Emergency Restoration Systems: Quick disaster management tools to fix damaged transmission structures

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:

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.

Conclusion

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.