Fluids, liquids or gases, that are used in electrical equipment serve three essential purposes. These fluids must provide adequate insulation, efficiently transfer heat from the source to the atmosphere and quench arcs that may develop in the equipment. To meet these requirements the selected fluid must consist of non-polar molecules that are chemically stable. Viscosity, specific heat and thermal conductivity are properties that must also be considered when evaluating the ability of the fluid to conduct heat. Several liquids and gases that possess these attributes have been used as dielectric materials in electrical equipment. These liquids, such as mineral oil, are produced from natural sources or are chemically synthesized. Silicone dielectric fluids and perchlorethylene are examples of synthetic products. In addition to the above required properties other factors that are considered in the selection of a dielectric fluid include: resistance to oxidation and other types of chemical degradation, stability at high temperature, flammablitiy, solvent properties, toxicity and bio-degradability, cost and availability. Oxidation of mineral oils results in acid production. Acid at any concentration attacks cellulose insulation and can lead to formation of sludge. Sludge can plug oil circulation pathways which can lead to overheating. Silicone fluids, in contrast, are resistant to oxidation and acid formation. High temperature stability is required so that the material does not undergo molecular degradation at anticipated operating temperatures. For safety reasons dielectric fluids should have high flash and flame points, especially if the equipment is located indoors. High flash point was a major factor that promoted the wide use of polychlorinated biphenyl (PCB) fluids before the environmental hazards were known. The selected dielectric fluid should not dissolve insulation materials or gaskets used in the construction of the electrical unit. It is for this reason that transformers that are retrofit with silicon fluids require replacement of the installed gaskets, that were not attacked by the original PCB fluid or mineral oil. Toxicity and bio-stability are increasingly important from an environmental and liability perspective.
展开▼
