MV GIS Operation and Preventive Maintenance Guide

Gas Insulated Switchgear (GIS) has become the preferred choice in medium and high-voltage distribution systems, highly valued for its fully sealed design, superior reliability, and extended service life. Although GIS is widely regarded as “low-maintenance” equipment, its continued high performance depends heavily on strict adherence to operational protocols and proactive maintenance practices. A thorough understanding of the GIS’s core advantages and characteristics, coupled with strict execution of its usage and maintenance guidelines, is fundamental to guaranteeing the equipment’s long-term safety and operational stability.

I. GIS Switchgear Overview

GIS, or Gas-Insulated Metal-Enclosed Switchgear (often used in MV applications such as Ring Main Units), is a critical component in modern power distribution systems. Its unique design utilizes an inert gas (such as SF6 or environmentally friendly gas mixtures) as the primary insulating medium, completely sealing all medium and high voltage switchgear components—including switches, busbars, and circuit breakers—within a grounded metallic enclosure. Thanks to the high dielectric strength of the gas, GIS significantly reduces the required insulation distance, leading to its renowned compactness. Equipment volume is typically reduced by 60% to 90% compared to traditional Air Insulated Switchgear (AIS), making it the ideal solution for space-constrained environments such as urban networks, subways, high-rise buildings, and large industrial facilities.

The core strength of GIS lies in its fully sealed, high-tightness design. High-voltage live parts are completely isolated from the external environment, effectively eliminating threats from condensation, dust, pollution, salt spray, and even small animals, demonstrating superior environmental adaptability. This robust reliability ensures the long-term stability of the equipment in harsh conditions, including deserts, coastal areas, and chemical plants. Furthermore, since internal components operate in a clean, dry, sealed environment, they are protected from external corrosion and oxidation, resulting in an extremely low failure rate.

Due to its exceptional safety and low maintenance requirements, GIS typically achieves maintenance-free or extremely low-maintenance periods exceeding 20 years, substantially extending the equipment’s overall service life. Leveraging these key advantages requires operators to deeply understand the specific maintenance requirements necessary to sustain the equipment’s performance over time.

II. Routine GIS Switchgear Operation and Maintenance Guidelines

To ensure the long-term safety, reliability, and stability of the GIS, operators and maintenance personnel must strictly follow inspection and maintenance procedures across these 8 key areas:

1. Gas Compartment Pressure and Sealing Monitoring (Critical Maintenance Point)

The performance of GIS fundamentally relies on its gas insulation.

• Routine Checks: The pressure of the SF6 or other insulating gas within the compartment must be checked periodically. The SF6 gas pressure gauge
  or density relay is the key monitoring tool.
• Critical Limits: If the pressure drops to the alarm threshold, the source of the leak must be found immediately, and gas replenishment must be performed; if the pressure drops to the lockout threshold (below the minimum safe operating pressure), the affected bay must be immediately de-energized and decommissioned.
• Leak Detection: Leakage is the primary factor affecting GIS longevity. High-precision gas leak detectors should be used for timely inspection to ensure optimal gas tightness.

2. Grounding System and Anti-Vibration Checks (Safety Assurance)

The reliability of the grounding system is the fundamental guarantee for equipment and personnel safety.

• Grounding Continuity: Periodically check the GIS grounding (earthing) connection points for looseness, corrosion, or poor contact. Ensure continuity of the enclosure, cable sheaths, and all accessible metallic parts to prevent touch or step voltage hazards during a fault.
• Anti-Vibration Measures: If the GIS experiences severe or abnormal vibration during operation, immediate action must be taken:
  1. Emergency Shutdown: Immediately stop the equipment’s operation.
  2. Detailed Investigation: Check if the vibration is caused by external mechanical shock, internal mechanism jamming, or abnormal contactor chatter. Resolve the fault before restoring service.

3. High-Voltage Cable and Termination Inspection (External Connection Points)

Although the GIS itself is highly insulated, external cable connection points remain potential weak links.

• Cable Condition: Routinely check the high-voltage incoming and outgoing cables:
  • Inspect the cable outer sheath for aging, damage, or cracking.
  • Utilize Partial Discharge detection equipment to monitor cable terminations for Partial Discharge activity, which is a typical indicator of insulation deterioration.
• Abnormal Sounds: If abnormal “hissing,” “crackling” sounds, or an ozone smell are detected, these are usually signs of corona or spark discharge. The equipment should be immediately taken out of service for inspection.

4. Operating Mechanism and Indication Device Checks

The functional accuracy and reliability of the operating mechanism and indicators are mandatory.

• Operating Mechanism:
  • Switching Operations: Periodically perform open and close operations to check the reliability of the GIS operating mechanism, the status of the spring charging motor, and the functionality of the mechanical interlocks.
  • Position Indication: Verify that the mechanical position indicators for the circuit breaker, disconnector, and grounding switch accurately align with the electrical signal.
• Voltage Indicating System: Check that the voltage presence indicator is functioning correctly, as it is a crucial reference for safe maintenance. The indication must be verified prior to any switching operation.

5. Enclosure Cleaning and Anti-Corrosion

Despite the excellent gas tightness, enclosure maintenance remains important.

• Surface Cleaning: Keep the GIS enclosure surface clean, promptly removing accumulated dust, oil, or corrosive substances, especially around terminals and control panels. Contamination can affect control circuits and heat dissipation.
• Anti-Corrosion Treatment: Check the metal enclosure’s anti-corrosion coating, particularly in coastal or chemical high-corrosion environments. Rust should be promptly removed and the protective paint touched up.

6. Environmental Safety Clearance (External Design Requirements)

• Safety Clearance: In cases of outdoor overhead line connections or specific open-air substations, strict safety clearance standards must still be observed to ensure personnel safety and prevent external flashovers.
  • Minimum distance between conductors and ground: Residential Areas: 7.5M; Non-Residential Areas: 6M; Areas with Traffic Difficulties: 5M.

7. Monitoring and Control Circuit Inspection

• Secondary Circuits: Check that the wiring of the protection, measurement, control, and signal secondary circuits is secure and that terminal blocks are not loose. Ensure the relay protection devices and monitoring units are operating correctly.
• Heater/Dehumidifier: Check that the electrical enclosure heater and dehumidifying units (if applicable) inside the control cabinet are functioning correctly to prevent moisture ingress to the control components.

8. Inspection and Operating Procedures

• Safe Operation: Strictly adhere to the principle of “test for voltage first, then apply ground”. The voltage indicators and voltage testers must be used for double verification.
• De-energized Maintenance: Internal GIS maintenance involving professional gas handling and high-voltage components must be performed by professionally trained personnel in strict compliance with the relevant Electrical Safety Work Regulations.

III. Conclusion and Operational Assurance Strategy

The superior performance of GIS stems from its system design focusing on gas-tight insulation, compact structure, and low maintenance needs. To translate these design advantages into long-term operational benefits, maintenance work must transition from a traditional “passive maintenance” approach to an upgraded “Condition-Based Preventive Maintenance” methodology.

Successful GIS operation and maintenance requires management to focus attention on three critical domains: insulation integrity (gas pressure and PD), mechanical operating performance, and external connection reliability. Through continuous, detailed monitoring, the operations team can accurately predict potential equipment risks and intervene before a failure occurs. This scientific operational assurance strategy not only maximizes the service life of the GIS and reduces life-cycle costs but is also essential for guaranteeing long-term grid reliability and optimized asset management.