Estudo do comportamento de cabos com cobertura isolante de redes de distribuição compactas frente as sobretensões impulsivas padronizadas

Abstract

The main purpose of this work is to investigate the influence of the polymeric insulation layer of covered cables on the impulse withstand voltage of a typical single phase structure used in compact distribution lines in Brazil. The development of this work includes experimental tests performed in a single-phase structure named CM2 of CEMIG, a Brazilian power utility, by applying the standardized double exponential voltage waveform (1.2/50 s) characteristic of lightning overvoltage events. The disruption mechanisms that happen inherent to the use of covered cable such as partial discharge and charge accumulation were also presented. In order to compare the effect of the presence of the cable insulation layer, tests using bare cable were also performed to represent a critical situation that happens when the cable insulation layer is totally damaged. Besides that, two sets of tests were adopted in this work aiming the evaluation of surface charge accumulation on the impulse voltage withstand of covered cable. In the first procedure, named group 1, after each impulse application a conductive earthed brush is used to remove charges on cable insulation layer. In the second procedure, named group 2, no charge removal was performed. Based on tests results, the lowest disruptive voltage responsible for the cable puncture was calculated as an average of breakdown voltage achieved in each sample. The mean voltage withstand using covered cable on CM2 structure is 239 kV and 219 kV respectively for positive and negative impulse voltage and disregarding the stored charges on the cable insulation. Experimental tests with bare cable provided a 90% disruptive discharge voltage around 133 kV and 161 kV with positive and negative polarity respectively, indicating a higher supportability provided by covered cables. Also, tests performed without the removal of stored charges indicated an increase on the voltage impulse withstand (mean voltages of 328 kV and 272 kV for positive and negative polarity respectively). Finally, it was proved the lower supportability of punctured cables compared with new cables, indicating a reduction on voltage withstand up to 28.9% , depending on the distance between the puncture and the support structure of the cable.