Understanding electrical trees in solids: from experiment to theory

A review of recent developments made in the understanding of the electrical tree mechanism is presented. The life of the tree is covered from initiation, through propagation, to long-term changes in shape. The initiation process is examined in terms of the injection of space charge and its ability to transfer energy to the polymer to create damage. Theoretical models for the processes involved are assessed in terms of the experimental data and an outline for the sequence of events in tree initiation developed. The inter-relationship between tree discharges, tree propagation, and tree shape is discussed. Theoretical models for these processes are evaluated in terms of their ability to reproduce experimental data, especially tree shapes and discharge sequences in time and space. The chaotic nature of the tree propagation mechanism is discussed both through experimental data and the results of a completely deterministic theoretical model. Some special features of electrical trees such as the existence of conducting trees, acceleration at long times and slow growth in thick insulation are briefly touched upon. Finally a summary of the state of the art is presented.