Charge motion in technical insulators: facts, fancies and simulations

Technical insulators, whether gas, liquid or solid, are largely characterized by unipolar ionic flow. The carriers are supplied by injection, or by bulk ionization, and the charge transport is space charge perturbed. The author discusses theoretical modeling of steady state, time dependent, and oscillatory conditions. Continuum equations have provided the basis for early models of corona currents. First he shows the validity of the Deutsch assumption in gases, and then extends the calculation to describe the situation far downwind. These equations also account for many effects in solids such as injection transients and open circuit decay. We discuss their application to electrode blocking capacitance, and to discharge currents in shorted samples. We note that even the simplest bipolar flows require many parameters to describe them. Next, we consider the problem of reconstructing distributions from acoustic pulse and similar measurements that necessarily contain instrumental broadening of the signals. It is then necessary to attempt to explain the observed evolution of the charge density by simulations. We discuss some of the problems associated with the algorithms, and with modeling the microscopic details of the carrier motion when the numerical approximation is necessarily mesoscopic. We mention some applications of the theory to approximation is necessarily mesoscopic. We mention some applications of the theory to measurements made with AFM probes, with the SEM mirror method, and with a Faraday cage. We mention the difficulties in 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 modeling) which need our future attention.