Charge motion in technical insulators: facts, fancies and simulations

Technical insulators are largely characterised by unipolar ionic flow. The carriers are supplied by injection, or by bulk ionisation, and the charge transport is space charge perturbed. Continuum equations have provided the basis for models of charge flow. First I show the validity of the Deutsch assumption in gases, and extend the calculation to describe the situation far downwind. The equations also account for many effects in solids such as electrode blocking capacitance, injection transients, open circuit decay, and short circuit discharge currents. Next, we consider the problem of reconstructing distributions from acoustic pulse and similar measurements that contain instrumental broadening of the signals. It is then necessary to explain the observed evolution of the charge density. We discuss problems associated with simulation algorithms, and with modelling the microscopic details of the carrier motion. We mention some direct measurements of deposited charge, and consider the problem of accounting for low frequency oscillations in cables. Finally, we outline the materials problems (identity of carriers and traps), the numerical problems (reconstruction, discrimination between the effects of various carrier types and dipoles, and deduction of parameters), and the logical problems (distinction between simulation and accurate modelling) which need our future attention.