Assessing Materials For Use As Outdoor Insulation

After surveying the numerous processes which can cause progressive degradation and eventual failure of outdoor insulation, the thesis summarises the principal mechanisms of failure by tracking and erosion. Various tests for comparing the relative resistance of insulating materials to tracking and erosion are discussed and the problems of interpreting the results for practical applications considered. After preliminary tests with the CTI drop method investigations were concentrated on the Inclined Plane test. It was soon apparent that the procedures detailed in current specifications were not sufficiently well defined to ensure reproducible results. In particular it is shown that air circulation and ambient humidity can appreciably affect the results with some materials. It is also shown that if the inductive impedance of the test transformer is high there will be a significant distortion of the test voltage waveform when severe discharges occur. Thus tests made in different laboratories and particularly with several samples tested in parallel may give differing results. Other factors which may appreciably affect test results have also been investigated. As the damage caused by the discharges must depend partly on their magnitude and partly on their recurrence frequency, it appeared important to monitor the changes in discharge behaviour and of the associated surface current and test voltage during several successive cycles. Special techniques have been developed for this purpose and the thesis discusses the significance of the results obtained on different insulating materials. In order to fully understand some of the information obtained from these results a degree of correlation between the electrical and physical characteristics of the discharges was required. A method was developed which allowed simultaneous recording of the discharge current wave and the discharges themselves. These results are presented in the latter part of the thesis and the final discussion is concerned with the mechanisms involved in determining the complex discharge pattern obtained.