Evaluation of the circular metal plate as a UHF coupler for partial discharge monitoring

UHF monitoring of partial discharges inside GIS structures requires the use of a coupler whose function is to transfer UHF signals from within the GIS cavity to an external measurement system. Resonances and reflections within the GIS extend the PD pulse in the time domain and modify its spectral content. Similarly the circular coupler must be regarded as a resonant structure rather than as a simple capacitor. This paper applies the theory of resonance in explaining the effect of the coupler on the UHF content of the partial discharge signal. Various disc sizes, dielectrics, spacings and coaxial probe locations have been investigated together with the effect of earthing of the disc at selected points