What is a gas-insulated ring main unit (GIS)? What’s the difference between it and traditional air-insulated ring main units (AIS)?

What are the common pitfalls during GIS ring main unit installation?

1. Inadequate gas tightness inspection: Before installation, use an SF₆ leak detector (or dry air dew point meter) to check the cabinet’s sealed points, especially flange interfaces and valve connections. Unaddressed leaks may lead to reduced insulation performance or equipment tripping later. In one project, a displaced gasket during transportation caused a gas pressure alarm just 3 months after installation, resulting in high rework costs.
2. Excessive cabinet level deviation: Core components inside GIS ring main units have high requirements for installation accuracy. The level deviation of the foundation channel steel must be controlled within 2mm/m; otherwise, the Circuit Breaker operating mechanism may jam. During installation, use a level to measure each point instead of relying on visual judgment.
3. Non-standard grounding: The cabinet shell and metal brackets must have double grounding, with a grounding resistance ≤4Ω. Some projects take shortcuts by using only one grounding wire, which may cause the shell to become electrified during thunderstorms, posing safety risks.

During daily operation, how to judge if a GIS ring main unit is working normally? What regular inspections are needed?

1. Daily/weekly inspection: Check the gas pressure gauge (or electronic pressure display) on the cabinet. If the pressure is lower than the rated value (e.g., ≥0.4MPa for SF₆ models, alarm if ≤0.35MPa), immediately check for leaks. Listen for abnormal noises (such as sizzling discharge sounds) from the cabinet—these may indicate insulation faults.
2. Quarterly inspection: Use an infrared thermometer to measure the temperature of the external terminal blocks. If the temperature exceeds 70℃, loose connections may be causing poor contact, requiring power-off tightening. Check if the opening/closing indicator of the operating mechanism matches the actual status to avoid misjudging equipment operation.
3. Annual in-depth inspection: Hire professional institutions to test gas purity (SF₆ purity ≥99.9%, moisture content ≤150μL/L). If purity decreases or moisture exceeds standards, replace the gas. For cabinets with remote monitoring, check communication normalcy to ensure real-time data upload to the monitoring platform.

If a GIS ring main unit alarms for low gas pressure, how to handle it?

1. Cut off power to ensure safety: If the alarm is accompanied by equipment tripping, immediately cut off the incoming power to the cabinet to avoid faulty operation. Even if no tripping occurs, prepare for power-off to prevent insulation failure and accidents.
2. Accurately locate leak points: Use an SF₆ leak detector (or soap bubble method) to focus on flange interfaces, valves, and pressure gauge joints. Common leaks occur at aged gaskets or loose bolts. If the leak is in the cabinet body (e.g., welds), it may be a manufacturing defect—contact the manufacturer for repairs.
3. Replenish gas or repair as needed: For small leaks (e.g., annual leakage rate ≤0.5%), replenish the corresponding insulating gas to the rated pressure and monitor the pressure weekly. For large leaks (e.g., pressure drop >0.05MPa within a month), first repair the leak (e.g., replace gaskets, tighten bolts) before replenishing gas to avoid repeated refilling costs.

For discarded GIS ring main units, how to handle the SF₆ gas inside? Can it be discharged directly?