Grounding transformer suppliers 1

[radug]

Hello,

I am looking for supliers of grounding gransformers, wye-open delta for high resistance grounding (10 A) of a 11 kV busbar. I have contacted different suppliers and have not been able to obtain anyone.

Does anybody now about suppliers in Europe for this type of transformer?


Thanks.

 

[Marmite]

You could try Stewart Transformers in the UK.

http://www.stewart-transformers.co.uk/page4.html

We buy earthing transformers from them. Not the same as you require, but the firm is a bit of a specialist and I'm sure they would be able to help out.

Regards
Marmite

 

[waross]

Have you considered using three distribution transformers? That may be the cheapest and quickest option. It is an "off the shelf" solution in many areas.
Consider also the price and quick availability of a replacement in the event of a failure in the future.

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

 

[radug]

waross,

It is a very small transformer: 11/0.272 kV, 63 kVA, Ynd11 with open delta and a resistor connected in the delta.

In a past project we were able to obtain one custom made from a supplier, but as it is not a "common product", I am looking for other suppliers.

 

[radug]

Marmite,

Thank you.

 

[slavag]

What about zig-zag grounding transformer?

 

[isquaredr]

You could try Cressall resistors

http://www.cressall.com/neutralearthing/neutralearthing160.html

 

[waross]

There should be many off the shelf distribution transformers available in that size range. The primaries should be rated for line to line voltage or greater, not line to neutral voltage.
Use three single phase transformers.
If you use a three phase transformer you may want to avoid a three legged core. The phantom delta effect may cause greater than calculated ground current. This may be avoided by placing the resistor in the primary circuit and closing the delta.

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

 

[radug]

waross,

All the information I have seen about earthing transformers is that they are or the zig-zag type or of the wye-open delta type, and that the resistor is placed in the secondary due to cost reasons.

I have not seen any reference about placing the resistor in the primary. Could you provide more info?

 

[waross]

10 amps at 11 kV requires an 1100 Ohm resistor. 10 Amps at 1100 Ohms requires a 110 kW resistor. This is not economically reasonable. That is the reason that the current limiting resistor is placed in the secondary circuit with a lower voltage so that the resistance and the power rating will be much less.
The primary wye point may be grounded through a distribution transformer as is done with generators to reduce the power rating of the resistor but with three transformers already in use the same result is obtained by placing the resistor in the transformer secondaries, in the delta.
If I were tasked with grounding your system I may proceed as follows.
1: Select a primary voltage for the transformers. The voltage rating must be a minimum of line to line voltage. If it is common in your area to use line to neutral connected transformers then select the next highest common voltage. A transformer rated for line to neutral connection on a system of 19 kV or higher will be suitable.
2: Select a KVA rating. The transformer must be capable of carrying 10 Amps on the primary winding. 10 Amps times the voltage rating of the transformer, or 10 Amps times 19 kV equals a 190 KVA transformer. I would be willing to allow a 150% transformer overload until the fault was cleared so that 125 KVA transformers would be suitable. If the next readily available voltage rating is 25 kV then the transformer size would be 250 KVA or 167 KVA.
Select a suitable distribution transformer that is readily available locally and compare the price of three of them to the price you paid for the last dedicated grounding transformer. You may be pleasantly surprised.
A three phase distribution transformer rated for the next highest voltage class may be suitable. In this case I would consider closing the delta and grounding the wye point through a single phase distribution transformer. That way a phantom delta will work to your advantage.




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