Electrical properties of palm oil and rice bran oil under AC stress for transformer application

Abstract

This paper presents the investigation on the dielectric properties (dissipation factor, relative permittivity, and resistivity) and AC breakdown voltage behaviour of Palm Oil (PO) and Rice Bran Oil (RBO). The dielectric properties experiment was carried out according to the IEC 60247 standard using an automatic ADTR-2K Plus instrument. The AC breakdown test was carried using two different setups (automatic oil tester and BHT AC Generator) using similar VDE electrodes with 23 mm in diameter and comply with IEC 60156 standard. The data of AC breakdown voltage were statistically analysed to predict the withstand voltage (1%) of oil samples by using Weibull distribution. The results show that RBO has lower DDF than PO in which will give better insulating oil capability than PO. For relative permittivity, PO has the highest value among the oil samples which gives advantages in terms of stress distribution between solid and liquid inside the transformer. The relative permittivity of PO is 2.176 closer to that of solid insulation (4.0) which leads to an evener stress distribution between solid and liquid materials. On the other hand, for resistivity, RBO has the highest value among the oils, and it is correlated well with the results in AC breakdown. In the AC breakdown voltage study, two different equipment which having da differences in the volume of oil, electrodes arrangement and as well stirrer were investigated. In the AC breakdown study, RBO has the highest breakdown voltage for both test setups. PO is the second-best in AC breakdown voltage while MO shows the lowest breakdown voltage among oil samples. The percentage difference between using BAUR oil tester and BHT AC generator setup is between 4% and 10% at a 2.5 mm gap distance which is comparable for both types of equipment. For various gap distances studies, the breakdown voltage is increased with the increase of gap distance between the electrodes, for all oil samples. The Weibull distribution of MO, PO and RBO has well fitted with the fitting line of AC breakdown data. From the Weibull distribution, the empirical withstand voltages equations with gap distances of PO and RBO are derived.