Gas-insulated transmission lines: the next generation of power transmission

August 5th, 2015, Published in Articles: Energize

 

High voltage underground cables for power transmission have been in use for many years and number of different technologies have developed over the years. Solid insulation cables have limits when it comes to the voltage that can be carried safely, and oil impregnated paper cable are also limited in the capacity. Gas insulated transmission lines (GIL) provide technical, environmental and operational features which make them a very good alternative wherever the transmission of extra high voltage (EHV) and extra high currents (EHC) is needed within restricted space, e.g. wherever overhead lines cannot be used.

For long distance transmission and in areas where there are no servitude restrictions, overhead transmission lines (OHL) remain the most economical and practical method of high power electricity transmission. In both urban and rural environments land disruption is greater when laying underground cables than when erecting overhead line towers and the cost of installing underground cables is much higher than the cost of erecting overhead lines.

When faults occur 400 kV underground cables are on average out of service for a period 25 times longer than 400 kV overhead lines. This is due principally to the long time taken to locate, excavate and undertake technically involved repairs. The maintenance and repair costs are also significantly greater [1].

In areas where it is not possible or practical to use overhead lines, such as urban or developed areas, industrial sites, and other places, underground cables have been used as the main means of transmitting high voltage electricity. Technologies commonly in use are paper insulated oil filled cables, which are capable of operating at voltages up to 300 kV, and the solid insulation cross linked polyethylene (XLPE), which operates at voltages up to 500 kV.

Both types have problems with maintenance, deterioration with time and failures of the insulating medium and in addition failures can have a substantial impact on the surrounding environment. High voltage cables are generally buried directly in the ground and are vulnerable to accidental mechanical damage. Jointing of both oil filled and XPLE cables is a complex process, and joints can be a source of failure.

Under fault conditions, between two and six weeks can be required to locate the fault or fluid leak and repair the cable [1].

Gas insulated transmission lines do not have these problems, as the insulating medium is gas and there is no need for physical layers of insulation, and GIL is becoming more and more popular as a means of high and medium voltage transmission in restricted areas. This system comprises aluminium conductors which are supported by insulators contained within sealed tubes. These can be installed above ground, in trench or tunnel installations. The tubes are pressurised with a N2/SF6 gas to provide the main insulation. The main advantage of GIL is that a higher cable rating can be achieved than with solid insulation cables and the terminations at the cable ends are less complex and less prone to failure. Fig. 1 shows a GIL system in the process of installation [2].

Fig. 1: GIL system in the process of installation [2].

Fig. 1: GIL system in the process of installation [2].

Although currently limited to relatively short runs (<30 km) in urban and developed areas, the technology is being developed for longer distances. Systems rated at up to 800 kV are in operation [3]. GIL is a new, future orientated technical solution for power transmission. The non-availability of right of way for new overhead lines in general and the further increase in demand of electrical energy will generate the need for high power underground transmission, even over long distances of 100 km and more.

The GIL concept was developed in the 1960s and Generation 1, the first operational system, was installed in the Wehr hydro power plant in 1974. The concept has been further developed into Generation 2 systems, with improved features, which reduces cost and simplifies installation.

Table 1: Characteristics of a typical GIL system [2].
Gas insulated transmission lines – typical technical data
Rated voltage 245 to 550 kV
Rated current Up to 4500 A
Rated short circuit current 63 kA/3s
Rated transmission load 2200 to 3900 MVA
Insulating gas N2 and SF6 mixture
Impulse withstand voltage 1050 to 1675 kV
Capacitance 55 nF/km
Resistance 10 mΩ/km
Inductance 220 nH/m
Electromagnetic field strength 1 µT
Outer diameter 375 to 512 mm
Weight per phase 50 kg/m

Construction

GIL consists of two concentric aluminium alloy tubes, the inner tube of approximately 180 mm diameter and having a cross-section of 5400 mm2 forming the conductor, and the outer tube of approximately 500 mm diameter forming the containing wall for the insulating gas and the support for the insulators. A tubular construction – instead of a solid – is used for the inner conductor to compensate for the skin effect. The outer tube is usually earthed. The construction is illustrated in Fig. 2.

Fig. 2: GIL construction [2].

Fig. 2: GIL construction [2].

A particle trap, which provides a field-less space, is located on the bottom of the enclosure housing. This trap ensures that any particles resulting from discharges, or contained in the enclosure are trapped at the outer wall and do not affect the insulation quality. Particles may occur in the GIL, despite all cleanliness measures. Due to the influence of the electric field and under the gravity any particle will move underneath that particle trap, and thereby be neutralised, before it can have any negative influence on the dielectric strength of the GIL. The particle trap enhances the reliability of the GIL.

GIL modules are designed and manufactured to be assembled on site, and are supplied in sections of 12 to 20 m in length. The inner and outer conductors, as well as the insulating posts are supplied separately and assembled on site. The tubes and conductors are welded together with a computerised orbital welding process. The seams are 100% ultrasonic tested to ensure that the system is absolutely gastight. The tube can negotiate bends with a radius of 400 m without distortion. For sharper changes in direction special “elbow” modules are used. The form of a straight section of welded tube module is shown in Fig. 3.

Fig. 3: GIL straight section module [4]. 1. Enclosure; 2. Inner conductor; 3a. Male sliding contact; 3b. Female sliding contact; 4.Concical insulator; 5. Support insulator

Fig. 3: GIL straight section module [4].
1. Enclosure; 2. Inner conductor; 3a. Male sliding contact; 3b. Female sliding contact; 4.Concical insulator; 5. Support insulator

Insulators

Support insulator/post insulator (Fig. 3, item 5) are placed at intervals of approximately 12 m. Pairs of support insulators made of epoxy cast-resin are arranged at an obtuse angle, and keeping the conductor centred in the enclosure housing. They are fixed to the conductor and slide on the inside of the enclosure housing to compensate the different thermal expansions of conductor and enclosure housing.

The bushing or conical insulator (Fig. 3 item 4), made of epoxy cast-resin is used as a fixed point for the conductor at regular distances and fastens the conductor position to the enclosure housing, i.e. the conductor will be kept in an axial direction and be prevented from torsion. The bushing is fixed to the conductor as well as to the enclosure. These bushings may be either gas-tight or perforated and have the ability to withstand mechanical stresses.

Sliding contact (Fig. 3, items 3a, b)

A sliding contact system is installed at each fixed point in order to compensate for the differences of thermal expansion between conductor and enclosure housing. Well proven multi-segment-contacts with silver plated contact surfaces are used.

Insulating gas

A mixture of nitrogen N2 and SF6 is used, the most common proportion being 80% N2 and 20% SF6. The gas is maintained at a pressure of 7 bar.

Characteristics

Typical characteristics of GIL systems are given in Table 2

Table 2: GIL systems installed worldwide 2010 [8].
Voltage level (kV) Length per phase (km)
72/145/172 38
245/300 33
362 15
420 110
550 90
800 3
1200 1
Total 290

Losses

The resistive losses of GIL are lower than OHL and other types of UGC, due to the larger size of conductor and lower resistance. Fig. 4 gives a comparison of total losses for OHL, GIL and UGC [3].

Fig. 4: Comparative losses for GIL, OHL and UG cables [5].

Fig. 4: Comparative losses for GIL, OHL and UG cables [5].

The dielectric losses of GIL are negligible. This reduces the operation costs and causes savings. Due to the large outer diameter, the heat dissipation is better than with cables and GIL normally do not require sophisticated cooling systems.

Reactive compensation

Due to low capacitance, GIL systems do not require phase angle compensation up to system lengths of 70 km.

Electromagnetic fields

The construction of GIL results in much smaller electromagnetic fields in the vicinity of the installation – as much as 15 to 20 times smaller – than with conventional power transmission systems. GIL is operated as a solid grounded system and the enclosures of a multiphase installation are connected with each other. The low electrical impedance of the enclosure pipe, due to its big cross section, allows a current induced in the enclosure as high as the conductor current. The induced electrical current in the enclosure is 180° phase-shifted to the conductor current. The result of the addition of the magnetic field of the conductor current and the enclosure current is the effective magnetic field which is reduced by more than 99%. This makes GIL a transmission system with very low extraneous magnetic fields (Fig. 5).

Fig. 5: EM field levels for different technologies [5].

Fig. 5: EM field levels for different technologies [5].

GIL can be laid in combined infrastructure tunnels together with other systems (e.g. close to telecommunication equipment). No special shielding is required even in areas which are critical with respect to EMC (e.g. airports or computer centres), or in populated areas. GIL systems satisfy the most stringent magnetic flux density requirements, for example, the Swiss limit of 1 μT [2].

High safety

If an insulation failure were to occur in a GIL, the fault arc would be safely enclosed within the outer housing, with no impact outside the enclosure. GIL are fire resistant and do not contain flammable material, or emit noxious fumes under fire conditions.  This means optimal protection of persons and environment.

Installation methods

The GIL can be laid aboveground on structures, in a tunnel, or directly buried into the soil like an oil or gas pipeline. The overall cost for the directly buried version of the GIL makes this the least expensive method for GIL installation. [6].

Direct in ground

In this application systems are coated with a continuous polyethylene layer to safeguard the corrosion-resistant aluminum alloy of the enclosure, providing protection of the buried system for >40 years. As magnetic fields are marginal in the vicinity of all GIL applications, the land can be returned to agricultural use with very minor restrictions once the system is completed.

Tunnels and troughs

Tunnels and troughs made up of prefabricated structural elements are another quick and easy method of GIL installation. The tunnel elements are assembled in a trench, which is then backfilled to prevent any long-term disfiguring of the local landscape. The GIL is installed once the tunnel has been completed. With this method of installation the land above the tunnel can be fully restored to agricultural use.

Above ground

GIL installation aboveground is a trouble-free option, even for extreme environmental conditions. GIL is unaffected by high ambient temperatures, intensive solar radiation or severe atmospheric pollution (such as dust, sand or moisture). Corrosion protection is not always necessary. For security and safety reasons above ground installation is only used in secure areas, such as industrial plant and power stations or substations. The outer surface of the GITL is grounded, which makes it safe to contact and simplifies mounting.

Fig. 6: The GIL installation at Ruacana hydropower plant [9].

Fig. 6: The GIL installation at Ruacana hydropower plant [9].

Existing installations

There are numerous installations at various sites around the world, ranging in length from 300 m to 21 km [7]. Table 2 lists the status at 2010 [8]:

The most interesting installation from an African perspective is that at the hydropower station at Ruacana in Namibia. Installed in 1977, this first generation GIL system is rated at 245 kV and 630 A, and has a total length of 800 m with a vertical length of 115 m (Fig. 6). The system has been operating satisfactorily since installation. This was the first vertical installation in a shaft of a hydropower plant, and showed that GIL could be installed in long vertical shafts. The conductors are held by conical insulators and the enclosure is fitted to the shaft wall by a steel structure. The GIL connects the high voltage transformer in the generator cavern to the substation at ground level [9].

HVDC application

GIL is currently used only for HV AC transmission, but development of the system for use with HVDC is currently under way. A particular application which is being considered is that of offshore wind sites. With the increasing number of renewable energy installations which need to be connected to the grid, GIL HVDC may become a viable technology in the development of networks to cater for the high transmission loads resulting from renewable energy [10].

References

[1]    National grid: “Undergrounding high voltage electricity transmission”, www.landsnet.is/uploads/1068.pdf
[2]    Siemens: “Gas insulated transmission lines”, www.energy.siemens.com/hq/pool/hq/power-transmission/gas-insulated-transmission-lines/GIL_e.pdf
[3]    H. Koch: “GIL – Gas insulated transmission lines”, Wiley, 2011.
[4]    D Kunze: “Gas insulated transmission lines-underground power achieving a maximum of operational safety and reliability”, Jicable 2007.
[5]    C Johnstone: “Undergrounding electricity transmission:Introduction to gas insulated line (GIL) technology”, National Symposium on Future Electricity Networks, London, January 2011.
[6]    E Eduvard: “Gas insulated transmission system design”, Electrical engineering portal, October 2012.
[7 ]    Dr. P Rudenko: “Gas insulated lines: the next generation of power transmission Technologies”, Electrical Networks of Russia, 2010.
[8]    H Koch: “Experience with second generation gas insulated transmission lines (GIL)”, www.jicable.org/wets03/pdf/wets03-1-06.pdf
[9]    IEEE: “Gas insulated transmission line (GIL)Applications”, IEEE PES substation committee, http://ewh.ieee.org/conf/tdc/D_08TD0791-GIL-Applications-2008-04-15-Hermann.pdf
[10]    Siemens: “Siemens develops gas insulation line for direct current”, www.siemens.com/press/pool/de/pressemitteilungen/2015/energymanagement/PR2015040183EMEN.pdf

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