Abstract
A widely used, effective method for diagnosing the insulation of a gas-insulated switchgear (GIS) is to detect partial discharges using signals in the ultrahigh frequency (UHF) band. To detect partial discharge signals that propagate inside a GIS, it is necessary to install a sensor inside or set on the outside of a GIS tank. For GISs that have already been constructed, it is effective to use external diagnosis using electromagnetic wave (EM-wave) signals that radiate (leak) from the insulation spacer of the GIS tank, and there is strong demand for such a diagnosis technology. Externally leaking EM-waves are often detected using an antenna, and multiple antennas supporting different frequency bands are commercially available. The accuracy of external diagnosis could be improved by comparing and evaluating the characteristics of various antennas. The insulation spacer of a GIS tank is usually installed on the flange and fastened with metal bolts. GIS tanks are made electrically continuous by these metal bolts and have the same grounding potential. EM-waves leak from the part of the solid insulator surrounded by these metal bolts and the GIS tanks. This leaking part forms a slot antenna and has a resonant frequency that depends on the spacing between the bolts in the circumferential direction of the flange. At this resonant frequency, the output of externally leaking EM-waves is higher and enables more sensitive measurement. In this study, the detection characteristics with different antenna positions and directions were compared in consideration of the number and positions of the bolts on the spacer, and the frequency characteristics of various antennas were obtained. To improve the detection sensitivity, a prototype of an improved dipole antenna was fabricated and its characteristics were evaluated. Based on the experimental results, an ideal diagnostic system based on the externally leaking EM-waves is proposed.
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