Transformer Stability tests

[mhamini]

transformer stability test

We have been doing Transformer Stability tests at our project.
The System details are:
Transformer - 34.5kV/.480kV,Delta-Star,3Ph,60Hz
The Transformer Secondary Neutral point is connected to Earth through a High Resistance Ground Unit.
A typical Transformer Differential protection scheme is applied.

For testing the diffrential scheme-we apply a reduced voltage of 480V on the Transformer Primary.
The Secondary bus is shorted. A current equivalent to the rated primary current is forced to flow through the transformer primary winding.

At this condition we measure the Secondary current.
However we noted that the measured Secondary current is much less than the rated Secondary current.
Does this mean that the % Transformer Impedance is not correct
Z=6.75% in this case.
Or is it that the % Transformer impedance is applicable to its rated primary Voltage and will vary based on the applied Primary voltage?

 

[jonw0224]

stability test of transformer

6.75% Transformer impedance means:

If you short the secondary terminals, you should be able to raise the primary voltage to 6.75% of the nominal voltage (2.33kV L-L) before you would see rated secondary current.

A better way to test it:

Short the primary. Gradually raise secondary voltage until you measure rated current on the primary. Then measure secondary voltage. % Impedance will be the measured secondary voltage over nominal voltage time 100. This way you: measure lower currents and use lower voltages. Good luck.

If you only have a 480V three phase source, then you can connect as you do and calculate % impedance as % nominal voltage divided by % full load current times 100. In your case, you used 1.391% voltage. If your %Z = 6.75 then you should have measured 20.6% of the full load current.