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Sunday, 5 October 2014

Towers for Extra and Ultra High Voltage Transmission lines

Towers in a Transmission Line are used  to....

Towers in a transmission line are used to support insulators, line conductors, and overhead earth wire. Extra High Voltage (EHV) and Ultra High Voltage (UHV) transmission lines have large mechanical loading because of bundle conductors, large air and ground clearances and considerable dynamic forces.   As the number of sub-conductors increase in a bundle of conductors, there may be enormous effect of vibration on conductors. 
" For example the 765 kV transmission line in India uses 4 sub-conductor bundle, similarly the proposed 1200 kV UHV AC line is to use 8 sub-conductors." 

To encounter this problem, the mechanical design of the transmission system, i.e. tower dimension, phase spacing, sub-conductor spacing, conductor height etc. are to be modified.

The cost of transmission line towers with fabricated steel members varies from 30 to 50% of the total cost of the line (both single and double circuit arrangement). These fabricated lattice structures are made up by bolting/riveting/welding the structural members. Foundations for tower structures may be large and costly, particularly if the ground conditions are bad for example towers to be erected in wetlands. RCC and PCC type foundations are used depending on the soil conditions. Foundations are specially designed for certain specific locations such as river crossings etc. 

" For a lattice tower 4 foundations are required for the tower legs that require lands up to 100 sq.mtr. The foundation of each tower of the 1200 kV, double circuit (D/C) test line at Bina (M.P.) consumed concrete in excess of 500 m3."   

Towers for High Voltage Direct Current transmission lines:

High Voltage Direct Current (HVDC) transmission lines have lesser conductors as compared to an EHV or UHV AC line; therefore, HVDC towers are simpler, smaller and cheaper. The design, manufacturing and erection practices of HVDC towers are the same as that of an EHV or UHV AC transmission line.  

Towers can be Categorized as...

Mainly there are three categories of towers used in any long EHV or UHV transmission lines; viz. rigid self supporting tower, semi-flexible and flexible towers.

Rigid self supporting towers
Self supporting transmission line towers have a broad base and rigid foundation and are preferred for voltages up to 765 kV EHV AC and ± 500 kV HVDC transmission lines (in India). These towers have high strength, low flexibility and are costlier. 

They are further classified on the basis of angle of the line at the tower. Accordingly they are:
·         Tangent towers,
·         Medium angle tower,
·         Large angle tower, and
·         Dead end tower.

Tangent towers, also called straight line towers, are used for normal spans and straight lines.  In tangent towers the conductor is tangential to suspension insulators and hence the name. Under normal conditions the tangent towers take up the downward load caused by the mass of conductors, insulators, hardware and wind pressure; but are not designed for loads caused by longitudinal or transverse pull of conductors.

Medium angle self supporting towers are used at locations where the line is deviating from the tangent by more than 15o. Large angle towers are used at locations where the line is having an angle of 30o or more. Figure 1 shows a Large angle tower.

Fig.1: A Large angle tower.

Angle towers have larger base and heavier construction and are therefore costlier. Dead end towers are erected at the end or at the beginning of the transmission line. They take up the entire pull of the conductors which terminate on them. Like the angle towers, dead end towers also have broader base and stronger members.    

Semi-flexible and flexible Towers

Narrow towers have higher switching surge withstand level and lower inductive reactance of the line. Hence, flexible or semi-flexible towers having higher switching surge withstand level and lower inductive reactance, because of their lesser width, are preferred for voltages of 1000 kV and above. 

Semi-flexible towers give saving in steel and are used for single circuit 3-phase transmission lines with horizontal conductor configuration.

In the flexible or guyed towers, guy wires are used to support the tower. These guy wires are held in tension between the cross-arm points and the anchor at the ground. In the normal V-shaped guyed structure, the two legs of the tower are pivoted on the same foundation, whereas in a chainnete structure the two legs are spaced apart. It consists of two lattice steel structures or legs on hinged foundation and guy wires and anchors are provided to hold the two legs.   

Fig. 2: A rigid self supporting type  tower used for 765 kV transmission line in India

Fabrication of Towers  

The process of fabrication of tower comprises of cutting, stamping, straightening, punching, notching, bending, and heel grinding. These structural members are usually galvanized. The galvanizing process starts with de-greasing and cleaning of members with sodium hydroxide solution and HCL solution respectively. The parts are then rinsed with water to remove the acid contents. The surface is prepared with synth flux and dried at hot plate. Now the members are dipped in 99.95% pure zinc bath maintained at 4500 oC to 4650 oC temperature. 

Lastly, they are quenched in water and di-chromating is done. Usually the thickness of galvanizing is 460 gm per sq. mtr. for members of thickness below 5 mm; above which the thickness is 610 gm per sq. mtr. At the end towers are tested for ultimate loading carried out on test beds as per IS:802 part-3.

In India many companies such as EMCO, Sterlite Grid (subsidiary of Sterlite Technologies Ltd.), Bajaj Electricals, IVRCL, KEC International etc. with a rich experience are involved in construction, erection and installation of transmission line towers up to 765 kV level. The towers used for 1200 kV D/C test line at Bina,  each of weight 400 tonne and height 130 mtr., was supplied by KEC International (RPG group company).