Synch. Generators with ungrouned Wye-Grounded Wye transformers

[sykimk]

Hello,

I have a trouble with my client due to the type transformer winding connections. Please refer to the following information and advise whether ungrounded primary wye-grounded secondary wye connection would not cause any problems.

1. Multiple synchronous Generators are all rated at 11kV and low resistance grounded (100A each)
2. Most of distribution transformers have Delta(primary)-Wye(secondary) windings with NGR (100A each), voltage rating is 11kV-6.9kV or 11kV-0.4/0.233kV
3. The client strongly wants to have ungrounded Wye(11kV)-grounded Wye(6.9kV) transformers (Yyn) with NGR (100A each) at secondary side for some specific locations to match phase sequence at all 400V system
4. All these transformers (Dyn & Yyn) primary side will be connected to 11kV generator bus.

Can anyone help me to make sure Yyn transformer will not cause any problems at 11kV generator bus as well as 6.6kV secondary bus of Yyn transforemer?

I am not sure but I think Yyn transformers would cause a problems with ground faults at both 11kV and 6.6kV although low resistance grouding is applied. I assume that ground fault currents at 6.6kV may not be limited to 100A and may be quite higher than 100A.

Any technical papers supporting this issue would also be apprecaited.

Thanks in advance.

 

[waross]

Your issues may not be ground related but neutral related. Although the terms are often used interchangeably there is a difference.
Running a primary wye transformer without a fourth wire connecting the neutral back to the source is not a good idea.
There are a few exceptions, that present other issues.
Does t work? Sometimes.
A wye primary with a floating neutral has similar (but not identical) issues as a 120:240 circuit with an open neutral.
There are a few effects that will lower but not eliminate the issues.
Some of these may be:
Saturation of the transformer phase with the highest voltage.
Phantom delta effects of some types of cores.
A delta winding will mitigate the issues but with some issues of its own.
This may be:
A delta secondary, which may be the transformer connection.
A delta tertiary winding.
A smaller wye:delta bank to provide a neutral for the main transformer.
A zig-zag transformer to generate a neutral for the wye primary transformer.
If you are not paralleling at the 400 Volt level you don't ned phase matching.
Consider a 1:1 delta/wye isolating transformer to match the angles.




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

 

[jghrist]

The ungrounded wye - grounded wye 11kV-6.9kV transformer connection will not provide a ground source for the 6.9kV system. You might consider a delta tertiary if you need to maintain a zero degree phase shift. The delta tertiary will provide a path for zero-sequence currents and will provide a ground source on the 6.9kV secondary.

See The Whys of the Wyes

http://www.geindustrial.com/publibrary/checkout/38652.30055.7762.44019/generic/GET-3388B.pdf