Partial Discharge Monitoring of C3F7CN/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 SF₆ equipment, but still in its infancy for the emerging SF₆ alternatives as the power industry transitions from SF_6­­. PD characteristics of 20% C₃F₇CN / 80% CO₂ gas mixture as a viable retrofill solution were extensively investigated over extended energization periods in a busbar demonstrator designed for SF₆ 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 SF₆ and 20% C₃F₇CN / 80% CO₂ 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, SF₆ showed a comparatively longer time-to-breakdown when compared to a 20% C₃F₇CN / 80% CO₂ mixture. Results obtained in a controlled laboratory environment demonstrate wider applicability to field measurements of C₃F₇CN based mixtures adopted for retrofill application in SF₆-designed gas insulated busbar.