DIRECT CURRENT BREAKDOWN CHARACTERISTICS OF TWO-ELEMENT MIXED OIL FOR POWER TRANSFORMER RETROFILLING

Abstract

Environmental concerns as well as utility advantages of the natural ester in retarding paper insulation ageing makes transformer insulation liquid development from a mixture of natural ester and mineral oil preferable over those of synthetic ester and mineral oil.  In the present study, mixed insulation liquid was developed from trans-esterified palm olein and mineral oil (MO). The DC breakdown characteristic was measured using a Glow 28720 DC-High Voltage (HV) generator. The results of the measurement were analysed using the Weibull probability distribution as a statistical tool. The natural ester shows a slightly higher Breakdown Voltage (BDV) compared to the MO with low uncertainty. Blend samples with high natural ester percentage composition with respect to the MO, show greater breakdown voltage values. Two blends with the first composed of 50% palm olein fatty acid methyl ester (POFAME) and 50% MO and the other having  75% POFAME and 25% MO both notably exhibited improvement in BDVs up to 6.3% and 5.7% respectively with respect to the MO. The improved breakdown strength is indicative of the potential implication of safe retrofilling of a transformer initially with mineral oil to methyl ester of palm olein as a natural ester and vice versa. Although the blend show improved breakdown strength compared with MO, further investigation of its physical, thermal and dielectric properties is required for its applicability in high-voltage oil-filled equipment.