I. Introduction
Partial discharge (PD) is an electrical discharge within an insulation system that does not completely bridge the electrodes under high electric field stress [1]. Generally, when there are voids or protrusions at the insulator interface in electrical equipment used under high voltage, PDs occurs as a result of concentration of electric fields in these areas. There are several types of PDs that can occur for different reasons [2], but one type can damage insulation, and eventually lead to dielectric breakdown accidents. Therefore, it is a demanding task to monitor PDs to accurately determine the insulation performance and to perform preventive maintenance.