I. Introduction
Transformers play an important role in the electrical power transmission and distribution networks. The first transformer in the world invented in the 1880s was of dry type [1]. The lifetime of a dry-type transformer mainly depends on the condition of insulation material. The insulation system under combined thermal, electrical, mechanical, and environmental stresses undergoes gradual deterioration, which may lead to serious accidents and great losses for utilities and customers. The insulation system of a dry-type transformer mainly refers to the winding insulation, and whose main material is aramid paper. Therefore, it is important to study the aging characteristics of the aramid paper. The aging characteristics of oil-paper insulation used in oil-immersed transformers has been studied systematically and the aging mechanism theoretical system of the oil-paper insulation was researched by many scholars [2]–[5]. And some research has been done on aging evaluation of the insulation of dry-type transformers [6]–[7]. However, few research has been done in the aging of aramid paper.