UHF diagnostics for gas insulated substations

Gas insulated substations (GIS) have been made reliable by good design, and the adoption of strict quality control procedures during their manufacture, assembly and testing. In spite of this care, defects which might lead to failure can arise. These could be free metallic particles in the chambers, some misalignment exposing sharp edges, vibration or thermal cycling causing poor contact to electrodes, and so on-and all of these give rise to partial discharge (PD) activity before breakdown occurs. The authors describe how most success in detecting these PD is being achieved with the ultra high frequency (UHF) technique. This has been developed over the past ten years in the UK, and the couplers it needs are now fitted as standard in all new GIS of the National Grid Co. and ScottishPower. The UHF technique can be used both during site commissioning tests and in normal service, and enables any defect which might be present to be found and located so it may be corrected before complete failure occurs