Investigation on Effects of Thermal Ageing on LDPE Based on Polarization and Depolarization Currents

This paper describes about ageing effect on the polymeric insulation through Debye model analysis. Polarization and depolarization current (PDC) measurement is a noninvasive technique to judge the condition of dielectrics used for high voltage insulation. Polarization current is the combination of conduction current and displacement current but the depolarization current is only dependent on dipole relaxation and so, no conduction current is involved in it. Therefore, the depolarization current directly depends on the response function which is further dependent on the inherent property of the dielectric. So, it is possible to analyze the condition of the insulation on the basis of PDC. In this paper, the Debye model parameters are estimated from the depolarization current. It is identified from the branch parameters that with increase of ageing duration the branch parameters as well as the time constant of each branch distinctly increased. An equation is derived which is based on released charge from the depolarization current. From the equation, parameters which can identify the ageing status could be observed.