How do I Size a Grounding Bank for a Delta Source?
How do I Size a Grounding Bank for a Delta Source?
(OP)
The situation I have is a 24.94-kV four wire grounded system normally serving load through a 24.94/12.47-kV delta/wye stepdown. Both the 12.47-kV and the 24.94-kV are four wire grounded wye systems. There is a desire to reverse the flow and serve several customers on the 24.94-kV side from a source on the 12.47-kV side. The existing stepdown bank is rated 1500 KVA and consists of (3) 500 KVA single phase transformers on a platform. There will be no path for ground current on the 24.94-kV side, so a grounding bank is required. I know that ZigZag is a better type of connection for grounding, but a bank of (3) grounded wye/delta connected single phase transformers are planned. How do I calculate the size the grounding bank?






RE: How do I Size a Grounding Bank for a Delta Source?
This will then guide you as to the size transformer to be used, based on available kVA ratings and impedances. Select transformer fusing that will protect the bank for uncleared ground faults, while coordinating with downstream relaying.
One word of caution, based on experience - if you are using completely self-protected transformers, make sure that the internal protection won't operate when the transformer is used for grounding service. You may also want to confirm with the manufacturer that the transformers can handle this duty. I have used this solution as a temporary measure, as the zig-zag approach is definitely considered a "better" permanent fix.
RE: How do I Size a Grounding Bank for a Delta Source?
And I think the delta-wye bank is probably the way to go for this size bank. The primary advantage of the zig-zag winding (I believe) is that it results in a much smaller transformer for the same fault current capbility. But in reality, virtually no one builds these any longer. It will be a special "one of" order and you will have trouble getting quotes. The delta-wye bank can probably be built out of available transformers, or easily (and competitively) obtained. It should work fine.
Good luck.
RE: How do I Size a Grounding Bank for a Delta Source?
RE: How do I Size a Grounding Bank for a Delta Source?
RE: How do I Size a Grounding Bank for a Delta Source?
Will the normal source for the 24.94 KV system be disconnected when you are backfeeding from the 12.47 KV system? From a protection standpoint, you need to define how much ground current you need to properly trip your relaying. I've bought wye delta systems from Postglover (www.postglover.com) - they were helpfull and seemed to understand the different options. I bought a 2400 Volt zig zag from Federal Pacific (www.federalpacific.com)- It was designed for a 2000A Postgolver resistor in the neutral, have since changed it to a 400A resistor because of a system change.
RE: How do I Size a Grounding Bank for a Delta Source?
Thanks.