MV Grounding Bank? 3

[deedo1124]

Working on a project were the utility company has requested a grounding bank be connected to the primary of their 12KV delta-wye transformer used as a generator step up for (3) 480V landfill gas generators. It will be located outdoors on a pad with the utility transformer and wired to the generator protection equipment. Can anyone reccomend a manufacturer who makes this type of equipment. Is grounding bank the correct terminology? Google searches come up empty.

 

[dpc]

They are probably talking about a grounding transformer to provide a ground source on the ungrounded 12 kV delta.

Historically, this has been done using zig-zag grounding transformer, but can also be done with a small wye-delta transformer.

Google up "grounding transformer" and see what turns up.

 

[RRaghunath]

deedo1124,
You say on primary of stepup trafo, that makes it 480V terminals of the trafo. Generator step-up transformers have delta on generator side (generally speaking) and that makes it on 480V side in your case.
If so, where is the need for artificial grounding - 12kV is Star with neutral point available and on 480V side, generator star point can provide grounding!!

May be you would like to clarify.

 

[waross]

Primary and secondary often get confused with GSUs. Do they want the ground on the 480 side or the 12kV side? Do they want a solid ground or a high resistance ground? Are they concerned with current limiting impedance or the ground grid and/or electrodes?

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

 

[deedo1124]

The grounding bank is for the 12Kv delta side of the transformer. The intent is to keep the generators from contributing to a fault on the utility side of the transformer. Can anyone reccomend a vendor for this type of equipment?

 

[davidbeach]

Any transformer with a 12kV grounded wye and a delta winding will work. Delta winding voltage doesn't matter as it won't be connected to anything. Transformer needs to be sized such that it won't be damaged while it feeds a ground fault on the 12kV system. That's a calculation of current vs. time and is highly dependent on the specific installation conditions. As for recommended vendor I'd go with who ever you normally buy your transformers from.

 

[waross]

Hi David.
I suspect that the OP is looking for a high resistance/impedance neutral grounding device. I suspect that the transformer is 480 V delta and 12kV wye. In this case do we have to be concerned with high side ground fault current through the transformer? The GSU may be existing and just need a grounding resistor.

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

 

[davidbeach]

My guess, particularly in light of "12Kv delta" is that we are dealing with a site that takes service at 12kV and has their own (or even utility owned) 12kV [Δ]-480Y/277V transformers and that the generators are connected solidly grounded to the 480V system. The utility, quite correctly, is looking for the 12kV system to remain effectively grounded in that interval between when the utility substation breaker opens and the generators are disconnected from the utility system. It is quite frankly a very expensive solution to very nasty situation that might exist for less than a minute over the entire life of the site. But so be it. I wouldn't allow anything less on our system.

A far better solution would be to connect the generators to their own transformers, [Δ] on the generator side and grounded wye on the 12kV side. Now they're not looking at a grounding transformer that is unneeded almost all the time. But when it is needed it better be there or there will be all sorts of trouble.

 

[waross]

Thanks David. That makes sense to me.

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

 

[rockman7892]

davidbeach

Can you explain the situation in your last post further. Do you mean between the time when the utility breaker opens and the generators are connected to the system. With utility and generators wouldn't one be closing as the other opened?

 

[dpc]

I think the assumption was that the generator was running in parallel with the utility and both breakers are normally closed in this situation.

 

[waross]

The utility has a grounded system. In the event of an event opening the utility breaker on the line that services the generator, the line will be fed from the generator momentarily and at this point the line wil be ungrounded. The utility will require a grounded system for the generator so that at no time will it be possible for the line to be energized but ungrounded. A 480V delta by 12kV wye transformer may be the simplest solution.

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

 

[davidbeach]

Yep, I was assuming parallel operation, otherwise the utility wouldn't be requiring the grounding bank. dpc & waross have filled in a lot of the details. What wasn't mentioned is that the utility circuit is likely to have many line-neutral/ground connected loads and arresters. When there is a ground fault on the line and no ground source - utility breaker is open and generators are supplying voltage through a delta winding - the voltage to ground of the unfaulted phases will approach nominal phase-phase voltage. The overvoltage of up to 1.7 per unit will cause significant damage and must be prevented; therefore the requirement for the grounding bank. I've required the same.

 

[rockman7892]

Could somebody please post a single line for this arrangment. I think I am understanding everything but having a hard time visualizing it.

 

[apowerengr]

See if the utility can replace the existing transfomrer with a Y-gnd Y-gnd transformer.

 

[RRaghunath]

deedo1124,

How big are your generators?
Is it necessary to parallel the generators with the grid?

If the generators are small compared to the 12kV source MVA of the utility, the, You should be able to convince the utility that the contribution (for 12kV system earth faults) from the generators is not much and seek waiver from the requirement of providing grounding trafo at 12kV.

 

[davidbeach]

raghun, it will never happen, at least not if my suppositions about configuration are correct. The problem is the voltage shift on a ground fault on a less than effectively grounded system. That ground shift poses an unacceptable risk to the utility system and to its other customers.

Without the grounding transformers the fault contribution by the generators, no matter the size, will be approximately zero and that is the problem. The grounding bank is necessary so that there will be ground fault current and the voltage will be stabilized.

 

[RRaghunath]

davidbeach,

I agree with you that the ground fault contribution is zero. With grounding transformer, the contribution from generators to the ground faults in the utility power system can be ensured. That is one part.

I am looking at a case when the utility fault level is large enough even without considering the contribution from these generators. If true, this would save the bother of retrofitting the grounding transformer.

The over voltages in 12kV system is not an issue here, I hope. The utility system must have 12kV neutral grounded at the source end.

Hope I could clarify.

 

[waross]

Even though the system may be large in relation to the generator, the distribution feeder to the generator may not be large in relation to the generator. In the event that the feeder breaker at the utility substation opened, the line may be energized by the generator. If the generator installation could not provide a ground path, the feeder would see the damaging overvoltages. The generator does not presently have a neutral, that's the point. The system neutral will have no effect with the feeder breaker open.
The problem arises after the utility feeder breaker opens on a ground fault and the feeder is still supplied by the generator.
With no generator ground, the generator will not contribute to a ground fault and will not trip on a feeder ground fault.

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

 

[davidbeach]

Right, Bill, you've got it. raghun, as Bill rightly states, with the substation breaker open it doesn't matter that the neutral is grounded at the substation and at every few poles, it has no reference to the generator.

The whole concern is during the few cycles between the utility substation breaker opening for a ground fault on the circuit and the generators separating from the circuit. Even with transfer trip the generators will be on the circuit for 1-2 cycles longer than the substation. That 1-2 cycles is long enough to do considerable damage.

We will never approve customer generation in parallel with a general use feeder circuit unless the generation always appears to be effectively grounded from the point of common coupling.

In the system of the OP, it would be much better for the generators to be behind a transformer that is grounded wye on the high side and delta (or even grounded wye) on the generator side.

I'd hate to have to make the economic justification for a grounding transformer that is only needed during fault conditions. But I will require that transformer without hesitation if that is the only way to ensure that the customer generation appears to be effectively grounded when seen from my circuit.