Dielectric properties of PVC leads

摘要

In this paper theoretical and experimental results are presented relating to the polarization which appears as a consequence of non-uniform temperature spacing in cable insulation. This temperature gradient has both a short term influence and a long term influence. In the brief time the gradient creates mainly a conduction inhomogenity. This means that the conductivity of insulation adjacent to the heated core is higher than conductivity of insulation at outer radius of the cable. After a long time operation the changes of conductivity became massive due to chemical reactions accelerated in the places with higher temperature. All these processes give rise to the interfacial polarization which is created in insulating material with non-zero gradient of conductivity or permittivity. In our work we try to identify the mentioned interfacial polarization in a specimen of commonly used PVC cable. It was found, that the polarization appears at frequencies of the order of a few mHz. Measurement in this scale requires a special apparatus as it is sensitive to any perturbation. Here we are dealing with the problem of separation of polarization part from the conduction part of dielectric data. Following our experiments the best way of date processing is the use of complex modulus formalism. Experiences with interfacial polarization induced by a temperature gradient will be utilized for monitoring the cable properties during a long-term ageing.