A study of fire-induced air-gap voltage breakdown characteristics under HVDC conditions.
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Date
2012
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Abstract
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.
Description
Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2012.
Keywords
Electric power transmission--Alternating current., Electric power transmission--Direct current., High voltages., Electric lines., Theses--Electrical engineering.