Tomoyuki Hikosaka

Analyses of electro-chemical characteristics of Palm Fatty Acid Esters as insulating oil

We have developed a new vegetable based insulating oil for transformers called PFAE (palm fatty acid ester). PFAE has 0.6 times less viscosity and a 1.3 times higher dielectric constant compared to mineral oil. This means that a PFAE-immersed transformer has better cooling efficiency and better insulation characteristics in paper-and-oil composite insulation systems, resulting in size reduction in comparison to conventional transformers immersed in mineral oil. Additionally, the flash point of PFAE is higher than that of mineral oil. PFAE also shows higher biodegradability when compared to mineral oil. PFAE would be much safer for the environment than mineral oil in the event of soil or water contamination. Moreover, electrical insulating oil in transformers used over several decades requires excellent durability. The oxidative stability of PFAE has also been determined to be superior to mineral oil.

Space charge behavior in palm oil fatty acid ester (PFAE) by electro-optic field measurement

For power transformer insulating oil, we focused on palm oil fatty acid ester (PFAE). It has satisfactory insulating performance, excellent cooling ability and superior biodegradability. Recently, investigations for application of PFAE to environment-friendly power transformers installations have been started. In this paper, the space charge behavior in PFAE was investigated from the measurement results of the electric field by using Kerr electro-optic method under dc voltages applications. For the field measurement, the Kerr constant of PFAE was identified first. Then, the electric field strength and its temporal change were measured in several PFAE / pressboard (PB) insulation systems. The electric field in PFAE was determined by the capacitive distribution at the moment of the voltage application, and decreased rapidly with time and reached the steady state determined by the resistive distribution. The transition time of the capacitive distribution to the resistive one and the strength of electric field in PFAE were much different from those in mineral oil. The influence of oil flow on the time transition of the electric field was also measured. Their differences were discussed with the charge behavior in liquids, based on electro-chemical properties of the oil. We discussed the difference of the flow electrification characteristics between PFAE and mineral oil, and its mechanism was discussed based on the charge behavior in oils.

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.

Gas generation of cellulose insulation in palm fatty acid ester and mineral oil for life prediction marker in nitrogen-sealed transformers

The authors would like to make 2 corrections to the above paper (1) The weight of the silicon steel should have been 2.1 g (page 421, line 15). (2) The data in Table 2 (page 423, on the left bottom) should have been as in the following Table.

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.

Charge behavior in Palm Fatty Acid Ester oil (PFAE) / pressboard insulation system under flow condition in power transformers

From viewpoints of low environment impact, palm fatty acid ester oil (PFAE) is potent insulating liquid for power transformers because of its high biodegradation performance. Moreover, PFAE has high permittivity, low viscosity, high thermal and chemical stability and high productivity. These characteristics well satisfy the requirement of electric insulation performance in power transformers. In this paper, electric field distribution in a PFAE / pressboard insulation system was measured with a Kerr electro-optic technique at various oil flow conditions under dc voltages and the charge behavior was investigated systematically. We discussed the difference of the flow electrification characteristics between PFAE and mineral oil, and its mechanism was discussed based on the charge behavior in oils.