Lifetime estimation of cellulosic insulation material in palm fatty acid ester for power transformers

Palm fatty acid ester (PFAE) is evaluated as a new biodegradable dielectric fluid for power transformers. Sealed SUS tank aging tests of high density pressboard (HPB) and thermally upgraded paper (TUP) in PFAE and mineral oil (MO) were performed at three different temperatures in an oxygen free environment. The transformer components are consistent with those of an actual power transformer (27MVA). Lifetime estimates were established for HPB and TUP, based on the results of degree of polymerization (DP) and tensile strength (TS) tests. Electrical properties of aged fluids were also measured. Lifetime estimates indicated that, the longevity of PFAE impregnated TUP aged at 110 °C would be 6.1 times, that for MO impregnated TUP, as DP450 was set as the end-of-life criteria. If 50% retained TS is set as the end-of-life criteria, estimates show the lifetime of PFAE impregnated HPB at 110°C would be 7.3 times that for MO impregnated HPB. Higher water content in the MO impregnated TUP or HPB resulted in lower retained tensile strength than PFAE impregnated TUP or HPB with the same average DP. Water and acid released from other cellulosic components during the aging process significantly accelerates the degradation rate of MO impregnated TUP or HPB, expanding the lifetime differences between the PFAE and MO impregnated TUP by several times, resulting in a much lower breakdown voltage of aged MO than that of aged PFAE.