Abstract :
Summary form only given. National Grid Company (NGC) have 15 years experience of operating EHV GIS substations and have 6 major installations with a further 3 on order. In order to further increase the reliability of GIS switchgear NGC has been active in evaluating alternative in-service diagnostic methods which are sensitive to deterioration of either the solid or gaseous dielectric. Work by NEI Reyrolle has resulted in the development of a UHF partial discharge coupler and practical site test techniques which have been successfully applied during commissioning of GIS substations. As a result of the developments in the UHF partial discharge technique NGC are now specifying that monitoring facilities should be installed in all new GIS substations. The author briefly discusses the basic principles of the technique and how the UHF signals are propagated within the CIS and subsequently detected. The applications of the technique, with reference to a 420 kV CIS substation, is discussed
