Dielectric Behaviour of Thermally Aged Epoxyester Varnish for Coverage of Large Generator Windings

Abstract

The primary objective of this study was to investigate the impact of elevated temperatures on the dielectric properties of epoxyester insulating varnish. This varnish, primarily used for its protective effects, forms a shielding layer that covers the insulation of windings within rotating machines, ensuring their resilience and uninterrupted functionality, particularly notable for its mechanical and chemical stability. The experimental setup involved the application of epoxyester varnish onto copper samples within a three-component insulation system, simulating the winding of a generator. To extend the scope of the investigation, thin layers of varnish were cast into silicone molds, creating samples for broadband dielectric spectroscopy. Thermal aging experiments were subsequently conducted at three distinct temperatures (150,175, and 200∘C) over varying exposure durations. Parameters including dissipation factor, relative permittivity, and insulation resistance were measured. Conducted findings revealed a decline in insulation resistance and observable color alteration, along with the onset of delamination of the insulating varnish, only at the highest temperature (200∘C). Broadband dielectric spectroscopy results indicated that the varnish remained uncured immediately after solidification (at 168 hours), followed by decreasing in relative permittivity. Visual inspections reveled discoloration and pronounced material brittleness during thermal exposure.