Our Generator Transformer#4 (250 MVA, 15.77/420 KV, YNd1, SL no 600490, Year of MFG-1994, Elect Spec No- 600717, OFF load Tap changer with 5 Taps, Make BHEL, BHOPAL) which we have to de-synchronize from the grid due to appearance of Buchholz alarm at around 10.55 hrs on 15.9.2014. All the electrical test result found matching with previous result except core to tank IR value is found to be zero by 500 V meggar. Even by multimeter the said reading is found to be 0.3 ohm. The yoke to tank isolation value by 500 V meggar is 0.05 M ohm and by multimeter it is 597Kohm. DGA result of oil shows presence of Acetylene to the tune of 9ppm. Even the gas in buchholz also shows presence of acetylene to the tune of 24ppm.
Mr. S. R. Kariyaree, Sr. Service Engineer, BHEL BHOPAL, reached KTPS on 18.9.2014, make a thorough inspection & then after getting clearance from Mr. C. P. Deshmukh, Sr. Specialist, TCB department BHEL Bhopal, drying out & oil filling, oil settling done maintaining schedule. It is to be mentioned here the oil BDV was 83 kv & moisture was 10.47 ppm at the end of the dry out cycle. Finally electrical testing done again on 29.9.2014 & results was again found o.k. Though yoke to tank IR value improved the core-tank remain in shorted condition as stated earlier.
Then as per clearance of Mr. M. K. Shoree, Sr. Manager, TXX, BHEL BHOPAL, charging of transformer in the form of step voltage build up in no load was adopted from 20.00 hrs on 29.9.14. Initially voltage was started from 2kv till 12kv no abnormality noticed. But when the GT Voltage reached 15.75kv (rated) immediately formation of gas in buchholz noticed & within few minutes the GT tripped through buchholz relay. Mr. C. P. Deshmukh was immediately contacted. He opined for testing of buchholz gas & DGA before making any final decision on that basis.
After receiving all informations M/S BHEL opined further charging of transformer not advisable & suggested for repair of the winding with insulation.