Influence of the Lightning Impulse Shape on the Electrical Stresses on Windings Insulation of Power Transformers and Shunt Reactors

摘要

Designing of power transformers and shunt reactors insulation is carried out so as to withstand the impact of lightning overvoltages in operation. After the developing of the transformer equipment design and its manufacturing the electric tests are carried out by application of the full and chopped lightning impulses which are applied to all terminals of the transformer. There are two fundamental problems within this approach. The first one is related to the difficulty of providing standard lightning impulse parameters in tests. For this reason the standards define the limits for possible deviations of these parameters, but the insulation calculation at the design stage is made based on the mean values. Since the deviation bands are wide enough, it leads either to insufficient or redundant testing. The report presents the results of calculations showing the influence of this difference in parameters values of full wave and chopped wave lightning impulses on the electrical stresses level. The second problem is related to the possible significant test voltage shape deviation. In the test conditions, the unit under test (transformer or reactor) affects the applied voltage shape and parameters of the impulse differ from standard values. In the last edition of IEC 60060-1 there is a k-factor approach proposed to compensate for the overshoot at the front of the full lightning impulse. The approach is based on empirical data on the electric strength of the insulation models affected by standard full lightning impulse and by impulses with superimposed oscillations on the front. However, for power transformers and shunt reactors the shape of the input voltage substantially determines the transients inside windings and particularly the voltages on the elements of longitudinal isolation. Despite all the advantages of this approach, it does not fully recognize the specifics of voltage oscillations inside windings of transformers and reactors, and therefore it is not entirely applicable to the transformer equipment. In the report the results of longitudinal insulation safety margins estimations are presented for the cases of full lightning impulse application with different overshoots at the front.