Investigation of impulsive breakdown of interfaces formed by ester insulating liquids and soild dielectrics

Due to the comparable dielectric behaviours, and significant enhancements in terms of environmental and safety performance, ester fluids are seen as a viable replacement for naphthenic liquids for use within insulating applications. In order to increase the adoption of ester liquids, and widen possible industrial applications, more extensive substantiation of their dielectric characteristics is required; particularly in how these dielectric liquids behave when exposed to impulse voltages. Furthermore, there is a lack of published work on how the breakdown properties of these liquids are altered when operating as part of a liquid-polymer insulation system. The present work details experiments conducted using impulse voltages of negative polarity to investigate the breakdown parameters (breakdown voltage and time to breakdown) of interfaces formed by dielectric liquids and a selection of polymer dielectrics. Samples of Nylon 66, Polymethylmethacrylate (PMMA), and Polyvinylidene difluoride (PVDF) were tested with: a synthetic ester, MIDEL 7131; a natural ester, Envirotemp FR3; and a naphthenic liquid, Shell Diala S4 ZX (used for benchmarking). Experimental results from this work show that, under the studied test conditions, the solid-liquid interfaces formed by the selected solid dielectrics and ester fluids exhibit comparable dielectric strengths to those formed with the naphthenic oil under investigation.