In-service measurements of UHF attenuation in a gas insulated substation

An attenuation survey was carried out for UHF signals in a 400 kV gas insulated substation (GIS). The GIS is fitted with UHF couplers that form part of a partial discharge monitoring system. An impulse train generator was used to inject 15 V impulses into the couplers at a repetition rate of 100 MHz. The output signals at couplers further along the GIS were recorded using a spectrum analyser. Signal attenuation was calculated by summing the input and output powers for each harmonic. Differences in attenuation between the three phases are evaluated. Results confirm that the losses cannot be accounted for simply by assigning a fixed attenuation to each GIS component. However, the spread of the measurements was less than might be expected for such a wide variation in signal paths.