The basics of Gas Insulated Substation (GIS) for students

GIS substation operating principles

GIS switchgear is totally capsuled, that is impervious to and distinguished from the external ambiance and other GIS substation equipment. This is a huge benefit from environment viewpoint such as ocean based oil rigs, particle or mist pollution sources. Nevertheless, because the gas isolated switchgear is totally capsuled, a needed visible disconnecting means cannot be directly accomplished.

The grounding and disconnect switches, needed in both air and gas insulated arrangements, will have view ports in gas isolated devices.

GIS has a reduced “footprint” than a corresponding air insulated substation, usually less than half the area. Even though a gas isolated substation will initially cost more than a similar air insulated substation, the economics may rationalize its installation where land is pricey, such as city centres.

GIS may also be rationalized when a low profile substation is required to “hide” a substation.

GIS substation operation

As a gas isolated switchgear element is isolated for servicing, it will be required to affirm the places of the grounding and disconnect switches. Since these switches are totally cased within the aluminum enclosure, it is essential for producers to allow for view ports. The view ports allow, by visual verification, to check the position of the different disconnect and grounding switches. In some situations, this can be completed with using just a flashlight.

In other situations, at strange access points, a camera with a light source supplied by the producer is handy.

The protective relays related with the GIS devices may or may not put in the same place. Since SF6 gas behaves as a vital insulator, it is required to keep adequate density within the GIS devices. Hence, there will be alarm and trip contacts from sensors for each gas separation to warn staff or isolate devices when the insulation integrity is insufficient.

Figure 1 – Gas density monitoring indicator

One of the advantages of GIS devices over its air insulated equipment is the minimal servicing that is needed of the GIS. This is mainly due to the breakup of the conductors and isolators from the outside ambience. Modern GIS devices have very low SF6 gas leakage rates.

The operation counter may help in finding out if any servicing will be needed on the mechanisms, but this is generally many years between maintenance.

Safety concerns

Staff safety holds a crucial priority status when servicing a GIS substation. The metallic enclosure of the high voltage spaces are grounded where direct contact is not possible, except at the external links. This safety element is underlying in the GIS arrangement.

Moving elements such as operation rods or motor drives are typically protected with protective plates or showed by colouring for greater safety. In case of an internal fault, pressure relief elements open the enclosing to free the hot gas to the surrounding internal elements.

To set up GIS inside indoor or outdoor substation safety regulations are further described in IEC 61936-1. Installation regulations are presented to integrate factory assembled and type-tested GIS equipment.

Demands of grounding, accessibility, fire protection, safety of walkways and other areas are described.

Watch Video – Installation of 380kV GIS substation

GIS substation equipment is manufactured and tested in accordance with IEEE C37.122 or IEC 62271-203 standards. All measurements must be finalized for the GIS to achieve certification. Prior to testing, GIS substation must be manufactured and assembled in a facility within controlled clean rooms. The design must successfully complete all types of testing and routine tests. GIS substation equipment undergoes testing in accordance with designated on-site tests post-installation, as specified in the referenced regulations.

Additional criteria for the GIS apply to external connections, installation procedures, and maintenance requirements. External connections are typically established using transmission overhead lines, cables, coils, transformers, or capacitor banks.

The installation and erection must be coordinated to mitigate risks to personnel and prevent damage to other equipment.

GIS design and erection demands

The GIS substation equipment must be arranged to provide operators with a clear perspective of the bay structure. Essential components for erection, operation, and maintenance must be readily accessible and pose no risk to the substation operator. Ladders and walkways must be provided if necessary. Provision for the arrangement and accessibility of devices and equipment, including cranes, ropes, and hooks, should be ensured.

Appropriate designs to connect the GIS to external connections are essential for safe on-site operations. Sufficient workspace is necessary, and all metallic constructions must be grounded.

Related Reading – Guide to erection and commissioning of MV/HV switchgear (Inspection, installation and assembly)

Guide to erection and commissioning of MV/HV switchgear (Inspection, installation and assembly)

Operation platforms and ladders

The substantial dimensions of high voltage GIS substations, typically at the 420 kV and 550 kV voltage levels, may necessitate the installation of platforms and ladders for operational and maintenance purposes. Platforms or ladders may be necessary to ascertain the position of the disconnect or ground switch via the viewport. Consequently, platforms or ladders ought to be integrated into the GIS framework.

The design of these ladders and platforms must ensure operational safety. Platforms are often affixed to the GIS, whereas ladders may be either permanent or detachable.

Figure 2 – SF6 Gas Insulated GIS Substation: Metalic ladder and platform

GIS monitoring (gas density)

Monitoring components in GIS substation equipment should be arranged and labeled for easy identification by color coding and/or numbering. Monitoring elements are employed for gas density measurement and are positioned in the gas space. The previous approach of employing gas pipes to connect the gas compartment to a central gas density control cubicle is obsolete due to the increased risk of gas leaks from these pipes and their fittings.

Contemporary GIS technologies typically have gas density readers that offer solely red or green indicators. Green indicates “acceptable, no gas loss“, whereas red signifies “unacceptable, gas loss“. The switchgear will be automatically shutdown by isolating the part from high voltage.

Figure 3 – GIS Gas Density Monitor

It is imperative that each gas compartment is distinctly and clearly marked for the servicing personnel. This guarantees that the gas area may be distinctly separated between the two gastight isolators of the gas compartment. Typically, allots are sown using external coloration.

GIS earthing

According to IEEE Standard 80, touch voltages may be more intense than step voltages in AIS equipment. The increasing number of devices in Gas Insulated Switchgear (GIS) and the presence of induced voltages on GIS enclosures need enhanced management of contact voltages in GIS equipment. To evaluate the maximum contact and step voltages occurring on GIS enclosures during a fault, it is important to conduct an assessment of the substation grounding system.

Utilize available earthing software to conduct assessments of maximum touch and step potentials, as well as ground potential rise.

In standard GIS applications, there are two grounding grids that constitute the grounding arrangement:

Arrangement #1 – the station earthing network, similar to standard AIS installations;

Arrangement #2[/highlight2] – the GIS earthing mesh, a closely spaced earthing grid (typically 3–5 meters) embedded within the concrete slab housing the GIS equipment. A perimeter earthing conductor encircling the structure connects the two earthing grids. The multipoint earthing system consists of short links between the GIS cases and the earthing mesh at approximately 10-meter intervals or according to the manufacturer’s specifications.

The optimal patterns for GIS earthing and bonding encompass the following:

1. All earthing conductors must be minimized in length.
2. The earthing mesh and connections must be positioned to conduct the system’s fault currents without exceeding thermal and mechanical limits.
3. All exposed earthing conductors must be protected from mechanical damage and positioned to avoid creating a “trip hazard” for operational personnel.
4. Sufficient earthing and bonding techniques, including additional conductors or voltage limiters, are required at all separations inside the GIS apparatus. This encompasses SF6 gas-to-air connections, SF6 gas-to-cable connections, SF6 gas-to-oil connections, and the points where GIB exits the building.
5. Ensure that all metallic building areas, GIS support structures, and GIS maintenance platforms are properly grounded.
6. The reinforcement steel in the GIS building floor must be connected to the GIS earthing mesh to ensure alignment of ground potentials.
7. All secondary cables must be insulated, with both ends of each cable’s shield grounded to reduce potential electromagnetic interference.