Modeling charge transport and linking charge dynamics with dissipative processes responsible for electrical ageing are a challenging objective for developing a mature approach in insulation design. Such an approach is exemplified here for polyethylenebased materials by introducing two models describing bipolar Space Charge Limited Current in transient and steady states. They rely on two classical descriptions of the distribution function of the energy levels of trap states -single trapping level or exponential distribution. Their predictions are discussed as regards the experimental behavior. They notably highlight the importance of recombination processes in explaining the sigmoidal shape of the steady-state current-voltage characteristic. Also, bipolar charge transport seems to be a necessary factor for the observed features of oscillatory charge packets. The energetic features of charge dynamics is reviewed with particular emphasis on recombination phenomena because the latter promote electronically excited states that are chemically reactive and could be involved in ageing reaction. The relationship between charge recombination and electroluminescence is highlighted through experiments and simulation. The spectral analysis of the emitted light advocates the existence of massive chemical/physical degradation in the electrical regime where recombination is a major factor of the Space Charge Limited Current (SCLC).

A bipolar model of transport intended to describe the behaviour of polyethylene under dc stress in steady state conditions is presented. The model is based on Poisson's equation and the conservation law of charges, with trapping, detrapping and recombination phenomena being taken into account. Electric field and carrier concentrations are formulated for double-carrier injection and boundary conditions are given by the Schottky injection law. This system of equations with partial differential functions is solved using a finite volume method. The current versus applied electric field characteristic is obtained and analysed for different parameter sets of the model in order to understand their role in the predicted macroscopic behaviour. To do so, we consider two approaches. In a first step, the simulation has been realized with symmetric parameters for positive and negative charge carriers in order to facilitate the interpretation. In a second step, asymmetric parameters have been considered in the model as it constitutes a more realistic situation. Results show that the recombination plays an essential role in order to obtain the two changes in slope, forming a sigmoid-like shape in the current–voltage characteristic as observed experimentally in this kind of material.

A charge transport model allowing the description of electroluminescence in polyethylene films under AC stress is proposed. The fluid model incorporates bi-polar charge injection/extraction, transport and recombination. The physics is based on hopping-mobility of electronic carriers between traps with an exponential distribution in which trap-filling controls the mobility. The computation mesh is very tight close to the electrodes-of the order of 0.4 nm allowing mapping the density of positive and negative carriers during sinusoidal, triangular and square 50 Hz voltage waveforms. Experiment and simulation fit nicely and the time-dependence of the electroluminescence intensity is accounted for by the charge behaviour. Light emission scales with the injection current. It is shown that space charge affects a layer of 10 nm away from the electrode where the mobility is increased as compared to the bulk mobility due to the high density of charge. The approach is very encouraging and opens the way to model space charge under timevarying voltages.