In many physical insulation systems it is often necessary to establish the approximate estimate of the breakdown voltage for nonstandard impulse voltages. The disruptive effect (DE) method, which is an analytical method, has some success in such estimations. However, it does not cater to waveforms that are of the bidirectionally oscillating impulse type. Such voltages occur frequently within transformer windings. A new method called "unconditionally sequential approach" is proposed in order to serve as a measure of insulation strength under bidirectionally oscillating voltages. The method is validated by experiments on test specimens. The utility of the method is shown by an assessment of tap stresses in a 50/25 MVA power transformer.Index Terms-Disruptive effect, internal transient response, nonstandard lightning waves, transformer.

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