Electrical tree initiation

This paper reviews the recent understanding of the initiation mechanisms of electrical trees and the phenomena occurring during the induction period in the discharge free condition. The importance of charge injection in electrical tree initiation is well established, not only under dc or impulse stresses, but also under ac stress. Electroluminescence can be used as a monitor for charge injection. Tree initiation from field enhancing locations under moderate stress proceeds through cumulative degradation reactions and cavity formation caused by injected charge. The dependence of tree initiation on temperature and the nature of dissolved gases, underline the importance of free volume processes, and therefore of polymer morphology. Electron avalanche type tree initiation is strongly influenced by field modification due to space charge and also by the size of the high field region.