Comparative studies on the aging of thermally upgraded paper insulation in palm fatty acid ester, mineral oil, and natural ester

A new biodegradable dielectric fluid for transformers, palm fatty acid ester (PFAE), with better cooling performance and oxidation stability than conventional mineral oil (MO) was developed in 2008. The aging of thermally upgraded paper (TUP) in PFAE was evaluated, comparing to MO and natural ester (FR3). The electrical properties of aged dielectric fluids were compared. Sealed aging SUS tanks containing copper, silicon steel, TUP, and PFAE, MO and FR3 (partly) in a nitrogen environment were aged at 130 °C-170°C for 20-120 days, with low concentrations of oxygen and moisture strictly controlled. Paper degradation after aging was determined using the degree of polymerization measurements. The gas production characteristics during the aging process were also compared. The results show that the TUPs in PFAE age at slower rates than those in MO, while at faster rates than those in FR3. Under the possibly low initial concentration of oxygen and water in the fluids in the absence of other material which may produce water during aging process, the limit service temperature for TUP in MO and PFAE with expected 30-year lifetime (DP450) would possibly reach 109°C and 113°C, respectively. Life estimation results show that the lifetime of the TUP in PFAE aging at 110°C would be at least 1.6 times the lifetime of the TUP in MO, if DP450 was set as the end-of-life point of the TUP. Aged PFAE exhibits highest breakdown voltage among the three types of dielectric fluid. Much lower acid value of aged PFAE than FR3 reveals that PFAE has better hydrolytic stability, which could ensure the safe operation of transformers. The aging mechanism is proposed.