Problems Faced in VT Ratio Test

You need to post this again using different identifiers for both primary and secondary terminals. Use H1, H2 and H3 for the primary and X1, X2 and X3 for the secondary.

Hi There.

Thanks for the tip.

I hereby edit my original question.

Hi Guys

We are in the midst of testing our 22,000V/110V VT and we are doing the VT Ratio Test.

When we injected 487V across H2-H3, the readings we obtained are as follows :

X1- Earth = 0V

X1 - X2 = 2.17V

X2- Earth = 2.46V

X2 to X3 = 2.46V

X3- Earth = 0V

X3 - X1 = 0.30V

Rightfully, the voltage readings that we obtain for X1-X2 should be 2.46V but we are getting 2.17V instead.

Also, Voltage readings obtained for X3-X1 should be 0V but we are getting 0.30V instead.

We have absolutely no idea whats wrong.

Appreciate if any kind soul can explain this phenomenom as we are in the mist of commissoning and is going to energize the 22kV panel soon..

__________________

What are the primary and secondary windings? Is it a delta priamry with a wye secondary?

There seems to be a difference between what you are actually testing and what you believe you are testing. Assuming that all of your tests are VRMS measurements:

X1-Earth=0V → X1=Earth

X1-X2= Earth-X2

Your measurements disagree with this conclusion therefore the measurements are somehow testing slightly different circuits. This probably is from either load changes or poor connections but a myriad of other possibilities exist.

There are such things as corner-grounded transformers. Sometimes intentional, sometimes not, and the difference can be crucial.

I get the strong impression you do not know anything about three phase power and single unit three phase transformers so unless you are testing it with a three phase feed ignore what ever readings you get on the other two of the three phases you are not supplying power to.

That is if you are only using a single phase power source test each phase independently and ignore what ever voltage readings you find on the other secondary taps until you are testing their specific primary and secondary phases.

The whole thread as a bit of an odour to it... when was the last time any of you saw a transformer with a 22 KV primary and a 110V secondary. I never have.

They are not allowed in Canada. I think you are restricted to using resistor divider networks for your high voltage monitoring because your technicians can't handle complex things like transformers.

A VT transformer as in Voltage Transformer which like a CT or Current Transformer is only used for monitoring purposes not load handling purposes which in the case of what the OP has is used to bring a 22KV line voltage down to 110 volts for voltage and power monitoring purposes similar to how a CT unit takes a high current and converts it to a low current for monitoring purposes.

Ya... ok. I guess that was meant to be funny.

And yes... they are alowed for HV monitoring. The term "VT" means a voltage transformer, does not automatically signfy protection and metering purposes.

Unless the OP comes back with the type of transformer we're wasting our time. Many VT's for MV are corner grounded open delta to keep the vector angles the same.

At first, release all connection on a Terminal block of VT.

Then try to inject voltage one by one (Phase - Phase, Phase - Earth).

I guess that there is some miss-connections on it.

It is quite common mistake at site.

Hope it help you.