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This book, Gas-Insulated Transmission Lines, covers various aspects of the gas-insulated transmission line (GIL) applications, including the description of the past and future installations and represents a very useful source of information for the analysis and design of GILs.

Gas-Insulated Transmission Lines

by Hermann Koch, Wiley-IEEE Press, ISBN 978-0470665336.

GILs can be applied when environmental or structural considerations rule out the use of overhead transmission lines. Dr. Hermann Koch, the author of this book, participated in all stages of GIL development and application. This book covers various aspects of GIL applications, including a description of the past and future installations, and represents a very useful source of information for the analysis and design of GILs.

The book consists of ten chapters including “Introduction” and “Conclusions.” In the introduction the author underlines the main features of GILs, namely, high transfer capability, very low electromagnetic field, no electric and thermal aging, and low losses as well as a high level of reliability and personal safety.

In the second chapter “History,” the author presents a brief description of the ac and dc power transmission line development up to the UHV level. The first (100% SF6) and second (80% N2 +20% SF6) generations of GILs are discussed in greater details with the overview of various installations. The fully encapsulated GIL design withstands various environmental conditions, requires only outside inspection, and is maintenance free. The first GIL using SF6 gas was energized in 1975, and since that time about 300 km of GILs have been installed around the world. The nominal voltage levels for these installations reached 1,200 kV with the current rating up to 8 kA and short circuit rating up to 100 kA.

The third chapter, “Technology,” covers the principles of gas-insulated high voltage equipment and the basis for each specific GIL design criteria. These parameters depend on the electrical field strength, maximum conductor temperature, required mechanical strength of the enclosures as a function of line nominal, and short-circuit currents. The author describes the characteristics of insulating gases, principles of gas mixture, best point determination, and normalized gas properties. He also provides a list of GIL verification tests and their results as well as an overview of the basic design parameters. Thermal design characteristics, details of GIL testing, and on-site monitoring are described for different GIL configurations. This chapter also covers quality control and diagnostic tools, planning issues including reactive power compensation, grounding and safety issues, corrosion protection, environmental limitations, and reliability issues. This chapter describes, in detail, the required steps in GTL production including a brief description of the commissioning and onsite tests.

In Chapter 4, “System and Network,” the author discusses different aspects of integrating GILs into the network and operating them as a part of the transmission system. The GIL’s main parameters and a comparison with the overhead lines are summarized. Thus the GIL inductance is about four to five times lower than the values for corresponding overhead lines. At the same time, the GIL capacitance is about four to five times greater than the corresponding values for the overhead lines. This results in much lower surge impedance and, therefore, much higher surge impedance loading for GILs in comparison with corresponding overhead lines. The actual GIL rating depends also on the conductor cross-section size, for example, a pipe enclosure equal to 0.5 m in diameter corresponds to about 3,000 MW loading capability. Dr. Koch stated that GILs, being a gas-insulated system, have similar switching characteristics to the conventional transmission lines. Indeed, although the physical capacitance of the GIL is greater than for overhead lines, the GILs are usually quite short, resulting in similar transient responses of the transmission circuits with and without GILs. It is also underlined that the system availability of the GIL is very high. Thus during more than 30 years of experience with about 300 km in operation, no failures leading to power flow interruption were reported and no aging effects were detected. At the same time, the metallic enclosure makes GILs very safe since an internal arc has no external influence. In addition it is possible to lower the basic insulation level for GILs in comparison with corresponding overhead lines by using surge arresters at the GIL terminals. GIL performance is monitored by measuring gas density, partial discharges, and temperature.

Chapter 5 describes the electromagnetic fields from GILs. It is illustrated that the strength of both fields, magnetic and electric, in the vicinity of the GIL is much lower than from the comparable overhead lines due to the solidly grounded low impedance enclosures.

Chapter 6 deals with economic aspects of GILs. The production and installation cost of GILs is quite high with the material and assembly representing about 80% of the total project cost. The cost of various GIL components such as maximum transportation length, use of elastic bending, and handling are analyzed in this chapter. According to the author, GILs might find some applications as a part of a transmission network despite its much higher initial cost.

Chapter 7 describes typical GIL installations designed and energized during the last 40 years. These GILs were installed mainly in densely populated or sensitive environmentally protected areas and in conjunction with hydropower dams and power plants. The total length of GILs in service equals about 300 km with nominal voltage from 72 kV to 1,200 kV. The largest number of GILs was installed at 420 and 550 kV voltage levels. Dr. Koch describes 15 various GIL installations. It should be underlined that GIL installations are usually custom designed, and therefore the details on the illustrated GIL are very useful, especially for planning and designing new GIL applications. The author also discusses possible future applications. For example, Dr. Koch states that GILs might be installed in traffic tunnels and along roads and highways when bringing EHV circuits into energy intense densely populated areas.

In Chapter 8 the author presents a general comparison between GILs and other transmission circuits. The author states that direct comparison between transmission circuits is quite difficult since technical and economical aspects vary from project to project. In addition to a general comparison of the various transmission systems, a number of parameters such as losses, magnetic field strength, short-circuit ratings, overvoltages, temperature limits, and site characteristics should be considered. Finally, the cost factor represents a very important parameter when various technologies are compared. The GIL becomes more competitive on the UHV level. Thus, for example, the GIL is about eight times more expensive than overhead lines on 420–550 kV levels and two to three times on the 1,000 kV level. Obviously, the GIL becomes more competitive with other transmission circuits at the increased power flow levels.

Chapter 9 deals with power transmission pipelines, which are represented by tunnel systems for connecting to mainland substations the collected offshore generation. In this chapter the author illustrates the applicability of the GIL in under-sea tunnel systems for delivering up to 8,000 MW from the offshore wind farms.

Finally, in the conclusion, Dr. Koch states that with the 35 years of positive experience GILs are ready to be applied when high power transmissions are required.

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