Abstract

Partial discharge (PD) detection under impulse voltage excitation is becoming increasingly important due to its high sensitivity in examining the insulation integrity. Assessing the potential weaknesses at the early stages helps in the maintenance of the transformer. In this context, Holo-Hilbert spectral analysis (HHSA) is proposed for a detailed assessment of PD signatures obtained during a lightning impulse (LI) test to locate the region of occurrence of PD in the transformer winding. The experiments are carried out on a 315-kVA, 11-kV/433-V transformer at different voltage levels, up to 50% of the basic impulse insulation level under laboratory conditions. Two PD setups: corona PD in air and corona PD in oil, are developed and used between different tappings of the transformer winding to inject PD. Laboratory experiments are carried out for 1) different voltage levels and 2) different gap distances between the electrodes and HHSA spectrum have been reported for the first time. The results obtained are compared with the traditional time-frequency techniques. The results reveal that the proposed HHSA can be used for identifying the region of occurrence of PD in the vicinity of transformer winding.