Nonconventional UHF sensors for PD measurements on GIS of different designs

As proven by service experience, GIS (gas insulated switchgear) show a good long-term performance. To exploit the service life as far as possible an assessment of the insulating condition of the equipment in question can be done using the UHF PD measuring technique. However, GIS of early designs are not equipped with any provisions for insulation monitoring so that two different types of nonconventional UHF sensors were developed which are applicable from outside the enclosure. The first sensor makes use of a field grading electrode inside a barrier insulator of a three-phase GIS. The second sensor is designed to be mounted outside the inspection window of a single-phase GIS. The sensitivity of the new nonconventional UHF sensors is compared to the conventional IEC 60270 method and to standard type UHF sensors according to the verification procedure proposed by CIGRE WG 15/33.03.05.