The effect of impulse rise–time on the breakdown of composite ester-polymer insulation

This paper reports and discusses the breakdown behaviors of composite insulation formed from insulating fluids and practical polymers, under test conditions representative of electrical stresses experienced within pulsed power machines. Interfaces formed from the synthetic ester fluid, MIDEL 7131 (M&I Materials, UK) and the natural ester fluid, EnviroTemp FR3, (Cargill, USA) and solid polymers (NYLON 6,6 and PMMA) have been stressed with discrete negative impulses until breakdown; with measured parameters compared to those gathered for identical composites utilizing transformer oil (Shell Diala S4 ZX). Dissimilarity in the behavior of the composite insulation systems has been observed; with those formed of the Nylon polymer shown to possess a tendency to experience bulk polymer breakdown rather than surface flashover; the predominant form of breakdown seen in tests conducted on liquidPMMA samples. Regardless of the type of breakdown experienced, both ester fluids offer comparable levels of dielectric strength to that of transformer oil under the test conditions applied. It can also be reported that a clear sensitivity to the impulse parameters (rise-time) was experienced by all PMMA interfaces, with a reduction in dielectric strength observed as impulse rise-time was increased; with no such trend witnessed in the behavior of composite systems utilizing the Nylon polymer.