Thermal failure of transformers

The thermal failure in high voltage equipment is little heard and is generally known to have the slow & deteriorating effect on the insulation system, which ultimately reduce the life of the equipment. In a unique classical case, 9 units of 100 MVA 230/110/11 kV oil filled auto transformers supplied to one of the electrical utility in India failed in quick succession due to oil starvation in a particular part of the winding leading to thermal failure of complete lot of 9 nos. transformers at sites. In spite of oil starvation, the transformer withstood the voltage and passed all the dielectric tests successfully, which concludes that the dielectric failure had not occurred. The failure due to thermal failure was further confirmed by the different degrees of blackening of cellulose insulation depending upon the time, it has put into service. In this paper, findings of the studies made and corrective measures taken to avert such failures have been discussed & detailed.