DC-field in solid dielectric cables under transient thermal conditions

Abstract

The electric field in a dielectric changes from capacitive grading immediately after the voltage application to, resistive grading under steady state conditions. In the case of a polymeric dielectric, the conductivity is known to be strongly dependent on both the electric field and the temperature, characterized by an activation energy in the range of 0.5 to 1.2 eV. The dielectric constant /spl epsi//sub 0//spl middot//spl epsi//sub r/ for an olefinic polymer is almost independent of the applied field and temperature, in contrast to the electrical conductivity /spl sigma/, which has a strong dependence on both temperature and field, in the parameter range of engineering interest for a DC cable. During normal operating conditions, load current will heat the conductor and generate a temperature gradient across the dielectric. This temperature gradient will change the electric field distribution in a resistively graded dielectric, with a time constant determined by field- and temperature-dependent carrier mobility, which controls the evolution of space charge towards a steady state condition characterized locally by the dielectric time constant.