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

Abstract

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.

Lightning caused overvoltages on power transformers recorded by on-line transient overvoltage monitoring system

“…Front and tail time of such overvoltages may be shorter or longer than the standard one defined by IEC standard, and the wave shapes may be very different from the unidirectional double exponentials used in laboratory conditions and may even be bidirectional [7]. Results presented in [8] indicate that transformer winding's response to certain oscillatory voltages can be worse than that under any form of aperiodic voltages, whether lightning impulse, chopped lightning impulse or steep front -long tailed switching surge. It was shown that terminal excitation at frequencies coinciding with any one of the winding's natural frequencies may lead to large voltage amplification inside a transformer and can cause severe stresses on insulation.…”

Electric stresses on transformer winding insulation under standard and non-standard impulse voltages

“…Heller and Veverka [8] also observed higher inter-turn and inter-disk insulation stresses for a chopped wave. Mitra et al [9] did computational studies on winding response of transformer to oscillatory voltages. They reported that the effect of oscillatory voltages is worse than under full lightning impulse, chopped lightning impulse or steep front long tailed switching surge.…”