Destruction free ageing diagnosis of power cable insulation using the isothermal relaxation current analysis

This approach describes the destruction free evaluation of the ageing behaviour of polymeric cable insulations using a new dielectric method. Ageing is influenced by electrical, physical, mechanical and morphological properties. All these changes have a strong influence on the residual strength and the lifetime of a power cable. The cable performance should be evaluated onsite. The destruction-free determination of the ageing behaviour is governed by the depolarization processes inside the insulation. After previous d.c. polarization of the cable insulation the transient part of the current is based upon the thermic release of carriers from traps. The analysis of the current from discrete trap levels hereby enables an evaluation of the ageing process inside the insulation. The Isothermal Relaxation Current (IRC)-Analysis uses the relaxation current measurement and a special mathematical calculation to gain the information about the changes of the polarization processes inside the insulation. The response of the polymeric insulation can be physically described and interpreted taking into account the phenomenon of interfacial polarization and the polymeric conduction behaviour. Therefore the development of an equivalent circuit was necessary to discuss these phenomena more explicitly. The experimental results were achieved during the DIN VDE 0273 A1/A2 (Entwurf) long term test of medium-voltage cables in different steps of ageing and degradation by a major German cable manufacturer.<
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