Partial Discharge Monitoring of C₃F₇ CN/CO₂ Mixture Retrofilled in Gas Insulated Busbar

Condition monitoring of gas insulated high voltage equipment is important to ensure reliable operation of electricity networks. Partial discharge (PD) monitoring is well established for existing SF ₆ equipment, but still in its infancy for the emerging SF 6 alternatives as the power industry transitions from SF ₆ . PD characteristics of 20% C ₃ F ₇ CN / 80% CO ₂ gas mixture as a viable retrofill solution were extensively investigated over extended energization periods in a busbar demonstrator designed for SF ₆ and rated to 420/550 kV. Protrusions of 3, 5, 10 and 15 mm needle lengths were used on the high voltage conductor for comparative PD investigations of SF ₆ and 20% C ₃ F ₇ CN / 80% CO 2 gas mixture. PD activity was measured with IEC 60270 (apparent charge) and ultra-high frequency (UHF) detection methods. The PD patterns recorded for all needle lengths were representative of a protrusion-on-conductor defect and the two PD detection methods were comparable. For identical defect lengths, SF ₆ showed a comparatively longer time-to-breakdown when compared to a 20% C ₃ F ₇ CN / 80% CO ₂ mixture. Results obtained in a controlled laboratory environment demonstrate wider applicability to field measurements of C ₃ F ₇ CN based mixtures adopted for retrofill application in SF ₆ -designed gas insulated busbar.