ESCOM OVERHEAD LINE TECHNOLOGY Course A Presentation by Levison Harrison ( Material Prepared By Gift Banda) Course content-Lecture 1 1. 2. 3. 4. 5. 6. General Safety Precautions Line survey Fundamentals of Line Design Construction of Overhead lines Stays and stay components Line materials & tools • Poles • Cross-arms • Conductors General Safety Precautions • There are two general safety rules that each lines personnel must follow before commencement of any work. These are extracted from the safety rules book: 1. RULE E 1 (1) Do not carry out any work (including maintenance, repairs, cleaning and testing) on any part of High Voltage Apparatus unless such parts are:a) Dead b) Isolated and all practicable steps taken to lock off from live conductors c) Efficiently connected to earth at all points of disconnection of supply to such apparatus, or between such points and the point(s) of work d) Screened where necessary to prevent danger; and Caution and Danger Notices fixed e) Released for work by the issue of an Electrical Permit – to- Work 2. RULE E 17 (1) • Before any wood pole is climbed, it shall be sounded. • No pole with a doubtful condition, shall be climbed without the permission of the competent person in charge. • No person shall climb any pole or tower unless he/she is wearing and makes proper use of an efficient safety belt and is under observation by a second person who is in attendance, preferably at the base of the pole. Line survey • Steps involved: There are basically two main steps involved in line survey which are selecting the route and pegging. 1. Selecting the route • This is the first step taken prior to line design and construction. • The main points between which the line is to be built will be known. • The intervening territory should be laid out on a scale large enough to show clearly all division lines, towns, roads, streams, hills, ridges, railways and bridges. • The line profile is drawn Some guide lines in selecting the route a) Select the shortest route practicable • The shortest line is naturally the cheapest, other things being equal. b) Parallel highways as much as possible • For easy delivery of materials for construction and maintenance • In rural areas, locate lines a short distance from the highway in order to avoid cutting trees. To preserve nature and prevent the cost of cutting trees. c) Follow section lines • Doing this causes less damage to public property • Right of way is purchased cheaper d) Route in direction of possible future loads • The line direction should be where there will be future loads provided the additional cost is not excessive. e) Avoid crossing hills, ridges, swamps and bottom lands • Hills and ridges subject the line to lightning and storms. • Swamps and bottom lands subject the line to floods. • Furthermore, delivery of materials as well as the construction of the line becomes difficult. • Extra guying is often necessary in swamps (additional cost!). f) Avoid paralleling telephone lines • Paralleling telephone lines causes disturbances or interferences in the telephone lines by induction and therefore requires transposition of the power conductors (additional cost!). NB: whenever crossing a private land, right of away has to be sort first before proceeding with pegging/construction works. 2. Pegging • It is the process of marking out the route of an overhead line. • The construction crew is required to do pegging of medium and low voltage lines. • Ranging rods are placed at the approximate distances apart and aligned visually in the direction planned. • The pegs are driven in at the established positions. • The same method used for ranging rods is used to measure angles and the positions of stays. • For High Voltage lines, pegging is done by surveyors and they use equipment such as theodolite or dumpy level. Fundamentals of Line Design 1. Distribution overhead lines • On public access roads, poles shall be placed on the side which is most free from foreign lines and trees. • Care being taken in cities and towns to keep the more important streets as free as possible from primary circuits. • The same side of the road shall be used throughout the length of the line wherever this is feasible. • Poles in streets should be set as close as possible to the sidelines of the streets. • The length of spans may be increased or reduced so as to make poles in line with property lines or fences, or to place them in positions satisfactory to adjoining property owners whenever this is practicable. • In locating poles on lot lines, either in the street or in the alley, care must be taken not to block driveways or the entrances to the garages. Furthermore, care should be taken not to obstruct doorways, windows, fire escapes, gates and runways. • Where lines cross private property, easements shall be obtained from property owners before any work is started. • Poles should be set on the junctions of streets to facilitate the installation of branch lines or stays. • Poles should be located to give the longest straight line between angle points using as much of the road as possible. 2. Transmission overhead lines • In locating poles or towers on transmission lines, the following general principles should be kept in mind:a) Select high places b) Keep spans uniform in length c) Locate to give horizontal grade • By locating poles or towers on high places, shorter poles or towers can be used and still maintain the proper ground clearance at the middle of the span. • By avoiding ravines and low places, a better footing is usually also obtained • Poles should not be placed close to places where the ground washes badly. i.e. along river banks, or edges of cuts or embankments Examples of Correct Routing Line is routed along the valley and the lower slopes of the hills or mountains and away from the lake, thus causing minimum interference with natural landmarks. Line is routed across plains but curves between hills or mountains, causing least disruption of natural lines. Right of way (Wayleave) • When the route of the line has been selected, permission must be obtained to run the line, or land must be purchased. This is known as The Right Of Way or Way Leave or Easement. • Most low or medium voltage pole lines do not have complete right of way, i.e. owners do not possess a continuous strip of land, but only the area on which the tower or pole is located. • In the case of high voltage lines, a continuous strip of land is often acquired. • The reason for acquiring the right of way much wider than the actual space necessary is to prevent tall trees from being blown into the line and to prevent damage from forest fires. Width of wayleave Voltage Level Width (m) Remark Supply 230/400V 5m Normal structure Supply/Distribution 11,000V 10m Normal structure Distribution 33,000V 20m Normal structure Distribution 33,000V 30m H-Pole Structure Transmission 66,000V 30m Transmission 132,000V 30m Transmission 400,000V 55m Reference: Electricity Act, 2012 Clearing the right of way • In clearing the right of way, all stumps should be cut low. • All logs and bush should be removed for a distance of at least 25ft (7.5m) under each conductor, in order that there may be ample room to assemble and erect poles or towers and later to string the line conductors. • Any under brush or piles of dead wood should be removed. In case they are left, they may catch fire and burn down the line or anneal the conductors and cause them to sag abnormally. • Permission should be obtained to cut down the tall trees immediately adjacent to the right of way. Construction of Overhead lines • Factors to consider in the design of an overhead line: a) Electrical properties b) Mechanical properties c) Environmental conditions a) Electrical properties • This is the first consideration in an overhead line design. • The current carrying capacity of the line must be sufficient for the required power to be transmitted without: i. Excessive voltage drop ii. Overheating • The insulation must be adequate to cope with the system voltage. b) Mechanical properties • Consider the mechanical factors that would influence the design: • Conductor material and supports must be strong to withstand the forces to which it is subjected. And both should be sufficiently durable to give satisfactory service over a long period of time. • The tension in the conductors should be adjusted so that it is well within the breaking load of the material. • This will mean in practice that an appreciable amount of sag must be allowed c) Environmental conditions • The first combination would be still air with high temperature whilst the second one would be low temperature with snow or ice coating. • Still air and high temperature provides the easiest condition. • In still air conductor is only acted by upon its own weight and if the temperature is high, the amount of the resulting sag will give a low tension. • Snow or ice coating and low temperature provides the worst condition. • Low temperature reduces the sag; snow or ice coating increases weight per unit length of the conductor. Determining size of line conductor Factors affecting conductor size • The size of a line conductor depends on the following factors: a) line voltage b) the amount of power to be transmitted c) the mechanical strength • For a given amount of power, the greater the voltage the less the current will be. The current in turn determines the size of the conductor. • Equally important is the magnitude of the load. For a given voltage, the larger the load, the larger the current (ohms law). • The conductor must also have sufficient mechanical strength to carry its own weight and any additional load due to ice or wind. • Other factors are: length of line, power factor of load, and length of span. Current Carrying Capacity • The amount of current flowing in a line could be determined if the line voltage and the power to be transmitted are known. CURRENT CALCULATION GIVEN PARAMETERS GIVEN APPARENT POWER AND VOLTAGE GIVEN REAL POWER AND VOLTAGE 1Ο΄ (AC) πΌπΏ = πΌπΏ = π ππΏ π ππΏ π₯ cos Ο΄ • Where: πΌπΏ is the line current (A) ππΏ is the line voltage (V) π is the apparent power (VA) π is the real power (W) cos Ο΄ is the power factor 3Ο΄ (AC) πΌπΏ = πΌπΏ = π 3ππΏ π 3ππΏ π₯ cos Ο΄ Span, Ground Clearance And Sag Span • The span is the distance between line supports; i.e. the distance between poles or towers. • The longer the span, the fewer will be the number of poles or towers and the number of insulators. • And also, the longer the span, the larger must be the conductor in order to carry its own weight and any additional load due to ice and wind. • The determination of the most economical span is a complex problem and involves balancing the increased cost of the increased size of conductor and tower against the saving due to the lesser number of towers and insulators. Ground Clearance • In the interest of public safety, there are some recommended minimum heights at which the conductor shall be strung above ground. • Ground clearance is the distance between the point of sag of a conductor and the ground. Sag • The sag of a conductor is the dip of an aerial wire between two adjacent poles or towers. An Illustration of Span, Ground Clearance and Sag Minimum Heights for overhead lines VOLTAGE LEVEL RAILWAY CROSSING ROADS ACCESSIBLE BY VEHICLES <650 V 7.6m 5.5m 11,000 V 7.6m 5.8m 33,000 V 7.6m 5.8m 66,000 V 7.9m 6.1m 132,000 V 8.5m 6.7m 400,000 V 9.7m 7.3m Factors Affecting Sag • The factors affecting the sag in a conductor are: a) b) c) d) the length of the span, the weight of the conductor, wind load and the temperature. • The temperature is a very important factor, because, when it is hot, the conductor lengthens and the dip is increased; and when it gets cold, the conductor contracts and tightens. • If the conductor is strung too tight while the temperature is high, the conductor may break when the temperature is low. Sag • The changes in sag due to expansion and contraction because of changes in temperature are therefore of much importance in the construction of lines, especially the stringing of wires. • In order to avoid excessive tension of conductors during cold weather, they must have the proper slack or sag. • Furthermore, in order that they may not dip below the clearance to the ground required during the hot season (summer time), conductors must have the proper tension. Profile and Span Plan • All lines personnel/supervisors must be able to read profile spans plans and identify symbols such as railway crossings, road crossings, rivers etc. on the plans. • The angle of the change of direction at the angle poles, and the angle at which the resultant stay wire should be placed, must be indicated. • When the line is long and straight, each 12th intermediate pole will be changed into a section pole. • Section poles will be used at all crossings. • The distance between poles is dictated by the terrain over which the line is erected. • In mountainous areas where there are many hills, the poles will be placed closer together than usual in order to maintain the clearance between the conductors and the ground. • The maximum distance between two poles of 10.8m (35ft) in length is about 120m. Power Linesman Tools • • • • • • • • • • • • • Winch/tirfor Pull lift Derrick Shovels Picks Axes Ropes Spirit levels Tape measures Augers 14lbs hammer 7lbs hammer Come along 2lb hammer Hacksaw frames Wire cutters Pipe wrench Skidding channels Rammers Crowbars Ladders Tool boxes Plumb bob Sag boards Dynamometer The setting depth of supporting part of the pole Estimation of multiplication of 1/6 by the length of the pole is sometimes used to determine the length of the support part. Stay component and make-off a) The use of stay • Stays are used to reduce the horizontal load placed on structures by the tension of the conductors and the earth wire. • Abnormal tension can also be placed on structures by broken conductors or during the stringing of the conductors. • The size of a stay should be such that its maximum strength is at least twice that of the maximum tension calculated for normal conditions. • Where stays are used for abnormal tension, caused for example, by broken conductors or adverse condition during stringing, the safety factor may be reduced. • It is recommended, however, that the maximum strength of the stay should nevertheless be one and a half times that of the maximum calculated tension for the normal conditions. Locating of stay • Where possible, the stay must be attached to the structure at the point where the tension is exerted and then extended in the opposite direction to the tension. • In addition, the stay must be attached to the structure in such a way that the clearance between the stay and any live part is not less than the phase – to – earth clearance for all design conditions. • Where possible, stays must not be located under or over communication lines, or in the close proximity of telephone poles. Location of stay wire Insulation of stays • Stays are usually installed at an angle of 45° or more, with reference to the ground. • It is therefore, possible to touch the stay while standing on the ground several meters from the point where it is anchored to the ground. This increases the danger of exposure to high voltage in the case of an electrical fault to the stay. • To eliminate this danger, a suitable insulator must be installed on lines. The insulator must be installed as high as possible off the ground, and far enough from the structures so that the stay is not electrified if a fault occurs. • In the case of metal structures with earth conductors, the section of the stay above the insulator must be electrically connected to the structures or the earth conductor. Stay wires and insulators • The material used to make stays is usually galvanized steel. • Different sizes of stays (stranded) are used. For Malawi network system, the following sizes are used: ο 7/14 and 7/12 are used for LV and MV ohl networks ο 7/8 is used for HV ohl networks • The thickness of the conductor used determines the size of stay wire to be used. • Double stays are used in certain structures. ο They are used at crossings (road or railway) and at the beginning and end of a line. • A double stay consists of two stay wires from the section pole to two stays in the ground, placed at a distance of one metre. • Stay insulators are usually spaced at 1.8m from the top of the pole. Types of stays 1. Line stay • It is installed at the beginning and end of a line, and also at section poles. • This stay holds straight the horizontal tension of the conductors. 2. Angle stay • It is installed where a line deviates to take a different direction. • It holds the angular tension of the conductors which they place on the structure. 3. Strut stay • a shorter pole is planted in such a way that the top of the strut presses against the main pole, as high up as possible. • The top of the strut is bolted to the main pole. • Strut stay is used in case there is no space for line/angle stay Strut stay Size of line pole (m) 9 10.8 12.3 13.5 Size of strut (m) 7.5 9 10.8 12.3 Ground distance between the pole and the strut (m) 2.7 3.1 3.7 4.6 Types of stays 4. Overhead/flying stay • It is used where a line runs close to a road. • Another pole is used on the other side of the road, planted at a depth determined by the length of the pole. • The length of the pole is such that the stay wire is 7.3m above the road surface. • A stay is then planted behind the pole and made off at the pole top. • A stay wire is spanned across the road from the main pole to a turn- buckle clamped to the pole. • The turnbuckle is used to take up the slack which is normally present. Flying stay Types of stays 5. Transverse stays • They are usually two, installed at the sides of an intermediate pole where sections are far away from each other, or where the setting of the pole is questionable – i.e. pole depth not done to standard size due to soil conditions.
