Study On DC Flashover Arc Development Characteristics And Dynamic Circuit Model Of Snow-Accreted Insulators

With the development goal of "double carbon",the rapid development of ultra high voltage(UHV)and extra high voltage(EHV)DC transmission lines in China has contributed to the severity of transmission line insulator snowing flashover hazards.Scholars at the domestic and foreign countries have successively researched the insulator covering snow problem from the aspects of flashover characteristics and arc development mechanism,and established a flashover model to predict the snow flash voltage.However,many research results are based on artificial climate laboratory conditions,which cannot correspond to the actual snow-covered insulator arc development characteristics.Furthermore,the existing snow-covered equivalent circuit model has some limitations on the characterization of the arc and the equivalence of the arc development process.Therefore,it is important to carry out snow-covered insulator flashover tests under natural conditions and to establish a dynamic circuit model that conforms to the actual arc development process of snow-covered insulators.This provides theoretical support and practical basis for the design of external insulation of high-voltage DC transmission lines in snow-covered areas as well as snowpack warning mechanisms.In this paper,the arc development characteristics of snow-covered insulators under natural conditions are studied through a combination of experimental and theoretical methods,and the corresponding dynamic circuit model of snow-covered insulators is established.The main research contents and conclusions are as follows:(1)The insulator snowing morphology and snowing growth process were analyzed and studied.The results show that the snow is mainly deposited on the windward side of the upper surface of the insulator skirt,and that it extends outward to a certain length at the edge of the skirt.With the increase of snowing time,the windward side of the snow cover increases most obviously.Additionally,the snow layer thickness exceeds LXY-70 in the late stage of snow accreting,because the snow deposited by FXBW-35/70 develops along the axial direction to the upper and lower sides and there is a bridging trend.(2)The DC flashover test was carried out on snow-covered insulators for both flashover performance and arc development process.It was found that the DC snowing flashover voltage of insulator decreases with the increase of snow coating water conductivity,and shows a negative power function relationship.Under the same snow accretion condition,the snowing flashover voltage of FXBW-35/70 is larger than that of LXY-70.According to the increase of snow cover thickness,the flashover path gradually evolves from along the leakage distance to along the dry arc distance,resulting in a decrease of 36.18% and 34.72% in snow flashover voltage of FXBW-35/70 and LXY-70 insulators,respectively.In addition,the DC arc development process when snow bridging FXBW-35/70 insulators adjacent to the umbrella skirt was further studied.(3)The equivalent snow layer conductivity and DC positive and negative polarity arc parameters inside snow were established by conducting a flat plate coating snow test.Furthermore,the arc voltage gradient under different media was studied,and Esnow+ > Eice+ >Eair+ was derived.For the discrepancy between air gap arc and along-surface snow arc during flashover,the dynamic arc mathematical model is improved and optimized in terms of both arc dissipation power and arc parameters.Further,the arc development criterion is optimized.(4)The corresponding single-arc and double-arc circuit models for DC flash of snow-covered insulators were established based on the improvement of the existing models and flash path analysis.The results show that the maximum relative error between the positive and negative polarity snowing flashover voltage calculated by the single-arc circuit model and the test value are 6.34% and 10.22%,respectively.The maximum relative error between the positive polarity snowpack flashover voltage and the test value calculated by the double arc circuit model is 11.73%,and the simulated leakage current waveform is in good agreement with the test measured waveform.Therefore,the single-arc and double-arc circuit models established in this paper can accurately simulate two kinds of arc development processes,respectively,the arc along the snow layer development for the high-voltage end and the arcs towards the opposite direction development for the high-voltage end and low-voltage end.