Electroluminescence technique to evaluate the effect of impulse tests on high voltage cables

EL (electroluminescence) in XLPE (crosslinked polyethylene) subjected to impulses of various amplitudes, shapes and repetition rates has been investigated. Defects that could occur in the polymeric insulation of underground HV cables were simulated by embedding semicon protrusions in XLPE to create highly divergent electric fields. Time resolved measurement of the EL pulses emitted during positive, negative or alternating polarity impulses has shown that the EL pulses are grouped into two distinct time periods between which there is no EL activity. The EL emission probability increased with fall time of the impulse voltage but was independent of the impulse rise time. A study of electroluminescence characteristics from unaged and aged XLPE, subjected to a pair of positive and negative impulses, indicated that the charge trapped in the polymeric insulation during aging plays an important role during impulse application and could affect insulation failure.