Comparative study of design inputs in design of overhead transmission line towers

Abstract

The Ceylon electricity board is the main electricity producer in Sri Lanka. The supply arm of the Ceylon electricity board is divided into three categories: Generation, Transmission and Distribution. The power lines having the capacity 132 kV and above comes under the transmission category. The transmission lines may be spanning over a few kilometres to hundreds of kilometres, depending on the power station location. The lattice towers are the main supporting structures in the transmission line. The cost of the transmission towers is ranging from 28 to 40 percent of the total cost of a transmission line project. Every year the power demand may increase by around 10 %. To have uninterrupted power supply the Ceylon Electricity Board must increase its supply capability simultaneously. With this huge power demand CEB must move to 400 kV lines in the near future. The scarcity of new corridors to build new lines demands increase the capacity of the existing lines by using the same corridors. Both requirements demand heavy towers. The design of the transmission towers and foundations is done according to the CEB specifications, which were produced long ago. The CEB specification is based on the probabilistic method with safety factors while the world is moving towards the reliability-based method. The CEB towers are known to be the heaviest towers in the region. The largest existing tower uses the maximum available angle iron as the leg members. When increase the tower capacity and the number of circuits to four circuits from two, star angles or combined angles must be used as the leg members. The foundations have to be designed either as raft foundations instead of single footings or pile foundations in fairly good soils. The towers need to be optimized in order to overcome the limitations in the design as well as to reduce the cost of the projects. The lattice towers are designed for decades and the optimum tower configuration have been already identified by the designers. The optimization of the design input is the next possible approach to design an optimized tower. The design inputs mainly depend on the code of practice used in the design. In this research, an attempt was made to identify the economic viability of the CEB design inputs against IS 802:1:1 (2015) and IEC 60826 (2017) design inputs. The objective of the research was achieved by identifying the different design inputs against different codes of practices and design two type towers, one line tower (TDL) and one angle tower (TD3) in PLS tower software by using the identified design inputs. The design was carried out according to the ASCE 10:97 and IS 802:1:2 (2016). To identify the economic viability of the design inputs the support reactions and the member capacity usages were compared using the output of the FEM. As per the results that the tower weight and the foundation size could be reduced by a significant margin if the design was carried out by using either IS or IEC codes instead of the CEB specifications.