World’s first mixed gas insulated hv line, in Geneva

What is claimed to be the world's first gas insulated high voltage transmission line of the second generation is rapidly taking shape in the outskirts of Geneva near the Palexpo exhibition halls. By January it will have been completed. 450 metres of overhead lines and pylons will then be dismantled and construction work on Palexpo's new hall, which the overhead line obstructs, will resume.

Siemens, the suppliers of the new link for clients eos (energie ouest suisse) are not claiming that second generation GIL is new technology. They are claiming that what they are doing in Geneva is an advance on existing technology, by using as the insulation medium SF6 and nitrogen mixed in a 20/80 ratio, that new techniques make on site construction and safety testing cheaper and quicker, and that their rule of thumb price of 5 million euros per km gives them a competitive edge in a growing market.

They have great hopes of the Swiss market in particular because GIL is seen as ideal for heavily populated areas or for special replacement jobs such as Palexpo (Figures 1, 2 and 3). And as it has been established that the common or garden gas main is acceptable to the public, it is assumed therefore that GIL will be too. The Swiss energy market is about to be deregulated, the relevant energy market legislation having already been passed; elsewhere in Europe this has resulted in a fall off in investment in anticipation of lower energy prices, resulting in cost pressures on suppliers of products and systems for power transmission and distribution.

Gas insulated line

According to accepted wisdom GIL is longer lasting, more reliable and less demanding in maintenance terms than other systems. Transmission losses are lower than overhead lines and even on longer routes power factor correction is not required. But it is too slow and expensive in the making and installation to be used where overhead lines are acceptable. Siemens sees the market as lying chiefly in line replacement or substitution where o/h line is not possible, for example near airports or buildings; or where there are environmental problems with cross country lines – in Alpine regions for instance where it is expected that cross-mountain overhead lines will continue to be banned; and for connections to high voltage substations, or to make new links to strengthen grid systems.

Gas insulated line is designed for transmission of high power ratings over long distances. Electrically it behaves in a similar fashion to overhead line, but has the largest cross section and therefore the lowest losses of any available transmission system, cable or overhead, reducing operating costs and wastage. Resistive losses are significantly lower than cables and o/h lines and dielectric losses are negligible. It has a low capacitative load and can therefore be used in lengths up to 100 km. The insulation medium does not age which reduces maintenance costs. Importantly, gas insulated line is electrically compatible with overhead line, so no interface circuitry is needed at the unions.

The safety factor is high because the metal outer is a reliable protection – even under shorted conditions – and because there are no inflammable components the cable can be brought into public places – the street, over a bridge, or through a tunnel.

Lightning strikes are also rendered impotent by the structure. The outer tube is grounded and therefore inert. Since a direct strike on the conductor is impossible, it is feasible to reduce the lightning impulse voltage level by using surge arrestors at the end of the GIL.

GILs have been operating for 25 years, the first application being a 700 m tunnel at Schluchsee in Germany where a cable fire had destroyed the existing line and its tunnel. GIL was selected as the replacement for its inherent safety qualities; the original line is still operating, with an uninterrupted service record, and remains, at 400 kV, the longest application at this voltage level anywhere.

Since then 100 km of GIL have been built around the world, 30 km of it by Siemens, at voltage levels between 135 and 550 kV. The long distance record to date is held by an installation in Japan which is 3.5 km long and carries 257 kV. In all 2500 km years have been clocked up worldwide, with an enviable service record caused mainly by the simplicity of the system; the welded two aluminium tube design with its non-degrading gas insulation has very few moving or wearing parts.

Insulation

The transmission capacity of a gas insulated line corresponds roughly to that of a gas pipeline. It can be laid in a tunnel or buried like a gas main; but the inner pressure is lower, around 6 bar, compared to 20 bar or thereabouts for a gas main.

Insulation of the live conductor, an aluminium pipe in the centre supported by ceramic formers, is a mix of 80 per cent nitrogen with 20 per cent SF6. SF6 is a non-flammable and eco-friendly gas, having almost no greenhouse impact. The Kyoto conference endorsed it as being one of the six gases least responsible for greenhouse effects.

Costs

Cost reductions are perhaps as important as technical developments. The major breakthrough has been achieved by component standardisation coupled with the use of automatic welding techniques, in particular the use of a computer controlled orbital welder that, employed on site, allows much faster laying of pipe sections. Trading on the flexible nature of the aluminium piping (it can achieve 400 m radius bends) allows the meeting of all design contingencies with only four components - a straight unit, an angle unit, a disconnection unit and a thermal compensator unit (Figure 4). The use of welded jointing throughout eliminates the necessity for connecting units. These factors have a positive impact on quality as well as reducing unit costs.

Second generation

The technology Siemens are calling “second generation” is not new, but it has been refined. Less SF6 reduces the cost; leakage rates have been reduced to the point where topping up with gas is generally not necessary by investing in on-site welding of the tubes and rigorous weld and leak testing techniques. Figures 5 to 7 illustrate the stages of this process. Outer tube sections are spiral seam welded and brought to site to be assembled into line sections in a prefabricated temporary factory. The spiral weld ensures an even stress loading on the line casing. Line ends are prepared for butt welding, before the entire section is sent to the tunnel access area for jointing to the already installed line using computer controlled orbital welding equipment. After the weld has been proved with ultrasonics, the new section is pulled through by a motor driven winch. During construction the line is mounted on rollers at each supporting bracket, and presents low enough resistence to allow the use of a manual winch. Once the entire line is completed, the rollers are replaced by pads, except at the central point of the line where it will be fixed, ensuring that thermal expansion is taken up equally by the sliding compensator units at each end. The final stage is high voltage testing of the line, carried out in an access pit specially provided for the purpose.

Monitoring

For long distance lines disconnecting units are placed at distances of 1200 to 1500 metre in underground shafts. These are used to separate gas compartments and to connect high voltage testing equipment for the commissioning of the GIL.