Failure Investigation of Gas-Insulated Switchgear at 110-500 kV Substation, Maintenance and Repair

A key part of the study covers outages across 30 GIS installations from 2020–2023. About 900 decommissioning requests were analyzed, split nearly evenly between scheduled and unscheduled outages. On average, each GIS bay is taken out of service every 18–24 months for approximately three days, largely independent of rated voltage. Statistical data show that current circuit breaker switching and mechanical lifetimes are rarely consumed; in practice, short-circuit currents are significantly below rated interrupting values. This means circuit breakers will not exhaust either switching or mechanical resources over the possible service life of 30–50 years.

Two detailed incident investigations illustrate the causes of GIS failures. These cases highlight vulnerabilities related to poor assembly, material quality, and defects in moving contact assemblies. International CIGRE data indicate that GIS failures predominantly fall into two categories:

inability to perform required switching operations (63%) and insulation dielectric breakdowns

(23%). The share of operational failures increases sharply after 15–20 years due to aging and corrosion of mechanical parts. However, in Rosseti’s installations, most breakdowns occurred within the first 2–4 years of operation rather than during the expected 15–20-year “critical age”.

Nevertheless, half of the equipment fleet will soon reach this age threshold, raising concern about future reliability. The study finds that current maintenance strategies—based on routine scheduled servicing— are insufficient for GIS approaching mid-life. Manufacturers recommend medium or major repairs every 12–20 years, but many utilities worldwide avoid these because they require opening the gas-tight enclosure, increasing cost and risk. Instead, the authors advocate transitioning to condition-based maintenance using diagnostic tools already available, such as partial discharge monitoring and SF₆ decomposition analysis. This shift would allow early detection of insulation degradation and mechanical wear before major failures occur.

Overall, the research concludes that while routine maintenance has been adequate so far, approaching mid-life GIS assets will require more sophisticated diagnostics and conditionbased repair planning to maintain reliability and prevent the anticipated rise in dielectric failures.