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.

No abstract

Space charge plays a significant role in long term electrical degradation of polymeric insulation in high voltage cables and consequently there is growing interest in the measurement of the energy dissipation of mobile and trapped charges in dielectric molecules. The dissipation process is associated with the emission of visible photons, a process known as electroluminescence (EL) and can be used, potentially, as an indicator for the initiation of electrical ageing of insulation. This paper presents the experimental work undertaken to examine EL from different ac applied voltages. Using a charge coupled device (CCD) detection system, the setup allows the determination of the light emission occurrence respect to the applied field. Phaseresolved EL measurements have been obtained using sinusoidal, triangular and square voltages.978-1-4244-2549-5/$25.00

Ac electroluminescence (EL) in polyethylene films is well documented on experimental grounds but a detailed model encompassing injection and transport is still to be strengthened. In particular, different papers report that the EL under ac sinusoidal voltage for both divergent and uniform field configurations is independent of a superimposed dc voltage offset, which is confirmed by our own measurements. Based on modelling, we show that the dc offset voltage induces a transient behaviour of the ac EL; where the symmetry between the EL peaks occurring during each half cycle of the ac voltage is broken but the same steady state is reached for any value of dc offset. The transient duration is short relative to the timescale of a typical EL measurement explaining why it has not been reported experimentally. It depends on the injection level, charge mobility and recombination efficiency. The model also explains the dependence of ac EL on the presence of homo-space charge near the electrodes.

Recently, a charge transport model allowing the description of electroluminescence in polyethylene films under AC stress has been developed. The fluid model incorporates bipolar charge injection/extraction, transport and recombination. The model has been very successful in explaining the timedependence of EL under 50 Hz sinusoidal, triangular and square voltage waveforms. In this work we discuss experimental measurements and simulations using different frequencies of the sinusoidal stress, voltage amplitude and offset.