Screening Of SF₆ Alternative Gases And Study On Discharge Characteristics Of HFO/N₂ Gas Mixture

With excellent insulating properties and arc extinguishing performance,SF₆is the most widely used gas insulation.However,due to its strong greenhouse effect,SF₆ was listed as a gas for which emissions need to be limited in the Kyoto Protocol adopted by Japan.Therefore,it is urgent to find a new eco-friendly gas insulation to replace SF₆.Gas insulation needs to have a high dielectric strength,while dielectric insulation of insulating medium depends on the development of electron avalanche.In order to quickly screen out SF₆ alternative gas,the microscopic dielectric parameters affecting the development of electronic avalanche were calculated based on density functional theory,the prediction expression of gas dielectric strength was established by regression analysis.Combining with the relationship between the microscopic dielectric parameters and gas molecular structure,the screening criteria of SF₆ alternative gas was proposed.According to the proposed screening criteria,121 kinds of gases were screened preliminarily,and 51 kinds of insulating gases were obtained.Then,dielectric strength of each gas after initial screening were calculated,with 0.8 times relative dielectric strength of SF₆ as the screening standard,insulating gases were furtherly screened.Considering the environmental friendliness and toxicity of gases,HFO has the potential to replace SF₆.Therefore,HFO gas and its gas mixture were selected as the research objects,and a comprehensive assessment on its dielectric strength characteristics was carried out.In the application of electrical equipment,insulation defects will lead to distortion of electric field.If the electric field intensity is too high,partial discharge will occur.Thus,research on partial discharge characteristics of HFO/N₂ gas mixture under typical metal protrusions was carried out,the results show that partial discharge inception voltage of HFO/N₂ gas mixture increases linearly or exponentially with the increase of pressure in power frequency positive and negative half cycles.The breakdown of insulating medium is main cause of power equipment failure.Therefore,power frequency breakdown characteristics of HFO/N₂ gas mixture were investigated.The results suggest that pressure,mixing ratio and electrode grap has significant effects on the power frequency breakdown voltage of HFO/N₂ gas mixture under slightly non-uniform electric field.The breakdown voltages of HFO/N₂ gas mixture increases linearly with the pressure under power frequency voltage,while breakdown voltages of pure HFO saturates with the pressure.The breakdown voltages of HFO/N₂ gas mixture increased exponentially or linearly with the increase of mixing ratio and electrode grap,respectively.The breakdown voltages of HFO/N₂ gas mixture increases exponentially and linearly with the increase of mixing ratio and electrode grap,respectively.Due to the homogenization of corona on the electric field,the breakdown voltages of HFO/N₂ gas mixture show three variation trends with the increase of pressure under extremely non-uniform electric field,namely,linear change,peak change and"N"curve change.The breakdown characteristics of HFO/N₂ gas mixture under impulse voltage were studied,the results show that 50%breakdown voltages of HFO/N₂gas mixture show a saturation growth trend with the increase of pressure under impulse voltage for sphere-plate gap.The"hump"phenomenon appeared as 50%breakdown voltages changing with pressure for needle-plate gap.In addition,due to the change of the electric field unevenness,50%breakdown voltages of HFO/N₂ gas mixture under impulse voltage show polarity effect.Insulating gas will decompose under partial discharge,the insulating property,toxicity and environmental-friendliness of decomposition products will directly affect the application of gas insulation.Therefore,the decomposition mechanism of HFO/N₂ gas mixture was analysed based on density functional theory,and decomposition products were predicted.The results show that HFO gas molecules will be decompose though different paths in the range of low temperature plasma under the action of different energy electrons,and different products will generate though free radical recombination,addition reaction and internal configuration conversion of molecule.The predicted decomposition products of HFO/N₂ gas are in good agreement with the measured results under partial discharge.Comprehensively considering partial discharge characteristics,breakdown and decomposition characteristics,40%HFO/60%N₂ gas mixture has the potential to replace 20%SF₆/80%N₂ gas mixture.