LUMINESCENCE IN CROSS-LINKED POLYETHYLENE AT ELEVATED-TEMPERATURES

Electrical treeing is often responsible for the breakdown of insulatin g materials used in power apparatus such as high-voltage transformers, cables, and capacitors. Insulation, such as crosslinked polyethylene (XLPE), used in underground high-voltage cables usually operates at te mperatures above ambient. This paper describes the characteristics of luminescence, emitted prior to electrical tree inception, at a crossli nked polyethylene-semiconducting material (XLPE-semicon) interface hel d above room temperature. Use of a sensitive light detection system sh owed that XLPE subjected to elevated temperatures emits luminescence e ven without voltage application. This light was attributed to thermolu minescence which decreased with the decrease in the concentration of t he crosslinking by-products present in the polymer. The spectra of the rmoluminescence were only in the visible range. On the other hand, ele ctroluminescence occurred when the XLPE-semicon interface was held abo ve room temperature and subjected to high electric stress. This light did not depend on the concentration of the crosslinking by-products an d the spectra of electroluminescence were in the visible and the ultra violet ranges. It is proposed that XLPE-semicon interface held at elev ated temperature and subjected to long-term voltage application initia lly emits both thermoluminescence and electroluminescence. As the cros slinking by-products exude out of the polymer, thermoluminescence decr eases with time and ultimately ceases, but electroluminescence occurs as long as the voltage is applied to the polymer. Although the intensi ty of electroluminescence emitted at high temperature was lower than t hat emitted at ambient, sufficient ultraviolet radiation was emitted. The ultraviolet radiation could photodegrade the polymer and lead to e lectrical tree inception.