Carmo Lança

Cross-linked polyethylene (XLPE) is currently widely used as an insulating material for power cables due to its good physical properties, however when in use it undergoes an electrical ageing process. Its ability to trap electric charge can give rise to space charge accumulation in the bulk of the polymer and produce localised electric stresses that can lead to cable failure, since the electric field will be increased above the design stress in some regions favouring the initiation of degradation there. In this work the PEA (pulsed electro-acoustic) method was used to compare the charge dynamics in three samples (XLPE cable peelings) aged in different ways (electrothermally in the laboratory, field aged in service and thermally aged in the laboratory). Very different transient behavior was found depending upon the ageing history. This is related to differences in the migration of chemical species in the insulation layer, which are known to act as charge traps. All materials showed heterocharge peaks when the space charge reached stability, the magnitude of which seems to be related to the severity of the ageing. (c) 2007 Elsevier B.V. All rights reserved.

An understanding of space charge build-up in the polymeric insulation of power cables is important in determining how aging occurs and progresses and, also in predicting cable lifetime. In this investigation electric-field induced space charge in peelings from XLPE (cross-linked polyethylene) cables was measured using two different methods: the pulsed electro-acoustic technique (PEA) and the combined procedure of isothermal and non-isothermal charging/discharging currents (FTSDC). These two methods allow the study of space charge in highly insulating materials. Also, since electric fields of different orders of magnitude are applied to the sample in the two methods, it is possible to analyze different characteristics of the space charge traps. Prior to the measurements the samples were subjected to conditioning to remove volatiles. Cable peelings from various brands aged under different conditions (including field aged and thermally aged samples) were studied as received from the manufacturers. Some of the samples have undergone further ageing in AC electric field (50Hz) for 1000h to see the influence of further ageing on space charge build-up. The results for the different types of samples are compared in an attempt to correlate different ageing parameters.