Study of the voltage stresses developed in grid connected EHV transformers under switching conditions in power grids

For many years high voltage lightning and switching impulses simulated in the laboratory have been used to ascertain breakdown strength of insulation materials used in the transformer. On the contrary, a transformer in actual field operation encounters voltage transients of rather complex and varying wave shapes. Transformers are complex assembly of coils around magnetic core having inductances and capacitances and thus, exhibit unique frequency response characteristics with a number of natural resonate frequencies. Electrical transients generated in power systems due to switching and other operations often traverse through transformers and can excite one or more of the transformer´s natural resonate frequencies resulting severe internal voltage amplification. It is important for the transformer designer to anticipate the problem and to know how the windings will respond to such system transients that can trigger internal resonances. In the present paper the authors have explored the occurrences of oscillatory switching transients in the Eastern Regional Grid of India and analyzed the behavioral characteristics of a 400/220 kV grid connected transformer under such transients. The results have indicated that the voltage stresses developed in the windings under certain switching conditions can exceed the stresses developed under 1.2/50 mus standard lightning impulses at transformer´s BIL voltage