Study on DC voltage breakdown characteristics of gap under fire conditions

Many HVDC transmission projects are in operation or under construction in the world for HVDC transmission´s unique advantages and many HVDC transmission lines inevitably across the forests with lush vegetations for long transmission distance. Hence, the frequent forest fires become a serious threat to the performance of transmission lines´ external insulation. The DC voltage breakdown characteristics study of gap in the flame conditions has an important meaning for DC transmission lines´ construction and safety operation. Gap breakdown in forest fire relates to many factors, such as flame temperature, conductivity and particles. Different vegetation´s combustion characteristics determine that the influence to trigger discharge of these factors is different, thus breakdown voltage has a great difference. In order to analyze the gap breakdown mechanism of DC transmission line under the forest fire conditions, using simulated forest fire, experiments study on the DC voltage breakdown characteristics of wire-plate gap under different influencing factors were carried out in this paper. Experimental results show that, the gap´s insulation strength in the flames decreases significantly, which is about 20% of that in the pure air. Gap breakdown has an evident effect of polarity characteristic under DC voltage, and breakdown voltage of positive polarity is smaller than that of negative polarity. The breakdown voltage of gap decreases about 30% after adding KCl in the wood crib flame.