Partial discharge characteristics of mineral oil compared with natural ester

In this paper, partial discharge inception voltage (PDIV) and partial discharge (PD) characteristics of mineral oil compared with natural ester were studies. Three typical PD types, corona discharge of the liquid insulations, surface discharge and internal discharge of the liquid impregnated pressboards were investigated. In case of corona discharge, both corona discharges from high voltage side and from low voltage side were simulated by using the needle-plane electrode configuration. The needle with the tip radius of 10μm, 20μm, 40μm respectively was used as high voltage electrode while the brass plane electrode with 45 mm diameter was used as the grounded electrode for simulation the corona discharge at high voltage side, and vice versa, for simulation corona discharge at low voltage side. The gap distance was fixed for 30 mm. Moreover, the mineral oil and natural ester were used to impregnate the pressboards which were 1.6 mm thick. Then, the impregnated pressboards were selected to experiment for surface discharge and internal discharge respectively. The surface discharge experiment was performed with the 40μm tip radius needle as high voltage electrode put on the ten pieces of 12 hour impregnated pressboards stuck together as a single insulation and the plane electrode with 45 mm diameter as grounded electrode. In case of the internal discharge, ten pieces of 6 hour impregnated pressboards were stuck together as a single insulation. A cylindrical void with 5 mm diameter and a height of 3.2 mm was simulated at the middle of the insulation. Then, the insulation was placed between the mentioned plane electrodes for the internal discharge experiment. PD test circuit was prepared following to IEC 60270. PDIV was examined by applying the step voltage to the high voltage electrode with the increase rate of 1 kV per step for the step duration of 1 minute until observing the PDIV. Then, PD activity was examined and recorded for 1 minute at 110% PDIV level. Form the test results, the natural ester and pressboards impregnated with natural ester provide higher PDIV than mineral oil and the mineral oil impregnated pressboards. Consequently, the PD amplitude at 1.1 PDIV of natural ester and natural ester impregnated pressboards are higher than that of mineral oil and mineral oil impregnated pressboards.