Sources of insulator charging and the induced electric field enhancement in gas insulated substations

Insulators used in electricity transmission equipment are generally assumed to bear no electric charges when they are designed. However, generation and migration of electric charges onto insulators do take place. It has been found that trapped charges are frequently left inside gas insulated substations (GIS) after de-energisation of an isolated section of busbar. The understanding of the sources of charging is important for taking measures to avoid insulator charging, or for adopting appropriate designs to contend with the charging process when it is difficult to avoid. The knowledge can enable one to deduce the locations and maximum values of insulator borne charges. Nonuniform bulk conduction mainly results in charge build-up inside the insulator whereas charges coming from outside of insulators deposit charges on the surface of insulators. Such information is vital for modelling the effect the insulator charging. This paper formulates some theoretical models of insulator charging and analyses data gathered from real systems to evaluate the likelihood of possible sources of insulator charging. A real example is given on how to incorporate the conclusions reached in electric field calculations using the finite element method.