Ramp Rates Effect in Ramp Method for Partial Discharge Inception Voltage Measurement in Mineral Oil

IEC 61294 regulates about Partial Discharge Inception Voltage (PDIV) measurement procedure in liquid insulation using the ramp method with speed 1 kV/s. This PDIV test method is assumed to give the PDIV test results higher than it should be. This paper aims to study the feasibility of the using ramp rates lower than 1 kV/s on PDIV measurement in mineral oil. Experiments were performed using test circuit complied with IEC 60270 standard. The needle–plane electrode configuration with a gap distance of 50 mm was used. Tests were carried out with the mineral oil, Nynas 4000X, with moisture content less than 10 ppm. Test voltage with ramp rate 0.25 kV/s, 0.5 kV/s, 1 kV/s and 2 kV/s were applied to the test vessel to examine PDIV value respectively. In addition to PDIV level measurement, Partial Discharge (PD) activities were also measured for 1 minute at each PDIV level. Moreover, the combine test method which is combination of ramp method and step method was applied to the test electrode system. The test results show that the usage of lower ramp rate in ramp method for PDIV level measurement produced lower PDIV level. The PDIV levels and PD activities obtained in the test using the ramp rate of 0.25 kV/s and 0.5 kV/s decreased insignificantly compared to the test results acquired from the test with ramp rate 1 kV/s. Test result comparison illustrates that the combine method is the better alternative method compared with reducing ramp rate for PDIV measurement in mineral oil.