Gas Insulated Transmission Line (GIL) System Design

Introduction to GIL

GIL systems are based on the success of SF6 tubular conductor technology, which has been around for several decades. Gas Insulated Transmission Line consists of a central aluminum conductor with a typical electrical cross-section of up to 5,300 mm 2.

The conductor rests on cast resin insulators , which centers it in the outer enclosure .

This enclosure consists of a solid aluminum tube, which provides strong mechanical and electrotechnical containment for the system. To meet the most recent technical and environmental requirements, the GILs are filled with an insulating gas mixture composed mainly of nitrogen and a lower percentage of SF 6 gas.

GIL has been developed to meet a wide variety of requirements for installation and operation. An installation process allowing prefabricated modules to be mounted at the installation site was instrumental in meeting this demand, thus allowing optimal adoption of the chosen routing.

This concept also has logistical advantages. All items such as tubes, angles, and special modules are lightweight and small enough to be transported by relatively light standard trucks.

technical data

The main technical data of GIL for 420 kV and 550 kV transmission networks are shown in the table below.

For 550 kV applications, the SF6 content or the casing pipe diameter could be increased. The nominal values ​​shown in the table below are chosen to meet the requirements of the high voltage transmission network of overhead lines.

Table – Technical data for 420 kV and 550 kV GIL transmission networks

Nominal voltage (kV)420/550
Nominal current (A)3150/4000
Lightning pulse voltage (kV)1425/1600
Switching pulse voltage (kV)1050/1200
Supply frequency voltage (kV)630/750
Short-time rated current (kA = 3s)63
Nominal gas pressure (bar)7
Insulating gas mixture80% N 2 , 20% SF 6

The power transmission capacity of the GIL is 2000 MVA, whether the tunnel is laid or directly buried. This allows the GIL to continue with the maximum power of 2000 MVA of an overhead line and bring it underground without any reduction in power transmission.

The values ​​comply with the IEC standard applicable to GIL, IEC 61640 .

Gas Insulated Transmission Line
Gas Insulated Transmission Line

Standard units

Figure 1 shows a straight unit combined with an angle unit.

The straight unit consists of a single phase aluminum alloy enclosure. in the enclosure (1), the inner conductor (2) is fixed by a conical insulator (4) and is located on supporting insulators (5).

The thermal expansion of the conductor towards the enclosure is regulated by the sliding contact system (3a, 3b). A straight unit has a maximum length of 120m made up of single pipe sections welded together by orbital welding machines.

If the change in direction exceeds what elastic bending allows, an angular member (shown in Fig. 1) is added by orbital welding with the straight unit. The corner element covers angles from 4 ° to 90 ° . Under normal landscape conditions, no angle unit is needed because the elastic flexion, with a radius of curvature of 400 m , is sufficient to follow the contour.

At distances of 1200-1500 m , disconnection units are placed in underground wells. Disconnect units are used to separate gas compartments and connect high voltage test equipment for commissioning of the GIL.

The compensation unit is used to accommodate the thermal expansion of the enclosure in sections not buried in the earth. A compensator is a type of metallic enclosure, a flexible mechanical section, which allows movement related to the thermal expansion of the enclosure. It compensates for the thermal expansion length of the housing section.

Thus, compensators are used in GIL in tunnels as well as in Gas Insulated Transmission Line trees buried directly .

The directly buried GIL enclosure is factory-coated with a multi-layer polymer sheath serving as passive protection against corrosion. After the orbital weld is complete, a final corrosion protection coating is applied to the site of the joint area. Because the GIL is an electrically closed system, no lightning pulse voltage can reach the GIL directly.

Therefore, it is possible to reduce the lightning impulse voltage level by using surge arresters at the end of the GIL. The integrated surge protector concept helps reduce high frequency surges by connecting the surge arresters to the GIL in the gas compartment.

For the monitoring and control of the GIL, secondary equipment is installed to measure the pressure and temperature of the gas. These are the same elements that are used in gas-insulated switchgear (GIS).

For commissioning, partial discharge measurements are obtained using the sensitive sensor very high frequency (VHF) measurement method .

An electrical measurement system for detecting an arc location is implemented at the ends of the GIL. The electrical signals are measured and, in the very unlikely event of an internal fault, the position can be calculated by the Arc Locating System (ALS) with an accuracy of 25m.

The third component is the compensator, installed in the enclosure. In the tunnel or underground version, the GIL enclosure is not fixed, so it will expand in response to thermal overheating during operation. The thermal expansion of the enclosure is compensated by the compensation unit.

If the GIL is directly buried in the ground, the compensation unit is not necessary due to the weight of the ground and the friction of the surface of the GIL enclosure.

The fourth and last basic module used is the disconnect unit , which is used every 1.2–1.5 km to separate the GIL in the gas compartments. The disconnect unit is also used to perform sectional high voltage commissioning tests.

An assembly of all these elements as a typical configuration illustrates a section of a GIL between two wells (1) . The underground wells house the disconnectors and compensation units (2). The distance between the wells is between 1200 and 1500 m and represents a single gas compartment. A directly buried corner unit (3) is shown by way of example in the middle of the figure.

Each corner unit also has a fixed point, where the conductor is attached to the housing.

Laying methods

The GIL can be installed above ground on structures , in a tunnel , or directly buried in the ground such as an oil or gas pipeline. The overall cost of the directly buried version of the GIL is, in most cases, the cheapest version of the GIL installation. For this installation method, sufficient space is required to allow accessibility to the work on the site.

Therefore, the directly buried laying will generally be used in an open landscape crossing the countryside, similar to overhead lines, but invisible.

Above ground installation

Surface installation of GIL is a hassle-free option , even for extreme environmental conditions. GILs are not affected by high ambient temperatures, intense sunlight, or severe air pollution (dust, sand or moisture). Corrosion protection is not always necessary.

Particularly high transmission power can be achieved with surface installation.

Tunnel installation

Tunnels made up of prefabricated structural elements are another quick and easy installation method from GIL. The tunnel elements are assembled in a trench, which is then backfilled to prevent long-term disfigurement of the local landscape.

The GIL is installed after the tunnel is determined. With this method of installation, the land above the tunnel can be fully restored for agricultural use. Only a negligible amount of heat is dissipated into the ground by the GIL. The system remains accessible for easy inspection and high transmission capacity is ensured.

Vertical installation

Gas-insulated tubular conductors can be installed seamlessly at any gradient, even vertically. This makes it a solution of choice, especially for cave hydropower plants, where large amounts of energy must be transmitted from the underground transformer to the switchgear and overhead line above ground.

Since GIL systems pose no fire risk, they can be installed in an accessible tunnel or shaft, but can also be used for ventilation.

Direct burial

Siemens also offers GIL solutions designed for direct burial. These systems are covered with a continuous layer of polyethylene to protect the corrosion resistant aluminum alloy of the enclosure, providing buried system protection for over 40 years.

As magnetic fields are marginal in the vicinity of all Siemens Gas Insulated Transmission Line applications, the land can be reused for agriculture with very minor restrictions once the system is complete.

High EM compatibility

Magnetic fields in microtesla (µT) for Gas Insulated Transmission Line, overhead transmission line, and cable (XLPE, cross-link) for a 400 kV dual system at 2 x 1000 MVA load, GIL, and cable laid at a depth of 1 m.

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