Investigation of pre-breakdown phenomena in polyethylene under high electric field with a streak camera

Needle-plane geometry samples were conditioned under AC voltage in order to grow an electrical tree at the tip of the needle. Observation of the different samples under an optical microscope revealed the presence of a small electrical tree (500 μm) at the tip of each needle. The samples were submitted to fast positive (20 ns/2 μs) impulse voltage in the rage of 30-90 kV. Light resulting from partial discharges was emitted from the region of the electrical tree, and lasted about 20 ns. The author presents a study of double emission phenomena resulting from the partial discharges activity under fast positive impulse voltage using a streak camera. It was observed that, under some conditions, a second light emission occurs 50 ns after the first one. These phenomena were correlated to an undershoot oscillation with a change of polarity which follows the first maximum of the impulse waveform. This second light emission could be related to charge accumulation from the preceding positive impulse voltage on the surfaces of the tree branches and in the bulk of the dielectric material