Research on the temperature dielectric spectrum of vegetable oil, mineral oil and their relevant oil-impregnated papers

Features of high flash point, high degradation rate and the reproducibility make vegetable oil possible to be a new kind of insulation oil, which is expected to replace mineral oil. Temperature has a great influence on the dielectric characteristics of insulation oil and the oil-impregnated papers, so with the increasing use of vegetable oil in transformers, it is necessary to study the temperature dielectric spectrum (TDS) characteristics of the vegetable oil-paper insulation to judge whether TDS can be used to evaluate the state of the vegetable oil-paper insulation system. In this paper, TDS of vegetable oil, mineral oil and their relevant oil-impregnated papers at 50Hz were measured and analyzed by using the Concept80 broadband dielectric spectrometer. It can be concluded that: relative permittivity (ε) of vegetable oil and mineral oil at 50Hz decreases when the temperature increases. However, ε of their relevant oil-impregnated papers at 50Hz increases firstly and then decreases with the increase of temperature. Besides, ε of vegetable oil and vegetable oil-impregnated papers is higher than that of mineral oil and mineral oil-impregnated papers respectively, which is helpful to miniaturize the transformer structure. Dissipation factor (tanδ) of both kinds of oil and their relevant oil-impregnated papers at 50Hz increases with the increase of temperature. The rising rate of tanδ of the vegetable oil is obviously greater than that of the mineral oil while tanδ of mineral oil-impregnated papers is slightly greater than that of vegetable oil-impregnated papers, which proves that vegetable oil can slow down the aging of transformer insulation. TDS of both ε and tanδ at 50Hz can be expressed by the cubic function. This provides a new method for using TDS to evaluate the insulation status of the transformer oil-paper insulation system.