119. Case Study of the Differential Relay Behaviour During Transformer Charging at NTPC Kahalgaon

The paper deals with the concerns in the area of transformer differential protection with respect to its stability during magnetic inrush conditions. Frequent tripping of the transformer because of the inrush while energizing the transformer is objectionable because it causes panic and delays in putting the transformer back in to service as no utility can afford to recharge the transformer without proper inspection/tests for trouble shooting and arrive at an Engineer's conclusion about the healthiness of the transformer. The experiences of mal operation of differential protection during charging a 600 MVA, 3 phase generator transformer at Kahalgaon Super Thermal Power Project of NTPC is shared in this paper. An effort is made to compare various methods used for restraining differential relay operation during magnetic inrush and also the setting provisions available in various relays. Paper emphasis the necessity of a well coordinated approach between the transformer manufacturers and the Relay developers to match the relay design/algorithm with the improvements in the transformer design field.