Assessing dielectric integrity of oil/paper insulated cable-GIS terminals using on-site techniques

Catastrophic failures of Gas Insulated Substations (GIS) involve both economical and logistical problems. Unexpected explosions and fires may produce power plant long unavailability periods, expensive reparation activities and, in extreme situations, might affect personnel integrity. In an effort to minimize any risk of failure either on the GIS itself or specifically on cable to gas transition terminals, high sensitivity non-conventional on-line partial discharges (PD) detection techniques have been carried out as a first diagnosis criteria. Once a higher PD value pattern is detected using on-line techniques, off-line PD measurements are carried out using a mixture of UWB and standardized PD detection methods. Experiences and results of on-line and off-line PD measurement and detection techniques are presented and discussed in this paper, emphasizing calibration and selectivity issues. When it is mandatory, internal inspection of faulty confirmed cable to gas transition terminals was carried out to validate the results of on-line and off-line techniques. Main results of internal inspection are showed and analyzed. Materials deterioration, on-site poor quality terminal manufacture and assembly and electric potential distribution inside the terminal were carefully analyzed. The systematic approach described in this paper allowed Mexican main Utility to assess the dielectric integrity of GIS and its accessories, preventing catastrophic failures and unscheduled repair activities.