Dielectric Properties of Diala D, MIDEL 7131 and THESO Insulating Liquids

Insulating oils can be used as dielectric materials in a wide range of high voltage applications. Most notable amongst these liquids are naphthenic mineral oils, which are used in elements of pulsed power systems, such as switches and capacitors. Dielectric liquids are also used as energy storage media in pulsed power systems. In this application the use of liquids with higher dielectric permittivities will allow high-density energy storage systems to be developed. A dielectric oil with an unusually high, static relative permittivity of ~15, THESO oil, has been recently developed by Tetra Corp. This paper compares the measured dielectric properties of three types of insulating liquids: Diala D (Shell Ltd), ester liquid MIDEL 7131 and the high permittivity oil THESO. The study includes measurements of their relative permittivities, their electrical conductivities and their impulse dielectric strength. The data obtained on the dielectric characteristics of these liquids will be important for the optimisation of their applications in high voltage and pulsed power systems.