Influence of space charges and content of oxygen on the luminescence light and dielectric aging effects

The influence of space charge in conjunction with oxygen on the characteristics of the electroluminescence (EL) emitted during the initiation of an electrical tree in low-density polyethylene (LDPE) was investigated. Measurements were done on both Lupolen 1812 DSK and BPD 4901 materials. Sensitive and extensive phase analysis of the EL light relative to the applied high voltage revealed a reproducible phase shift behavior, depending on voltage, stressing time and frequency. The ratio (I_-/I₊) of the light intensities, emitted in the negative and positive halfwaves depended strongly on the applied voltage and stressing time. The observed slight differences between Lupolen and BPD revealed the influence of morphology and chemistry on the distribution of electron and hole traps. An increase of frequency resulted in a decrease of the phase shift, in both the frequency and voltage domain. Spectral analysis revealed that the EL light was emitted in the range from 350 to 800 nm, reaching its maximum at about 550 nm. During the evolution of electrical trees, the degradation products of LDPE could be identified