# Determination of the Operating Temperature of the Gas-Insulated Transmission Line +
## 1 Introduction  
The growing demand for electrical energy requires a continuous expansion of the electric power infrastructure. Today, it has become increasingly popular to generate electricity in distributed energy sources, both on photovoltaic farms and in small co-generation units. Large-scale wind power plants or photovoltaic farms are usually far away from load centers and need to be connected with an electric power grid. The installation of the overhead lines is not possible everywhere, especially in cities or across seas; therefore, underground solutions for electric energy transmission are needed. Almost 50 years of using gas-insulated lines (GILs) in electric power infrastructure make them good candidates to be used as high-power underground transmission systems [1; 2; 3; 4; 5; 6; 7; 8]. GILs were introduced to power engineering in the late 1960s and are widely used today with high reliability. High-current GILs were often used in power generation and substation facilities as power transmission lines. Their main area of application is as a connection between the generator unit and its step-up transformer. Today, GILs are applied in many projects around the world where high-powered transmission with high reliability and maximum availability is required. The sizes of the projects are constantly increasing: from system lengths of a few meters, in the past, to several dozen kilometers, today. High-current busduct systems are usually gas insulated single-pole systems and tubular aluminum or copper conductors encapsulated in the coaxial aluminum screen. The insulating medium in the first generation of GILs was SF\({}_{6}\) (sulfur hexafluoride) under pressures ranging from 0.29 to 0.51 MPa (at 20 \({}^{\circ}\)C). In the next generation of GILs, SF\({}_{6}\) has been replaced with a mixture of nitrogen and SF\({}_{6}\). The operation pressure is dependent on the mixture temperature and is specific in particularinstallations. Due to environmental protection requirements, the mixture of N\({}_{2}\) and SF\({}_{6}\) is preferred in applications where large gas quantities are used and where dielectric performance is desired [9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20]. The simple design of GILs has proven to be highly reliable over the last 50 years of operation. Until recently, no critical failure has been reported from any installation in the world [18]. GILs have several advantages, such as low transmission losses and high transmission efficiency, low capacitive load, low electromagnetic field, high reliability, no electric and thermal aging, low environmental influence and a high level of safety concerning surrounding equipment or nearby personnel [21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31]. GIL technology is increasingly becoming popular in the bulk transmission of electric power across long distances. The high-current basducts can be installed above ground, in a tunnel, trench or directly buried. These installation opportunities offer a large field of possible applications inside substations and power plants or even outside as cross-country installations. The simple design and high safety level of GILs allows for their installation in traffic and railway tunnels [7; 8; 18; 19; 20]. GILs can be installed in a street or railroad tunnel because they do not pose a threat to other users or elements of the tunnel. They can also be fixed on the tunnel roof or laid in a separate tunnel, which is often used as an emergency exit tunnel. Moreover, GILs low capacitive load makes it possible to build busducts lengths of up to 800 km without any reactive power compensation. For these reasons, very long tunnel systems can also be equipped with GILs [18]. The use of traffic tunnels with GILs is now under investigation in different parts of the world. In Europe, interconnections between Germany, Austria, Switzerland, Italy and France are planned to improve the traffic flow and to allow trade of electrical energy.