特高压GIL非均匀热气流特性与三支柱绝缘子绝缘裕度分析

Non-Uniform Gas Convection in UHV-GIL and Insulation Margin Analysis for Tri-Post Insulator

特高压(UHV)气体绝缘输电管道(GIL)导体通流发热,使绝缘气体SF6在温度梯度场下形成对流,导致三支柱绝缘子的沿面绝缘裕度与设计产生差异,威胁特高压GIL的安全运行。为探究气体对流效应对特高压GIL三支柱绝缘子沿面绝缘性能的影响,该文建立了水平敷设的1100kV GIL三支柱绝缘子电场-温度场-流体耦合仿真模型,考虑电场和气体密度的影响,计算了三支柱绝缘子放电起始电压。结果表明:大负荷条件下(8000A)GIL导体温度升高53℃,周围气体密度和介电强度下降15%,电场强度设计基准需同比例降低15%以保证足够的绝缘裕度;负荷导致放电起始电压降低11.6%,由于气体受热上浮,绝缘子上支柱周围的气体密度和介电强度均低于底部区域,更容易发生气体放电。因此,气体对流效应对特高压GIL绝缘性能的影响不能忽略,需要针对运行工况对结构及参数作进一步优化。

In recent years,gas insulated transmission lines(GILs)have been widely applied in power systems because of their excellent performances including high transmission capacity,low power loss,strong environmental adaptability,etc.Under the current carrying condition,the joule heat generated from the central conductor can raise the conductor temperature to 70~80℃,causing a radial temperature gradient between the conductor and the shell.Under temperature gradient,SF6 gas convection is formed,changing the designed insulation margin of tri-post insulators,which threats the safe operation of UHV-GIL.At present,more attention is paid to the problem of electric field distribution during checking the insulation performance of UHV-GIL,and the influence of thermal-fluid field distribution on gas dielectric strength is seldom considered.Therefore,this paper analyzed the insulation margin of the UHV-GIL tri-post insulator considering the non-uniform gas convection effect.The simulation model of the electric-thermal-fluid field in a horizontally laid 1100kV GIL was established to explore the effects of the temperature and the gas density distributions inside the GIL on the breakdown strength of the insulation gas.Then,the insulation margin analysis for the tri-post insulator was conducted based on the current reference values of the designed electric field considering the gas convection effect.Finally,the discharge inception voltage based on the volume-time theory was calculated considering the electric field and the gas density.Results show that under the large current-carrying condition(8000A),the GIL conductor temperature reaches 78.1°C,reducing the local gas density and the dielectric strength by 15%.To keep the insulation margin,the reference value of the designed electric field should be lowered accordingly.The revised allowable electric field strength considering the gas convection is 20.4 kV/mm in the SF6 gap,is 10.2 kV/mm on the insulator surface and is 3 kV/mm inside the insulator and on the insert surface,respectively.At the peak time of the lightning impulse voltage,the tangential electric field along the tri-post insulator is concentrated in the post leg region of the insulator,whose maximum value is 8.9 kV/mm and is below the revised allowable electric field strength.While the normal electric field along the tri-post insulator is concentrated in the spherical area of the insulator,whose maximum value is 18.5 kV/mm.The synthetic field strength in the SF6 gap is up to 20.5kV/mm at the connecting sleeve,which exceeds the revised allowable electric field strength under the large current-carrying condition.Under the rated power frequency voltage,the electric field strength on the insert surface is 3.25 kV/mm,which does not meet the electric field design specification.The load induces the discharge inception voltage to drop by 11.6%.Gas discharge easily occurs around the upper post of the insulator,because the warm SF6 gas with low density and dielectric strength goes up by buoyancy.Since the gas convection has an evident impact on the insulation performance of GIL,the structure and parameter of the tri-post insulator should be adjusted to satisfy the current carrying condition.The research results are expected to provide references for the optimization design of GIL tri-post insulators.