A comparative study of insulator materials exposed to high voltage AC and DC surface discharges
Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2009.
The rising worldwide popularity of HVDC applications for power transmission purposes increases the need to study the performance of commonly used insulation materials when exposed to this voltage type. The aim of this study is thus to compare several insulation materials according to their resistance to erosion and tracking when exposed to surface discharges of HVAC and HVDC voltages of both polarities. The materials are tested according to the Incline Plane Test method described in the IEC 60587 standard, using a test voltage of 4.0 kV (rms). An important aspect of this project is the development of a bipolar DC source capable of delivering the required test voltage and current for both positive and negative polarities, while performing in accordance with the specifications set down in the standard. The design is intentionally kept modular in order to make it adaptable for future installations. Since the standard describes two different test methods, i.e. the constant tracking voltage method and the stepwise tracking voltage method, a comparison is done between the two methods in order to determine which one delivers best results for the test samples. The comparison shows that the methods deliver comparable results, but the constant tracking voltage method is preferred due to its superior test procedure. Finally, four different polymer insulator materials are tested and successfully compared under the effects of HVAC and HVDC of both polarities. The test materials include a RTV silicone rubber coated ceramic, two different HTV silicone rubbers and an EPDM rubber. The comparison is done according to the results obtained for a number of evaluation criteria. The results show that the RTV silicone rubber coating exhibits the least erosion for an AC voltage, but shows extensive erosion for a negative DC voltage. The HTV silicone rubbers exhibit only minimal erosion when exposed to an AC test voltage, but develop strong erosion under a positive DC voltage which can result in sample failure. The EPDM experiences its least erosion for a negative DC voltage, but shows maximum erosion severity for the AC voltage.