Using inclined plane test to compare tracking on silicon rubber under HVAC and HVDC.

Abstract

This dissertation presents research work conducted on high-temperature vulcanised (HTV) silicon rubber electrical tracking and erosion performances under HVDC and HVAC. The aim was to evaluate the performance under outdoor environmental conditions. Failure of polymer insulators is an area that has not been researched thoroughly in the past decade. The aging mechanism of insulators is of paramount importance in manufacturing and design. Outdoor conditions like humidity, moisture, pollution affect the durability of most insulators. Silicon rubber is widely used in power distribution and transmission insulation networks. It is also being used in power devices such as metal oxide surge arresters as the electrical insulation. Its advantages are that it is lightweight, low cost, easily processed, has good dielectric and hydrophobicity properties and has better pollution performance in outdoor insulation systems. The first part of this research looks at the procedure for testing following the normative recommendations as per IEC 60587: 2007 standard. Constant tracking voltage method was implemented in the inclined plane test for electrical tracking and erosion. A voltage of 4.5 kV was applied for all +DC, -DC and AC testing. The failure criteria as per IEC-60587 were followed. The most monitored criteria were that the leakage current magnitude should be below 60 mA. An increase in leakage current magnitude and duration of discharges were observed under +DC test conditions. The tracking time was recorded to be less under –DC than in +DC. The average current under +DC increases with the voltage application duration, however under AC the current does not increase. Under –DC voltage the current flow was observed to be less intermittent. Sample surface degradation mechanism was seen to be different in +DC, -DC and AC. A data logger monitored the leakage current measurements. An average leakage current of 9.40 mA was found under +DC. An average leakage current under –DC was measured to be 8.92 mA. Under AC voltage an average current of 11.90 mA was recorded. The second part of the study looked at the quantitative analysis techniques of silicon rubber. Silicon rubber test samples were carried out using scanning electron microscope (SEM) with Energy Dispersive Spectroscope (EDS), Fourier transform infrared microscope (FTIR) and transmission electron microscope (TEM). These physiochemical tests results were comparable for both DC and AC, it was concluded that the insulator deterioration was due to surface tracking and dry-band arcing discharges which resulted in the erosion of the test samples. The work reveals that +DC is more aggressive for the test samples tested. It was concluded that under +DC conditions tracking is more severe as compared to –DC and AC.