Electroluminescence due to impulse voltage in cable-grade XLPE

Abstract

Underground power cables in service are inadvertently subjected to impulses generated by lightning and switching surges that are superimposed on the ac voltage at which they operate. HV transients caused by lightning and switching operations not only radiate large electromagnetic fields but also impose additional stresses on the insulation and could initiate deterioration which can continue under normal operating conditions. Electrical breakdown of HV cables is a local phenomenon and electrical aging at local sites in the polymeric insulation occurs by molecular dissociation of the polymer and formation of new chemical bonds. This process usually involves the electronically excited states of the molecules that give rise to radiative phenomena, such as electroluminescence (EL). Analysis of EL can help to clarify the degradation mechanisms that occur at points of electrical stress enhancement and lead to cable breakdown. The EL technique is several orders of magnitude more sensitive than the commonly employed partial discharge detection and can provide a better understanding of the various time dependent mechanisms such as space charge injection, trapping and decay that can lead to insulation failure. The characteristics of EL in crosslinked PE subjected to impulse voltage, are described in this paper. Impulses having the same polarities as the half cycles of the ac voltage on which they are superimposed give rise to the largest number of EL pulses. Also, more EL pulses are emitted when impulses are applied at the peaks of the positive and negative half cycles than at the zero crossing of the ac voltage. This suggests that the amount of charge injected and trapped into the polymer plays a crucial role for EL emission.