An empirical model of fire-induced airgap voltage breakdown characteristics under HVDC conditions

The classical reduced air-density model resulting from an increase in temperature affirms the degradation of the dielectric strength under such conditions. This paper discusses the effect of fire on air as a gaseous insulator acting between adjacent HVDC poles and between the poles and the ground plane. Laboratory experiments were conducted using the rod-rod gap configuration up to a maximum of 50.0cm and 300
⁰
C in air gap length and temperature respectively. It was observed that the dielectric strength of air reduced by as much as 55% and 50% from ambient temperature conditions for positive and negative polarities respectively. As per the literature available, a higher negative polarity dielectric strength was observed when compared to the positive polarity. The introduction of particles in the air gap was investigated and found to have a limited influence on the breakdown voltage. Subsequently, the study concluded that this influence is negligible under practical conditions. Hence, only the effect of high temperatures within the air gap can practically be assumed to influence the voltage breakdown levels. An empirical model is developed to approximate the dependence of the breakdown voltage of an air gap at high temperatures for both the positive and negative polarity. This model postulates the dependence of the breakdown voltage at high temperatures to a parameter x (defined as the product of the relative air density at a specific temperature and air gap spacing).