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UHF Couplers for GIS - Sensitivity and Specification

A novel calibration system is used to measure the sensitivity of UHF couplers used for partial discharge (PD) detection in gas insulated substations (GIS). The technique employs a gigahertz transverse electromagnetic (GTEM) test cell to generate a subnanosecond electric field step at the coupler aperture. The frequency response of the coupler is obtained from its time domain response using FFT processing. Results for internal and external couplers are presented and the effect of the coupler mounting recess is investigated. A possible approach to formulating specifications for UHF couplers is then described.