Spacecharge, breakdown and lifetime estimation of polyethylene insulation systems

Abstract

Ever increasing demands on long haul submarine telecommunication cable systems is leading to ever increasing system voltages. Associated with these increased voltages is an increased risk of high voltage breakdown, and a reduced system lifetime expectancy. This has resulted in a greater need to evaluate and improve insulation system materials and overall system design, whilst gaining a greater understanding of the high voltage and ageing mechanisms present within the polyethylene insulation. Prominent within the high voltage and ageing mechanisms is the formation of space charge, which can add significantly to the electrical stress in localised regions. The influence of space charge formation and decay mechanisms in polyethylene insulation systems is investigated using the Pulsed Electro Acoustic (PEA) method, (at room temperature). Both bulk limited, and electrode limited mechanisms are discussed with reference to the obtained experimental results. Significantly comparisons are made between the bulk insulation, (i.e. typical of extruded insulation), and injection moulded material containing amalgamation zones, (i.e. typical of joint insulation). The high voltage performance of a real optimised submarine telecommunication cable system is then tested and characterised. The implications and applications of the generic material space charge characteristics on the performance of a real insulation system are then considered. Implications considered include reviews on proof and qualification testing protocols, as well as lifetime and reliability estimation of systems using the inverse power law model (IPM) and Weibull statistics. Finally, evidence is offered in support of threshold mechanisms existing within the insulation system suggesting that applicability of models such as the IPM may not be valid across all ranges of electrical stress.

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