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Requirement of modern survey techniques for expeditious planning & implementation of tansmission line projects

Basic Concept of an EHV transmission line
An EHV Transmission line generally links a Generating station and a sub-station or two sub-stations or two Generating stations as may be necessary. The basic requirement of a transmission line is to transfer the desired amount of electric power from generating stations to load centers.

The transmission lines are designed in such a way that the power carrying conductors are supported on steel structures erected along the route. The power conductors are insulated from the transmission line structure through necessary insulation systems. The necessary safety clearances as per Indian Electricity Rules (1956) as amended from time to time are to be maintained from any earthed object in the vicinity of the transmission lines. The clearances from the line conductor to the ground as well as to adjacent objects are ruled by the sag of the conductor that takes the form of a catenary, which increases with the span length between two adjacent transmission line structures. The atmospheric conditions like ambient temperature, solar radiation, wind velocity etc as well as power flow influences the sag. The conductor also swings under heavy wind and it is required to maintain clearances under such conditions. Hence placing of intermediate supports at appropriate intervals becomes essential. Further, the line has to take deviations from the straight line route between connecting generating and/or substations to maintain necessary clearances from permanent establishments, to minimize involvement of reserved forests, minimize major crossings, avoid wild life sanctuaries, archaeological monuments etc.

The major transmission voltages in the country, right of way requirements and power transfer capability is shown at Table-I. The transmission line requires a right of way along its route to keep minimum clearances from earthed objects. All objects are to be cleared within this zone. A representative figure showing details of right of way requirements for 765kV S/C line is shown at Fig-1.

S.No	System Voltage	Power Flow per circuit	ROW requirement
1.	132kV AC	85 MVA	27m
2	220kV AC	200 MVA	35m
3	400kV AC	800 MVA	52m
4.	800kV AC	2500 MVA	85m
5	500kV HVDC Bipole	2000 MW	52m

Why Surveying is important
Surveying is an important aspect of transmission line due to the following:

The objective is to optimize the cost of transmission line based on following considerations:

• Shortest route clearing various objects
• Minimum number of river crossing towers
• Accessibility i.e from approachability for construction as well as from law and order point of view.
• Selection of optimum foundations based on following information
  Type of soil
  Type of terrain : Benching and rivetment requirements, requirements of leg extensions
  Loose hills especially in young Himalayan region, areas prone to landslides
  Area of submergence as well as prone to river meandering
  It helps in ensuring the statutory clearances with respect to following:
• Power line crossings, Railway crossings, road crossings etc.
• Clearances from habitation
• Environmental clearances
  Minimum forests, minimum forests density
  Historically important areas and monuments
  National Parks and wild life sanctuaries

The surveys to collect the above information are carried out progressively at different stages of the project due to various reasons.


Fig. 1

The various stages in a transmission line project requiring map study/ surveying and the type of surveys associated are detailed below:

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