What is GIS Equipment? Composition, Characteristics & Structural Components

Gas-Insulated Switchgear (GIS) is a type of enclosed, high-voltage electrical apparatus widely used in modern substations. By enclosing all vital components within an earthed metal tank filled with Sulfur Hexafluoride (SF₆) gas as an insulating and arc-extinguishing medium, GIS offers superior reliability, compact space requirements, and robust performance in power transmission systems.

1. What is Gas-Insulated Switchgear (GIS)?

GIS integrates major electrical components into a single, compact, grounded metal enclosure. Its primary functions within power systems include the control, protection, and measurement of substation apparatus and transmission lines.

• Integrated Components: Circuit breakers, disconnectors, earthing switches, current transformers, surge arresters, voltage transformers, busbars, and bushings or cable terminals.

• Operating Mechanism: Switches and disconnectors are typically motor-driven. When closing, the motor rotates to drive the operating rod while simultaneously storing energy for opening. This ensures that even if the motor power supply is lost, closed switches or disconnectors can still be safely opened.

GIS Component Code Reference List

• CB – Circuit Breaker
• DS – Disconnector
• ES – Maintenance Earthing Switch
• FES – Fault Making Earthing Switch
• BUS – Busbar
• CT – Current Transformer
• VT – Voltage Transformer
• LA – Lightning Arrester (Surge Arrester)
• LCP – Local Control Panel
• BSG – Air Bushing
• CSE – Cable Terminal Box

2. Properties and Characteristics of SF₆ Gas

Sulfur hexafluoride SF₆ is the backbone of gas-insulated equipment. Understanding its physical and chemical properties is essential for safe operation and maintenance.

• Physical Properties: Pure SF₆ is a colorless, odorless, tasteless, non-flammable inert gas with stable chemical properties at ambient temperatures. Its gas density is 5.1 times that of air.

• Dielectric & Arc-Extinguishing Strength: At a pressure of 0.29 MPa, the dielectric strength of SF₆matches that of transformer oil. Its arc-extinguishing capacity is 100 times greater than air, making it the industry standard for electrical insulation.

• Decomposition & Toxicity Risks: Under electric arcs, corona, spark discharge, or high temperatures, SF₆ decomposes. When combined with moisture, these byproducts form corrosive electrolytes. Highly toxic decomposition products (such as SF₄, S₂F₂, S₂F₁₀, SOF₂, HF, and SO₂) can irritate the skin, eyes, and mucous membranes. Heavy inhalation can lead to dizziness, pulmonary edema, or fatal consequences.

3. Structural Components of GIS Equipment

A GIS setup consists of several core components housed within its metal-enclosed modules:

3.1 Spring-Type Circuit Breaker (CB)

• Function: Capable of making, breaking, and carrying normal operating currents, as well as handling specified abnormal currents (such as short-circuit currents) within strict time limits.

• Structure: Features a three-pole common-tank structure. The three pole arc-extinguishing chambers are arranged in a triangular configuration inside the metal enclosure, paired with a highly reliable, tight operating mechanism.

3.2 Disconnector (DS)

• Primary Function: Provides electrical isolation with specified insulation clearance and a visible open-break indication. In the closed position, it carries normal operating and short-circuit currents.

   

• Secondary Function: Handles switching operations during grid configuration changes (the CB must always be opened prior to DS operation). It must be capable of interrupting busbar circulating currents and small capacitive currents during frequent operations.

3.3 Bushing-Type Current Transformer (CT)

• Transfers current data to measuring instruments, meters, and protection/control devices. It steps down high-voltage large currents into low-voltage small currents and sends trip signals to secondary protection systems during short-circuit faults.

3.4 Voltage Transformer (VT)

• Steps down high grid voltages to safe, low levels for accurate system voltage measurement and comprehensive grid protection.

3.5 SF₆-Air Bushing

• Acts as the vital interface for connections between the GIS system and external outgoing overhead lines or circuits, typically built from ceramic or silicone composite insulating materials.

4. SF₆ Gas Quality Supervision & Maintenance

To ensure operational safety, routine monitoring of SF₆ gas quality and moisture levels is mandatory:

1. Pre-Commissioning Testing: Re-test moisture content and gas leakage rates.

   • Arc-extinguishing chambers: Maximum 150 ppm.

   • Other gas compartments: Maximum 250 ppm.

2.Operational Monitoring: For new or overhauled equipment, test every 3 months until moisture levels stabilize, then transition to annual testing.

• Arc-extinguishing chamber compartments: Must remain below 300 ppm.

• Other compartments: Must remain below 500 ppm.

5. Abnormal Conditions & Emergency Handling in GIS

• Pressure Monitoring & Alarms: SF₆ density relays monitor gas pressure in each compartment. If a seal fails and pressure drops to the alarm value, a "low gas pressure" signal triggers immediate investigation. If pressure drops to the blocking value in a circuit breaker compartment, the control circuit automatically disconnects, and all opening and closing operations are strictly prohibited.

• Accidental Gas Leaks & Tripping:

• Report sudden GIS tripping or major gas leaks to the dispatch center immediately.

• Do not operate switches or disconnectors arbitrarily.

• Wait until SF₆ gas has fully dissipated and the area is thoroughly ventilated before starting patrol inspections.

• Inspectors must approach from the upwind side, wearing appropriate gas respirators to check for abnormal discharge sounds, leaks, or vibrations.