Effect of voltage waveshape and pressure on streamer breakdown criterion for predicting flashover voltage of polymeric insulators

The flashover voltage (FOV) of an outdoor insulator varies with atmospheric conditions and the voltage wave-shapes. When altitude increases, the FOV decreases due to various atmospheric factors, mainly the pressure. The FOV of the insulator changes under normal operating conditions and transient over-voltages. As the flashover is dominated by streamer mechanisms, this study addresses the application of the streamer breakdown criterion to predict the FOV under different waveshapes and pressures. The Streamer breakdown criterion requires three parameters: electric field distribution, the relation between electric field and effective ionization co-efficient and the appropriate streamer initiation constant (K). The electric field distribution is computed using a FEM based electrostatic solver and the corresponding effective ionization coefficient is calculated. Though the computa-tional procedures of the first two parameters are well defined, uncertainty exist in the value of K, as K depends on the type of gas, electrode geometry, gap distance, pressure, voltage waveshapes, etc. Though many studies have been carried out on configurations under AC, DC and LI only with different ranges of K, reports on the effect of voltage waveshape on the same configuration at different pressures are lacking. Hence, FOVs of an 11 kV polymeric insulator under DC, AC, LI, SI and the steepest VFTO for various pressures (0.75-1.013 bar) are measured and exact Ks are extracted. The necessity of the voltage waveshape-specific K range is inferred and quantified for accurate prediction of FOVs and also inferring the possibility of prediction of FOV without doing any experiments. The applicability of the K range for different arc lengths is verified using SiR samples.