 141 and IEEE 142 are both very good references with much material on system grounding for low and medium voltage applications.