In Japan the demand for electric power increases annually, the power sources become larger and more high capacity transmission lines are needed. The cost of constructing transmission lines with upsized bundled conductors and taller towers continues to rise, which accounts for the increasing percentage of the capital investment of electric utilities. The effective and integrated use of land is important to Japan's population, which now exceeds 126 million people. Transmission lines are subject to severe construction restrictions and environmental regulations, which have forced some new circuits to be routed in mountainous areas, increasing construction costs, construction time and the number of staff required.
Tohoku Electric Power Co. Inc. (TEPCI), Sendai, Japan, supplies electric power to one-fifth of the inhabited area in Japan. Because the customer density is less than half the national average, the transmission system per customer is large compared with other electric utilities in Japan. Therefore, unit transmission line costs are more significant to TEPCI. For this reason, TEPCI has developed new extra high voltage (EHV) construction technologies that will be used on the Yamagata trunk line, now under construction.
Profile of Yamagata Trunk Line The Yamagata trunk line is a 53 km (33 mile) long, double-circuit, overhead transmission line. One hundred thirty-four towers are being built to meet the increasing power demand in Yamagata City and the surrounding area. This line, scheduled for commissioning in June 1999, is designed for 500 kV, but will initially operate at 275 kV. In the long term, the circuit will be upgraded to form part of TEPCI's 500-kV transmission system. The line is being routed through a mountainous district with altitudes ranging from 300 m-800 m (1000 ft-2600 ft) and an annual snowfall of 3.5 m (11.5 ft). The average tower height will be 79 m (260 ft) and the weight will be 79 t (87 tons). The ratio of tension type to suspension type towers is 7:3, and the average span length is 397 m (1300 ft).
The following design rationalization techniques and new technologies will improve workability and reduce construction and labor costs: -Finned low-loss electric conductor. -Mobile pile driver assembled on site for a tower foundation work. -Climbing Crane to construct towers. -Prefabricated stringing method.
Finned Low-Loss Conductor The electric conductor used on the line is a finned low-loss conductor developed in collaboration with Hitachi Cable, Ltd. This low-loss conductor is a thermal-resistant, steel reinforced (SBTACSR/UGS 530 mm2 or 0.820 inch 2) aluminum alloy conductor. It consists of thermal-resistant aluminum alloy wire rated at 150oC (302oF) allowing continuous temperature and ultra-high strength galvanized steel wire with a tensile strength of 200 kg/mm2 (19,800/lb/inch2). This conductor uses a thermal resistant aluminum alloy conductor (TAL) with conductivity over 60%. The wire is compressed to increase the space factor from the existing 75% to 90%. The steel wire improves the tensile strength, reducing steel wire cross-section and increasing aluminum cross-section. It reduces transmission loss 20% compared with the existing TACSR conductor with the same outer diameter.
In addition, a 1.5 mm by 1.5 mm (0.60 inch by 0.60 inch) fin is attached in a spiral to the surface of the conductor to reduce the probability of snow accretion to the conductor in mountainous regions (Fig. 1). This fin replaces the snow-resistant SR rings used for the same purpose on aluminum conductor steel reinforced (ACSR) conductors. This redesign saves material and construction costs.
Mobile Pile Driver In mountainous districts where the bearing strata of foundations are deep, and narrow and inadequate roads prevent transportable pile drivers from accessing tower positions, a concrete pier foundation is being constructed. This method requires large quantities of concrete, soil and sand to be excavated. The process is costly. Therefore, Sato Kensetsu Kogyo Co., Ltd. and Yamato Kiko Co., Ltd., in cooperation with TEPCI engineers, has developed a mobile pile driver, which reduces costs without comprising quality and safety standards.
Designed with bolted assembly, the pile driver comprises 14 units, the heaviest weighing only 2.4 t (2.6 tons), allowing transportation by either cableway or helicopter. The pile driver is 3.5 m (11.5 ft) wide and can climb a 22% gradient making it easy to drive on uneven sites inaccessible to conventional pile drivers. Using extended outriggers with this equipment assures stabilized piling work. The outriggers can drive 500 mm (20 inch) dia steel piles. The advantages of using the pile driver are:
-Simple transportation of equipment via 3.6 t (4-ton) truck. -Frequent application of piled foundations in mountainous regions. -Reduction in use of temporary access roads. -Replacement of pier tower foundations, reducing the construction time and cost for foundation work (approximate 20% savings).
Climbing Crane The Climbing Crane (Fig. 2) is the latest method introduced by TEPCI to simplify the construction of EHV transmission line towers and to overcome the utility's shortage of young, skilled construction workers. The crane consists of a mast with a superstructure and jib that can rotate horizontally through 360 degrees. The height of the mast is increased by raising the bottom with a hydraulic system or a wire rope. The tower construction procedure starts with the crane being self supported in the center position of the tower and assembly continues in stages by extending the mast, which is vertically supported, by installing horizontal stays on the tower legs. This process is repeated until the top of the tower is assembled. The crossarms are then installed either in a block or are partially installed at given positions on the tower. The crane can remove the tower smoothly as the mast is lowered and removed from the base of the tower.
Because the crane has a 32.7 t (36-ton) lifting capacity and 2.5 t (2.8 ton) nominal load, some of the assembly work can be performed on the ground, which means that less work is done on the tower. When the crane is dismantled, the heaviest unit weighs 0.9 t (1.0 ton), which is easily transportable by cableway or heli-copter.
Prefabricated Conductor Stringing Traditionally, conductors of the requisite length (determined from span lengths and tower heights) are strung between towers. A series of adjustments, conductor cutting, tension clamp compression and insulator installation are then completed on the tower. The Prefabricated Stringing Method (PSM) uses a conductor whose length is precisely calculated and then equipped with tension clamps while on the ground. The conductor is then strung between the towers and attached to insulators. Fine adjustments for sag complete the stringing. PSM is safer because it reduces the amount of tower work at high elevations. The method also assures quality control on tension clamp compression and reduces the construction period. Two new technologies, sticky reflective tape and protectorless clamps, further reduce construction costs. (Figs. 3 & 4)
The Outlook TEPCI has successfully re-engineered the construction of EHV transmission lines to overcome a shortage of skilled workers. The new material technologies and working practices developed in this process have significantly reduced electrical losses, saved construction costs and established a range of improved safe working practices for construction workers. This will ensure that TEPCI remains competitive in the energy market in Japan, where there are strict demands from customers for reliable electricity and low rates.
Yugi Kubota is manager of the Power System Engineering Department of Tohoku Electric Power Co., Inc. in Japan. During his 20- year career he has worked in all sectors of the company's transmission facility. Academically qualified at the University of Tokyo, Yugi is a member of the Institute of Electrical Engineers of Japan (MIEEJ). As a member of CIGRE, he is on the Japanese Panel of CIGRE SC22-Overhead Lines.