How do I Size a Grounding Bank for a Delta Source?

[polyphase]

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?

 

[peterb]

I suggest that you tackle this from an impedance perspective - what value of transformer impedance (in Ohms) will result in the level of ground current that you need to ensure selective relay or recloser operation.
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.

 

[dpc]

As peterb indicates, you should decide how much ground fault current you need and for how long. This will give you an idea of the transformer impedance (in ohms) that you need, and you can then translate that to a kVA rating based on nominal transformer impedances.

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.

 

[peterb]

dpc, you may very well be correct as regards availability & pricing of small zigzag transformers - my application involved temporary replacement while the old zigzag unit was being rewound.

 

[busbar]

Minor aside: One thing to watch for may be existing 24.9kV grounded-wye/delta banks contributing zero-sequence current in a SLG-fault situation. This wouldn’t necessarily be a ‘minus’ for the system at hand—it may possibly be used at an advantage.

 

[RJF]

Just found these forums today - facinating amount of information available.

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 (

http://www.postglover.com)

- they were helpfull and seemed to understand the different options. I bought a 2400 Volt zig zag from Federal Pacific (

http://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.

 

[patpat]

Hi! Can you suggest a web literature on sizing and principles of a grounding bank.

Thanks.