A study on the ageing of polymeric materials in the presence of space charge

Abstract

The use of polymeric power cables for high voltage direct current HVDC has become increasingly popular for renewable power sources such as offshore wind farms. The long term reliability of polymeric cables operating under DC conditions is still unknown and therefore of concern to the industry. However, there is no an explicit consideration of how the injected charges can cause effects to the prediction of the life-time. Thus, it is important to develop an ageing model can assist of understanding the effect of these charges on the insulating material.

Regarding to the polymeric material that are used as an electrical insulation, the presence of space charges could be the consequence of material degradations that are thermally activated and accelerated by the presence of electric field. The dynamics of space charge, therefore, can be potentially used to characterise the material. In this direction, an ageing model in which parameters have clear physical meanings has been developed and applied to the material to extrapolate the lifetime. The kinetic equation has been established based on charge trapping and detrapping of the injected charge from electrodes. The local electromechanical energy stored in the region surrounding the trap reduces the trap depth with a value related to the electric field. At a level where the internal electric field exceeds the detrapping field in the material, an electron can be efficiently detrapped and the released energy per detrapped charge can cause a weak bond or chain scission i.e. material degradation. The model has been applied to the electro-thermally aged low density polyethylene LDPE film samples.

To evaluate the damages structure during ageing process, a simulation work was performed on the developed ageing model to investigate the susceptibility of such parameters that can cause an explicit effect to the ageing process. The simulation work is performed on a two-dimensional square grid that is assumed to represent a part of the insulating material. The mesh structure is divided using the finite element method. Based on the nature of polyethylene, its structure is semi-crystalline with a spatially varying morphology. Consequently, each bond in the grid is assigned a set of parameter values. One of these parameters is the critical fraction of trapped charges C*, which needs to be reached in order to fail a bond. It is chosen at random values from a range centred on the characteristic value obtained from the experimental results. This indicates that the insulation life at varying parameter C* is lower than its characteristic value.

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