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.

 

[deedo1124]

Gentlemen,

The utility is providing a 1500KVA 12KV delta to 480V Y pad mounted transformer. They are not interested in supplying a different type. There are 3 generators each rated at 480V 300KW. Attached is a cut sheet of what we found for the potential ground fault sensing equipment. They want the ground fault equipment pad mounted. Does any one have any suggestions for alternative ground fault sensing equipment. I have only found this one vendor and the lead time is to long. The other option is to mount the equipment on a pole but this may not meet their requirements as all other equipment is enclosed pad mounted type.

 

[deedo1124]

Here is the system one line

 

[davidbeach]

You might well be better off taking service at primary, lower rates and all that, and then putting in a grounded wye-grounded wye transformer. In that case you wouldn't need a separate grounding bank as the issue it solves won't be present.

 

[waross]

I agree with davidbeach. grounded wye, grounded wye.
Avoid a grounded wye primary/delta secondary(480). This connection may overload your transformers and will back feed full voltage to a single open primary phase. (Subject to impedance drops).

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

 

[deedo1124]

One Last time,

The generators are located at a muncipal landfil. The utility company is providing a 1500KVA, 12Kv to 480V transformer delta primary (12Kv) wye secondary (480V) There is no negotiation this is what they are providing. Utility will be buying the power from the company that is installing the generators and gas equipment at the site. The utility company requires a ground fault sensing on the primary side to trip the generators and protect the grid. This is being done with a "grid protection package" which is a Beckwith relay provided by the microturbine vendor. The ground fault sensing will be done using three 12KV instrument class Pt's wired in a broken delta formation. The utility refers to this as a "ground bank". This equipment needs to be installed in a pad mount enclosure. It will be wired to the loop bushings on the main transformer. I have recieved a wealth of information but no one has adressed my main question. Can any one recomend a vendor who would build this enclosure and provide the PT's and other equipment in a package? The only one I have found thus far is Elliot Industries. Design is approved equipment is purchased. Only open item is the "ground bank". If any one can reccomend a vendor I would appreciate it.

Thanks,

jd

 

[davidbeach]

Oh, well that's certainly not what "ground bank" says to me, but that's their term. I don't have any ideas about where to find what you need, does the utility have any suggestions? So they aren't looking for you to provide a ground source for the line but simply a way to measure the voltage to detect a ground fault; simple enough.

Not the way I'd require it done, but that isn't my system.

 

[waross]

David, do I read this properly? To detect grounds don't the 12kV PTs have to be connected in grounded wye configuration?
If so, should they be full 12kV line to line rated PTs with the secondaries at 69 volts?
I can't understand how a ground would be detected unless the primary has a ground reference.
If the PT wye point is grounded, then when one phase is grounded the voltage will rise to 12kV on the healthy phases and the secondary voltage will rise to 120 volts.
Although the normal voltage across the PTs will be 6928 V, with a grounded phase this will rise to 12 kV and would saturate and destroy 6928 V PTs.
.

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

 

[davidbeach]

The VTs could be grounded wye-grounded wye or they could be grounded wye-broken delta. I'm not familiar with the Beckwith relay listed; if it has four voltage inputs then three will be used on the 480V system and the one remaining will be connected to the broken delta and will measure 3V0 from the 12kV system. Remember, broken delta is three windings, unlike open delta which is two windings, and sums the three voltages to produce 3V0.

On a system that could become ungrounded, you are absolutely correct that the VT primary voltage needs to be the line-line voltage. This scheme has been regularly used on grounded systems as a polarizing source for directional relays and in that case the VTs can be line-neutral rated as the voltage shifts are not as severe - much more a loss of voltage on the faulted phase than a voltage rise on the unfaulted.

 

[apowerengr]

Malton, Kuhlman, Shallbetter, Maysteel can all construct the metering/relaying enclosure - outdoor, padmounted, etc. As other's mentioned - the use of the term "ground bank" for a set of PT's providing ground fault sensing for the case when the utility and its ground source are isolated from their feeder and the landfill generators are backfeeding the fault is not a new concept. There are several ways to handle it.

I'd start the mfg. list in the reverse of the order listed if time is the primary driver for the installation.

 

[ausphil]

deedo,

try not to get too stroppy about people not answering your question, when you yourself did not explain until your 5th post, what you actually wanted.

the line of thought that was explained throughout this thread was completely valid with the limited, if somewhat misleading information that you supplied.

in your first post you asked the question "grounding bank the correct terminology? Google searches come up empty.". Maybe this should have triggered you to explain up front what you wanted in a bit more detail.

It took 9 days to work out what you asked, and one day to answer the question. I think that the contributors deserve a little more than attitude mister "One Last time,"

ausphil

 

[deedo1124]

Thanks to all for posting and sharing your knowledge on the subject.

I am in the process of getting competetive quotes for a custom pad mounted unit. In addition to the quote from Elliot Industries, Maysteel and Power Design Inc. have previously built similar units and are quoting their solution.

Your help has been invaluable.