Study On Insulation Characteristics Of Transformer Oil At Low Temperatures

UHV transmission has ultra long-distance, large capacity, low loss power transmission capacity, which can improve the efficiency of development and utilization of resources, relieve environmental pressures, and save valuable land resources, with significant economic and social benefits, in line with our national conditions and national energy development strategy. As a vast country, average annual extreme minimum temperature is generally about -40℃in Northeast and Northwest China. Low temperature environment affects the electrical properties of transmission and distribution equipment to a certain extent. Power transformer is one of the major electrical equipment of the power system. And transformers almost all use transformer oil as the dielectric and cooling medium. At present, researches are relatively less on insulation characteristics of transformer oil at low temperatures. Although some scholars have studied related contents many years ago, with technological development, the performance index of oil is also changing, and in recent years as the development of UHV technology, insulation structure changes have also raised new requirements for transformer oil, so further study on insulation characteristics of transformer oil at low temperatures has important engineering significance.Based on reading a lot of literature of this expertise, the various factors and their relationships affecting transformer oil insulation characteristics were systemically described. Electrical performance of transformer oil has many influence factors, in which primary factors having a greater impact on dielectric strength of transformer oil include water content in the transformer oil, gas content, impurity content, temperature, flow velocity and so on. This paper focus on the influence of low-temperature on the breakdown voltage for different ranges of water content, gas content, impurity content of the transformer oils.A test platform of study on transformer oil insulation characteristics was built and reasonable test methods and test procedures were developed. Karamay DB—45 transformer oils were selected as the specimens. Oil samples of different quality indexes were obtained after processing by the vacuum oil purifier, that is, different water content, and particulate pollutant and so on.. Each kind of oil sample was carried out experiments on insulating properties at low temperatures (-20℃～10℃). The test items include breakdown voltage, relative permittivity and dielectric dissipation factor. A large number of first-hand experimental data was obtained. By the statistical analysis of test results, the temperature dependence of insulation characteristics of transformer oil at low temperatures was investigated.①The relationship between the breakdown voltage of transformer oil and temperature is complicated. The curves of breakdown voltage vs. temperature show "U"-type features. That is with decreasing temperature the breakdown voltage decreases first and then increases. The lower the water content and particulate pollutant, the more unobvious the features.②Water content is the main influence factor of breakdown voltage of transformer oils.③In general, the values of the transformer oil breakdown voltage measured under test conditions at room temperature do not represent their breakdown voltage level at low temperatures.④If the oil samples meet the requirements of the current EHV/UHV transformer oil standards, the impact of temperature on the breakdown voltage is very small at low temperatures.⑤The permittivity and the dielectric loss factor of transformer oils little change in the range of -20℃～10℃.The development process of the instantaneous breakdown of transformer oil was recorded by using the high-speed camera. According to works liquid dielectric "bridge" breakdown theory, combined with the breakdown process captured, using the finite element method, by analyzing the water, impurities and other kinds of factors on breakdown voltage at low temperatures, the breakdown mechanism of transformer oil at low temperatures was further discussed.