Transformer Exitation

By excitation we can detect two types of faults.

1. Shorted turns in any limb.

2. Any short in the core. By short in the core, I mean we have insulation sleeve and insulating washers in core clamping bolts so that the clamping or tightening bolt does not short circuit the core.

The batter way to check above is by not simultaneously energising all limbs and measuring magnetising current, but just excite any one core and measure induced voltage in other two. This Test is called CORE BALANCE TEST and is a good diagnostic tool in hands of Maintenance Engineers. I had also used this to find which limb of transformer winding has a short circuit.

That means energise, say phase A alone, connect voltmeters via PTs across other two phases and measure induced voltage.

Under healthy condition of winding and core, you shall have practically same induced voltage if both Phase B and C and in magnitude 50% of applied voltage on phase A. (B will have negligibly higher induced voltage due to shorter magnetic path in case of core type transformer. In case of shell type construction of core both will have same voltage).

If you find that Voltage induced in B phase is less than C phase, the B phase has either inter turn short or the insulation of bolt is damaged on this limb. shorted bolt is also equivalent of singe turn short around core.

You can further verify results by exciting other limbs, one at a time and measuring induced voltage in remaining two.

Otherwise, if you have excited the core with 10kV or 5kV 50HZ AC supply, and want to know whether the current is within limit - Just open the Factory test report in which you will find Open Circuit curve of the transformer and compare the results.