Effect of Dielectric Relaxation of Epoxy Resin on Dielectric Loss of Medium-Frequency Transformer

Abstract

Epoxy resin (EP) is a typical and widely used insulating material for medium-frequency transformers (MFTs). The dielectric loss of EP is one of the key variables in the design and operation of MFT. In this article, six types of epoxy/anhydride systems are prepared, and the dielectric loss of MFT is calculated by finite element method (FEM) analysis based on broadband dielectric spectroscopy measurements. The material dependence of the dielectric loss of MFT is investigated using dielectric spectroscopy and molecular dynamics simulations over a wide temperature and frequency range. The results show that the variation of dielectric loss with output power is attributed to the temperature characteristics of the imaginary part ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\varepsilon ^{\prime \prime }$ </tex-math></inline-formula> ) of complex permittivity. Compared to the dielectric loss at no-load state, the one at rated power is reduced by 5–10 W. The anhydride hardener containing benzene ring or a methyl substituent has an inhibitory effect on dielectric loss, and the EP cured by methyl nadic anhydride (MeNA) has the lowest dielectric loss (16.2 W) at rated power. The dielectric loss and overload characteristics of MFT are closely related to the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\beta $ </tex-math></inline-formula> -relaxation and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\alpha $ </tex-math></inline-formula> -relaxation of EP, respectively. The acid anhydride curing agent containing benzene ring or a methyl substituent reduces the temperature corresponding to the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\beta $ </tex-math></inline-formula> -relaxation peak by more than 50 °C compared to the one without. The increased rigidity of the chain segment of EP makes the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\beta $ </tex-math></inline-formula> -relaxation move to the low-temperature side, which is beneficial to suppress the dielectric loss. This article not only serves as a bridge among dielectric loss of MFT, dielectric spectroscopy, and molecular structure of EP but also lays a solid foundation for the selection of insulating materials for MFT.