Injection, diffusion, conduction and space charge under high DC voltages in cable-grade low density polyethylene

Abstract

The thesis presents investigations on fundamental research topics and proposes new analytical results for conduction and space charges considering, injection, diffusion and structural features in a comprehensive manner. The theoretical results are experimentally verified and are found to be accurate. Conventional theoretical conduction models fall short of addressing issues related to conductivity of polyethylene, since these models were based on famous energy band theory which was developed to understand mechanism of conduction in inorganic crystalline solids such as silicon, germanium etc. These extensively used conventions hinder proper understanding of polyethylene/ low-density polyethylene (LDPE), since whole material is seen as uniform internal structure. Inaccuracy of these famous conductivity models shows deficiency in understanding the effect of different regions of material on conduction. This deficiency drove us to contemplate the structural component and its effect on conduction mechanism. In case of LDPE, along with the energy band theory, physical or chemical disorders, termed as local charge trapping states having energy just below the conduction band of polyethylene, are introduced. The thesis proposes a novel mechanism for conduction in LDPE, based on energy band theory and internal structural features. On experimental part, volumetric current measurements with time have been done for temperatures 303K, 323K and for each temperature, the external voltage is varied from 2kV to 20kV with 2kV step. Each voltage is applied for duration of 10 hrs. Geometric mean of data of current of different samples, at the end of 10-hour duration, has been taken as the value of current for each case. Experimental conductivity of the material has been calculated on the basis of this geometric mean. Remarkable matching of results of experiments with the results of new theory of conduction has been reported in the thesis. Further, homocharge, particularly, positive charge near anode has been observed inside the dielectric material (LDPE) in several experimental results by many researchers. However, the phenomenon is yet to be understood theoretically and mathematical analysis is pending, although, simulations have been reported. The past simulations on evolution of space charge in LDPE, with time and space, using bipolar charge transport model considered only drift component of charge carriers inside the dielectric, neglecting diffusion. However, experimental results suggest significant concentration gradient in space charge distribution. The thesis presents comprehensive drift and diffusion based analytical models, for the time/space evolution of space charge and electric field in dielectrics using Maxwell’s equations, current continuity equation, conduction and injection mechanisms. The proposed analytic models of electric field and space charge distribution entwine charge injection at electrodes and charge transport inside the medium, which apparently have never been reported until now. Remarkable agreement between the analytical and experimental results has been observed. Using the model, the role of conduction, injection, diffusion on homocharge accumulation has been put forth.