Circuital Modeling of Polarization and Depolarization Currents in Polymeric Materials Under Low Electric Fields

This article proposes a circuital model that describes polarization and depolarization currents (PDCs) observed in thin samples of polymeric materials under the application of an average electric field up to 30 kV/mm. The model employs the least possible number of parameters to fit the experimental measurements. A single carrier with a negative charge is considered. The polarization curves are modeled by a power law to account for transport assisted by shallow traps. A model for the deep traps with a Gaussian distribution of the density of states (DOS) describes the depolarization curves. Experimental measurements made on various materials with different charging and discharging times suggest that a portion of the deep traps is filled within a few seconds, while the time needed to release the trapped charges is in the order of several hours. The agreement between experimental and simulated currents confirms the validity of the proposed model that can be seen as a generalization of the extended Debye model.

I. Introduction

Electric systems are in the middle of a paradigm shift that is bringing them from a predominant ac operation to a hybrid ac/dc one, stimulated by the penetration of renewable energy sources and electrified mobility. In this situation, polymeric insulation may suffer intrinsic and/or extrinsic aging phenomena that must be known and accounted for at the design stage, for the sake of insulation system reliability [1]. Circuital models can help to predict the stress levels causing such phenomena and their phenomenology. For example, they are used to calculate the partial discharge inception voltage in insulation, under ac or dc, which is fundamental to rule out the extrinsic accelerated aging mechanism that can drastically reduce electrical apparatus reliability [2], [3]. Another example is provided by one of the basic diagnostic tests assessing the quality and reliability of insulation systems, that is, the polarization index, from which it is possible to relate variations of polarization properties to aging and other types of phenomena [4]. Here, circuital models can help in the interpretation of the experimental results, providing a conceptual framework to understand the causes of the changes in the dielectric properties.