The installed generating capacity in the Indian Power System is approx. 100,000 MW at present. In spite of having such capacity, there is 12 per cent deficit in peak demand and 6 per cent deficit in energy. In order to bridge these gaps in demand and supply, Government of India has evolved an ambitious power development programme. In this programme more attention is given to development and strengthening of transmission systems so as to supplement the shortcomings of the past. The programme envisages addition of about 100,000 MW generating capacity along with matching transmission network within the next decade. The transmission system, thus, to be planned and implemented in the next decade is of the order of 60,000 circuit kms in the EHV range and the investment in the sector is of the order of Rs. 80,000 crores. In order to execute such magnitude of transmission system, which is of much higher order than those totally implemented in the last 5 decades, precise planning, costing, scheduling etc. would be required. Optimum deployment of resources also would be of prime target in implementing these transmission systems. As transmission lines have to traverse the length and breadth of the country, for evacuation of power from generating stations to load centers and beneficiary states, the topographical & geographical nature of the terrains play significant influence in the project cost and implementation time. Hence, it is essential that at the planning stage itself various alternative routes and technical solutions for transmission lines be examined in detail. For undertaking such studies, one of the major requirements is obtaining adequate information regarding physical constrains, environmental factors etc. along the route so that optimum solutions are identified. Subsequently, during implementation of the project, it is required to obtain elaborate details about terrain, soil conditions, constraints etc. of the route for proper resource planning, costing etc. as well as reduction in implementation time. Presently, conventional methods of survey like walk over survey, preliminary survey and detailed survey are carried out at various stages from conceptualization of the project to implementation, which are time consuming tasks. As manual conventional methods are used for these surveys, which are conducted in not so friendly terrains, there is possibility that the results do not reflect adequately the true picture the terrain in great details. Presently, there are new means available to conduct route survey using remote sensing, aerial survey, GPS based survey etc.
These techniques are said to have edge over conventional techniques in terms of credibility of data but are expensive, require lot of expertise, specialized equipment and may have certain other limitations.
This paper details out the basic concepts of an Extra High Voltage (EHV) transmission line specifically relevant to surveying, type of surveys presently done at various stages of a transmission line project, their limitations etc. The Paper is being presented to a gathering of experts of GIS, GPS etc. The idea is to make them aware of specific requirements of a transmission utility like POWERGRID so that if fast and more accurate solutions are possible, the experts can address the same to streamline appropriate survey systems for power transmission applications.
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
- 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
It helps in ensuring the statutory clearances with respect to following:
The surveys to collect the above information are carried out progressively at different stages of the project due to various reasons.
The various stages in a transmission line project requiring map study/ surveying and the type of surveys associated are detailed below:
System Planning Stage
The transmission system planning is done considering the load demand and the projected additions in power generation capacity at various places of the country. For effective power transfer with good reliability, security and economy different alternatives are studied at the system planning stage; for example, different voltage level of transmission lines, different routes of transmission lines, different connecting points etc. Therefore the routing and the cost of transmission line projects depends on factors like terrain conditions (plain, hilly), soil conditions, river crossings, power line crossings, railway crossings, reserved forests, agricultural fields etc through which the line traverses. At present, general routing and estimation of cost of each alternative at planning stage is done with reference to certain empirical rules as well as forest and physical maps. As these maps are not updated frequently, percentage error in assessment may be large. Even though this may not largely affect the selection of alternatives it will be prudent to identify more accurate and faster methods of collecting details at planning stage itself so that a large number of possible alternatives can be examined. The requirement generally would be to have updated information on maps so that the planning engineers can study various line routes and then identify and recommend cost effective solutions.
Fig. 2: Flow Chart of the Methodology for Route Identification
Project Feasibility Stage
After system planning studies and identifying suitable transmission system Feasibility Reports for the transmission line projects are to be submitted for techno-economic clearance from the CEA and investment approval from the Government. At the project feasibility stage, detailed elaboration of the project is done for the technical details, BOQ, cost and implementation schedules of the project.
Presently, reconnaissances involving the following types of studies/ surveys are carried out prior to submission of feasibility reports:
- Map study
- Walkover survey and / or
- Preliminary survey
The types of surveys adopted at feasibility stage are generally ruled by time constraints for fast track projects. Reconnaissance forms the most important aspect of transmission line survey as the cost of the transmission line is influenced to a great extent by the route chosen. Selecting the best possible route can derive the following benefits:
- Line construction cost can be brought to the minimum.
- Material Estimation and procurement can be done fairly on realistic basis.
- Any possible delay/hindrance likely to come during the execution of the work can be avoided after taking due care of various statutory provisions during the course of selecting route alignment.
- Proper planning can be done for termination of lines at switchyards keeping provision for future lines etc.
- Approvals from PTCC, Railways, Civil Aviation, Forest authorities etc. can be obtained faster.
- Preparation of Master Network and fixing construction/erection targets can be done on realistic basis, which will help in the judicious planning of materials flow, cash flow and manpower requirements.
- Appreciable time can be saved during construction of line, if selection of River Xing points, route along hill sections and power line xings etc., are properly made.
Map Study: After drawing various feasible alternative routes of transmission line within 10kms of the bee line on the topographical maps (1:50000 scale) of Survey of India, a comparative study is done on the basis of the following data:
- Route length.
- Nos. and type of angle points in each proposal indicating the angle of each deviation as measured on the map.
- Nature and number of major crossings.
- Deviation in the line due to civil/military aerodromes and other industrial installations.
- Approach to the line in general for construction.
- Reaches through protected or Reserved Forests
- Continuously long stretches in paddy fields.
- Close parallelism with telecom and Railway block circuits.
Walk-over survey is carried out on these routes. Walk over survey means going over the area associated with the alternative routes proposed and collecting features observed other than those existing on the map. In addition the indication on following features are also checked :.
- Communication lines
- Power lines
- Expanding villages and towns
- Rich gardens and plantations
- Reserved forests and high tree areas
- National Parks & Wild life sanctuaries
- Archaeological monuments
- Aerodromes, radar centers etc.
- Steep sloping terrain, Areas prone to land slides, soil instability etc.
- Prohibited areas declared under statutory regulations
Preliminary survey: On completion of walkover survey proposal of the most suited route is further studied before taking preliminary survey. The main objective of preliminary survey is to transfer the route to the ground with such deviations as may be necessary as per field constraints. It involves generally fixing of angle points of the towers, route alignment, identification of major crossings, general classifications of soils, measurement of route length etc. Conventional instruments like tapes and theodolites do these. Based on these results, the Bill of Quantities (BOQs) of a transmission line are estimated and cost estimates are prepared. The preliminary survey does not include detailed soil investigations for locations along the route.
Project Execution Stage
On completion of preliminary survey and approval of Feasibility report, the detailed survey of the route is carried out . The detailed survey consists of accurately determining the number and types of towers along with extensions, special towers required, number and types of foundations , special foundations required etc. It generally involves the following actions:
- Plotting and profiling on a scale of 1:200 (vertical) and 1:2000 (horizontal)
- Final alignment and pegging of locations
- Trial pit excavations
- Detailed soil investigations wherever required
The final output of the detailed survey is in the form of tower schedule showing the type of tower with associated angle of deviation, span length, wind and weight spans, associated foundations along with its classifications, geological and geotechnical, data and any crossing involved in that span.
Limitations of Conventional methods of map study/ surveying
Apart from the considerable time involved, the following limitations are generally associated with conventional methods of surveying:
- The topographical maps used for walkover survey and preliminary survey can be very old and do not contain recent changes in inhabitation pattern, vegetation coverage and water bodies etc.
- The process of elimination is done at the ground itself which increases the number of angle towers and length of line.
- The surveying staff do not have birds eye view of the present ground condition This may particularly result in large inaccuracies in estimation of civil works (benching and revetment) quantities in terrain involving large undulations.
- There is every possibility of error in recording the ground data and subsequent transfer on the route map
Probable solutions for fast and reliable surveying
As a first step following procedure can be explored at each stage of transmission line project:
Route planning and fixing of alignment: This can be achieved through the following procedure:
- Data Input: Survey of India maps, Satellite data, railway maps, land use maps, settlement maps etc are used as input for data base preparation
- GIS: Above inputs through GIS are used to update the Survey of India topo sheets. Other specific maps may also be created.
- Using computer run algorithms or linear programming techniques optimal transmission line route may be finalized.
- On this output from GIS domain post field work is done and locational latitude and longitude of the deviation points are identified on ground using Geographical position system (GPS).
Further in terrain having large undulations, stereoscopic satellite imageries may be used selectively to interpolate the contours and digital terrain model can be used to select the route in hilly regions
Aerial photography can be used as a reliable tool to record the precise elevations and plan measurements of the selected route. This may involve taking following actions
- Requesting Ministry of Defence for granting permission for carrying out aerial photography
- Arrangement of flight on selected route
- Transferring of photographs to studios for processing and detailed measurements
- Authentication through ground verification
It is said that camera never speaks lie. That is precisely the greatest advantage of aerial photography in detailed survey application w.r.t traditional methods. Further once the ground profile in digital form is obtained the estimation of various types of earthworks involved viz. Benching, revetments etc. can be made using software.
Appropriate techniques for obtaining soil conditions, sub soil conditions, water tables, treacherous terrain conditions etc for accurate estimation of civil works needs to be explored.
POWERGRID’s experience in new surveying tools
POWERGRID in its effort to use new surveying tools have had taken a trial project with National Informatics Centre (NIC) for use of satellite imagery in preliminary survey for 400kV D/C Monubulu (near Nellore) to Sriprembdur transmission line (line length of approx. 184 kms). The major objective was to develop a methodology to use the satellite data using IRS LISS (resolution of 23.5 metres), and PAN(resolution of 5.88 metres), datasets, in conjunction with topographical maps on a single platform, in order to update the natural and cultural features of the routing of transmission line. A flowchart of the methodology used for route identification is shown at Fig-2
Three alternatives routes were identified by NIC using satellite imagery . These routes were then compared with the other three alternative routes identified by POWERGRID using conventional surveying methods (map study and walkover survey). Various algorithms and weightage techniques were used for comparison purposes with environmental impact being a major factor taken in account for comparison purposes. It was found that a saving of 1.5 kms (approx.) that is around 1 % could be achieved after comparing the best route from conventional and new method . Being first case of its kind to be given to NIC, the time taken was around six months instead of three months as expected earlier. The work with NIC gave the following additional important points to be looked into for further applications:
- There may be some error involved in using satellite imagery for preliminary survey due to the “shadow effect”. In the above case it was found sometimes difficult to differentiate between forests and other greenery. Thus exact boundaries of forests could not be demarcated
- Resolution plays an important part in interpreting satellite images. Power line crossings could not be identified in the satellite imageries of 23.5m/5.88m resolution. Higher resolution may improve the visual content may reveal ground realities more closely.
- Ground profile and subsoil data cannot be accessed through satellite imageries.
- Digitisation of complete topographical maps rather select features would provide a complete replica in integrated environment.
- Satellite data of resolution of 1 metre will be preferable . However the overall cost economics shall have to be worked out since this data will be more costly but will completely avoid field works.
- Symbols of physical features in satellite imageries are not easily recognizable by transmission line engineers.
Survey by Photogrammetry
POWERGRID also has undertaken a pilot project along with National Remote Sensing Agency (NRSA) for survey of 10 kms. Stretch of 400kV D/C Kolhapur-Mpausa transmission line on a trial case. The NRSA has taken up the survey and generated 1:15,000 scale maps with 1 mtr. contours for 10 kms. section of the line using 1:10,000 scale aerial photographs for 1 km corridor width. The output has been taken as digital data on topographical maps on 1:15,000 scale with 2 mtr. contour interval. The digital mapping of the corridor with the accuracy specified above has been achieved by the survey, however, appropriate software and tools for using the contours for tower spotting, estimation of civil works quantities etc. would be further required. One of the major constraints has been the exercise has taken more time than a conventional survey.
POWERGRID in its endeavor for an accelerated power development and formation of national grid in the country has been instrumental in adopting various ways and means to reduce project implementation time. Over the last decade, it has been demonstrated that a transmission line project can be constructed with 30 per cent less time than the time taken in the last decade through standardization of designs at project conceptualization stage itself. POWERGRID’s present endeavor is to reduce the total time involved from project conceptualisation to project approvals and to commissioning. The major constraints observed during conceptualization of the project is that the conventional ways of surveys and estimation of project costs consume considerable time by which the system planning, project feasibility and their approvals are taking more time. One of the major requirements at project conceptualisation and feasibility stage is to draw-up correct project definition, cost and implementation programme for which elaborate survey inputs within limited time would be of great help. The modern methods of survey using GIS, GPS etc. will be of more help in defining the project requirements precisely at project feasibility stage itself. With accurate project definition at feasibility stage itself, it would be possible to do correct project cost estimation, optimum resource planning, accelerate construction programme etc. Further, the cost and time overrun of projects can be kept minimum. In this connection, the following are identified as the major requirements:
Project Conceptualization Stage
Techniques are required to be developed to evaluate various line routes using inputs from digital topographical maps.
Project Feasibility Stage
Detailed mapping of the right of way, ground profiling, identification of tower locations along with geographical and geo-technical details of the location, site constraints etc. for evolving project definition in finer details.
Project Construction Stage
Detailed contouring of undulated terrain for estimation of benching and revetment quantities.
The basic concepts of an Extra High Voltage (EHV) transmission line specifically relevant to surveying, type of surveys presently done at various stages of a transmission line project, their limitations and probable solutions for faster and more accurate surveying have been presented. Further POWERGRID’s experiences in adopting satellite imagery, photogrammetry and GIS to surveying have also been presented.
- Report on ” Routing of EHV trans2mission line from Monubulu to Sriperumbdur” Remote sensing and GIS division, NIC, India
- Specifications for Survey of 400kV transmission line, POWERGRID
- Preliminary survey report of Monubulu to Sriperumbdur , POWERGRID, 1985