Estimation of UHF signal propagation through GIS for sensitive PD detection

Defects in SF₆ gas-insulated switchgear can result in breakdown of the gaseous insulation. Therefore, it is important to detect defects, such as protrusions and particles, in time and to determine their risk for a possible breakdown. For on-line PD detection, the UHF method has proven to be a sensitive method. Unfortunately, the propagation path of the electromagnetic waves from the location of the defect to the UHF coupler influences the magnitude of the UHF signals: the UHF signals attenuate, mainly due to reflections at the different components. As a result, several UHF couplers are required to reach enough sensitivity. In this contribution a transmission line model has been used to estimate the signal reduction of each GIS component. This has been used to determine coupler locations to reach certain sensitivity, which has been verified using the CIGRE sensitivity check.