The use of an auxiliary spark gap placed across the surge arrester of a medium voltage transformer.

Abstract

A possible lower cost alternative to medium voltage line arresters and parallel-connected surge arresters is the use of parallel-connected spark gaps across a metal oxide surge arrester. The function of the spark gap is to protect the transformer when the surge arrester fails. Clearly the breakdown voltage characteristics of such a gap need to be carefully co-ordinated with the transformer insulation and those of the arrester. Eskom (Electricity Supply Commission of South Africa) is the national electrical utility that provides the generation, transmission and distribution of electricity in South Africa. The majority of Eskom's electricity reticulation is done with either 11 kV or 22 kV electrical overhead networks. An unacceptable number of Eskom's pole mounted power transformers on these networks have failed over the past few years. The high failure rate of Distribution transformers in Eskom, South Africa has previously been highlighted and investigated in an MSc thesis, the most recent being the thesis completed at the University of KwaZulu-Natal by Chatterton [6]. The thesis proposed possible solutions to the problem but experienced high implementation costs and particular technical issues before widespread implementation could prove viable for the Distribution System. The average transformer failure rate for the Distribution Eastern Region for the twelve month period taken as a moving average was calculated to be 5.19 % per annum at the end of November 2005 and 3.84 % at the end of November 2006. (Eskom Eastern Region Plant report, November 2006). International norms seem to indicate that a transformer failure rate of between 0.5% and 1.0% per annum is acceptable, Chatterton [6]. The reason for the increased failure rates during 2005 was attributed to incorrect Ground Lead Disconnect (GLD) specifications by one of the major surge arrester manufacturers. The incorrect GLD specifications have resulted in premature and nuisance operations. These were triggered by low intensity lightning storms as a result of the lower threshold trigger values. These premature operations have left numerous transformers vulnerable for periods as long as six months and have resulted in the transformer failures increasing from 2.4% quoted by Chatterton [6] in 2002 to 5.19% in 2005 and a reduction to 3.84 % in November 2006 once the problem was identified and the GLDs corrected. See annexure A, Figure A1 of the Plant report for November 2006 for performance details. Hence, this manufacturing flaw and the GLD's sensitivity to specification necessitate further investigation into the spark gap as back-up protection. The spark gap therefore becomes more viable than line or double surge arresters due to its cost effectiveness and robustness. This thesis was based on an idea proposed by Eskom's (Industrial Association Resource Centre) IARC. The aim was to investigate the technical feasibility of using a spark gap to grade with a distribution class surge arrester whilst the surge arrester was operational. The purpose of the spark gap was to act as back-up protection when the arrester fails. Experimentation was undertaken via simulation using the FEMLAB software to model the most suitable gap and geometry for a given rod diameter. The breakdown characteristic of the rod was well understood and verified. Thus, the results obtained from the simulation were compared against the laboratory experiments for the same rod diameters and tip shapes used in the simulation. The results have been analyzed to determine whether the spark gap is a feasible solution for use with surge arresters to protect the transformer from induced strikes following arrester failure.A possible lower cost alternative to medium voltage line arresters and parallel-connected