A methodology to predict the lightning insulation strength for distribution transformers by applications of reduced lightning standard impulse voltages

The lightning withstanding of transformers are based on a set of impulse voltage applications with 1.2/50 μs shape, in accordance with international standards. The standard lightning impulse test is a destructive test, in which the transformer insulation strength is predicted by comparisons between reduced and rated levels for full and chopped impulses. In this paper a different method named "progressive impulse test" is proposed to provide estimation with respect to the voltage level in which evidences of insulation failure appears. Modifications on oscillation frequency of current waveforms may indicate sparks or short-circuits between turns, layers or between windings and the ground. By comparisons among these current waveforms it is possible to predict the insulation performance, as well as manufacturing quality process without the necessity to submit the transformer to all lightning impulse sequence test.