Investigation on the voltage stresses developed on transformer insulation under non-standard terminal excitations

Grid connected EHV transformers experience different types of high voltage surges when in service. Transients generated due to switching operations, lightning strokes, power system faults etc have significant impact on the transformer windings and their insulation. To investigate the effect of developed voltage stresses on the winding insulation of the transformers under various types of terminal disturbances, a number of standard and non-standard wave shapes like lightning impulse, chopped lightning impulse, steep-front long tail switching surge and oscillatory transient over voltage have been simulated. A comparison of the stress conditions in the main and tap changer windings and their insulation structures have been made when the transformers are subject these terminal disturbances. The results of the present study should provide comprehensive idea about how transformer insulations are subject to voltage stresses under different kinds of terminal excitation and this knowledge should be helpful in optimizing the insulation design of high voltage transformers.