Dielectric windows improve sensitivity of partial discharge detection at UHF

To detect partial discharges (PD) in equipment such as gas insulated substations and power transformers, UHF couplers can often be fitted at dielectric apertures (windows) in the metal cladding. Waveguide theory is used to show how the window structure affects PD detection sensitivity in the UHF band. By increasing the relative permittivity of the propagation medium, the effective diameter of the window can be increased, leading to improved UHF signal transfer. A given sensitivity to PD can therefore be achieved using a smaller, more robust window, which is particularly important when the insulation is a liquid or gas under pressure. Alternatively, if the window size is fixed, filling it entirely with a dielectric material can improve PD detection sensitivity. A window designed specifically for coupling UHF signals is proposed as an alternative to the use of internal couplers