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Technology | November 2013

China's biggest transmission line

Siemens completed the world´s longest and highest-capacity high-voltage gas-insulated transmission line connection in China. Power Today reports on a technological marvel.

Siemens completed the construction of a high-voltage gas-insulated transmission line (GIL) for China Three Gorges Project Corporation at the country's second largest hydropower plant ´Xiluodu´. It is located in the southwest of China on the , Jinsha Jiang River at the Xiluodu dam. The project consisted of the installation of a total of seven three-pole systems, each with a length of 620 m. The single-tube tube length is 12.5 km, making this the world's longest GIL.

This system features a second world record designed for a power capacity of 3,900 megavolt ampere (MVA) at a voltage of 550 kilovolt (kV) and a rated current of 4,500 ampere (A), this is the world's highest-capacity connection using GIL.

In order to transport electricity from the dam to the population centers in the eastern part of the country, power must first travel from the turbines in the power plant cavern to the overhead transmission lines, a distance of about 460 m. Seven three-pole GIL systems, which are inserted vertically in four shafts to the right and left of the dam, each transmit 3,900 MVA of power safely and efficiently to the overhead power lines.

Why GIL?
Using a GIL instead of a high-voltage cable has the added advantage of its special design that eliminates any risk of fire. GIL tubes are not flammable. In the event of a malfunction, they do not represent a fire load and thus pose no hazard to operating personnel. The tunnel of the insulated high-voltage lines can also be used as ventilation and service shafts for personnel. Optimal values for electromagnetic compatibility, increased earthquake resistance, a leak-proof design due to welding style, and the fact that GIL´s do not age, round out the system's broad benefits.

Additionally, the GIL tubes are welded over their entire length. This not only increases operational reliability but also gives the GIL tube flexibility, so it can adapt to any changes in the tunnel wall that occur over time.

The welding of the GIL´s aluminum tubes is an innovative high-tech process that requires clean-room conditions. The latest welding and ultrasound methods ensured the required quality was achieved, and allowed rapid installation of the gas-insulated lines.

The work
The individual GIL modules, each of which is about 11.5 m long, were welded directly to each other in a vertical position inside the installation shafts, and then pushed upward, section by section, in the tunnel. Various branch-offs and horizontal sections extend the overall length of the individual three-phase systems to 620 m.

´In the Xiluodu project, we completed the world's longest vertical GIL installation," said Project Manager Mathias Schreiber, Siemens Energy. ôThis is also the first time technical figures such as 550 kV of operating voltage and 3,900 MVA transmission power per system have been involved. We're proud that we completed this massive project in such a short time under complex installation conditions at the job site.´

After the first of the seven GIL systems passed high-voltage testing in March 2013, three more systems were commissioned in July. As a result, the China Three Gorges Corporation, both a utility company and the power plant operator, was able for the first time to send hydro-generated electricity to a distant consumer location and sell it there. The final two GIL systems were completed in September 2013.

GIL Technology
GIL technology is a further development of tube conductor technology. A gas-insulated extra-high-voltage line consists of an aluminum conductor tube and an aluminum enclosing tube. The GIL is suitable for connecting load centres and urban and industrial centres over a transmission length of a few km, as well as for longer distances with no need to take measures for reactive power compensation. GIL lines can be routed over any terrain, including steep inclines or´as in the case of hydropower plants with dams vertical sections. GIL technology is especially suitable for laying underground or in tunnels over short transmission lengths. Gas-insulated high voltage power lines are part of the Siemens´ environmental portfolio, which earned the company revenues of nearly EUR19 billion in fiscal 2008, accounting for about a quarter of Siemens´ total revenues.

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