Analysis of Transformer Impulse Test Signals using Transfer and Coherence Functions

Transformers are major and expensive equipment in a power system, which alters the magnitude of system voltage without changing the frequency. The transformer comprises of primary and secondary windings isolated by an insulation system. A transformer’s Insulation system shall provide isolation between transformer components at various voltage levels. In oil-filled transformers, the insulation system comprises both oil and cellulose, whereas in dry-type transformers, the epoxy based insulation system is in use. Insulation is the weakest link in transformers. Hence, considering all operating conditions, a transformer’s insulation system shall be well-designed and manufactured. The lightning impulse test is a type test that evaluates the integrity of the transformer insulation system. This paper discusses three different analysing techniques used to detect failures during impulse testing of transformers. Failure analysis during real-time impulse testing on four distribution transformers of different ratings using the analysing techniques is presented as case studies.