LV Generator Grounding & Protection principles

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sooria365

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Hi All,

I am a new member to this forum. I would appreciate if any one can advise me on the following:
Application: 3 x captive generator (1680kVA each) with step-up transformer and connected to utility grid

1) What is the industrial norm for grounding an LV AC generator neutral? solid or isolated or through a resistance?
2) what is the recommended practice for protective relays in above cases?
3) There is a suggestion to go for device 59 (over voltage) relay fed from broken-delta on generator end and also for the HV side of transformer. Is it the correct practice?

Thanks in advance,
 
sooria -
You have definitely picked a non-trivial task. I suggest starting with IEEE 141 (red book), IEEE242 (buff book), and Blackburn, Protective relaying. This will get you a good start in understanding the physics.

I will attempt to answer your questions, but (mild disclaimer follows):
Without having the complete scope and specifics - I could be easily way off base. By choice, I'm not a designer. I'm a field dog. My work generally involves getting an installation, designed and build by others, to perform as needed by the customer.

sooria365 said:
...
1) What is the industrial norm for grounding an LV AC generator neutral? solid or isolated or through a resistance? ...
I tend to like impedance grounded. Boosts the reliability, cuts down on the damaging fault currents.

sooria365 said:
...
2) what is the recommended practice for protective relays in above cases? ...
There are a couple of common methods - The voltage levels and customer needs will dictate.

One can set up the relays to take a generator off-line if a GF shows up. If you are generating at 13.8kv then you want to get the gen off-line right away.

If you are generating at 480V and if continuity of power is required, then set the relays to annunciate only - and do an orderly shutdown on the gen. Then go look for the GF immediately.

sooria365 said:
...
3) There is a suggestion to go for device 59 (over voltage) relay fed from broken-delta on generator end and also for the HV side of transformer. Is it the correct practice? ...
Generating at 480V or 13.8kv? Utility is likely above 13.8kv - maybe 69kv or 138kv. I don't know where you are going to sync - Utility to xfm cb, gen cb?

There will be pts everywhere: Utility side of CB, transformer HV, transformer LV, gen. They are needed for the sync circuits and the protective relays.

Open-deltas for gen pts work fine and are fairly common (480V or 13.8kv). The HV (utility) pts will likely be wye connected and center-grounded. The gen open-delta pts will be corner grounded. Depending on where you are syncing there could well be phase shift issues to deal with.

There will be a dozen or so relays on the gen and the xfm. As for the specific question, you will want a 59 on the gen cb. You will also likely want a 59 on the utility cb.

Let us know how it turns out. It is an interesting project.

carl
 
Thanks very much Carl.
I will try to get hold of the reference books you have stated. As these generators are to be synchronised with Utility at 10kV, I am not sure what the utility requirements are with respect to protection, metering & control.
I shall keep you posted of the outcome (may not be soon due to lot of formalities with Client / Utility)
Cheers!
 
coulter said:
sooria -
You have definitely picked a non-trivial task. I suggest starting with IEEE 141 (red book), IEEE242 (buff book), and Blackburn, Protective relaying. This will get you a good start in understanding the physics.

I will attempt to answer your questions, but (mild disclaimer follows):
Without having the complete scope and specifics - I could be easily way off base. By choice, I'm not a designer. I'm a field dog. My work generally involves getting an installation, designed and build by others, to perform as needed by the customer.


I tend to like impedance grounded. Boosts the reliability, cuts down on the damaging fault currents.


There are a couple of common methods - The voltage levels and customer needs will dictate.

One can set up the relays to take a generator off-line if a GF shows up. If you are generating at 13.8kv then you want to get the gen off-line right away.

If you are generating at 480V and if continuity of power is required, then set the relays to annunciate only - and do an orderly shutdown on the gen. Then go look for the GF immediately.


Generating at 480V or 13.8kv? Utility is likely above 13.8kv - maybe 69kv or 138kv. I don't know where you are going to sync - Utility to xfm cb, gen cb?

There will be pts everywhere: Utility side of CB, transformer HV, transformer LV, gen. They are needed for the sync circuits and the protective relays.

Open-deltas for gen pts work fine and are fairly common (480V or 13.8kv). The HV (utility) pts will likely be wye connected and center-grounded. The gen open-delta pts will be corner grounded. Depending on where you are syncing there could well be phase shift issues to deal with.

There will be a dozen or so relays on the gen and the xfm. As for the specific question, you will want a 59 on the gen cb. You will also likely want a 59 on the utility cb.

Let us know how it turns out. It is an interesting project.

carl
Hi Carl,

Thanks very much for your quick response. I will try to get hold of the books you have stated. The generators are rated 400V and being hooked up to 10kV utility lines through a YnY0 transformers. The utility requirements for paralleling these generators are still not clear. I will keep you updated.

Cheers!
 
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