Research On Insulation Property And Breakdown Mechanism Of CF₄、N₂ And Their Mixtures Under Refrigeration Temperature Range

SF₆ which has high global warming potential index and high liquefaction temperature is the most widely used gas insulation material in power system.Because of these remarkable defects,the usage of SF₆ is greatly limited under extremely cold region.And the research on SF₆ substitution is mainly focus on room temperature.In order to make up this gap,several gases was selected in this paper aiming to find suitable substitution for SF₆ under low temperature.Moreover,the influence of temperature on gas discharge mechanism is still uncertain at low temperature.In this paper,refrigeration temperature environment was created and the insulation property of CF₄,N₂ and their mixtures was tested.Finally,the influence of temperature,pressure and electronegativity on gas discharge mechanism under low temperature is revealed.The main contribution are as follows:Tremendous of liquefaction temperature of SF₆ substitute was investigated and mixture gas dew point calculation method was studied.This essay uses cubic equation combined with two-parameter model in theorem of corresponding states to estimate dew-point of CF₄ and C3F8 and their N₂ mixtures.Finally,CF₄ and its N₂ binary gas as the research object.Subsequently,the influence of temperature decreasing on mixing ratio was studied by using Van der Waals equation.The result shows that temperature decreasing can lead mixing ratio monotone decrease.During refrigeration temperature range,the mixing ratio amplitude of variation of CF₄ and its N₂ mixtures is 7.4×10-4,which can be ignored in engineering and theory study.In order to study the insulation property of CF₄/N₂ hybrid gas,a low-temperature-gas-insulation test system was designed and build.And this test platform has realized bidirectional sealing of positive and negative pressure for the first time.Insulation property of CF₄/N₂ hybrid gas was tested under the condition of refrigeration temperature range.The test result shows under condition of high pressure,AC impulse and certain mixing ratio and molecular number,the breakdown voltage of CF₄/N₂ hybrid gas doesn’t change with the temperature adjusting.Moreover,20%CF₄-80%N₂ mixtures has low liquefaction temperature,low global warming index and good insulation property.So it is the greatest potential mixing ratio during refrigeration temperature range.And under condition of low pressure,DC impulse and certain mixing ratio and molecular number,the breakdown voltage of CF₄/N₂ hybrid gas fluctuates with the temperature changing.The vibration situation is related to the electronegativity strength of the test gas.For strong electronegativity gas SF₆,the breakdown voltage increases with the temperature decreasing.For weak electronegativity gas CF₄,the breakdown voltage decreases with the temperature decreasing.For Electricneutrality gas N₂,the breakdown voltage fluctuates with the temperature decreasing.Lennard-Jones model combining reaction rate theory were used to analysis the influence of temperature on gas comprehensive cross section.Numerical calculation shows that,temperature decreasing can lead the comprehensive cross section increasing.Under condition of high pressure,the initial cross section is extremely big,and electron gain pretty low energy from electric field,so the temperature changing has less influence on breakdown voltage.But under condition of low pressure,the initial cross section is small and electron can gain a lot of energy from electric field,so the temperature changing has remarkable influence on breakdown voltage.Aiming to analyze under condition of low temperature,low pressure,cooling process has diverse influence on breakdown voltage of SF₆ and CF₄ COMSOL mutiphysics was used to simulate the rate of reaction particle concentration.And the result shows that the growth of attachment reaction rate is higher than ionization reaction rate for strong elecronegativity gas SF₆.For weak elecronegativity gas CF₄ the growth of ionization reaction rate is higher than attachment reaction rate.The study of this paper is an extension of SF₆ substitute.Gas insulation problem of extremely cold region can be solved to a certain extent.Simultaneously,the influence of temperature on gas discharge mechanism was analyzed form experimental,emulational and numerical calculation aspects.And the results has important guiding significance for gas insulation design in extremely cold regions.