By Satyajit Ganguly, Managing Director, North East Transmission Company Limited (NETCL); Rajesh Gupta, Chief General Manger, Powergrid, Ex-Director Project, NETCL; and Harshal Malewar, Deputy Manager, PMS/Project, NETCL
Transmission towers are prone to collapse under heavy wind loads. Due to practical limitations in design and constraints of resources, transmission line towers cannot be constructed to withstand all types of calamities. Consequently, tower failures are being observed during natural calamities such as storms, typhoons, strong winds, heavy rains, floods, earthquakes and landslides as well as man-made disasters such as sabotage.
Earlier, these breakdowns were being attended to through conventional restoration methods, which involved debris removal, casting/repairing of concrete foundations, arrangement of all tower parts and line material, erection and stringing. These activities required a minimum of four to six weeks, depending on the nature of the failure, mainly due to the settling time required for cement concrete in foundations. Consequently, power systems were prone to long breakdowns. This, coupled with the fact that the transmission system in the country has low redundancy, led to serious issues in attending to tower collapses or line diversions.
A unique method being used to overcome tower collapse problems is the emergency restoration system (ERS). This comprises temporary towers that are primarily used in electrical transmission lines. The ERS has a modular construction, made of aluminium alloy or hot dip galvanised structural steel or a combination of both. Aluminium alloy is preferred for its light weight. These quick-erect, corrosion-free aluminium towers can be raised in the field in less than a day and are used to address outages/trippings in power lines during emergencies, maintenance requirements, line diversions and various other applications. It is a proven technique to deal with disaster management in the transmission sector.
Causes of tower failure
The high wind velocity during storm, cyclone and local phenomenon of whirlwind, etc., might have exceeded the wind speed for which the tower is designed. This type of wind is difficult to predict. Theft/Sabotage of tower members, generally theft of secondary members (connected with one or two bolts) of towers by the local people makes the tower structurally weak, which ultimately leads to failure during high speed wind/storms/whirlwind/cyclone/avalanche, etc. Sometimes, proper protection has not been provided for the foundation of towers in steep slope/hilly terrain. Many times, landslides cause erosion of soil below the foundation, which, in turn, causes the failure of foundation and subsequently, failure of towers. The tower foundation’s failure (located near a riverbank) is due to erosion of soil below the foundation by flash floods or change in the river course.
Advantages of ERS
The advantages of using ERS are numerous, including no requirement for a foundation, bypassing of critical towers, reduction in inventory, standardised design as per IEEE1070, modular design, besides being very user friendly, quick restoration of collapsed portion of transmission line, no civil engineering work is required, besides avoidance of high political and social cost.
Components of ERS
The ERS structure is designed for easy handling and transportation. The column section is the heaviest component. ERS includes insulators and conductor hardware. Polymer suspension and post insulators are of high strength and low weight, which can be easily transported to difficult terrains, including hilly areas. All components can be easily transported by open trucks to any nearby location and after that they can be shifted to various tower locations by head-loading.
The major components of a typical ERS are:
- Foundation plate: It supports the tower by distributing its weight evenly on the ground. Its weight is 250 kg and provides 2.32 square metres of bearing surface and can be pinned to normal ground using 25 mm diameter reinforcing rods of about 1,500 mm length. The foundation plate is designed to rest on the ground surface with anchors or metal stakes to avoid sliding. It is made of lightweight, high-strength material.
- Gimbal joint/Articulation base: The first section above the foundation plate is the gimbal joint. It can be rotated in four major axes to allow horizontal column assembly. It allows the tower to move under various loads in order to avoid bending. The gimbal joint and the required column section can be bolted together.
- Column section mast section: It includes nine openings on each side to allow attachment of a wide range of accessories such as swivel guy plates, insulator brackets, platforms, etc. These are made of lightweight aluminium alloy, high-strength material and available in different lengths. They can be erected one by one to get the required height of the structure. The column may have diagonal bracings and positioned in a manner to facilitate tower climbing.
- Box section: (If required) shall be of such design that it allows attachment/ mounting of insulators and guy wires to the structure. It shall be assembled between column structures and shall have predetermined holes on the sides to allow attachment of insulators and guy wires. Suitable provisions shall also be provided on the ERS tower for installation of earth wire as required.
- Guy plate/Swivel guy plates, box section/connecting box and provisions for earth wire: The design of the guy plate shall be such that it shall allow attachment of insulators and the guy wires to the structure. It shall be assembled between two column structures and have predetermined holes to allow attachment of insulators and guy wires. Depending upon the requirement, the angle of the guy plate shall be 0/0, 0/45 or 45/45. It shall be made of light weight, high-strength material. The guy plates may be of different types for different suppliers. The swivel guy plate is used to attach guy wires and guy strain insulators to towers.
- Composite insulator strings, hardware fittings and guy insulators: Light-weight, high-strength polymer insulators are being used for easy handling and quick assembly. Its weight is about 10-15 per cent of the conventional porcelain insulators.
- Anchoring the assembly system of the ERS: Depending on the prevailing soil conditions (soft, hard, normal), different anchoring arrangements could be required. Cross-plate anchors are used for normal soils, whereas screw-type anchors are used for loose/marshy soil and rock-anchoring systems are used for hard rock terrain. Proper selection of anchoring arrangement is necessary for the stability of the structure.
- Erection of ERS structure: The ERS can be erected without heavy equipment using a small, lightweight gin pole or boom attached to one side of the structure. A crane provides the simplest method for erecting the ERS structure. Even a fairly small crane can be used to erect the structure. Other methods can be winch-line method and helicopter method.
- Configuration of towers and software: The ERS can be erected in various configurations, in line with instructions given by manufacturers in the manual as well as with the PLS CADD and PLS-CADD/PLS-Pole/LW+Mast software by providing inputs. Generally, a 400 kV ERS contains a total of 256 mast sections of 2.904 metres, by which erection unit can create a total of 16 ERS towers with 50 metre normal height. The total height for a 400 kV ERS is generally up to 50 metres. A special ERS tower up to 92 metres has been erected by Powergrid in the 800 kV HVDC Raigarh-Pugalur transmission line, over Tungabhadra river crossing a span of 1 km (see picture).
Skilled manpower is required to install the ERS structure and proper safety measures are to be taken. ERSs are shipped in maritime containers that act as a way of mobilising as well as storing the material. The ERS has a container storage system, where the small hardware and equipment tools and plant are nicely stored in bins and shelves to quickly identify and take the material out.
The ERS is an effective disaster management tool for damaged transmission lines. Damaged/fallen transmission structures can be replaced in a few hours, depending on the nature and depth of the damage. Proper planning not only maximises the restoration of efficiency but can also minimise inventory levels. For utilities, having an effective emergency restoration plan can help control the financial impact of losses due to weather-related power outages. By using an ERS, high revenue loss and penalties can be saved by utilities and damaged or diverted transmission lines can be restored at the earliest.