Construction EHV Transmission Line

Transmission of Power
What is Power Transmission ?
Power transmission is the movement of energy from its place of generation to a location where it is applied to performing useful work.
Electric power is normally generated at 11-33 kV in a power station. To transmit over long distances, it is then stepped- up to 400kV, 220kV or 110 kV as necessary.
 Power is carried through a transmission network of high voltage lines. Usually, these lines run into hundreds of kilometers and deliver the power into a power grid.

What is Power Transmission ?
The grid is connected to load centres (cities) through a sub-transmission network of normally 33kV (or sometimes 66kV) lines.
These lines terminate into a 33kV (or 66kV) substation, where the voltage is stepped-down to 11kV for power distribution to load points through a distribution network of lines at 11kV and lower.

Efficiency of transmission Line
Whatever may be the category of transmission line, the main aim is to transmit power from one end to another.
Like other electrical system, the transmission network also will have some power loss and voltage drop during transmitting power from sending end to receiving end.
Hence, performance of transmission line can be determined by its efficiency and voltage regulation.

Every transmission line will have three basic electrical parameters.
The conductors of the line will have electrical resistance, inductance, and capacitance.
As the transmission line is a set of conductors being run from one place to another supported by transmission towers, the parameters are distributed uniformly along the line.

Power sent from sending end - line losses
=
Power delivered at receiving end.

The transmission lines are categorized as three types:
1)Short transmission Line – the line length is up to 80 km.
2)Medium Transmission Line – the line length is between 80km to 160 km.
3)Long Transmission Line – the line length is more than 160 km.

Transmission Lines
Extra High Voltage
110 kV, 132 kV, 220 kV, 400 kV
Ultra High Voltage
765kV
High Voltage Direct Current
±500kV

Transmission Line Execution
Survey
&
Profile Plotting

Transmission Line Survey
A transmission line is one of essential infrastructures of the power supply system.
In the site evaluation process for those facilities, it is necessary to carefully consider not only technical issues, but also the impact on natural environment, the influence on local communities, and various regulations.
To achieve optimum line length, minimise rocky and water terrains, reduced tower angle cut points, ROW issues etc; it is necessary to have detailed survey, profile of transmission line route.

Transmission Line Survey
Why Surveying Is Important ?
 To optimize cost of transmission line
 Line length, number of locations, deviations.
 Minimum river crossings.
 Minimum forest areas.
 Accessibility, right of way considerations.
To assess route constraints and do construction planning.
 To ensure statutory clearances
 Ground clearance.
 Horizontal/Right of way clearance.
 Clearances from power lines, railway lines, road crossings etc.

UHV Transmission Lines - 765 kV , Requiring Large ROW

Transmission Lines through Agricultural fields

Transmission Lines in Hilly areas

Transmission Line River Crossings

Multiple Transmission Lines Through Same Corridor

Transmission Line Survey :
Project Conceptualization / Project Feasibility Stage:
 Faster and accurate techniques required to evaluate various line routes .
Pre-Construction Stage:
 Route alignment, Detailed Mapping of right of way, ground profiling, finalisation of tower locations.
 Detailed contouring of undulated terrain for estimation of benching and revetment quantities
Project Construction Stage
Check Survey only

Detailed Survey – Modern Techniques
Route Alignment using latest satellite imageries superimposed on Survey of India Topographical Maps.
Digital terrain modeling in hills using contouring data.
Detailed Survey using GPS, Total stations or ALTM.
Digitized contouring at undulated / hilly tower locations.
Identification of Route constraints.
Identification of infrastructure details .
Tower spotting & optimization of locations using commercial software like PLS CADD.
Estimation of BOQ & Preparation Of Survey reports.
Soil Investigation.

Global Positioning System
Total Station
PLS CADD

Detailed Survey – Modern Techniques Global Positioning System
The Global Positioning System (GPS) is a space-based satellite navigation system that provides location and time information in all weather, anywhere on or near the Earth, where there is an unobstructed line of sight GPS satellite.
It is a modern technology that improves survey engineering to Global orientation.

Detailed Survey – Modern Techniques Total Station
A Total Station is a modern surveying instrument that integrates an electronic theodolite with an electronic distance meter.
A theodolite uses a movable telescope to measure angles in both the horizontal and vertical planes.
Coordinates of an unknown point relative to a known coordinate can be determined using the total station.

PLS CADD Power Line Systems –
Computer Aided Design and Drafting
PLS-CADD is the most powerful overhead power line design program.
PLS-CADD is a sophisticated three-dimensional engineering model.
The model can be viewed in a number of different ways: profile views, plan views, plan & profile sheets, 3-D view etc.
PLS-CADD supports both automatic and manual spotting.

PLS – CADD software - Some views

Transmission Line
Sample route Profile through Google Earth

Transmission Line
Sample route Profile

Topo Map
Sending end SS
Receiving end SS
AP 1
AP 2
AP 3
AP 4
Bee Line

Digitized Toposheet
Survey of India Topo Sheet
Corresponding Digitized Map indicating relevant information

Computerized Route Optimization by Study of Alternative Routes
Bee Line
Existing Line
Alternative 3
Alternative 2
Alternative 1

Digital Terrain Model – Computerized Three Dimensional Image

Transmission Line Survey:
The detailed survey is taken up on the line route approved by the concerned Electricity board.
Detailed survey should cover:
Fixing of Alignment
Fixing of Line Points
Fixing of Angles of deviation
Fixing of Angle Points
Fixing of Direction Points
Measuring of distance between Angle Points
Levels at every 10 M interval or where there is level difference of 30 Cm

Detailed survey should include:
Details of Roads
Details of Villages
P & T Lines Crossings
All Power Lines Crossings – LT to 765 kV
Railway Crossings
River Crossings
Agricultural wells, Field bunds & Earth bunds etc.
Measuring Soil resistivity at 1 Km interval
Trial Pits
Crop & Tree enumeration

Detailed survey Obligatory Points
AVOID:
Reserve Forest Area
Military Firing Ranges
Aerodromes
Inhabited and thickly populated areas
Hilly terrain
Marshy, Low lying & Submersible areas
Higher & 90 degrees angle turnings
Oil & Gas storage areas

Transmission Line Profile:
Detailed Profile should be
From the details of route survey, route plan and ‘Profile’, is prepared. Profile is also termed as Longitudinal profile or route profile.
The profile is prepared and plotted paper rolls of graphed tracing paper.
The profile shall progress from left to right.
A typical profile is enclosed.

Detailed Route Profile should be
Graphical representation
Preferably be in the Scale of
X-axis 1 Cm = 20 M
Y-axis 1 Cm = 2 M
Plot Plan of Alignment of Line from AP to AP
Plot Distances
Plot Levels with respect to distance

Detailed Profile should include & indicate
All Power Lines crossings: LT to 765 kV
All P & T Lines crossings
All Railway crossings
All River crossings
All Roads: Cart Tracks to National Highways
All Villages
All Agricultural wells
All Field bunds & Earth bunds
All Ponds & Lakes
Type of Soil
Crops & Trees

Tower Spotting
Identify the position of Terminal Tower.
Start from Terminal Tower.
Match the tower footing curve with the position of TT.
Adjust the position of template.
Ensure Ground Clearance.
Identify the position of next tower.
Move the template to the next tower.
Repeat the procedure.

Tower Spotting
NORMAL SPAN:
It is the design span
ACTUAL SPAN:
It is the actual distance between two adjacent towers
NULL POINT:
It is a point in a span where the position of the conductor is lowest (or) It is a point in a span where the sag is maximum
WEIGHT SPAN:
The distance between two adjacent null points
WIND SPAN:
It is the distance between two centre points of adjacent spans.

Transmission Line Profile – SAG TEMPLATE

Transmission Line Profile – Span Limitations
SPAN
110 /
132 kV
220 kV
400 kV
NORMAL SPAN
320 M
350M
400 M
WIND SPAN
Both
320 M
350 M
400 M
Single
192 M
210 M
240 M
WEIGHT SPAN
Both
400 M
450 M
600/800 M
Single
240 M
270 M
300/400 M

Transmission Line Profile – Clearances
Details
110/
132 kV
220 kV
400 kV
Ground Clearance
6.1 M
8.5 M
9.0 M
LT to 132 kV and
P& T Lines
3.1 M
4.6 M
5.5 M
220 kV
4.6 M
4.6 M
5.5 M
400 kV
5.5 M
5.5 M
5.5 M

Transmission Line Profile – Clearances
Details
110 /
132 kV
220 kV
400 kV
Highway Crossing
9 M
9 M
9M
Railway Crossing
14.6 M
15.4 M
17.9 M
Railway Crossing Span (Max)
2/3 of NS
2/3 of NS
2/3 of NS
Distance between Tower & Railway Track (Min)
TH+6 M
TH+6 M
TH+6 M

Transmission Line – Tower Schedule
Location No.
Type of
Tower
Angle of Deviation (degrees)
Span (M)
Left Wt. Span (M)
Right Wt. Span
(M)
Total Wt. Span (M)
Remarks
SS
Boom
-
-
-
-
Boom at sending end SS
70
1 TT
S
35 L
-
75
-
Terminal tower at sending end SS
210
2
P
-
135
150
285
300
3
R
22 R
150
130
280
Near village
280
4
P+3
-
150
125
255
33 kV Line crossing
250

Check Survey
Marking of Tower Location
Forward & Backward Line Points
Pit Marking
Excavation

Transmission Tower Foundation
Type of loads on foundation :
The foundation of towers are normally subjected to three types of forces. These are:
a)The compression or downward thrust.
b)The tension or uplift.
c)The lateral forces of side thrusts in both transverse and longitudinal directions.

Depending on the type of soil and the presence of surface water table , four types of foundation will be used for each type of tower location.
Normal dry type :
To be used for location in normal day cohesive or non- cohesive soils.
Partially sub-merged type :
To be used at locations where sub soil water table is met between 0.75 metre below the ground line.

Fully sub-merged type :
To be used at locations where sub-soil water table is met at less than 0.75 metre below the ground line.
Wet type :
To used for locations:
1.Where sub-soil water is met at 1.5 m or more below the ground line.
2.Which are in surface water for long periods with water penetration not exceeding one metre below the ground
3.In black cotton soils.

In addition, depending on the site conditions, other types of foundations may be introduced suitable for:
Intermediate conditions under the above classification to effect more economy, or
- For locations in hilly and rocky areas.
- For locations where special foundations (well type or piles) are necessitated.

Testing of soil
It is desirable to undertake testing of soil for all the tower locations and report should be obtained about the sub-soil water table, bearing capacity of soil, possibility of submergence and other soil properties required for the correct casting of casing of foundations.

Tower Foundation:
1.Excavation
2.PCC
3.Stub setting
4.Template Alignment
5.Concreting
6.Curing
7.De-shuttering & Template removal.
8.Back- Filling

Excavation

Excavated pit on hard rock

Water at Excavated pit
Dewatering

Stub setting & Template Assembly

Base levelling with sand filling

PCC

Raft Concrete

Stub Concrete
Stub Footing concrete

Transmission Tower Erection
Method of Tower Erection
There are four main methods of erection of steel transmission towers which are described below:
1)Build-up method or Piecemeal method.
2)Section method.
3)Ground assembly method.
4)Helicopter method.

Build Up Method of Tower Erection
This method is most commonly used in India for the erection of 66kV, 132kV, 220kV and 400kV transmission line towers.
This method consists of erecting the towers, member by member. The tower members are kept on ground serially according to erection sequence to avoid search or time loss.
The erection progresses from the bottom upwards. The four main corner leg members of the first section of the tower are first erected and bolted with the stub.

The cross braces of the first section which are already assembled on the ground are raised one by one as a unit and bolted to the already erected corner leg angles.
The cross braces of the first section which are already assembled on the ground are raised one by one as a unit and bolted to the already erected corner leg angles.
The members / sections are hoisted either manually or by winch machines operated from the ground.
After the tower top is placed and all side lacing are bolted up. Cross – Arms are erected as complete unit .

Section Method of Transmission Tower Erection
In the section method, major sections of the tower are assembled on the ground and the same are erected as units. These Units are erected with the help of mobile cranes.
Ground Assembly Method of Tower Erection
This method consists of assembling the tower on ground, and erecting it as a complete unit. The complete tower is assembled in a horizontal position on even ground.
In India, this method is not generally adopted because of prohibitive cost of mobile crane, and non-availability of good approach roads to tower locations.

Helicopter Method
•In the helicopter method, the transmission tower is erected in section. Sometimes a completely assembled tower is raised with the help of helicopter.
•This method is mostly used where access to the tower location is limited.
1

Tower erection - Finishing Works
All nuts shall be tightened with one to two threads shall be projected outside the nuts.
Punching after nut tightening and tack welding shall be done along with bolt and nut together to ensure that the nuts are not loosened in course of time.
The joints shall be painted with zinc paint on all contact surfaces during the course of erection to avoid rusting.
The finally erected tower shall be truly vertical after erection. Tolerance limit for vertical shall be one in 360 of the tower height.


Tower Erection in progress

Electrical Transmission Tower types and design
The main supporting unit of overhead transmission line is transmission tower.
Transmission towers have to carry the heavy transmission conductor at a sufficient safe height from ground. In addition to that all towers have to sustain all kinds of natural calamities.
So transmission tower designing is an important engineering job where all three basic engineering concepts, civil, mechanical and electrical engineering concepts are equally applicable.

A transmission tower consists of the following parts.
1)Peak of transmission tower
2)Cross arm of transmission tower
3)Boom of transmission tower
4)Cage of transmission tower
5)Transmission Tower Body
6)Leg of transmission tower
7)Stub/Anchor Bolt and Base plate assembly of transmission tower.

Peak of Transmission Tower
The portion above the top cross arm is called peak of transmission tower. Generally earth shield wire connected to the tip of this peak.

Cage of Transmission Tower
The portion between tower body and peak is known as cage of transmission tower. This portion of the tower holds the cross arms.

Cross Arms of Transmission Tower
The portion from bottom cross arms up to the ground level is called transmission tower body.
This portion of the tower plays a vital role for maintaining required ground clearance of the bottom conductor of the transmission line.

Design of Transmission Tower
During Design of transmission tower the following points to be considered in mind.
1.The minimum ground clearance of the lowest conductor point above the ground level.
2.The length of the insulator string.
3.The minimum clearance to be maintained between conductors & between conductor and tower.
4.The location of ground wire with respect to outer most conductors.

Transmission Tower
To determine the actual transmission tower height by considering the above points, we have divided the total height of tower in four parts.
1.Minimum permissible ground clearance (H1).
2.Maximum sag of the conductor (H2).
3.Vertical spacing between top and bottom conductors (H3).
4.Vertical clearance between ground wire and top conductor (H4).

Types of Transmission Tower
According to different considerations, there are different types of transmission towers.
The transmission line goes as per available corridors. Due to unavailability of shortest distance straight corridor transmission line has to deviate from its straight way when obstruction comes.
In total length of a long transmission line there may be several deviation points.

According to the angle of deviation there are four types of transmission tower.
1)A – type tower – angle of deviation 0 to 2
2)B – type tower – angle of deviation 2 to 15
3)C – type tower – angle of deviation 15 to 30
4)D – type tower – angle of deviation 30 to 60

As per the force applied by the conductor on the cross arms, the transmission towers can be categorized in another way.
1.Tangent suspension tower and it is generally A - type tower.
2.Angle tower or tension tower or sometime it is called section tower. All B, C and D types of transmission towers come under this category.

Apart from the above customized type of tower, the tower is designed to meet special usages.
1.River crossing tower.
2.Railway/ Highway crossing tower.
3.Transposition tower.
These are called special type tower.

Classification of Transmission Tower
Based on numbers of circuits carried by a transmission tower, it can be classified as:
1.Single circuit tower
2.Double circuit tower
3.Multi circuit tower.

This method is most commonly used in India for the erection of 66kV, 132kV, 220kV and 400kV transmission line towers.
This method consists of erecting the towers, member by member. The tower members are kept on ground serially according to erection sequence to avoid search or time loss.
The erection progresses from the bottom upwards. The four main corner leg members of the first section of the tower are first erected and bolted with the stub.
The members / sections are hoisted either manually or by winch machines operated from the ground

Section Method of Transmission Tower Erection
In the section method, major sections of the tower are assembled on the ground and the same are erected as units. Units are erected with the help of mobile cranes.

Ground Assembly Method of Tower Erection
This method consists of assembling the tower on ground, and erecting it as a complete unit. The complete tower is assembled in a horizontal position on even ground.
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.

Cross Arms assembled on ground for erection

Cross Arms Erected

Stringing
Stringing overhead conductors in transmission is a very specialized type of construction requiring years of experience, as well as equipment and tools that have been designed, tried, and proven to do the work.

Medhods of installing
There are four methods that can be used to install overhead transmission conductors:
Slack stringing
Semi-tension stringing
Full-tension stringing
Helicopter stringing

Slack stringing
This type is normally limited to lower voltage lines and smaller conductors.
The conductors are normally placed on “Reels” or “Jack Stand” and it is unreeled from the drum and dragged along the ground by vehicle or pulling device.
This method is typically used during construction of new lines where Right Of Way is readily accessible.

Semi Tension method of stringing
Semi tension methods are merely an upgrading of slack stringing, but do not necessarily keep the conductor completely clear of the ground, or the lines used to pull

Full Tension method of stringing
This is a method of installing the conductors in which sufficient pulling capabilities on one end and tension capabilities on the other, keep the wires clear of any obstacles during the movement of the conductor from the reel to its final sag position.
This ensures that these current-carrying cables are “clipped” into the support clamps in the best possible condition, which is the ultimate goal of the work itself.

Stringing with helicopters
This is much more expensive per hour of work, but can be much less expensive when extremely arduous terrain exists along the right-of-way and when proper pre-planning is utilized.
Although pulling conductors themselves with a helicopter can be done, it is limited and normally not practical.
Maximum efficiency can be achieved when structures are set and pilot lines are pulled with the helicopter, and then the conductor stringing is done in a conventional manner.

Transmission Line
Component of Transmission Line

Transmission Line
Components of Transmission Line

Common Tools Required for Stringing

Material Handling - Conductors
Handling, Loading /Unloading ,Transport & Storage

Handling, Loading /Unloading ,Transport & Storage
1.Handling and transporting of the conductor and accessories shall be carried out in such a manner as to minimize the possibility of damages from abrasion through rough handling or dirt and grit.
2.The drums should always be transported in vertical position with the cable ends fixed to prevent cable from slackening.
3.The drum should not be stored on its side under any circumstances whatsoever.
4.The ends of the cable should be sealed to prevent water penetration.
5.Loading and unloading are performed so that the drum remains in vertical position and the sides of the drum are not damaged.

Avoid storage like this

Stringing Procedure
The stringing procedure is broadly divided into the following steps:
Paying out & stringing of Conductor.
Paying out & stringing of Earthwire.
Final sagging of Earthwire and conductor.
Clipping & Fixing accessories.

Steps of Stringing
Proper Guying
Insulator Hoisting.
Paying out of pilot wire & Conductor.
Rough sagging of conductor.
Final sagging of conductor.
Clipping & Spacering.
Finishing activities.
Jumpering.
Final Checking.

Guying
Before commencing of stringing, the angle towers where the stringing is to be started must be provided with guy supports.

Guying
Precautions which should be taken at the time of guying?
The guys used generally are 20 mm steel wire rope. The guys are attached to the tower at the tip of the cross arm , to the strain plates with suitable D-shackles.
The guys are anchored in the ground at an angle of 45 deg. or less from the horizontal, attached to dead end anchors .
The guy wire is attached to the dead end anchor wire with the help of turn buckles of 10 tons capacity.
Excessive tightening of guy should be avoided. It is advisable to tighten the guy progressively at the time of rough sagging of the conductor.

Guying – Anchoring on ground

Insulator Hoisting
Single / Double suspension insulator strings are used on suspension towers and single /double tension insulator strings are used on angle and dead end towers. This is generally indicated in the tower schedule.
Double suspension insulator strings are used lines on Suspension towers of Railway, River and Power Line Crossings only.

Suspension Type Insulators
Consist of a number of porcelain discs connected in series by metal links in the form of a string.

Suspension Type Insulators
The conductor is suspended at the bottom end of this string while the other end of the string is secured to the cross-arm of the tower.
These insulators have a number of interconnected porcelain discs, with each unit designed to support a particular voltage.
Together, a system of these discs is capable of effectively supporting high voltages.

Strain Type Insulators
When there is a dead end of the line or there is corner or sharp curve, the line is subjected to greater tension. To relieve the line of excessive tension, strain insulators are used.
However, for the high voltage transmission lines, strain insulator consists of an assembly of suspension insulators as shown.

Strain Type Insulators
Strain type insulators are horizontally suspended suspension insulators.
When the tension in lines is exceedingly high, at long river spans, power line, railway & road crossings, two or more strings are used in parallel.
Strain insulators are typically used for high voltage transmissions.

Silicon Rubber Composite Insulators
Composite insulators with silicone rubber sheds offer advantages over traditional ceramics:
Improved safety for personnel and equipment.
superior pollution performance due to hydrophobic surface condition.
Excellent seismic performance.
Low weight & Flexible design
Short delivery times

Insulator fitted with Roller and Pulley for conductor Payout

Insulator Prepared for Hoisting

Payout
The paying out of conductor is done generally between two tension towers. In between two tension towers, there could be either zero or one or more suspension towers.
A pilot wire is used to pull the conductor. The pilot wire is initially laid through the centre wheel of the roller.
The pilot wire can be laid and joined with pilot wire connectors or it can be pulled from one side of the section.
Scaffolding shall be provided for P&T and road crossing before paying out of the pilot wire.

Stringing Procedure
Drum Scheduling - Basics
1.For effective utilization of the conductor and to prevent Wastage of the conductor.
2.Details of Tower schedule, Receipt of drum details, Standard length of each drum.
3.Knowledge of Usage of cut lengths for Jumpers and Short Spans.
4.All the joints or splices shall be made at least 30 metres away from the tower structures.
5.No joints or splices shall be made in spans crossing over main roads, railways and small river tension spans.
6.Not more than one joint per sub conductor per span shall be allowed.

Transmission Line
Tower Schedule

Transmission Line
Sample Drum Scheduling

Paying out Earthwire
Earthwire drum on turn table& unreeling in progress for payout.

Precautions before conductor Payout

Conductor Payout through Suspension Towers

Conductor Passing through rough terrains

Conductor Payout for each Phase

Conductor Rough Sag in progress

Conductor tensioning using come along Clamp and four sheave pulleys

Transmission Line Work Completion

Construction EHV Transmission Line

Construction EHV Transmission Line

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