Partial discharge characteristics of cavities with different appearances and positions in solid insulation

A cavity is a typical defect in a solid insulator which may cause the appearance of partial discharge (PD) activity in power equipment [1–4]. It is mainly caused by imperfections in the manufacturing process as well as by the creep during equipment operation [5]. In epoxy resin insulators, the creep brings irreversible deformations that increase with time at elevated temperatures and external stress [6,7]. Once a cavity has been formed, the PD can lead to insulation failure. Many studies on PDs in void defects, a type of gas-filled cavity, have been performed by pasting together three epoxy laminates and demonstrated factors that affect their intensity, including void size, shape and interface roughness [8–12]. However, such a test void is quite different in its structure and composition from what we further consider in this article, i.e., cavities appearing during actual insulator operation. Moreover, the pressure in a real cavity is usually very low, because of air leakage and polymerization shrinkage, whereas the test void typically remains under atmospheric pressure. Considering the above mentioned differences, we prepared experimental model insulators, containing various low-pressure cavities. The insulators were manufactured by means of automatic pressure gelation (APG) technique by changing the pressure and temperature during the resin pouring process, which allowed for simulating a more realistic operating environment.