How transformer DC winding resistance testing can cause generator relays to operate

DC winding resistance testing of a power transformer increases the residual flux of the transformer. If it is not demagnetized following the testing, the transformer retains a high level of residual flux. Therefore, when the transformer is returned to service, the voltage required to drive the transformer into saturation is decreased. When a generator step-up transformer is returned to service following this testing, significant magnetizing current is drawn as the field voltage is increased. This large magnetizing current has the potential to cause sensitive generator protection elements to incorrectly operate. In this paper, a real-world event and simulations are used to study the impact of dc winding resistance testing on the magnetizing current of a transformer. Solutions are provided to mitigate possible unintended operations.