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Partial discharge detection using a UHF band signal is a well known advanced insulation diagnosis method in gas insulated switchgear (GIS), and has been well studied. In contrast to conventional diagnosis with lower frequencies in the kHz range, UHF band signal above the cutoff frequency has been detected with higher-order modes that only appear in electromagnetic signal propagating inside the GIS tank. This is because the wavelength of UHF band signals is comparable to the GIS tank size. The authors had observed the characteristics of such higher-order electromagnetic waves with a focus on the resonance characteristics of the TE11 mode, using a disk-shaped UHF sensor with the sensor extending into the inside of the tank. The purpose of this paper is to investigate the propagation characteristics of higher-order mode waves in a coaxial GIS model. Considering application to actual equipment, it was investigated that the output of a sensor with an arch-shape not extending into the inside of the tank, which has less influence on the propagation mode of the inner electromagnetic wave. In the frequency domain below the cutoff frequency of the TE11 mode, the output characteristics were almost independent of the installation position of the UHF sensor, but in the higher frequency domain the output power displayed discontinuous increases at some frequencies. It was also studied that the circumferential dependence of sensor output. For higher-order modes resonant characteristics appeared that depended on the tank length, and it was recognized that the electric field distribution inside the tank influenced the output of the UHF sensor at resonant frequencies. Further, it was found that installing a spacer inside the tank shifted resonant frequencies and the influence of the spacer consistent with the relationship between the spacer position and the electric field distribution inside the tank.

Propagation Characteristics of Higher-order Mode Electromagnetic Signals in Coaxial GIS Model with Various Conditions of Arch-shaped UHF Sensor