Extended Application for the Impulse-Based Frequency Response Analysis: Preliminary Diagnosis of Partial Discharges in Transformer

Frequency response analysis (FRA) with low impulse voltage signals (IFRA) is useful in diagnosing faults like the mechanical displacements and inter-turn shorts in transformers. However, they fail to diagnose the presence of any source of partial discharges (PD) across the oil-paper insulation. PD is happening only above its inception voltage level for the applied signal, thereby changing the loading effect of the insulation system and becoming detectable. If FRA tests are extended carefully up to high voltages, they can be even useful for the preliminary diagnosis of the potential sources of PD. Investigations were not carried out so far to get an insight into this aspect. Motivated by this, an investigation was done, first time, on a 315 kVA, 11 kV/433 V transformer. IFRA was carried out carefully at various voltage levels, up to 50% of the Basic impulse insulation level. The IFRA results of the transformer under its healthier condition, under the emulated inter-turn shorts and, the PD sources across the major-insulation were, observed and compared. Low-voltage IFRA is found sufficient, only for diagnosing the inter-turn shorts. When IFRA is extended to moderately high voltage, it is found useful in the preliminary diagnosis of the PD sources.