I. Introduction
Partial discharge is one of the main factors that lead to the insulation deterioration in GIS and even affect the safe and stable operation of GIS. The UHF detection method has been widely used in PD detection in GIS in recent years because of its strong anti-interference ability and facilities for condition monitoring [1]. The UHF sensor is the device that receives the electromagnetic wave signal excited by the PD source [2]. According to the installation position, it can be divided into two types: internal sensor and external sensor. The internal sensor is installed in the interior of GIS, which has the characteristics of high detection sensitivity and strong anti-interference ability. However, it is usually manufactured together with the GIS body, and can not be flexibly repaired and replaced. According to the characteristic that UHF electromagnetic wave can leak from the basin insulator of GIS [3], the external sensor can detect the partial discharge in GIS, which has the advantages of not affecting the internal electric field of the equipment, flexible installation, maintenance, and portability. Therefore, the external sensor has been widely studied in recent years.