Charge distribution and electroluminescence in cross-linked polyethylene under dc field

The intent of this paper is to cross-correlate the information obtained by space charge distribution analysis and electroluminescence (EL) detection i n cross-linked polyethylene samples submitted to dc fields, with the object ive to make a link between space charge phenomena and energy release as rev ealed by the detection of visible photons. Space charge measurements carrie d out at different field levels by the pulsed electro-acoustic method show the presence of a low-field threshold, close to 15-20 kV mm(-1) above which considerable space charge begins to accumulate in the insulation. Charges are seen to cross the insulation thickness through a packet-like behaviour at higher fields, starting, at about 60-70 kV mm(-1). EL measurements show the existence of two distinct thresholds, one related to the continuous exc itation of EL under voltage, the other being transient EL detected upon spe cimen short circuit. The former occurs at values of field corresponding to charge packet formation and the latter to the onset of space charge accumul ation. The correspondence between pertinent values of the electric field ob tained through space charge and EL analyses provides support for the existe nce of degradation thresholds in insulating materials. Special emphasis is given to the relationship between charge packet formation and propagation, and EL. Although the two phenomena are observed in the same field range, it is found that the onset of continuous EL follows the formation at the elec trodes of positive and negative space charge regions that extend into the b ulk prior to the propagation of charge packets. Charge recombination appear s to be the excitation process of EL since oppositely charged domains meet in the material bulk. To gain an insight into specific light-excitation pro cesses associated with charge packet propagation, EL has been recorded for several hours under fields at which charge packet dynamics were evidenced. It is shown that current and luminescence oscillations are detected during charge packet propagation, and that they are in phase. The mechanisms under lying EL and charge packets are further considered on the basis of these re sults.