Measurement of electric field induced crack growth in polymeric insulation using laser interferometry

In recent years a mechanical interpretation has been placed on the mechanism of electrical breakdown in polymeric insulation, with particular reference to the mechanism of electrical treeing. Electrical trees frequently develop from existing voids and delaminations in solid insulating materials in a manner not dissimilar from that of the propagation of cracks. The propagation of cracks has therefore been investigated in a polymeric wedge-shaped sample under the action of the vibrating Maxwell forces induced by the application of alternating voltage. The measurement of the microscopic growth of such a controlled crack has been achieved by applying the technique of laser interferometry. Wedge samples have been tested with a 10 kV, 50 Hz voltage applied between parallel electrodes across the mouth of the wedge for a period of eight hours. The growth of the crack at the tip of the wedge was monitored and the average crack growth rate was calculated. The results of this work have shown that there is appreciable crack growth during this period of voltage application which suggests that the vibrational Maxwell forces could have an effect on the creation of free space from voids or delaminations in practical solid insulation. The creation of free space is the basis for tree growth from such voids and delaminations, when they become sufficiently large to sustain partial discharges within them. A comparison has also been made between the crack growth in wedge samples and the adhesive bond strength between the insulating components which comprise the wedge tip. The technique of laser interferometry permits an accuracy of crack growth measurement of approximately one micron