By Edward Casserly
“While there is no magic list of properties or values that define an insulating liquid that will ultimately perform for 30 years or more, many decades of experience have shown what has worked over the years.”
Dielectric mineral insulating liquids have changed dramatically over the decades due to advances in refining technologies. The original oils were refined at a lower severity compared to today’s mineral oils, but they met the basic requirements as understood at the time. Due to higher power requirements, higher loads, and smaller footprints, today’s insulating liquids must meet the more stringent requirements in today’s standards. The refining processes used to manufacture these liquids, as well as the specifications in the standards and how they relate to performance, will be discussed in this article.
Introduction
Dielectric mineral insulating liquids have been around since the dawn of transformers. The first U.S. patent which disclosed the use of waxy paraffinic mineral oil, wherein the wax was considered an asset since it absorbed the heat from the core as the wax melted, was granted to Elihu Thomson of the Thomson-Houston Electric Company in 1890. The first U.S. patent which disclosed the use of naphthenic mineral oil was granted to Harold Maitland of Sun Oil in 1926. Insulating liquids from paraffinic mineral oils could only be used in tropical climates since the residual wax resulted in high pour points. The isoparaffinic insulation liquids with acceptably low pour points were available after the development of new catalysis technologies were commercialized in the 1980s and 1990s. More recently, gas-to-liquid (GTL) insulating liquids, which are highly isoparaffinic, have been produced from natural gas.
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