Light emission and subsequent tree inception due to polarity reversal of the local field in polymeric insulation

The authors describe the treeing characteristics of normal and degassed low-density polyethylene under AC, DC, and half-rectified AC voltages. By individually controlling the amplitudes of the positive and negative half-cycles of the AC voltage, it is shown that the polarity reversal of the local field in the polymer plays a crucial role for light emission and subsequent tree inception. If light is not emitted, then treeing does not occur, because it is impossible for the unipolar injected charge to gain sufficient energy to cause impact ionization or break bonds of the polymer chain. The injection of electrons and holes and their recombination at luminescent centers gives rise to light having spectra in the visible and the UV (ultraviolet) ranges. The UV light photodegrades the polymer and leads to the formation of an electrical tree