Experimental Study on the Evolution of Dielectric Properties of Impregnating Varnishes with Thermal Aging

Impregnating resins are used in order to provide a mechanical and an electrical insulating support to windings. This work aims to study the thermal aging of two polyester-imide resins (Class H), which are made with reactive diluent based on styrenic and methacrylate monomers. Electrical characterizations such as dielectric complex permittivity (real (e') and imaginary (e”) parts), DC conductivity and dielectric breakdown are measured at 180°C on virgin and aged samples in order to assess the evolution of the insulating properties with the thermal aging of 240°C. The complex dielectric permittivity (e', e'’) and the DC conductivity (σ DC ) show a decrease of values, while, the dielectric breakdown increases after few hours of thermal aging. These findings suggest an improvement of the resins insulating properties. Indeed, the initial significant increase on dielectric properties of polyester-imide varnishes with temperature due to a thermal runaway and/or to space charge mobility are no longer observed after aging. Finally, the infrared spectrometry using the Fourier Transform (FTIR) and Differential scanning spectrometry (DSC) are performed to assess the variations of the chemical structures. A possible competitive processes of both oxidation and cross-linking causing respectively degradation and structural densification could product additional charges and/or alter charge mobility that explain such electrical changes of the varnishes behaviors.