Study On Effect Of Fog On Discharge Performance Of Short Air Gaps And Insulators

According to operation experience, discharge of external insulation in fogconditions poses a significant threat to the safety and stable operation of power grid inChina. The flashover performance of insulators wetting by steam fog in a climatechamber has been extensively studied by researchers. However, there is little researchon the flashover performance of transmission line external insulation influenced bynatural fog and its conductivity. Therefore, the investigation of the ac flashoverperformance of insulators and air gaps in fog conditions and their influential factors cannot only make the fog flashover well understood, but also provide very importantreference for the prevention of flashover of transmission lines in fog conditions and thesafety operation of the power grid.Supported by the National Basic Research Program of China (973Program:2009CB724503) and based on the existing studies at home and abroad, a large numberof tests have been carried out in the laboratory and Xuefeng Mountain test station ofChongqing University to investigate the flashover performance of rod-plane air gapsand several types of transmission line insulators in fog conditions. The mainconclusions in the thesis are as follows:According to the meteorological observation at Xuefeng Mountain test station, themonthly average foggy days and the characteristic parameters are obtained. By usingthe least square method, the visibility formula combining the effect of liquid watercontent and droplet concentration is proposed. With the use of ultrasonic fog nozzles,the artificial fog is generated and applied to investigate the ac flashover performance ofceramic and glass insulators in fog conditions. The test results obtained by the methodof steam fog and artificial fog proposed in this thesis are compared. The effect ofdifferent polluting methods on the flashover performance of composite insulators in fogconditions is analyzed. The deviations Δ1%of ac flashover voltage between quantitativebrushing method and spraying method are ranged in7.1%-9.3%at various SDDs.Moreover, the differences on exponents a characterizing the influence of SDD on theflashover voltage are negligible for the specimen whichever polluting method is used.Based on the tests in artificial chamber and Xuefeng Mountain test station, thispaper analyzed and summarized the influence of the fog characteristic parameters(water content, fog-water conductivity or temperature) and environmental factors on the AC discharge voltage of air-gap various separations. The results show that as the waterconcentration ranges from1to3g/m3, the discharge voltage increases by5.1%.However, as the water concentration ranges from3to4g/m3, the discharge voltage hasno obvious change. As the fog-water conductivity increases from100to5150μS/cm,the discharge voltage decreases by2.7%～9.1%.According to the test results and analysis, the effect of the fog-water conductivityon the ac flashover voltage of insulators in the area with heavy fog is put forward. It isproposed that the fog-water conductivity does not affect the flashover voltage if thecontamination degree on insulator surface is not smaller than0.15mg/cm2. While the acfog flashover voltage decreases by4.1%-25.6%with the fog-water conductivityincreasing from0.01to3.0mS/cm if the contamination degree is not larger than grade0.06mg/cm2. This means the effect of fog-water conductivity can be ignored in thecondition of severe pollution. On the contrary, the effect of fog-water conductivityshould be taken into account in light pollution conditions. The flashover voltage of theinsulator should be corrected for the clean or slightly polluted insulators in fog areaswith high conductivity and high concentration. The correction coefficient K ranges from1.0to0.8as the fog-water conductivity γ increasing from0.01to3.0mS/cm. The effectof fog-water conductivity on the flashover voltage of hydrophobic insulator is less thanthe hydrophilic insulators.According to the test results, the wetting characteristics of the surface pollutionlayers of insulators and condensation growth model are analyzed. The model tocalculate the equivalent salt deposit density reflecting the fog-water conductivity andpre-polluting salt density is established. Based on that, the formula to calculate the acflashover voltage of insulators in fog conditions is proposed by considering the effect offog water conductivity and equivalent salt deposit density. It is shown that thecalculation results have a good agreement with the test results, and the difference issmaller than6.8%. The fog flashover voltage gradient of the leakage distance of fourdifferent types of insulators under various fog water conductivities are investigated inthe0degree pollution condition. The results can provide valuable reference forcontamination classification of insulators under regions with the severe contaminationand heavy fog.According to a mass of fog flashover results of insulators under the natural fogconditions at natural testing station of Chongqing University, the flashover voltageformula is proposed by considering the effects of the salt density and ambient temperature. It is shown that the calculation results are consistent with the test resultsbased on Qinghai-Tibet railway of Chongqing University. It is proposed the duration ofarc throughout the composite insulator in condensation ceter was significantly lowerthan ones of the freezing fog flashover due to the fact of the change of the surfaceroughness of insulator sheds, which leads the arc melt the frozen fog droplets on thesurface of shed and extends its flashover time. Moreover, the discharge arc presentsvery irregular shapes bends, and has red burning flame or sway violently characteristics.