Transformer stability test, ct in neutral

[struja]

Hello....
I have some doubt and would appriciate if somebody could help me. If we would like to do transformer stability test on ynd11 transformer with one CT conected in neutral of the transformer (wye HV side), and inject 400v on LV side of transformer and make 3ph short circuit (to ground) on HV side, is it neccesary to disconect neutral of the transformer during stability test. And why?

Thanks all...

 

[krisys]

The test should simulate the actual condition, to the extent possible. Hence do not disconnect the neutral.

 

[struja]

Thanks for your replay.
Maybe I did not explane what exactly is my concern.
If CT in the neutral is simple EF Protection, (not some REF Protection which is comparing current from the neutral of the power transformer, and the lines). And if we make 3ph to ground short ciruit out of zone for TR diff Protection, but still CT in neutral will see this fault current and it could trip before we check stable conditions. Is my thinging ok, or I am wrong?
Please see attached picture.
Thank you all for help.
BR.

 

[krisys]

If you are simulating an earth fault, certainly the Earth fault relay connected to transformer neutral will sense the fault and operate.

Yes, you should short the transformer neutral CT.

 

[struja]

Thank you krisys, this helps me a lot.....

 

[jose0007]

I couldn't help notice that you mentioned
"inject 400v on LV side of transformer and make 3ph short circuit (to ground) on HV side,"

As far as I know, during short circuit test of transformer, LV side is shorted and Voltage is injected on HV side.

 

[DTR2011]

@Jose0007 -This is a stability test for transformer protections.

Additionally, if REF is to be tested, a low resistance (NGR) can be placed in series with one HV leg - earth, to create an imbalance, which I believe would truly prove REF. Have the other two legs directly connected to earth for REF tripping. This will cause some neutral current to flow, which can be measured against phase CT's.

 

[jose0007]

By stability test, I assume it will be differential stability. And as far as my testing experience goes, we short lv side only.

 

[slavag]

Not needed disconnect nothing.
3-ph SC to ground is zero in neutral.

Both direction is possible: feed 400V on the HV and short LV and vise versa.

 

[jose0007]

Just to reiterate my point:

http://www.eng-tips.com/viewthread.cfm?qid=135797

 

[panter]

Struja,

I support the opinion of Slava and there is no reason to do anything like disconnection or good reason for REF tripping during the test .
If you make short circuit before CTS on 132kV side you will have differential current in differential relay and it should be good test of CTS polarity .

Both direction is possible: feed 400V on the HV and short LV and vise versa but on HV feed required current will be smaller and easy to made .
Good luck

 

[struja]

Thank you all for your comments.....

I agree that 3ph and 3ph to ground are basicly same thing.....
I am just wondering, let say for the attached picture, (is's easier to explane with picture ) next few things:
1. Will there be any current flowing through CT3 (maybe some percentage), or this is totally impossible?
2. Is it possible that neutral of transformer could be some return path for 3ph to gnd short circuit
currents and that all current in that case could flow back through the transformer (''----'' line in the picture), like for
1ph-gnd fault?
3. Relay connected to CT3 i just basic earthfault Protection (no restricted, no directional...just simple EF).
thx

 

[DTR2011]

This is a pretty standard primary injection set up. As long as the voltages are balanced and the earthing conductors are equal size and length (impedance), you will expect negligible earth current.

If you want to prove the In CT, insert an impedance in one earthing leg (3 phase), or simply connect only one phase to earth.

As it it quite standard to make various measurements with a phase angle meter, you can check for yourself. Given that Isc HV is ~17A, I would not worry too much about EF tripping at that level. Again, if it were me, I would have a single phase fault connection, in order to prove that all protection devices operate as designed. It is generally easy enough to disable the main trips and monitor the relays for operation, while also making necessary amplitude and phase angle measurements. If you have performed all of the pre checks on the secondary circuits, relays and instrument transformers, primary injection is simply a way of declaring that the system is 100% correct. If there are any HV VT's remove the short circuit and primary test the VT circuits.