Natural Ester Filled Furnace Transformers

Electrical arc furnaces for steel production are powered by special liquid-filled power transformers which have to master high temperatures, numerous load cycles and frequent load changes, including severe overloads. For controlling the melt, the transformer output power must be regulated, which is usually done by on-load tap-changers (OLTCs). OLTCs for furnace applications have to perform a very high number of operations under high loads and so need frequent maintenance. Vacuum-type OLTCs have been found advantageous for furnace applications as they feature long maintenance intervals and require less oil change compared to non-vacuum type (arcing-in-oil) models. Electrical furnace applications need high operational reliability and environmental safety. It appears logical to fill furnace transformers with a less-flammable liquid, e.g. natural or synthetic ester. Regarding the carbon footprint, natural esters are advantageous to be used as insulating liquid for such applications. Due to deviating properties compared to mineral insulating oil, natural esters must be thoroughly evaluated regarding their electrical, thermal, physical and chemical suitability. It has been found that high oleic sunflower ester is the best choice for such demanding furnace applications, due to its well-balanced properties. The limited oxidation stability of natural esters requires closed (sealed) oil conservators to prevent the liquids from oxidative deterioration by ambient air ingress. As the tap-changer oil compartment is also filled with natural ester liquid, it must be verified that no free gases are produced during the load-switching operations, because they cannot not escape through the closed OLTC conservator. This paper describes the approach and effort of an Italian steel-producing company to meet the future challenges of increasing steel production under tightened environmental constraints. Transformer, tap-changer and liquid manufacturer present adequate solutions which are the result of thorough considerations, calculations and tests. Design rules for vacuum-type tap-changers for furnace applications in closed environment are given.