I. Introduction
Insulating paper is the most widely used solid insulation material in high-voltage power equipment, often assuming the dual role of insulation and mechanical support. Usually, oilpaper insulated power equipment is designed for a long life, and during its operation, the insulating paper will gradually age under the action of electrical, thermal, mechanical, chemical and other stresses [1–2]. Continued operation of power equipment when the insulation paper is seriously aging can induce equipment failure or even cause major accidents such as fire and explosion [3]. Therefore, it is important to accurately determine the aging state of insulating paper for power equipment maintenance, decommissioning decision making and ensuring the safe and stable operation of power system.