High voltage plant monitoring through oil/paper analysis

In order that plant maintenance staff can plan outages, and replacement/refurbishment of equipment that is not up to agreed standards, analysis of the condition of the insulation system of high voltage plant has to be carried out. In the case of oil-filled high voltage plant the component of the insulation system which is easiest to access, and which comes into contact with most other components, is the insulating fluid. For this reason, oil analysis is the front-line condition monitoring tool for oil-filled high voltage plant. The detection of degradation by-products-be they solid, liquid or gaseous-from the oil is usually indicative of both a deterioration in the properties of the oil and the presence of a detrimental stress mechanism within the overall insulating system of the plant, be it thermal, electrical or chemical. Where the oil is in contact with other component materials in the insulation system, e.g. metals, ceramics, paper and/or polymerics, there are additional complexities added to the degradation processes which can occur. The various components can both affect and be affected by the species generated by the degradation of the oil, e.g. acids. The range of analysis methods applied to the oil, to determine the condition of the insulation, investigate: the gases evolved from oil discharges or hot spots, the gases evolved from polymer and paper degradation, the solid by-products of polymer and paper degradation, the metallic or organometallic by-product materials from conductor erosion/interaction, and the liquid by-products of degradation of water, acids and sludge. Hydrogen and a variety of hydrocarbons, e.g. methane, ethane, ethylene and acetylene, are generated in mineral oil by the various stresses prevalent in high voltage plant. Oxygen and nitrogen are also found in oil filled components but these are thought to come from contamination by atmospheric gases. In addition, as indicated above, some gases are associated with polymer and paper degradation (e.g. carbon dioxide, CO₂). By measuring the levels of the various species present and, in the case of gases, the ratios of specific gases, it is possible to give an indication of the type and extent of the degradation occurring within the high voltage plant. The majority of the current methods applied to the analyses, and the interpretative techniques used, have been utilised in the industry for many years. The development of modern technologies, and improvements in traditional techniques, has provided additional information which raises the question of the suitability of traditional analyses for determining the condition of plant insulation