Photo-Inspection between Cross-Linked Polyethylene and a Semiconducting Layer

Compatibility between a cross-linked polyethylene layer and a semiconducting polyethylene layer in a block sample, simulating a solid dielectric cable, is investigated by use of a photo-multiplier to detect light emission at the interface. The application of a photo-multiplier to detect the quality of contact at the interface provides superior sensitivity to the conventional discharge detection method which, itself, is sensitive only to actual treeing discharges and consequently is valid as a detector only after the start of electrical treeing. The semiconducting polyethylene layer used as an electrode in the experiments is sandwiched between sheets of low-density polyethylene (M. I. = 1.0) and heated with dicumyl peroxide to achieve chemical cross-linking. As determined from the output of the photo-multiplier, the treeing inception voltage increases with the quality of the contact. A temperature dependency of the contact is also detected by the photo-multiplier in the range above room temperature where light emission shows a maximum at about 60°C, suggesting that contact becomes poorest at about that temperature. The effect that mechanical drawing of the test samples has on light emission at the interface is also investigated at various temperatures by means of the photo-multiplier.