ROUTING OF TRANSMISSION LINES

Transmission line routing requires a thorough investigation and study of several different alternate routes to assure that the most practical route is selected, taking into consideration the environmental criteria, cost of construction, land use, impact to public, maintenance and engineering considerations.To select and identify environmentally acceptable transmission line routes, it is necessary to identify all requirements imposed by State and Federal legislation. Environmental Reports for Electric Projects That Require Environmental Assessments.” State public utility commissions and departments of natural resources may also designate avoidance and exclusion areas which have to be considered in the routing process.

Maps are developed in order to identify avoidance and exclusion areas and other requirements which might impinge on the line route. Ideally, all physical and environmental considerations should be plotted on one map so this information can be used for route evaluation. However, when there are a large number of areas to be identified or many relevant environmental concerns, more than one map may have to be prepared for clarity. The number of maps engineers need to refer to in order to analyze routing alternatives should be kept to a minimum.

Typical physical, biological and human environmental routing considerations are listed inTable -1. The order in which considerations are listed is not intended to imply any priority. In specific situations, environmental concerns other than those listed may be relevant. Suggested sources for such information are also included in the table.

For large projects, photogrammetry is contributing substantially to route selection and design of lines. Preliminary corridor location is improved when high altitude aerial photographs or satellite imagery are used to rapidly and accurately inventory existing land use. Once the preferred and alternative corridors have been selected, the engineer should consult USGS maps, county soil maps, and plat and road maps in order to produce small scale maps to be used to identify additional obstructions and considerations for the preferred transmission line.

On smaller projects, the line lengths are often short and high altitude photograph and satellite imagery offer fewer benefits. For such projects, engineers should seek existing aerial photographs. Sources for such photographs include county planning agencies, pipeline companies, county highway departments, and land development corporations. A preliminary field survey should also be made to locate possible new features which do not appear on USGS maps or aerial photographs.

As computer information systems become less expensive and easier to use, electric transmission utilities are using Geographic Information Systems (GIS) to automate the route selection process. GIS technology enables users to easily consolidate maps and attribute information from various Sources and to efficiently analyze what has been collected. When used by routing experts, automated computer processes help standardize the route evaluation and selection process, promote objective quantitative analysis and

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help users select defendable routes. GIS tools have proven very beneficial to utilities whose goals are to minimize impact on people and the natural environment while selecting a constructible, maintainable and cost effective route.

Final route selection, whether for a large or small project, is a matter of judgment and requires sound evaluation of divergent requirements, including costs of easements, cost of clearing, and ease of maintenance as well as the effect a line may have on the environment. Public relations and public input are necessary in the corridor selection and preliminary survey stages…..

LINE ROUTING CONSIDERATIONS

Physical Source

Highways NHA(National highway authority) maps Streams ,rivers and lakes ICID(International commission on irrigation and

drainage) maps Railroads Pakistan railway dept maps Airstrips CAA(Civil aviation authority of Pakistan) maps, Topography ( major ridge

lines ,floodplains etc) Pakistan flood broadcast dept maps

Transmission lines & distribution lines

NTDC ,WAPDA maps

Pipelines,(water, gas) SNGP(Sui northern gas pipelines) , SSGP(Sui southern gas pipelines) maps ans WASA(Water and sanitation agency) maps

Occupied buildings Local tax maps, land use maps, local GIS maps Biological Source

Woodlands Forestry dept of Pakistan maps Waterfowl, wildlife refuges areas,

endangered Species & critical habitat areas

WWF(World wildlife federation) reports

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Human environmental Source Range land Cropland Urban development Industrial development Mining areas Recreation areas National parks

Satellite mapping, country planning agencies, state soil Conservation service

Mining bureau, bureau of land management NRCS(National resources conservation service)

CRITERIA FOR ROUTE SELECTION:

The route of a transmission line is decided from the following main considerations

a) Shortest length, hence least capital cost.b) Ease during construction and ease in maintenance of the line (route near roads for Easy approach & accessibility).c) Requirement of future loads (sub stations) near the proposed route so that the line Can be easily connected.d) Required separation distance from parallel communication lines (P&T, Railways Etc.) for meeting the conditions of induced voltage for obtaining PTCC approval.e) Avoiding of forest areas as well as wild life sanctuaries.f) Cost of securing and clearing right of way (ROW).g) Maintaining statutory distances from Airports / Helipads.

The following areas are to be avoided as far as possible while selecting the route of the line.

a) Tough inaccessible areas where approach is difficult.b) Towns and villages, leaving sufficient margin for their growth.c) Areas subject to floods, gushing nalas during rainy seasons, tanks, ponds, lakes, etc. And natural hazards.d) Wooded areas with high trees or fruit bearing trees involving payment of heavy compensations for cutting of the trees. e) Swamps and shallow lands subject to flood, marshy areas, low lying lands, river and earth slip zones, etc. involving risk to stability to foundations.f) High hillocks / hilly areas / sand dunes and areas involving abrupt changes in levels and requiring too many long spans.

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g) Series of irrigation wells.h) Rifle shooting areas and other protected areas such as army / defence installations and ammunition depots.i) Areas which involve risk to human life, damage to public & private properties, Religious places, cremation grounds, quarry sites and underground mines, gardens, orchards and plantationsj) Areas which will create problems of right of way and way leaves.k) Buildings / Storage areas for explosives or inflammable materials, bulk oil storage tanks, oil or gas pipelines, etc.

The route of the transmission line is to be so located that, as far as possible, it is protected from high winds and falling trees & branches. In hilly tracks, the line is to be routed, as far as possible, along the side of the hills or through valleys rather than over high points. However, a route of the line very close to steep slopes of hills be avoided as far as possible as there may be difficulty in obtaining lateral (side) clearance to ground for conductors. Also, there may be overhanging / loose boulders which may roll down and damage the line.

It is desirable to take the line as near the paths and roads as practicable without unduly increasing the length of the line so as to facilitate transportation of material during construction and the patrolling / maintenance of the line. Where the line cannot be routed near paths / roads economically, care shall be taken to see that easy access is possible at every 5 to 8 km. It shall be ensured that all angle / tension points, particularly in the case of 400 kV lines, are approachable to facilitate easy transportation of stringing equipment during construction and for maintenance / breakdowns.

In hilly / mountainous type of terrain or in thickly populated areas, it is generally not advisable to attempt a direct route or try to locate towers in long spans. Small angles of a few degrees cost a little more and add little to the length of the line. Suspension towers (A – type) can be provided for line angles of upto 2 degrees and small angle towers (B – type) can be provided for angles upto 15 degrees

In general, large angles in the line are to be avoided wherever possible. The magnitude of the angle be small as far as possible and should never be more than 60 degreesTOWER ERECTION:

The towers shall be erected on the foundations not less than 14 days (if OPC (optimal proximity correction) has been used)/ 21 days (if PPC has been used) after concreting or till such time that the concrete has acquired sufficient strength. The towers are erected as per the erection drawings furnished by the manufacturers to facilitate erection. For the convenience of assembling the tower parts during erection operations, each member is marked in the factory to correspond with a number shown in the erection drawing. Any damage to the steel and injuring of galvanizing shall be avoided. No member shall be subjected to any undue over stress during erection.

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There are three main methods of erection of steel transmission towers which are described as below:

a) Built up method or piecemeal method. b) Section method. c) Ground assembly method.

Built Up Method:

This method is most commonly used for the erection of 132 kV, 220 kV and 500 kV transmission line towers due to the following advantages:

a) Tower materials can be supplied to site in knocked down condition, i.e., in pieces which facilitates easier and cheaper transportation. b) It does not require any heavy machinery such as cranes, etc. c) Tower erection activity can be done in any kind of terrain and throughout most of the year. d) Availability of workmen at cheaper rates.

This method consists of erecting the towers member by member. The tower members are first set out and kept on the ground serially according to erection sequence to avoid time loss due to searching for them as and when required.

In order to maintain speed and efficiency, a small assembly party can be sent ahead of the main erection gang for sorting out the tower members, keeping the members in correct position on the ground and assembling those panels on the ground which can be erected as a complete unit. The main corner leg members are prepared by fitting all cleats / plates for joints & bracings and step bolts.

The erection progresses from the bottom upwards. The four main corner leg members of the first section of the tower are first erected and kept in position by fixing temporary rope guys. More than one leg section of each corner leg may be bolted together at the ground and erected in case they are short in length and light in weight.

The cross bracings of the first section, which may be assembled on the ground, are raised one by one as a unit and bolted to the already erected corner leg angles. The first section of the tower thus built and horizontal struts (belt members), if any, are bolted in position.

For smaller base towers / vertical configuration towers, one derrick / gin pole is used. For wide based towers and if one assembled section / panel of the tower is to be erected, then two derricks / gin poles are placed, one each on the top of diagonally opposite corner legs. These are guyed using ropes and temporary ground anchors.

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For assembling the second section of the tower, the derrick / gin pole is placed on the top of one corner leg. First, the leg members of the second section are hoisted and assembled. The temporary rope guys are shifted to the legs of the second section when they are being raised for erection. The legs of the second section / storey are kept in position by fixing the temporary rope guys. The bracings of the second section are then hoisted and assembled.

The derrick is then shifted to the corner leg member on the top of the second section to raise the parts of third section of the tower in position for assembly. Derrick(s) / Gin pole(s) and the temporary rope guys for the leg members are thus moved up as the tower is built up. This process is continued till the complete tower is erected.

The stages in this method of erection are shown in Appendix – A and Appendix – B. Cross – arms are assembled on the ground. The bird guards and hangers for suspension towers are fitted on the cross – arms. A rope is passed through a pulley fixed on the tower peak. The cross – arms are raised up with this rope and fixed to the main body of the tower. For heavier towers, a small boom is rigged on one of the tower legs for hoisting purposes.

The members / sections can be hoisted either manually or by pulling with a tractor or by winch machines operated from the ground.

Section Method:

The major sections of the tower are assembled on the ground and the same are erected as units. Either a mobile crane or a derrick / gin pole is used. The derrick / gin pole used is approximately 10m long and is held in place by means of guys on the side of the tower to be erected.

The two opposite sides of the tower section of the tower are assembled on the ground. Each assembled side is then lifted clear of the ground with the derrick / gin pole and is lowered into position on bolts to stubs or anchor bolts. One side is held in place with props or rope guys while the other side is being erected. The two opposite sides are then laced together with cross members and bracings / diagonals, and the assembled section is lined up and made square to the line.

After completing the first section, the derrick / gin pole is set on the top of the first section. The derrick / gin pole is made to rest on a strut of the tower immediately below the leg joint. The derrick / gin pole has then to be properly guyed into position.

The first face of the second section is raised. To raise the second face of this section, it is necessary to shift the foot of the derrick / gin pole on the strut of the opposite side of the

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tower. After the two opposite faces are raised, the bracings on the other two sides are fitted and bolted up.

The last lift raises the top of the towers. After the tower top is placed and all side bracings have been bolted up, all the guy are removed except the one which is to be used to lower the derrick / gin pole.

Sometimes, one whole face of the tower is assembled on the ground, hoisted and supported in position. The opposite face is similarly assembled and hoisted and then the bracing angles connecting these two faces are fitted. The cross – arms are assembled and erected in the manner given.

Ground Assembly Method:

This method consists of assembling the tower on the ground, and erecting it as a complete unit. This method is not useful when the towers are large and heavy and the foundations are located in arable land where assembling and erecting complete towers would cause damage to large areas or in hilly terrain where the assembly of complete tower on slopping ground may not be possible and it may be difficult to get the crane into position to raise the complete tower. This method is not generally adopted because of non-availability of good approach roads to tower location.

For this method of erection, a level piece of ground close to the footing is chosen for the tower assembly. On slopping ground, however, elaborate packing of the low side is essential before assembly commences.

The complete tower is assembled in a horizontal position on even ground. The tower is assembled along the direction of the line to allow the cross arms to be fitted. After the assembly is complete, the tower is picked up from the ground with the help of a crane and carried to its location and set on its foundation.

Tightening of Bolts & Nuts and Punching of Threads: All empty holes are to be filled in with nut and bolt of appropriate size and a spring washer.

All nuts shall be tightened properly using correct size spanners. Before tightening it should be ensured that filler washers and plates are placed in relevant gaps between members, bolts of proper size and length are inserted and one spring washer is inserted under each nut. In case of step bolts, spring washer shall be placed under the outer nut.

The tightening shall be carried on progressively from the top downwards, care being taken that all bolts at every level are tightened simultaneously. It is advisable to employ four persons, each covering one leg and the face to his right.

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The threads of bolts shall project outside the nuts by one to two threads and shall be punched at three positions on the top inner periphery of the nut and bolt to ensure that the nuts are not loosened in the course of time.

If during tightening, a nut is found to be slipping or running over the bolt threads, the bolt together with the nut shall be changed outright.

Tack Welding of Bolts & Nuts:

Tack welding is got done of all the nuts & bolts from the ground level upto bottom cross arm level, or as specified in the contract.

The threads of all the bolts projecting outside the nuts shall be welded with the nuts at two diametrically opposite places. The length of each welding shall be at least 10 mm, or as specified in the contract.

After welding, cold galvanizing paint (Zinc rich paint having at least 90% percent zinc content) shall be applied to the welded portion. At least two coats of the paint shall be applied.

FIXING OF ACCESSORIES:

The U – Bolts for earth wire suspension hardware are fitted on the top plate of the suspension towers.

The supports for the anti – climbing device are fitted on the main corner legs of all the towers. The anti – climbing devices (Flats with edges cut to a sharp point) are installed after the stringing work has been completed.

The number plates are fitted at the place provided for them on the face of the tower. Wherever there are roads near the tower, these should be fitted on the face from which they can be seen from the road.

The phase plates are fitted on the holes provided for them on the top leg of the cross – arms if the phase sequence is known at the time of erection of towers. Otherwise, these are fitted after the phase sequence is finalized.

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Checking the Verticality of Erected Towers:

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The finally erected tower shall be truly vertical after erection and no straining is permitted to bring it in alignment.

The verticality of the tower is checked using a theodolite placed away from the tower but in the longitudinal and transverse center lines of the tower.

The tolerance limit for verticality shall be one in 360 of the tower height

DESIGN OF TRANSMISSION LINE

DESIGN PARAMETERS:

There are four parameters which may effect the design of transmission line which will be discussed one by one .the names are given as follow:

a) Height of poleb) Span length c) Foundations d) Angles

Height of pole: Greater the height of pole greater will be the span length. So it is the advantage of greater height but at the same time greater height also increases the cost of pole, depth of the pole , and at the same time cost on foundations. So it is required to keep economical height keeping the minimum ground clearance ground clearance is the minimum distance between the conductor and the ground which should be maintained. Height of poles is different for different areas i.e. in hilly areas the height is specified according to the hilly edges and the height of hills to keep the minimum ground clearance.Similarly in forests the height of poles is very important and during the pole erraction height of different trees should be kept in mind to minimize the chance of any misshape .for rough planes the height of poles is constant and the usually ground clearance is about 6 to 7 m. similarly for transmission line passing over the roads and constructional areas the height of pole is very important and ground clearance in this case is more than normal clearance.

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Heights of poles for different voltage levels

Span length: Span length is the second important parameter which may alter the design of transmission line. Span length is the distance between two towers so greater the span length lesser will be the cost of transmission line. That why it is required to keep maximum span length while keeping the minimum ground clearance .span length is different for different areas i.e. in hilly areas span length is usually kept lesser than in planes and it varies from pole to pole means if one pole is on the edge of a hill then the next pole will be at greater span length as compared to the pole which is at the start of cliff.

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Normal span length maintaining ground clearance

Foundations: Type of the foundation depends upon the area through which the transmission line is passing through .i.e. for hilly areas there is no need of deep foundations and hence normal depth of 2-3 meters is sufficient. Similarly for planes having muddy soil the depth is about 5 meters and it is done by boaring the soil. In coastal areas the depth is mor than that of in planes as the sandy soil is more unstable than that of muddy soil. Similarly for a transmission line passing through the areas near the earth quake zones has most flexible foundations and hence having the most strength. As the design and construction of foundation is varied the cost for the pole foundation also varies. Some cost specifications for different foundations are given as follow to understand the cost variation of foundations:

Normal foundations - $2500 Special foundation - unstable soil $5000 Rock anchor - rocky ground $5000 Piled - mangroves, coastal flats $10000

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3-D view of pole foundation

Angles in T.L:

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The economical design for a transmission line from generation plant to consumption area is no doubt a straight line but some times during transmission line erraction we need to bend the direction of transmission line .these bends may be due to different reasons as given : 1) For future constructional areas. 2) Due to natural prohibited areas i.e. rivers, ditches, hills or cliffs, flood zones. 3) Due to man prohibited areas like airports, radio transmission areas etc.

To avoid these areas we use angle towers .the maximum allowed bend angle in transmission line is 45 degree. The bend sangles above 45 degrees are uneconomical and hence not allowed. On the basis of angles these towers may be classified into three categories as given follow: a) 0 to 1degree = normal towers b) 1 to 3 degree = angle tower c) 3 to45 degree = dead angle towers

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Dead angle tower

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CLEARANCES TO GROUND, TO OBJECTS UNDER THE LINE AND AT CROSSINGS

General: Recommended design vertical clearances for agency financed transmission lines of 230 kV and below are listed in the Tables 4-1 through 4-3. These clearances exceed the minimum clearances calculated in accordance with the 2007 edition of the NESC. If the 2007 edition has not been adopted in a particular locale, clearances and the conditions found in this report should be reviewed to ensure that they meet the more stringent of the applicable requirements

Clearance values provided in the following tables are recommended design values. In order to provide an additional cushion of safety, recommended design values exceed the minimum clearances in the 2007 NESC.

Assumptions

Fault Clearing and Switching SurgesClearances in tables 4-1, 4-2, 4-3, and 5-1 are recommended for transmission lines capable of clearing line-to-ground faults and voltages up to 230 kV. For 230 kV, the tables apply for switching surges less than or equal to 2.0; for higher switching surges on 230 kV transmission lines see the alternate clearance recommendations in the NESC.

Voltage: Listed in the chart that follows are nominal transmission line voltages and the assumed maximum allowable operating voltage for these nominal voltages. If the expected operating voltage is greater than the value given below, the clearances in this bulletin may be inadequate. Refer to the 2007 edition of the NESC for guidance.

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Design Vertical Clearance of Conductors: The recommended design vertical clearances under various conditions are provided in Table 4-1. Conditions Under Which Clearances Apply: The clearances apply to a conductor at final sag for the conditions ‘a’ through ‘c’ listed below. The condition that produces the greatest sag for the line is the one that applies. a. Conductor temperature of 32°F, no wind, with the radial thickness of ice for the applicable NESC loading district. b. Conductor temperature of 167°F. A lower temperature may be considered where justified by a qualified engineering study. Under no circumstances should a design temperature be less than 120°F. c. Maximum design conductor temperature, no wind. For high voltage bulk transmission lines of major importance to the system, consideration should be given to the use of 212°F as the maximum design conductor temperature. According to the National Electric Reliability Council Criteria, emergency loading for lines of a system would be the line loads sustained when the worst combination of one line and one generator outage occurs. The loads used for condition "c" should be based on long range load forecasts. Sags of overhead transmission conductors are predicted fairly accurately for normal operating temperatures. However, it has consistently been observed that sags for ACSR (Aluminum Conductor Steel Reinforced) conductors can be greater than predicted at elevated temperatures. If conductors are to be regularly operated at elevated temperatures, it is important that sag

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behavior be well understood. Current knowledge of the effects of high temperature operation on the long term behavior of conductors and associated hardware (splices, etc.) is probably limited; however, and a clear understanding of the issues involved is essential. The Electric Power Research Institute (EPRI) has prepared a report on the effects of high temperature conductor and associated hardware. The traditional approach in predicting ACSR conductor sag has been to assume that the aluminum and steel share only tension loads. But as conductor temperature rises, aluminum expands more rapidly than steel. Eventually the aluminum tension will reduce to zero and then go into compression. Beyond this point the steel carries the total conductor tension. These compressive stresses generally occur when conductors are operated above 176 °F to 200 °F. Greater sags than predicted at these elevated temperatures may be attributed to aluminum being in compression which is normally neglected by traditional sag and tension methods. AAC (All Aluminum Conductors) and AAAC (All Aluminum Alloy Conductor) or ACSR conductors having only one layer of aluminum or ACSR with less than 7 percent steel should not have significantly larger sags than predicted by these traditional methods at higher operating temperatures.

Altitude Greater than 3300 Feet: If the altitude of a transmission line (or a portion thereof) is greater than 3300 feet, an additional clearance as indicated in Table 4-1 must be added to the base clearances given. Spaces and Ways Accessible to Pedestrians Only: Pedestrian-only clearances should be applied carefully. If it is possible for anything other than a person on foot to get under the line, such as a person riding a horse, the line should not be considered to be accessible to pedestrians-only and another clearance category should be used. It is expected thatthis type of clearance will be used rarely and only in the most unusual circumstances. Clearance for Lines Along Roads in Rural Districts: If a line along a road in a rural district is adjacent to a cultivated field or other land falling into Category 3 of Table 4-1, the clearance-to-ground should be based on the clearance requirements of Category 3 unless the line is located entirely within the road right-of-way and is inaccessible to vehicular traffic, including highway right-of-way maintenance equipment. If a line meets these two requirements, its clearance may be based on the "along road in rural district" requirement. To avoid the need for future line changes, it is strongly recommended that the ground clearance for the line should be based on clearance over driveways. This should be done whenever it is considered likely a driveway will be built somewhere under the line. Heavily traveled rural roads should be considered as being in urban areas. Reference Component and Tall Vehicles/BoatsThere may be areas where it can be normally expected that tall vehicles/boats will pass under the line. In such areas, it is recommended that consideration be given to increasing the clearances given in Table 4-1 by the amount by which the operating height of the vehicle/boat exceeds the reference component. The reference component is that part of the clearance component which covers the activity in the area which the overhead line crosses. For example, truck height is limited to 14 feet by state regulation, thus the

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reference component for roads is 14 feet. However, in northern climates sanding trucks typically operate with their box in an elevated position to distribute the sand and salt to icy roadways. The clearances in Table 4-1 are to be increased by the amount the sanding truck operating height exceeds 14 feet. In another example, the height of farm equipment may be 14 feet or more. In these cases, these clearances should be increased by the difference between the known height of the oversized vehicle and the reference height of 14 feet.

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