220KV Bushing Test Tap is Not Closed During Charging of Transformer

"what is the technical reason behind it"?

Incompetence!

Couldn't the transformer manufacturer say, either in the manual or over the telephone?

Several things could happen if the test tap cap is not threaded into place on a HV bushing when energized:

1. The test tap layer(s) of the capacitive paper wrappings of the bushing core could see excessive voltage gradients. Since the cover grounds the test tap point, the test layer could have a voltage across it much higher than it normally sees during testing. Test voltage on a bushing in a Doble insulation power factor test, for example, is seldom over 10kV, while in service the bushing sees 220kV/√3. This could damage the capacitor layer due to "pinholing" or electrical discharge.

2. Dirt, moisture, and contamination could enter the test tap, and possibly contaminate the capacitive layers. This could lead to deterioration of their electrical qualities and cause bushing failure. Water and electricity seldom mix well.

So make sure you put the cap back in place after you finish your bushing tests on the transformer!

Thank you very much.......

Hello Muthukumar,

I am quite sure in this case the differential protection relay for 220kV Transformer should trip, seeing one phase harmonics to be missing. Usually a common practice in Siemens, ABB and other reputed manufacturers is to set the harmonics value at 15% with a cross block time of 60 to 100ms.