E4014 Construction Surveying

Transmission Line Surveys

Objectives

– provide an understanding of the principles involved and methods used for surveying transmission lines

– describe factors that affect the location of transmission lines

– describe the basic principles of the survey and design of transmission lines

Objectives (cont)– understand the survey & design

required for a rural extension line

– display a knowledge of the surveys required for high voltage lines

Introduction

• Three main types of transmission lines

(a) rural extension of the distribution network • ( low & high voltage )

(b) extra high voltage lines carried on towers

– similar design principles apply to types (a) and (b)

– factors are more critical and methods more accurate for type (b)

Introduction• Third type of transmission line

( c ) urban distribution network

Power Supply System• Generation

• Transmission

• Distribution

Power Supply System

• Generation

– electrical energy is generated at the power stations using 3-phase a.c. generators

– voltage and frequency are controled producing a moderate voltage of, say, 13.8 kV

Power Supply System

• Transmission– high voltage transmission lines are used to transfer electrical energy from the power stations to the load centres

– usually 3-phase overhead conductors suspended from insulators attached to towers, and called PRIMARY TRANSMISSION lines

– 275kV used in Qld, 330kV in NSW and 500kV in Vic

Power Supply System

• Transmission (cont)– power transformers are used at

generating stations to step up or step down the voltages

– choice of transmission voltage depends on the power to be transmitted and the distance involved

Single Wire Earth Return System ( SWER )• Low cost, low maintenance method

of supplying power to isolated (country) areas– An isolating transformer separates the main

high voltage system ( 11kV or 33kV ) from the SWER line ( 12.7kV to 19.1kV )• separates the different earthing systems

– a single conductor overhead line feeds a distribution transformer, which steps-down the voltage to 2 circuits of 250V, or 1 circuit of 500V

Single Wire Earth Return System ( SWER )

– The SWER system uses a unique method of passing the return current through the earth• the resistivity of the earth must be within a

certain range for the system to work effectively

Basic Principles of Transmission Line Design• Ideal route between the point of

power production and the point of consumption is a straight line

• Design Parameters and regional constraints affect the location of power lines

Design Parameters

• Height of Poles or Towers

• Span Length

• Foundations

• Angles

Design Parameters• Height of Poles or Towers

– minimum height of transmission lines above the ground, for road crossings etc is laid down by an Act of Parliament

– most economic design attains this ground clearance and no more

– more height increases span length, but• increases cost of pole or tower• increases depth of pole, or• increases foundation costs of tower

Design Parameters• Span Length

– aim is to have as few poles or towers per kilometre

– poles should be placed to maximise span length, whilst maintaining the minimum clearance

Design Parameters• Foundations

– location of poles or towers may affect the type of foundations required

– cost of towers in 1978• normal foundations - 4 legs, 5m deep, 0.5m

deep $2500• special foundation - unstable soil $5000• rock anchor - rocky ground $5000• piled - mangroves, coastal flats

$10000

Design Parameters• Angles

– the pole or tower must be able to withstand the tension of an indirect pull• poles will need to guyed on the half angle• angle towers can be up to $20000 more

expensive than an ordinary support tower

– essential to keep bends to a minimum

Design Parameters• Summary of Design Parameters

– Height of Poles or Towers

– Span Length

– Foundations

– Angles

Constraints• Built up areas

• topography– mountains, forests, rivers, swamps,

flood zones

• recreational areas– national parks, development areas

Constraints (cont)• mining areas

• cultivation areas

• future uses– highways, railways, airstrips

• environment– aesthetics, erosion

Rural Extension Surveys• Extend the power transmission

network to small settlements and farm dwellings

• may carry high or low voltage– low voltage < 650 volts– high voltage > 650 volts

• normal extension is 11kV or 22kV high tension line

Rural Extension Surveys• Proposed extension is designed by an

electrical engineer and plotted on existing topographical and cadastral maps– shows location of all properties to be served– shows proposed location of transformers etc

• actual route location and design functions are carried out in the field by the SURVEYOR

Rural Extension Surveys• Aim for

– minimum number of poles per kilometre

– minimum number of bends– minimum ground clearance (6m at

49°C)– minimum clearance over

• road cuttings• railways• telephone wires

Rural Extension Surveys– minimum cut angle between

telephone and power lines is 45° and parallel lines must be a minimum of 30m apart to avoid interference and power loss• before undergrounding of power and

telephone lines power lines were often placed 30 - 100m away from the phone line which was often in the road reserve

– advantages - easy connections to sheds and houses set back from road

– disadvantages - cultivation, access

Rural Extension SurveysSurvey & Design

• Basic survey problem is to obtain a profile of the land over which the transmission line is to traverse and to negotiate obstacles unforeseen in the proposed design - trees, hedges, buildings etc.

– clinometer and chain– total station– GPS

Rural Extension SurveysSurvey & Design

• traverse along proposed centreline from the last pole position

• note chainage and RL of grade changes and features

• continue past the next pole position• plot profile

• horizontal and vertical scales are different

• governed by sag templates provided by Electricity Authority

Rural Extension SurveysSurvey & Design

Rural Extension SurveysSurvey & Design• by using the sag template the position

of the next pole is determined

• place a peg at the pole position• only after the chainage and RL of the point

have been compared to the plot

• surveyor should do a reconnaissance survey of the route before hand so that the topography is known to some extent

Rural Extension SurveysSurvey & Design

• angles are read • where a bend occurs• where a spur or branch line occurs

• place a peg at the bend and at the half angle

• poles will have to be guyed

Rural Extension SurveysSurvey & Design• if line is traversing private property and easements

are required, then occupation is also located• running chainage to fence, angle along fence and distance along fence to

cadastral corner

Rural Extension SurveysSurvey & Design

• surveyor has measured • the profile of the line• the running chainages and RLs• the angles at bends

• an accurate plot of the final design is made for construction, easement and record purposes

High Voltage Lines on Towers• Basic design factors are the same

• the following are critical– cost factor– foundations– length of route– aesthetics and environment

High Voltage Lines on TowersInitial Design• most suitable route is chosen

based on• voltage, conductor, tower specification• topographical maps• cadastral maps• town planning schemes• aerial photos• local Authorities advice• other Government Department’s advice• ground reconnaissance

High Voltage Lines on TowersInitial Design• report on the route is prepared

• costing• scheduling• approval• environmental impact assessment• comments of interested Public Authorities

High Voltage Lines on TowersInitial Design• route maps are prepared

• 1:10 000• latest cadastral information• number of lines proposed• easement widths• current land use

High Voltage Lines on TowersInitial Design• contact is made with the owners

and occupiers• negotiate easements and access tracks• property owners ( and other interested

parties ) may object• property owners may state their intended

future use of the land

• if objections justify an amendment then the centre line may be re-routed

High Voltage Lines on Towers Survey of the Route• high accuracy required

• control survey for whole of the route

• centre lines are cleared, surveyed and marked

• further public reaction may be forthcoming and the route may be deviated again

• keep lines to a minimum height for aesthetic reasons

High Voltage Lines on Towers Survey of the Route• surveyor locates towers and access

tracks• appreciate land usage and erosion• report all relevant circumstances

particularly those affecting overall location

• field data must show enough data for the towers to be located and their positions to be checked in the office

Urban Distribution Network• Transmission lines are entirely

located in the road or street reserve• span length is not as important

– poles are place to service as many consumers as possible

• design and survey is usually carried out by electrical engineers and their staff

• underground transmission conduits– at least 5 times more expensive