Dielectric liquids for subsea high voltage equipment

Pressure compensated high voltage power devices filled with a dielectric liquid are being developed for supplying electrical power to subsea factories extracting oil and gas from deep sea reservoirs. The dielectric breakdown behavior of a liquid improves at high pressure so the thermal properties will determine the reliability of the power device and the size of the pressure compensated enclosure. The selection of a suitable dielectric liquid is therefore critical. In this article, the appropriate Figures of Merit are established to compare the thermal performance of dielectric liquids for subsea use. Four different commercially available dielectric liquids (a natural ester, a synthetic ester, a mineral transformer oil, and an isoparaffinic liquid) are evaluated using a thermal network model representing natural convection circulation in an electrical power device at 300 bar. The isoparaffinic liquid performs best, closely followed by mineral oil.