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.

Space charge behavior in palm oil fatty acid ester (PFAE) 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.

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

From the viewpoints of less flammability and environment protection, a new type of electrical insulating liquid for power transformers is strongly required. Low viscosity silicone liquid (20 cSt) is suitable for the requirements owing to the following excellent features; high flash point, low environmental impact concerning pollution and high chemical stability. In this paper, we investigated the charge behavior under dc voltage application by direct measurement of the electric field using Kerr electro-optic method. At first, Kerr constant of 20 cSt silicone liquid was investigated. Then, the time dependence of electric field strength in silicone liquid / pressboard (PB) composite systems was measured. The time constant of the decay was much larger in silicone liquid than in mineral oil. We discussed the charge behavior based on the physical properties of silicone liquid and we suggested that the influence of negative charge injection from a negative electrode?s surface in silicone liquid played a significant role because of the high volume resistivity.

Investigation of charge behavior in low viscosity silicone liquid by kerr electro-optic field measurement

From viewpoints of low environment impact, some kinds of environment-friendly insulating oils for power transformers have been developed. This paper deals with the discussion of charge behavior in these new oils by an electro-optic field measurement. Several potent insulating oils for power transformers have been proposed because of some excellent features not only on electrical but also chemical and economical points of view. In addition, the charge behavior in the insulating oils should be clarified for the insulation design of power transformers, because the space charge in oils strongly influences the transient and steady state electric field in high voltage apparatus. We compare the electric field characteristics in the insulating oils each other, and discuss the space charge behavior.

Charge behavior and field measurement techniques in different kinds of insulating oil for power transformers

By using Kerr electro-optic method, charge behavior in oil / pressboard (PB) composite insulation system, which is the basic insulation structure of a transformer, was investigated under DC voltage. Right after the DC voltage application, electric field became the capacitive distribution, and had a strong value which was determined by permittivity of materials. The field strength decreased gradually with time as charge in oil was accumulated on PB. When the voltage polarity was reversed, the applied electric field was superimposed by the electric field formed by the accumulated charge on PB, and very strong field appeared right after the polarity reversal. This could cause the discharge in the oil gap even when the independent applied voltage was insufficient to initiate discharge. It was found out that the critical value of the discharge initiation at the polarity reversal was determined by the breakdown field strength of the oil gap.

Study on dielectric breakdown at DC polarity reversal in oil / pressboard-composite insulation system

In this paper, we measured the electric field in transformer oil/pressboard composite system, using a Kerr electro-optic method to discuss the electric field distribution and charge behavior induced by flow electrification. We investigated electric field in both uncharged and charged oil, under different flowing conditions. From the experiments, we could quantitatively estimate and analyze the time transition of electric field by charge accumulation on the pressboard. Finally, we propose a charge behavior model in flowing transformer oil.

Study on charge behavior in flowing transformer oil/pressboard composite insulation system using electro-optic field measurement

Electrostatic charging may occur in oil immersed power transformers because of a flow electrification phenomenon. In order to discuss the charge behavior, we directly measured DC electric field in the oil/pressboard composite insulation system using Kerr electro-optic field measurement method under oil flowing conditions. As a result, even if no voltage was applied, the electric field emerged because of space charge in oil and charge accumulation on pressboard. We changed the flow rate of oil, the arrangement of pressboard barrier, and the applied voltage as parameters. We discussed experimental results from the viewpoint of charge behavior and generated field distribution.

Y. Nakagami

Papers

Study on charge behavior in flowing transformer oil/pressboard composite insulation system using electro-optic field measurement

Investigation of charge behavior in flowing transformer oil/pressboard composite insulation system by electro-optic field measurement