TF Diff Stability Test

[Prot123]

Hi All,
Can someone shed some light on TF diff stability test shorting the HV and LV winding to bypass TF impedance. How is the injected current controlled as I believe there will be theoretically 0 Z.
The reason to go in this path is because of not having enough current with the standard stability test setup.
Thank you.

 

[thermionic1]

I've performed stability tests feeding the LV side with a 3 ph source, but never heard of shorting the windings together. In that my case the HV side is shorted to ground 3 phase.

I don't think what you have described will work, as you want the TX ratio, CT ratio and Diff relay to all operate in unison. If you pass 100A through the HV CT side and then LV CT side you are missing the transformer ratio as well as the possible transformer phase shifts.

I'm planning a primary injection test on a large station with 3 500 MVA auto's soon. In our case, the HV & LV CT's are all in the dead tank breakers and of course the Xfmrs are auto so no phase shift. We will test one breaker and a half bus at a time.

If you are lacking enough current, there are a few things to check. Does the Xfmr have an LTC? - Try tapping it for more current. Are you using a genset as a source? Try forcing the voltage up. In extreme cases, a step up transformer can be placed in series with your injection side.

SEL relays are accurate down below 100mA. Assuming digital relays, you can check the accuracy of your relay with a relay test set prior to testing. We also have digital phase angle meters that are accurate down to ~10mA.