Tower costs typically range from one-fourth to half of total transmission line costs. As such, design optimisation could result in reasonable savings. Consequently, new designs, solutions and construction techniques for transmission towers are garnering significant interest from utilities and the industry.
Right of way (RoW) is one the most important considerations while undertaking transmission tower construction. RoW is important in densely populated areas, forest areas, areas with high land cost, areas with non-availability of the required conventional width of RoW, areas with narrow constrained geographies with no other alternative and areas dedicated to conserving flora and fauna. Appropriate tower design and configuration strategies can help address RoW issues to a large extent.
Power Line takes a look at some of the emerging tower designs and new construction practices…
Given the high population and infrastructure density of most cities and towns, tower structures need to be compact. In addition to self-weight, towers have to withstand forces like strong winds, earthquakes and snow loads. Therefore, transmission towers should be designed taking into consideration both structural and electrical requirements for a safe and economical design.
Further, tower designs must take into account the challenges that arise due to the nature of the terrain (hilly and snowy terrain), area-specific issues (particularly in deserts, forests and gorges), river crossings, space/RoW constraints, height restrictions, and project location (near bird sanctuaries or airports).
Tower configuration depends on a number of system and design parameters such as the required voltage level, circuits, number of conductors in a phase, wind characteristics, terrain and design philosophy. The configuration of the tower is done by first fixing the outline of all the towers as per Indian Standard requirements (IS 802). The selection of the best outline and bracing system patterns contributes to developing an economical and safe design of a transmission tower.
New tower foundation designs aim to customise structures in accordance with the terrain and reduce the construction time. To this end, precast and grillage foundations, which are factory-made solutions, as well as monopole foundations, which are compact, lightweight and visually more aesthetic, are considered viable solutions.
Transmission lines on monopoles have a similar line configuration as lattice tower-based lines (freestanding framework tower), except for the basic structure body. Monopoles have sleeker bodies made from polygonal tubular sections with a tubular cross arm arrangement for fixing tension or suspension clamps on it. Monopoles can be effectively used to resolve RoW issues. The structure helps offset RoW constraints and resolve the issues related to corridor width as the structure has a small footprint. A transmission line using a monopole can be accommodated on highway curbs/centre verges. Monopoles can be used near substations situated in urban areas where a large number of incoming and outgoing lines are connected, and this can prevent the congestion of lines.
One of the key tower structures that addresses the RoW issue is the multi-voltage multi-circuit transmission tower (MVMCT). MVMCTs with a narrow base may gain significant traction in India with regard to economic issues and minimisation of legal issues pertaining to the land. As compared to conventional towers, MVMCTs help in huge cost savings.
Amidst growing environmental concerns, bird-friendly towers are largely being deployed near bird or wildlife sanctuaries. In these towers, all conductors are placed at a single level. In order to avoid the collision of birds with the conductors, bird-diverting flags are provided on the top of the conductors. Besides, extra tall towers are used at such locations to protect wildlife and prevent deforestation.
Similarly, in airport-friendly towers, conductors are placed at relatively low levels. Low-height towers are also commonly used for power transmission near airports. Meanwhile, guy-type towers are used when there is not enough time for erection. However, for such towers, the RoW requirement is more and they are unsuccessful in situations where conductors are found impaired.
Another type of tower structure is the chainette in which the insulators are hung on ropes and the structure is further compacted with the help of chains. As a result, the tower’s width and RoW requirements are reduced. The main advantages of chainette towers are their lightweight as compared to conventional self-supporting towers, lower costs and reduced erection time. However, this type of tower structure is only suitable for flat ground terrain.
One of the most important developments in the cost optimisation area has been the use of tube profiles in place of angular profiles. Tube profiles have a less number of elements and members. The use of tubular profiles helps in the faster erection of transmission towers and reduces the overall tower weight by 15-20 per cent. Tubular profiles also attract less wind load and indirectly increase the strength of the tower.
Another key development in tower design is to have fewer sections and bends so as to reduce the inventory and lead times. In the entire manufacturing process, bending is the most crucial activity and the entire process is slow. Bending of heavy angles is difficult and every assembly has to be checked for accuracy. Therefore, instead of bending the main angles, it is advisable to terminate on plates, without sacrificing the structural safety.
New trends in tower manufacturing
India is among the largest hubs for tower manufacturing other than China, Saudi Arabia, Turkey, Iran, South America and North America. The manufacturing process has gone through many innovations that have resulted in increased efficiency and reduction in cycle time. Traditional manual machines have been replaced with computer numerical control machines, which do multiple operations like cutting, stamping, punching, drilling and notching. Master cutting or nesting software optimises the cutting plans to reduce wastage and leads to the procurement of optimum lengths and quantity of sections for batch production. Over the years, manufacturing plans have shifted from manufacturing in metric tonnes to completed towers.
The industry has now started using composite materials such as fibre-reinforced polymers (FRPs) for building towers. FRPs are gaining popularity in the transmission and distribution industry since they have a high strength-to-weight ratio, better insulation capability, low maintenance, affordability, ultra-violet resistance, rust and rot resistance, and flame-retardant properties. Pultruded FRP (P-FRP) towers and cross-arms are also being used as replacements for ageing wood poles in remote and extremely humid locations, as composites do not absorb moisture or swell up. Also, since P-FRPs are non-conductive, insulator strings are not required in the towers, resulting in cost savings.
Surveying helps in the selection of the optimum tower foundation depending on the topography, terrain and type of soil. It also helps in determining the minimum number of towers required to be constructed to build a transmission line. Besides, it facilitates the selection of the shortest possible route to optimise the cost of the transmission line. Conventional methods of surveying such as walkover survey, preliminary survey and detailed survey take a considerable amount of time and have several other limitations as well.
Modern survey techniques like light detection and ranging (LiDAR) and drone survey help overcome most of these limitations. LiDAR technology uses laser distance measuring to conduct topographic mapping with the help of an aircraft. LiDAR beams transmitted from the aircraft are able to function in cloudy conditions and can even penetrate dense forests and provide high accuracy at much higher speeds. Such automation helps in optimising costs and time, and is far more efficient than manual surveys. On the other hand, high quality aerial images and precise measurement can be achieved through drone surveys. Good visual data of each location helps in better decision-making. The information gathered through drone surveys is highly beneficial for assessing potential site locations, designing site layouts, generating 3D visualisations and making RoW estimations.
The way forward
With the changing requirements and development of new technology, tower designs and configurations are also evolving. The new designs and models, which deal with RoW issues, are installation friendly, compact, lightweight and visually more aesthetic, features that customers would be looking out for.
Moreover, over the next few years, investments by transmission utilities are expected to rise significantly. While at the interstate level investments will be made in the creation of evacuation infrastructure, at the intra-state level, the key focus will be on the upgradation and modernisation of state networks. Since expenditure on towers accounts for a major part of the total investments in any transmission project, technology providers and transmission equipment players are likely to see significant opportunities in the next few years.