CT on the buried delta of Y-Y transformer

[cesgibis]

Hi,
In continuation to thread thread238-372076, I am working on a project where we are buying a 120 MVA Y-Y 220/33 kV transformer with a buried delta winding. The primary is solidly earthed, while the sencondary is earthed through a resistance to limit the earth fault current to 400A.
I have been advised that we should earth a corner of the delta winding and install a CT on the connection to earth. Can anyone please explain:
- Why should we be doing this ? Is this for protection purpose or monitoring, or etc...
- What will this CT measure and what type of current it will it detect ?
- What should the ratio of the CT be ?

Thanks for your help.

 

[davidbeach]

Connecting the corner to ground limits the voltage stress on the insulation of the tertiary. Monitoring current in that connection will allow detection of a winding ground fault elsewhere in the tertiary winding. You'd have to know what magnitude of current might pass through the CT before you can determine the necessary ratio.

 

[waross]

With a grounded wye primary, the delta winding will have a high circulating current in the event of any zero sequence currents on the secondary. In the event of a two phase line to line fault on the secondary the current in the delta will be limited by the transformer impedance. Any voltage or phase angle imbalance will cause a circulating current in the delta.
The circulating current will act to oppose or correct the condition causing it.
If I were to instal a CT on a buried delta, my first choice of location would be IN the delta to monitor circulating currents.
If I was directed to monitor current to ground, I would prefer a zig-zag or wye-delta grounding scheme rather than corner grounding.
A zig-zag transformer is generally a special design and custom build.
A wye-delta scheme may utilize distribution transformers which should be available "Off the Shelf". This is particularly advantageous in the event of a grounding transformer failure sometime in the future.
If you do corner ground you may wish to consider grounding through a fairly high impedance so that an inadvertent ground on the delta winding doesn't cause a rapid failure of the transformer.
In the event that you detect a ground fault in the delta winding you may be advised to trip out instantly to avoid the fault level
currents that may be a result of rapid winding failure.

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

 

[cesgibis]

- If we install a CT in the delta to monitor the circulating currents, Is it possible to calculate the amount of circulating currents in this delta winding. Would any current circulate in the absence of L-L or L-E fault on the secondary ? what about third harmonic current ? are these permanently circulating in the delta ?
- In case the CT remains on the link from the corner to earth, does this CT see any current in case of earth fault on the secondary of the transformer ? or will currents circulate in the delta only ?

 

[waross]

Think of the delta winding as a combination of a single transformer and an open delta transformer.
If there is any voltage unbalance or phase angle error on the primary (or both), either the voltage or the phase angle of the single transformer will not match the voltage across the open delta.
When the delta is closed the error will cause a circulating current in the delta that will be limited by three times the impedance of the primary:delta winding pair.
This situation is unique to a wye delta transformer or bank with the primary wye point connected to the source neutral. Grounding most often provides a connection to the source neutral.

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