On observation of deterioration of tan-δ value to 1.08 %, i.e., surpassing the limiting value of 0.7 % as defined in IEC60137 for the 420 kV OIP bushing for a 315 MVA, 400 / 220 / 33 kV
transformer located at substation Katni (India), a decision was made to replace the bushing. The replacement was risky, and it was a threat of causing the catastrophic failure for the transformer itself and
the colossal loss to the neighbouring equipment and structures
