A study of fire-induced air-gap voltage breakdown characteristics under HVDC conditions.
This dissertation investigates the role that is played by high temperatures of air gaps on the breakdown voltage levels under DC positive and negative polarity applied voltages. Due to past experience of AC transmission lines tripping as a result of sugar-cane fires that occur under these lines during cultivation seasons, this study was initiated to investigate this effect under DC applied voltages. Results were obtained from laboratory work conducted and these were closely analysed to understand the behaviour of air gaps under these conditions. A 17mm2 square-cut brass rodrod electrode configuration was used to carry out these tests at the various air gap temperatures. These were induced by a gas burner for both the positive and negative polarities at 200C – 3000C for the 10 mm – 150 mm air gap range and 200C – 1500C for the 200 mm – 500 mm air gap range. Later particles were introduced into the air gap to determine the subsequent behaviour. These were introduced vertically from the top into an air gap via a vibrating micro sieve mechanism to regulate the consistency of the introduction of these particles in the air gap. A reduction of 55% and 50% was observed on the breakdown voltage under positive and negative polarity applied voltages respectively from ambient conditions to 3000C. Additionally the breakdown behaviour of both negative and positive DC was found to be linear which is similar to the AC case. However, air gaps subjected to positive DC applied voltages were found to portray an inferior dielectric strength as opposed to the equivalent negative DC polarity. The study found that the effect of particles in the air gap is practically negligible and that for practical purposes, only the temperature effect plays a role due to the reduced air density at high temperatures. Empirical models for both the positive and negative DC polarities have been proposed by the study that incorporate the effect of the temperature in the air gap to enable the determination or prediction of the breakdown voltage level at various temperatures. These models may be utilised for DC transmission line design for servitudes in areas that are known to be prone to fires.