Charge transfer and tree initiation in polyethylene subjected to AC voltage stress

An electrical tree is grown in the region of a dielectric subjected to an intensified local electric field. It can be artificially generated in a specimen by using a needle-to-plane electrode configuration. It is characterized by an incubation period which usually tends to decrease with applied voltage. During this period, charge carriers (electrons and positive holes) move back and forth repeatedly between the electrode and the dielectric. This repeated charge transfer is believed to be mainly responsible for tree initiation. Two main contributions can be conceived: the space charge buildup and the attack on the polymer chains by high-energy charge carriers. A space-charge distortion model is proposed for the initiation of an electric tree in polyethylene. This model can also include material aging processes initiated by injected carriers. It can explain ultraviolet light emission and morphological effects which are experimentally observed in association with tree initiation