Hidenobu Koide

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.

DC field measurement in Oil/Pressboard composite insulation system by electro-optic Kerr effect

An insulating oil/pressboard (PB) composite insulation is generally utilized in oil-immersed power transformers. In such a transformer, charges accumulate at oil/PB interface which changes the electric field distribution depending on time and oil condition. In this paper, we report on directly measured dc electric field in oil/PB composite insulation system using the Kerr electro-optic field measurement technique. From measurement results, we propose a physical model for the charge behavior. We have calculated an electric field distribution using finite element method (FEM) based on the charge movement. We compared the results of measurements and calculations and confirmed that the proposed charge behavior model explains well the experimental results in the case of the polarity reversals as well. As a result, a significant validity of the charge behavior model was confirmed.

Fundamental property of electric field in rapeseed ester oil based on Kerr electro-optic measurement

In the future, environmentally friendly insulating oil such as rapeseed ester oil is expected to be utilized as a substitute for mineral oil for power transformers. In addition, electrical insulation design for transformers becomes more important regarding size reduction and enhancement of electric field stress. The fundamental property of the electric field distribution in the rapeseed ester oil and in the oil/solid dielectrics composite insulation system should be clarified. In this paper, we have directly measured the electric field in the rapeseed ester oil with parallel-plane electrode and the rapeseed ester oil/pressboard (PB) composite insulation systems under DC voltage application using the Kerr electro-optic technique. From the results, we compared measurements of electric field and charge behavior for the rapeseed ester oil and the mineral oil and confirmed the applicability of the rapeseed ester oil for use in transformers

Charge behavior in flowing oil in oil/ pressboard insulation system by electro-optic field measurement

The electrical insulation structure of most power transformers is a combination of insulating oil and solid materials. One of the crucial problems we face in transformer operation is flow electrification, which occurs at a flowing oil/solid material interface. Thus, for the transformer insulation design, we need to clarify the now electrification phenomena. In this paper, we measured the electric field strength in an oil/pressboard composite insulation system using an electro-optic method of the Kerr effect. We obtained the time variation of electric field distribution in both flowing uncharged and charged oil. It is notable that we could quantitatively clarify the electric field distortion in flowing charged oil by flow electrification. Furthermore, we measured the leakage current from divided electrodes and derived a charge density distribution along the flow direction. Finally, we quantitatively discussed the electric field distribution and the charge behavior from the measurement results.

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.

Kerr electro-optic field measurement in palm oil fatty acid ester transformer insulation system

A palm oil fatty acid ester (PFAE) has good insulation performance, excellent cooling ability and superior biodegradability. Recently, investigations for application of PFAE to power transformers have been started. In this paper, the electric field in PFAE was measured in situ by using Kerr electro-optic method under dc voltages application. At first, Ken-constant of PFAE was identified. Then, electric field strength E and its temporal change were measured in several PFAE / pressboard (PB) insulation systems. Electric field E in PFAE showed the capacitive distribution at the moment of dc voltage application, and decreased rapidly with time and finally the resistive field distribution was obtained. The transition time of the capacitive distribution to the resistive one and the value of E in PFAE were different from those in mineral oil. Their differences were discussed with the charge behavior in liquids, based on electro-chemical properties of the oil.

Charge behavior in flowing charged oil in transformer oil duct

The electric field in flowing charged oil in an oil/pressboard (PB) composite insulation system was directly measured by an electro-optic Kerr effect. We discussed the charging mechanism and proposed the charge behavior model based on the measurement results. We compared the results of measurements and calculations and confirmed the validity of charge behavior model.

Charge behavior in flowing charged oil/pressboard composite insulation system based on electric field measurement

Electrostatic charging may occur in oil immersed power transformers because of a flow electrification phenomenon. In this paper, we directly measured the time variation of electric field strength in the flowing charged oil using an electro-optic Kerr effect to clarify the charge behavior in the flowing oil/pressboard (PB) composite system. We discussed the measurement results from the viewpoint of charge behavior. Finally we found that the charging process is dependent on the three different electric fields, i.e. applied field, space charge field and counter field by accumulated charges, and is finalized by the balance of these fields.

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.