Optimization and Design of the 400 kV Double Circuit Overhead Line Crossing the Hoogly River in West-Bengal, India

摘要

This paper summarizes part of the studies performed for the optimization and design of the overhead line crossing of the Hoogly river near Kolkata. This crossing consists of a double circuit 400 kV line transmitting 600 MW power and will include the tallest towers in India. Due to environmental constraints, various crossing locations were assessed involving central spans of 1500, 1600, 2000, and 2300 m.This paper covers the methodology to arrive at the most economical combinations of conductors and tower heights for this project with a special emphasis on the option of central span of 1567 m that has been finalized for this project.Various conductor types and sizes were selected to be at least as conductive as the remaining the 400 kV line. as a starting point, the total aluminium area of the river crossing twin bundle conductor was assumed equivalent in conductivity to the line's bundle totalling 1057 mm~2 of A1 aluminum. For this purpose, and in addition to A1 aluminum, two other alloys were studied: A2 alloy (53% IACS) according to IEC 60889 and A4 alloy (57 % IACS) according to Canadian Standards.After the first round of conductor studies, the most promising conductor types were mainly A2/S3A (Aluminum alloy wires reinforced with extra high strength galvanized steel sires) or A2/SA20 (Aluminum alloy wires reinforced with aluminum clad extra high strength steel wires). The former type of wires was preferred based on indications that little corrosion was reported on older ACSR conductors in the area. In presence of corrosive conditions, greased S3A wires, or SA20 wires would have been used.Various wire standings were studied such as 54/19, 38/19, 38/37, 40/37, and 42/37. Their impact on suspension tower weight and heights was also assessed and discussed in this paper.As regards the design criteria, a high reliability level III (i.e. Return period of wind loads equals to 500 years) was selected for this crossing as per IEC 60826. If normal overhead lines were designed according to this reliability level, no additional overload or safety factors would have been used, except in caseswhere towers are not full scale tested. In such case, an overload factor of 1.1 would have been used tocompensate for the lack of experimental validation of the design. Due to the fact that river crossing towerscannot be tested, and in order to reduce the risk of the suspension tower failure in case of a phase failure,three overload factor of 1.5, 2.0 and 2.5 were considered for the tower design. It is reminded that innormal lines, the design approach where suspension towers are designed to fail before the conductor is theopposite of this requirement for the river crossing because replacement of a river crossing conductor is alesser problem than replacing a failed 230 m suspension tower.The results of the above studies are presented in this paper and the crossing is currently being built and isscheduled to be completed in 2014.