Gas insulated transmission lines (GILs) are composed of pipes that house conductors in highly insulative sulfur hexafluoride (SF6) gas, which have high load-transfer capacity. The development of GILs in Japan started in 1964 when the late Professor Setsuo Fukuda at Tokyo University researched compressed gas insulation. The university, the Central Research Institute of Electric Power Industry (CRIEPI) and cable manufacturers jointly produced a demonstration GIL.
In 1979, Tokyo Electric Power Co. Inc. (TEPCO) commissioned Japan's first GIL, a 160-m (525-ft) double-circuit bus bar in a 154-kV, 2000-A substation. However, because GILs were much less flexible than cables and not capable of accommodating a displacement of terrain, they were only used for short transmission lines (such as a few hundred meters for bus ties in Japan's substations).
On a global front, in 1974, Siemens A.G. in Germany installed a GIL in a tunnel at Wehr hydropower plant, which was the world's longest GIL at that time.
The Decision-Making Process
To meet growing electric power demand, Chubu Electric Power Co. Inc. (CEPCO) needed to construct an additional thermal power station adjacent to the suburbs of Nagoya City and a transmission line capable of carrying 3000 MW of electricity from the power station to the substation. Because an overhead transmission line would have to be routed through the suburban area, CEPCO considered an underground cable system for the 3.3-km (2.1-mile) route between the planned thermal power station and the substation. If 275-kV XLPE cables had been used for this high-capacity transmission system link, at least five circuits (including one spare circuit) of the largest 2500-mm2 copper-cored XLPE cable — each with transmission capacity of some 800 MW per circuit — would have been required.
The overall costs of laying XLPE cables was excessive when considering the costs of substation components, such as gas-insulated switchgear (GIS) at both ends of a cable and line reactors for cable capacitance compensation. In contrast, the use of a GIL would make it feasible to construct the large-capacity transmission link. It was possible to use conductors greater than those available in conventional 275-kV XLPE cables, because they allowed conductors to be larger in cross-section than XLPE cables. Furthermore, the installation of a GIL with transmission capacity of about 3000 MW per circuit would allow a double-circuit link to be installed.
Subsequent cost-benefit studies confirmed that when the cost of all substation components were taken into account, the expense of installing GILs would be less than the use of XLPE insulated cables. However, various pre-construction problems had to be solved before the large-capacity GILs could be installed, including:
The difficulty of installing the GILs composed of large-bore aluminum pipes in a gently curved, long tunnel.
Because the GILs were to be conveyed to site by truck, the length of each GIL section was limited; therefore, several sections would need to be connected in the tunnel. Thus, a low-cost connection method had to be developed.
Because the transmission lines were to be laid in a dusty environment in a tunnel, contaminants might lower the insulating reliability of GILs.
The earthquake-resistant design and performance of GILs had to be taken into consideration.
To solve these problems, CEPCO initiated a program to design a GIL system for a long tunnel application and to develop engineering technologies for manufacturing GILs. Japanese manufacturers Mitsubishi Electric Corp., Furukawa Electric Co. Ltd. and Sumitomo Electric Industries Ltd. supported the utility in its work.
Outline of the Shinmeika-Tokai GIL
A view and profile of the Shinmeika-Tokai GIL are shown in Figs. 1 and 2. This transmission line was wholly laid in a tunnel connecting the Shin-Nagoya Power Station in the suburbs of Nagoya City to Tokai Substation. The tunnel was built approximately 30 m (100 ft) below the public road and had four bends (the minimum bending radius being 150 m [490 ft]). Shafts were placed at each end of the tunnel, and a ventilation hole was provided in the middle of the tunnel. With an internal diameter of 5.6 m (18.3 ft), the tunnel was divided vertically into two sections. In the upper section, two circuits (single-phase design) GILs were installed, and in the lower section, a pipeline for liquefied natural gas was installed to supply Shin-Nagoya Power Station.
Design Features of the GILs
Figure 3 shows key characteristics of the GIL, while Fig. 4 has a cross-sectional view of the GIL.
The single-phase design GIL was selected to achieve ease of installation of the transmission lines in the gently curved tunnel and to accommodate the maximum diameter of manufacturable extruded aluminum pipes. The basic dimensions of GILs were determined based on current capacity derived from Fig. 3.
To develop the insulation design, the following factors were examined: the allowable electrical stress in SF6 gas and insulators, the electric field of inner enclosures to prevent particles or metal foreign objects from floating up, and measures to make particles harmless.
The main components of GILs are described below:
- Conductors and Enclosures
Extruded aluminum alloys, which are widely used in aluminum pipe bus conductors at substations, were chosen as materials for conductors and enclosures. They satisfy conductivity, machinability and economic efficiency requirements. Considering the enclosures had to be transported to site by vehicles, the length of each section of the enclosure was 14 m (46 ft). Thus, some 1500 sections and joints are included in the GIL tunnel.
- Insulators and Particle Traps
Heat-resistant epoxy resin was selected as the material for insulators to meet thermal design requirements, with consideration to electrical and mechanical properties and casting workability of epoxy. Relatively inexpensive post-type and tri-post-type insulators were installed in the standard sections of GILs, and cone-type insulators were used in some parts of the lines to form gas sections.
In the Shinmeika-Tokai GIL, there was the possibility that metal foreign objects (for example, particles of aluminum powder generated by contact of the enclosures when GIL sections were connected) might pollute the GIL. To avoid this, the GIL sections were connected in a strictly dust-controlled environment and by the highly precise connecting method that was developed for this particular application. Moreover, to prevent metal foreign objects from any sections from adhering to insulators, particle traps capable of catching these objects and making them harmless were fitted inside the enclosures where the insulators were installed.
- Plug-in Contacts
Two types of plug-in contacts were used: a standard-type plug-in contact, used to connect conductors in the field; and a long-type plug-in contact, capable of absorbing the thermal expansion of conductors and accommodating a seismic displacement. The long-type plug-in contacts with a sliding stroke of ±130 mm (±5.1 inches) and standard-type plug-in contacts (Fig. 5) with a sliding stroke of ±30 mm (1.2 inches) were specially developed. To absorb a seismic displacement, achieve ease in connecting conductors and secure workability of contacts, these plug-in contacts were manufactured so that their angles were axially variable within the limit of ±1.5 degrees.
- Aluminum Bellows
Bellows absorb the thermal expansion of enclosures and a seismic displacement. In addition, because the solid-bond type was adopted in the Shinmeika-Tokai GIL, an induction current flows through enclosures in a reverse direction to the conductor current. To secure adequate current-carrying performance, aluminum bellows were developed (Fig. 6). The aluminum bellows have an expansion stroke of ±120 mm.
- Enclosure Joints
For enclosure joints, expanded pipe joints with a ±1 degree variable angle performance and with high levels of air tightness were adopted. As expanded pipe joints allowed the tip of 14-m (46-ft)-long enclosures to move by 30 cm (12 inches), an error in the installation of units on a support in the field could be easily corrected. Double seals were provided at the unit joints to prevent metal foreign objects generated by the friction of adjacent sections when inserted from infiltrating the sections, and also to prevent spatter from flying into the sections during welding. After the sections were inserted into the joints, a fully automatic welding machine welded the joints.
Technologies to Facilitate Installation of the GIL
The tunnel afforded limited working space and, as it was located below the road, it had a 3-D gentle line configuration with a mixture of curves and gradients. In the summer, the tunnel became humid. Accordingly, the following technologies were developed:
GIL Layout Suitable for a Long Tunnel
In a thermal expansion design for GILs, fixed supports were arranged at intervals of 56 m (180 ft), which were regarded as one thermal expansion unit. Long-type plug-in contacts to absorb the thermal expansion of conductors and seismic displacement and aluminum bellows to absorb the thermal expansion of enclosures were installed in each fixed support. Obtuse-angled units were placed along the curves in the tunnel.
Development of Efficient Construction Technologies
To assure ease and quality of work, as well as work efficiency in the limited space of the tunnel, construction machines and equipment moving on the tracks laid in the tunnel were used. Installation work was carried out as follows:
Via the shafts at both ends of the tunnel, the sections were hoisted by gantry cranes and carried into the tunnel where they were conveyed in transporting cars to the designated support positions.
At the installation positions, the units were temporarily placed by lifts fitted to the transporting cars onto the supports. To connect conductors and enclosures, the units were centered by adjustment equipment capable of fine-tuning positions.
Next, conductors and enclosures were connected in a dust-controlled clean booth (Figs. 7a and 7b). Resin-coated tools and measuring instruments were used to prevent metal foreign objects from being generated by contact with the sections, in addition to preventing the sections from being damaged.
Enclosures were welded in equipment where humidity was regulated to stay below 80% and the wind velocity was less than 0.5 m/s (Fig. 8). The tungsten inert gas (TIG) used in the welding process limited spatter. For enclosure welding, an automatic welding machine with computer-controlled tool-up speed, current value and aluminum welding rod feed-out speed was developed. During welding, the machine continuously rotated circumferentially welding the 1500 points of enclosure satisfactorily. Following the welding, each joint was X-rayed to ensure the GILs were properly connected.
Pressure tests were performed to verify the welded enclosures were airtight. To overcome the problem of using several off-the shelf cylinders to supply the dry air, a carriage with an air dryer and filter to remove foreign objects was designed. This development proved to be a major labor-saving device.
GIL Commissioning Tests
The commissioning test plays an important role in checking the integrity of the GIL. Anticipated defects include metal foreign objects that adversely affect insulation performance; plug-in contacts that may loosen, affecting the electricity conduction; and improperly installed GILs. Hence, ac withstand voltage tests, partial discharge measurements and load current tests are undertaken.
AC withstand voltage tests are undertaken in conjunction with partial discharge measurements. However, because the majority of the Shinmeika-Tokai GIL is laid in the tunnel 30 m (100 ft) below the ground, it provides an environment in which external noise is low and a partial discharge can be measured with high sensitivity. The sensors used include the ultrahigh frequency (UHF) antenna system capable of catching electromagnetic waves generated by a partial discharge and the foil electrode system, which detects an electric current with impedance. Figure 9 illustrates the UHF antenna system and the foil electrode system. To measure the electromagnetic waves leaking from the GIL, the UHF antenna sensor was installed at the insulation cylinder in the enclosure and the foil electrodes were fitted on both sides of the insulation cylinder. Detection impedance was connected with the foil electrodes on both sides of the insulation cylinder, and a closed circuit was formed through electrostatic coupling between the conductor and enclosure to detect an electric current. To sensitively measure a partial discharge with both systems, noise in the tunnel was measured with a spectrum analyzer, a frequency band (narrow bandwidth) with a high signal-to-noise ratio was selected, and a partial discharge was measured at this frequency band.
The insulation cylinders are installed at eight positions in the tunnel: six positions within the length of the tunnel and at two positions near the joints at each end of the GIL. The GIL was subjected to a dummy pulse, similar to a partial discharge, and the pulse attenuation characteristic in the GIL was measured. This pulse attenuation characteristic enabled the spacing of the insulation to be determined so that the sensitivity to the detection of a partial discharge signal would have a margin greater than 3 dB, relative to ambient noise levels. Because the area near the joints of the GIL is just below ground level, the external noise is higher than inside the tunnel. Thus, AE sensors were installed in these sections, enabling the partial discharge to be measured (Fig. 10).
AC withstand voltage tests were done six times on each single phase of the GIL. A discharge locator was installed to find the position of a partial discharge. This equipment discharge detects a partial discharge with sensors, determines the difference in pulse arrival times and locates the position of the partial discharge from the previously measured pulse propagation speed. These tests confirmed that insulation levels within the GIL were acceptable.
Loading current tests consist of a heat-cycle whereby an electric current of 5100 A (80% of the rated electric current) is passed between the conductor and enclosure in each phase for eight hours during a 24-hour period. The heat-cycle test checks the integrity and ability of the complete GIL installation to absorb the thermal expansion, the expansion of each bellows, the deflection of the sections at the curve positions, and the movement on the supports. Repeating three heat cycles on the GIL and measuring changes in the heat cycles validated the measured expansion.
To detect unusual heat generation due to the loose connection of plug-in contacts, the temperature of enclosures was measured with a fiber-optic temperature profile sensor installed on the surface of enclosures in each phase of the GIL. As a result, the expansion measured during the three heat cycles was found to have come within the design limits. The surface temperature of enclosures was within acceptable limits of the design value, and no unusual rise in temperature was observed at the plug-in contacts.
CEPCO designed a GIL suitable for installation in a long tunnel. Then, together with three Japanese cable manufacturers, developed the engineering technologies, product quality control and testing methods for the completed GIL transmission link. After two years of construction of the 3.3-km (2.1-mile) Shinmeika-Tokai GIL, the world's longest GIL was commissioned in 1998. To date, this transmission line has been fault-free, operating in accordance with the design standards for the past six years.
Naoki Takinami received a BS degree from Meiji University, Tokyo, Japan, in 1984, and joined Chubu Electric Power Co. Inc. the same year. He has been engaged in the development and installation of EHV underground transmission lines and GILs, and is currently manager of the Transmission Lines Section. Takinami is a member of the IEE of Japan.
Shin-ichi Kobayashi received a BS degree from Yokohama National University, Yokohama, Japan, in 1989. He joined Chubu Electric Power Co. Inc. the same year. He has been involved in the installation of EHV underground transmission lines and GILs, and currently works as assistant manager of the Electrical Engineering Section. Kobayashi is a member of the IEE of Japan.