Study On Effect Of Salt Fog On Flashover Performance Of Contaminated Insulators

In coastal area and islands, salt fog is formed due to disturbance of seawater. Salt fog is a special form of contamination, called type B pollution. It’s a contamination of high conductivity which quickly deposits on insulator surface, resulting in the condition where the insulator changes from an acceptably clean, low conductive sate to flashover in a short time and then returns to a low conductive state when the event has passed. Very few of studies on effect of salt fog on flashover character of contaminated insulators is carried out. Therefore, the research on flashover character under salt fog condition can make the flashover mechanism understood and provide reference for prevention of contamination flashover of power transmission lines and reliable operation of power grid in coastal area and islands.In this paper, cause of formation of salt fog and influence factors are analyzed. Based on the results of natural contamination on the surface of insulators in an island, artificial pollution tests on different type insulators have been carried out under salt fog condition. The results and conclusion are followed:In the artificial pollution tests, salt fog is generated from a fog system recommended in IEC 60507. The flashover voltage of a polluted FXBW4-110/100 insulator is much higher than that of a XWP2-70 or LXHY5-70 insulator string of 7 unit under the same condition. The flashover voltage Uf of insulators decreases with the increase of equivalent salt deposit density(ESDD) and salt fog water conductivity by a power function with different negative influence index for ESDD and salt fog water conductivity. The influence index for salt fog water conductivity is greater than that for ESDD.With certain non-soluble deposit density(NSDD), the relationship between the flashover voltage of suspension insulator(string) and equivalent salt deposit density(ESDDr), salt fog water conductivity(20s) can be expressed as20a cf ESDDU Ar s- -=. The magnitude of index c is greater than that of index a and A, a, c are determined by the type and material of insulators.In the flashover process of contaminated insulators, salt fog provides water for contamination layer and conductive ions. The dry zone can be destroyed by salt fog water. However salt conductive ions in salt fog also increase surface leak current and improve the formation of dry zones. Arc extinguishment and recovery are observed in AC flashover process of contaminated insulators under sat fog condition. Porcelain and glass insulator string with 7 units discharge with yellow-white color and composite insulator discharge with red-white color mixed with other colors.An improved model for insulator flashover is established taking air arc into account. When local arc extends to the critical length, flashover is inevitable without connecting the whole creepage distance of contaminated insulator. The calculation result is consistent with phenomenon in tests.