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What Is Gas Insulated Switchgear/Substation

Gas Insulated Substation (GIS) and Gas Insulated Switchgear (GIS Switchgear) Overview

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1. Overview of Switchgear

In a Gas Insulated Substation (GIS), switchgear is a combination of fuses, switches, and circuit breakers designed to protect, isolate, and control electrical equipment.

High-voltage switchgear, which is used to operate electric machinery and motors, was developed in the 19th century. In power distribution systems, switchgear is typically installed on both sides of power transformers to ensure safe and reliable electricity transmission.

In industrial applications, switchgear and transformers can be housed together in a single enclosure known as a Unitized Substation (USS).

2. What is Gas Insulated Switchgear (GIS)?

Definition

Gas Insulated Switchgear (GIS) or Gas Insulated Substation (GIS Substation) is a metal-encapsulated switchgear that contains high-voltage components, including circuit breakers and disconnectors. GIS is a gas-insulated system that can be safely used in compact spaces.

Applications of GIS Switchgear

GIS switchgear is commonly used in:

  • Urban areas where space is limited
  • High-rise buildings and rooftops
  • Offshore platforms and industrial plants
  • Hydropower stations

Advantages of GIS Technology

GIS technology offers:

  • High reliability and safety
  • Compact design and space efficiency
  • Environmental friendliness
  • Voltage range from 72.5 kV to 1200 kV to meet different power needs

GIS switchgear components are enclosed in a gas-tight metal casing, which allows them to be installed in very confined spaces.

GIS gas insulated switchgear

3. Why is SF₆ Gas Used in GIS?

Role of SF₆ in GIS

Sulfur hexafluoride (SF₆) is widely used in GIS because of its excellent insulating and arc-extinguishing properties.

Properties of SF₆ Gas

  • Strong insulation for phase-to-phase and phase-to-ground protection
  • Non-toxic, inert, and non-flammable
  • Higher dielectric strength than air, making it an ideal insulating medium
  • High density (5 times denser than air), preventing ionization and electric discharge
  • Low solubility in water, ensuring long-term stability

SF₆ gas is pressurized within the switchgear to prevent breakdowns and maintain insulation performance.

Why is SF₆ Better Than Other Insulation Mediums?

Compared to air or oil insulation, SF₆ gas:

  • Has higher dielectric strength
  • Suppresses arcs faster, reducing equipment wear
  • Allows for a more compact switchgear design

SF₆-insulated switchgear is essential for building substations in restricted spaces.

4. Components of Gas Insulated Switchgear (GIS)

(1) Busbar

  • Definition: A busbar is a conductive strip made of copper or aluminum used for power distribution.
  • Function:
    • Connects incoming and outgoing power lines
    • Transfers electricity between generators, transformers, and switchgear

(2) Circuit Breaker

  • Definition: A circuit breaker is a mechanical switching device that interrupts electrical flow in case of faults.
  • Function:
    • Automatically trips during overloads and short circuits
    • Protects electrical equipment by disconnecting faulty sections
    • Restores power easily after faults are cleared

(3) Current Transformer (CT)

  • Definition: A current transformer steps down high AC currents to measurable levels for monitoring and protection.
  • Function:
    • Reduces high line currents to safe levels for measurement
    • Isolates high voltage systems from measuring instruments
    • Detects fault conditions like sudden current surges

(4) Voltage Transformer (VT)

  • Definition: A voltage transformer steps down high voltages to measurable levels.
  • Function:
    • Enables energy meters to monitor high-voltage service connections
    • Converts primary high voltage to a lower secondary voltage for safe measurements
    • Helps in fault detection and system protection

(5) Power Cables

  • Definition: Electrical cables are used to transmit power between substations and end users.
  • Function:
    • Connects switchgear, transformers, and loads
  • Types: Overhead or underground installation, depending on requirements

5. Advantages and Disadvantages of GIS

Advantages of Gas Insulated Substation (GIS)

  • Compact Design: Occupies less space compared to conventional substations
  • Reliable and Flexible: Suitable for urban areas, high-rise buildings, and offshore platforms
  • Minimal Maintenance: Gas-sealed components reduce wear and tear
  • Environmental Protection: Encased in a sealed system, making it resistant to dust, humidity, and extreme weather

Disadvantages of GIS

  • Higher Cost: More expensive than Air Insulated Switchgear (AIS)
  • Complex Maintenance: Requires specialized tools for gas handling
  • Gas Handling Challenges: SF₆ gas needs careful management due to environmental concerns
  • Extended Downtime for Repairs: GIS failures may take longer to diagnose and fix

6. Applications of GIS

GIS technology is used in:

  • Urban Power Distribution – Compact design for crowded cities
  • Hydropower Plants – Resistant to moisture and harsh conditions
  • Offshore Power Stations – Withstands marine environments
  • Industrial Areas – Suitable for high-pollution environments

7. Working Principle of GIS Switchgear

GIS consists of several key components working together:

  • Circuit breakers interrupt faults
  • Disconnectors isolate power
  • Busbars distribute current
  • Transformers step voltage up or down

In a GIS system, these components are enclosed in a gas-tight metal housing filled with SF₆ gas to ensure insulation and arc suppression.

8. Installation and Maintenance of GIS

Installation Process

Proper installation and commissioning ensure the long-term reliability of GIS systems.
Key steps include:

  • Foundation inspection to ensure stability
  • Busbar and cubicle connections verification
  • Electrical grounding and cable installation checks
  • Insulation testing before power-up

Maintenance Process

GIS switchgear requires periodic maintenance to ensure reliable operation.
Common procedures include:

  • Circuit breaker and transformer testing
  • Partial discharge detection
  • Gas leakage monitoring
  • Voltage and current transformer calibration

Regular inspections and diagnostic tests help prevent failures and extend equipment lifespan.

9. GIS Testing Procedures

Since GIS is a critical part of the power network, it must be tested rigorously to ensure reliability.

Standard GIS Testing Methods

  • AC Voltage Testing – Ensures insulation integrity
  • Partial Discharge Testing – Detects insulation weaknesses
  • Lightning Impulse Testing – Verifies surge protection performance

On-Site GIS Testing

  • GIS test setup for 123 kV systems
  • High-voltage tests on medium voltage GIS
  • Multi-channel partial discharge monitoring
  • Resonant circuit testing for 380 kV GIS

Thorough testing ensures that GIS switchgear operates safely under all conditions.

Conclusion

Gas Insulated Switchgear (GIS) is a highly efficient, space-saving, and reliable solution for modern electrical networks. Despite its higher cost, GIS offers superior performance, safety, and low maintenance requirements, making it the preferred choice for high-voltage substations worldwide.

Professiona Switchgear supplier and manufacturer 

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