Continuous on line relative tan delta monitoring of high voltage insulation.
The thesis discusses the findings of an investigation into the use of novel condition monitoring techniques for oil-paper insulated high voltage equipment as used on the Eskom Main Transmission System. This research into the monitoring of the condition of high voltage (RV) insulation was undertaken because of the high failure rate of high voltage current transformers (eT's) and transformer bushings on the Eskom Transmission National Grid. These failures reached unacceptably high levels in the 1990's. The extent of failures has been quantified and was the driving motivation behind this research. Techniques for the condition assessment and condition monitoring of oil-paper filled high voltage equipment have traditionally focused on off-line techniques, in particular off-line tan delta measurements. This requires that the equipment be removed from service temporarily, but at widely spaced intervals before a measurement may be taken (typically every 3 to 6 years). Such techniques will not be able to detect faulty equipment where the insulation integrity deteriorates rapidly, resulting in catastrophic failure with risk to both adjoining equipment and personnel. The need for an on-line technique for detecting deteriorating insulation prior to failure was identified in the early 1990's and various systems were developed. This research investigation has focussed on assessing the use of on-line relative tan delta monitoring of RV insulation and compares this to off-line monitoring. In particular, the ability of such a relative tan delta measurement system to detect deteriorating oil-paper insulation has been assessed. The investigation has included the design, construction and commissioning of a dedicated test facility located at Eskom's Tugela substation. This test facility is unique in the world. This test facility has resulted in a number of experiments that have provided invaluable insight into possible failure modes of oil-filled high voltage equipment and the ability of on-line techniques to detect rapid failure modes has been carefully assessed. Further assessment of the on-line monitoring systems was also undertaken at various Eskom operational installations. The results of these tests and operational monitoring are addressed in this research. The research work and its findings are assessed against published literature and global activity in this important area.