Charge Distribution and Electroluminescence in Xlpe under Dc Field

The intent of this paper is to cross-correlate the information obtained by space charge distribution analysis and electroluminescence (EL) detection in cross-linked polyethylene samples submitted to DC fields, with the objective to make a link between space charge phenomena and energy release as revealed by detection of visible photons. Space charge measurements carried out at different field levels by the pulsed electro-acoustic method show the presence of a low-field threshold, close to 15 to 20 kV/mm, above which space charge begins to accumulate considerably in the insulation. Charges are seen to cross the insulation thickness through a packet-like behavior at higher fields, starting at about 60 to 70 kV/mm. EL measurements show the existence of two distinct thresholds, one related to a continuous excitation of EL under voltage, the other being transient EL detected upon specimen short-circuit. The former occurs at values of field corresponding to charge packet formation and the latter to the onset of space charge accumulation. The correspondence between pertinent values of the electric field obtained through space charge and EL analyses provides a support to the existence of degradation thresholds in insulating materials. Particular emphasis is given on the relationship between charge packet formation and propagation, and electroluminescence. 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 electrodes of positive and negative space charge regions that extend into the bulk prior to the propagation of charge packets. Charge recombination appears to be the excitation process of EL since oppositely charged domains meet in the material bulk. To gain an insight into specific light excitation process associated with charge packet propagation, electroluminescence 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. Mechanisms underlying EL and charge packets are further considered on the basis of these results.