DC inclined-plane testing of silicone rubber formulations

The inclined plane test for tracking and erosion resistance of materials, IEC-60587, has become a routine tool for researchers developing high voltage outdoor insulation materials. The test is designed for ac applications and many laboratories have many years of experience with the technique. DC power systems are becoming more important and competitive in the market place. Often insulation systems designed for AC applications are utilised on dc systems. However, there is clear evidence of behavioural differences between identical materials under AC and DC stresses. An equivalent test to IEC-60587, has been developed and utilised for the dc case. A computer-based monitoring system is used to study materials beyond the criteria set by IEC 60587 and this system is reviewed in the paper. A number of different formulations of silicone rubber have been evaluated using the rig described. In this paper particular attention is given to the nature of electrical discharges under different test conditions and how this influences material degradation. Measurements show that the sample degradation mechanism is very different for positive and negative HVDC, while positive DC has shown to be the most aggressive for the materials tested.