On site oil test for a transformer using sweep frequency response analysis

Oil filled transformers are widely used to step up or step down voltages in power transmission and distribution systems. A periodic maintenance of the transformers includes checking oil level, chemical analysis and dielectric strength (break down voltage) test taking oil sample and sending it to laboratory. Sometimes it becomes impossible to check the oil level without opening the transformer. In this paper the method of sweep frequency response analysis (SFRA) has been proposed for on-site and instant test of oil condition without requiring the transformer to be opened and sending oil sample to a remote laboratory. Notably SFRA has long been used to diagnose transformer winding faults such as mechanically displaced and shorted or open turns. So this paper proposes application of SFRA for the first time ever for spot monitoring of certain conditions such as oil level and contamination in emergency situations. The experimental results obtained from SFRA of a healthy transformer and two transformers having oil related problems were compared. In this way a guideline regarding the frequency range where a deviation from the response of the healthy transformer may suggest oil related problem, has been formulated. The conclusion has also been supported through a PSpice simulation.