Apparent mobility and its relationship with ageing in polymeric insulation materials

Apparent mobility obtained from space charge decay measurement has been recently used as an ageing marker to assess degradation of the insulation materials. The apparent mobility of charge carriers becomes slower after experiencing electrical ageing. In this paper, we have measured space charge decay in polymeric insulation material using the pulsed electroacoustic technique and studied apparent mobility features in a wide range of the applied fields and poling times. It has been found that the increase in the applied field and poling time produces more deep trapped charges, leading to a decrease in apparent mobility with time. For a specific time, the detrapping process is dominated by the charges of specific trap levels, resulting in a time dependent apparent mobility. In addition it has been found a sudden increase in apparent mobility at high fields, indicating that a precaution has to be taken when assessing the ageing using the method.