Paper Title
Surface Flashover Characteristics of Composite Insulator under Lightning Impulse Voltage Superimposed on Pre Stressed DC

Nowadays, high voltage DC (HVDC) transmission lines are being used for power transmission over long distances due to its numerous advantages over the conventional HVAC such as higher efficiency with larger current carrying capability and lower electrical losses. To support the conductors, insulators of different shapes, types and materials are being used. In order to improve the reliability of insulators, many research studies regarding breakdown, flashover, etc have been conducted on overhead line insulators under DC, AC and impulse voltages only. The impulse breakdown characteristics are of fundamental importance in the insulation coordination studies. The critical voltage wave shapes for the insulation is the lightning or switching impulse voltage. In practical, surge overvoltage appears on the electrical equipment while it is stressed under DC or AC working voltage. Hence, for a short time the equipment is subjected to surge voltages superimposed upon the operating DC or AC voltages. Furthermore, no systematic study has been carried out to study the effect of surge voltages superimposed on the operating DC voltages. This paper presents surface flashover characteristics of overhead line composite insulator under lightning impulse (LI) voltage superimposed on pre-stressed DC voltage. It is found that for the same polarity of LI and pre-stressed DV, the breakdown occurs at lower voltage. On the other hand, when the polarity of the pre-stressed DC and the LI are opposite, flashover occurs at higher voltage. Keywords - Composite insulator, lightning impulse, flashover, breakdown, composite overvoltage.