Assessing the resistance of polymers to electrical treeing
The susceptibility of polymeric insulating materials to breakdown by ‘treeing’ has impeded their utilisation as insulation for high voltage cables. This review surveys the results of tests used to investigate tree initiation and propagation and discusses factors which affect the resistance of materials to breakdown by treeing and the mechanisms involved. Several test procedures can provide reproducible results, provided the processing of the test samples and conditioning before tests' are carefully controlled. However, changing the electrode geometry or the morphology of the polymer can greatly affect the electric stress required for tree initiation and the subsequent time to breakdown. The stress at which tree initiation occurs, in a given polymer, depends on the waveform, frequency and time of application of the test voltage, and also on whether the voltage is applied continuously, or periodically interrupted. Usually tree initiation and breakdown occur at lower voltages as the ambient temperature is raised, but the effects of humidity are more complex. In view of the many factors which may affect the treeing resistance of polymers it would seem expedient to recommend standard test conditions so as to facilitate the assessment and comparison of polymers and the effects of additives. Preferably such a recommended test should take account of operating conditions in cables; thus it should be suitable for tests at the maximum temperatures used in cable operation. It may also be wise to demonstrate that additives which are beneficial in small injection or compression moulded samples are equally effective in extruded insulation, where the morphology of the polymer may be very different.
