Study On AC Flashover Characteristics Of Nature Icing Suspension Insulator (Short) Strings

Icing on insulators of transmission lines is one of the important reasons leading to the reduction of security of power grid operation.Although a large number of experimental studies on flashover characteristics of ice-coated insulators have been carried out by domestic and foreign researchers,they are mainly carried out under artificial simulation conditions.By comparing artificial icing with natural icing,it is not difficult to find that there are great differences between artificial icing and natural icing due to the influence of environmental factors such as temperature,wind speed and humidity,which will affect the flashover characteristics.Therefore,in order to improve the reliability of power grid operation and reduce the accident rate of ice flashover,it is of great significance to carry out natural ice flashover test research.In this paper,three typical suspension insulator strings(short)were naturally iced at Xuefeng Mountain Nature Icing Test Base.Through monitoring the environmental parameters,the natural icing characteristics of insulator strings and AC flashover characteristics of contaminated ice-coated insulator(short)strings under different test conditions were obtained.The electric field simulation calculation model is established,and the influence of icicle length,water film conductivity and ice layer conductivity on the electric field distribution along the surface of ice-coated insulator string is analyzed.The development path of flashover arc is studied in combination with the development process of flashover arc photographed by field test.The conclusions are as follows:(1)The basic condition of natural icing of insulator strings is the existence of temperature and wind speed which can freeze water droplets.Simple freezing rain icing and freezing rain fog alternately icing can make insulator strings form rain hoop and mixed hoop icing forms respectively,and the degree of icing on the windward side of insulator strings is more serious than that on the leeward side.In the process of icing growth,the growth rate of the diameter of unbridged icicle is slower than that of bridged icicle,and the thickness of icing on windward side of umbrella skirt will gradually increase,but the trend will slow down.The number of icing edges of insulators is positively related to the diameter of insulators.The "big-small-big" umbrella skirt structure can effectively restrain the growth of icing;the icing thickness on the surface of double umbrella insulator is obviously larger than that on the bottom surface;the icing weight of insulator string is seriously affected by its umbrella structure.(2)The AC flashover voltage of suspended insulator(short)strings with natural icing decreases gradually with the increase of SDD,and satisfies the negative power function relationship,the influence characteristic index b is between 0.240 and 0.350.With the increase of d,AC icing flashover voltage decreases gradually,which accords with negative exponential function,the characteristic index c ranges from 0.268 to 0.355.There is a negative power function between icing flashover voltage U50 and ice stress product ISP,and the characteristic index p is between 0.432 and 0.472.The critical flashover current increases with the increase of contamination degree before icing,and decreases linearly with the increase of icing degree.Under the same SDD,the AC flashover voltage of naturally iced insulator strings is lower than that of polluted insulator strings(3)Through filming the field tests of mixed and glaze ice-covered insulators,the arc development and variation during the discharge process of insulators with different ice-covered morphologies were obtained.The arc mainly develops along the surface of the ice layer under the mixed ice cover,while under the condition of glaze ice-covered,the arc will appear more serious drifting arc phenomenon.During the development of arc,there exists the phenomenon of quenching and re-burning,and the partial arc will appear randomly on the windward side of insulator strings.When the complete flashover occurs,the arc channels are basically located on the windward side of the insulator string.Compared with the two types of icing,the electrical performance of insulators decreases more when the ice is covered by glaze.(4)The voltage drop ?U of the first ice edge air gap at the high voltage end of the iced insulator string is much larger than that of the other air gap,and ?U of the first ice edge air gap at the high voltage end increases with the increase of the ice edge length,while ?U of the other ice edge air gap decreases with the increase of the ice edge length.The change of water film conductivity will affect the surface electric field of ice-coated insulator strings,but it will not change the distribution of electric field;the ice layer conductivity has no substantial effect on the electric field distribution of ice-coated insulator strings.