Midal Cables Ltd.Bahrain - Kuzhippalathil OHCL Installation (doc... · MIDAL CABLES – BAHRAIN...

Midal Cables Ltd.Bahrain

MIDAL CABLES – BAHRAIN

Overhead Aluminum Conductor Installation Guide

P a g e | 1 www.midalcable.com Doc.dtd. 110416

1.Introduction

This document is released as a guide which provides suggestions and recommendations of various methods , equipment and tools which is found to be practically adopted in the field for the installation of overhead transmission and distribution lines and can be used for the successful stringing of conductors manufactured and supplied by Midal Cables. This document is to be read in conjunction with IEEE Std 524: “Guide to the installation of overhead transmission line conductors” for optimizing the conductor installation techniques. It may be recognized that each and every installation set-up is different and there will be situations where customized techniques will be required and could be different from the methods given in this document as well.

The types of conductors covered under the scope of this guide include ACSR,ACSR/AW, ACSR/TW,AACSR, AACSR/AW,AACSR/TW, AAAC , AAAC/TW , ACAR and AAC. For all other special type of conductors , respective documents from Midal Cables shall be asked for.

It is strongly recommended that , the conductor used in a given pull or sagging section shall be from the single supplier.

2.Handling

The prime importance is to avoid any damage to the conductor surface and protect the integrity of the conductor. The conductors are not supposed to be rewound from its original shipping drum to another drum in the field. The conductor drums are to be always kept with the flanges upright on a solid surface and conductor shall not be dragged across bare ground, rocks ,fences or other structures. It is strongly recommended to refer to Midal’s Conductor handling and storing instructions document for additional information.

3.Conductor Stringing Methods

The possible methods of stringing include Tension /Semi tension and slack and layout. Any of these methods can be used for installing Midal conductors in distribution lines . However for transmission line installation the tension method is the recommended option. All these methods are dealt in detail as follows.

In Tension method, the conductor is kept and pulled into position under tension during the stringing process. In this method conductor is kept clear off the ground and obstacles preventing it from getting surface damage as well as clear from the energized circuits. This method involves a puller arrangement on one end of the line section and a tensioner at the

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other end. The conductor drum is to be kept behind the tensioner. In general a pilot line will be pulled into the travelers first which will be then used to pull the pulling line made of steel or synthetic rope. The pulling line is used to pull the conductor through the sheave till reaches the puller. Conductor is threaded through the bull wheels of the tensioner. The back tension is created and controlled by the bull wheel tensioner. An helicopter or ground vehicle can be used to pull /lay out the pilot line or pulling line.

Semi Tension method is very similar to tension method but very low ground clearance is maintained. The conductor is directly pulled from the drum into the spans and tension is maintained with small breaking force on the conductor drum pay off

In Slack method conductor drums are mounted on stands or jacks placed either on ground or a transporting vehicle. The conductor is dragged along the ground. When the conductor is dragged past the structure, at each structure, the conductor is lifted and placed in to a stringing sheave ( traveler) before proceeding to the next pole. This method is possible only if the terrain is easily accessible to a pulling vehicle but conductor surface condition is possible to be compromised.

Lay out method is similar to slack method. The lead end of conductor is tied off and the vehicle mounted with conductor drum travels down the line section, paying out the conductor as it drives on. Later the conductor is lifted and placed into the stringing sheave at each structure,

4.Tensioners

Multi groove bullwheel tensioners are recommended to be used. The number of grooves in the bullwheel must be sufficient to prevent the outer layer of wires from slipping over the underlying layers. In order to wrap the conductor into the grooves two wheels in tandem are used. Tandem wheels are aligned with an offset of approximately one half the groove spacing.

Midal manufacture conductors normally with right handed lay. All right handed lay conductor should enter the front tensioner wheel on left most groove and exit from the top right-most groove of the back side wheel ( fig. 1 ). For the left handed lay conductors (mostly of French standard or earth wire), the conductor should enter the right hand groove and got pulled off from the left while facing in the direction of pull. This is necessary to avoid the loosening of the outer layer strands as the conductor passes over the bull wheel.





In line with IEEE524 , Midal does not recommend the use of V-grooved tensioners for its multilayer conductors ( conductor having more than 7 wires/or 8 wires). Also all type of tensioner grooves shall be preferably provided with some elastomer /rubber lining for protecting the conductor from getting surface damage during stringing .

5.Recommended Bull wheel dimensions

The recommended dimensions of the tensioner wheel shall be estimated from the following scheme.

Fig .1

Fig .2a.





6.Pullers

The pullers are required to use when tension method is used for conductor stringing. Two types of pullers are possible; bullwheel type and drum-trype (or reel-type). The pulling and braking systems should operate smoothly and should not cause any sudden jerking or bouncing of the conductor. Each system should be readily controllable and capable of maintaining a constant tension.

The puller and tensioner capacities should be based on the conductor type, span length, terrain, and clearances required. Recommended stringing tensions in general is about 50% of sag tensions and shall be ensured not to exceed the sag tensions during stringing. The Annexure D,F,H & I of IEEE 524 may be referred for the method of calculating the required capacity for both puller as well as the tensioner.

7.Travellers

It is always recommended to use larger sheave diameters which will help to have higher bending radius for the conductor. This will reduce the strain and relative movement of strands in the conductor. Also the higher sheave diameter minimizes the strand notching while conductor is pulled . For estimating the minimum recommended sheave dimensions, the following table shall be referred.

Fig .2b.





Minimum Diameter at bottom of Groove Ds

For Typical pulls Ds=20Dc-200 ( in mm) For pulls >3000m Ds=20Dc-100 ( in mm) For pulls over uneven terrains or large angles/elevation changes Ds=20Dc Note: for multi layer transmission line conductors, Ds must be minimum 16 x Dc

Groove Radius Rg Aluminum layers : 1 or 2

3 >3

Minimum Rg =0.55 Dc Rg =0.55 Dc Rg =0.55 Dc

Maximum Rg =1.1Dc Rg =0.75 Dc Rg =0.625 Dc

Minimum Groove Depth Dg Dg=1.25 DC

Groove flare angle Gf 15o < Gf < 20o

Travelers can be of any material in general but the consideration shall be given to the weight. It is very important to inspect the stringing sheave lining condition, as worn or torn linings should not be used. A worn out lining at the bottom of the groove means that the conductor will not track true and will have uneven pressure applied to it as it passes through the sheave. Stringing sheaves should roll smoothly when the conductor is being pulled through. A stringing sheave that bounces or does not spin at a constant speed indicates bad bearings. When this occurs, the conductor pull should be stopped, and the defective stringing sheave shall be replaced. Urethane or neoprene-lined stringing sheaves have been successfully used in the installation of bare overhead conductors. For multilayer

Fig .3.





conductors, bare aluminum stringing sheaves are not recommended as they may cause surface scratches.

8.Conductor Grips

These are devices designed to permit the pulling of conductor without splicing on fittings eyes, etc. It permits the pulling of a continuous conductor where threading is not possible. The designs of these grips vary considerably and a wide variety of conductor grip styles and tie-down products available in the industry. Always consult with the grip manufacturer for the correct selection so that the grip type and configuration is sized for the diameter and type of conductor being installed and is capable of holding the conductor to the highest tension that is anticipated during the sagging operation.

There are two basic types of mechanical grips that are used to secure the conductor during the pulling, sagging or dead-ending operation. The Klein (Chicago) and Crescent types of grip are open-sided rigid body with opposing jaws and swing latch that pinch the conductor between the jaws. The pocketbook type of grip, often referred to as come-along, suitcase, or four bolt, incorporates a bail attached to the body of a clamp that folds to completely surround and envelop the conductor. Bolts are then used to close the clamp and obtain a grip. A woven wire grip (often referred to as a basket, wire mesh, Kellem®, or sock) sized to fit over the conductor and a suitably sized swivel link should be used for tension stringing. To prevent the grip from accidentally coming off the conductor, two punch-lock steel bands should be applied at the open end of the grip over the woven wire grip and conductor. Tape applied over the bands will protect the bullwheel tensioners and stringing sheave lining from the steel bands.

Fig .4. Conductor Grips





Note : while stringing shaped wire conductors ( ACSR/TW,AACSR/TW & AAAC/TW ) additional gripping mechanism may be required to lock the round wire core in order to prevent it from slipping .

A washer and compressed steel sleeve shall be attached to the leading end of the conductor as illustrated in fig.5

The aluminum strands are to be cut to a length approximate to that of the steel sleeve from the end of the conductor. Thereafter inserted a steel washer on the exposed steel core to press against the cut ends of the aluminum strands. The washer outside diameter should be equal to the conductor diameter. Thereafter steel sleeve shall be compressed over the steel core. 9.Procedure for stringing

9.1.Site Selection : Accessibility, terrain, angles in pull section, location of usable dead ends, length of conductor to be strung, available conductor and line lengths, puller capacity, snub structure loads the physical area needed for placement of equipment and ability to give adequate grounding system are the main factors to be considered in selection of pull, tension, snub structure, and splicing sites.

The location of puller , tensioner and intermediate anchor sites are to be selected in a way not to over load the snub structures. A pulling line slope of three horizontal to one vertical from traveler is highly recommended. Also important is that the pulling line to enter the machine at a minimum horizontal angle.

Pulling Grip

Sleeve crimped on core

Steel Sleeve

Conductor

Steel washer





The conductor drum stand and tensioner shall be positioned such that minimum lateral angle between conductor is maintained as it approaches the bullwheel and the plane of rotation of the wheel so that the conductor does not rub on sides of the groove. Typically a maximum fleet angle of of 1.5° from the plane normal to the conductor reel axis and a back tension of approximately 4.5 kN is recommended for large conductors in order to avoid bird caging problems. The problems of bird caging are prominent in large conductors with more than two aluminum layers and hence extreme care is suggested to this effect not to damage the conductor.

9.2.Protective Grounding

Adequate grounding must be established at all sites. The methods required and equipment used will be determined by the degree of exposure to electrical hazards around the equipment as well as the pull section and the soil conditions at the site. All equipment, conductors, anchors, and structures within the work area must be bonded together and to the ground electrode.

Fig .5





Prior to pulling in any conductor or conductive type pulling lines, two running grounds should be installed , one between the reel stand or conductor tensioner for conductor and the first tower, and one between the puller for the pulling line and the last structure. These grounds must be bonded to the ground previously established at each site.

9.3.Stringing

The pulling line which is pulled under tension shall be connected to the conductor through a swivel link, or to bundle conductors through swivel links and a running board. Swivels must not be used to connect lengths of pulling line. A rope connector should be used for this purpose.

Swivel links shall be of sufficient rated working load to withstand loads placed on them during tension stringing. They should also be compatible with the travelers being used so that they can pass through without spreading or damaging the sheaves. These special line stringing swivel links are clevis type and are compatible with woven wire grips and swaged steel pulling lines.

It is strongly recommended to use conductor of only one manufacturer and one drawing stock (rod-type cast or rolled) in a given pull, and preferably in any given sagging section. This precaution will help avoid significantly different conductor sag characteristics.

Pulling speed is an important factor in achieving a smooth stringing operation. The field experience is that Speeds of 5 to 8 km/hr usually provide a smooth passage of the running board or connecting hardware, or both, over the travelers; whereas slower speeds may cause significant swinging of the traveler and insulator hardware assemblies. Higher speeds create a potential hazard of greater damage in case of a malfunction.

The maximum tension imposed on a conductor during stringing operations should not exceed that necessary to clear obstructions on the ground. It may be necessary, under certain circumstances, to string the conductor near sag tension to clear crossing structures such as poles over highways, roads, or existing distribution lines. If stringing tensions are greater than 10% of the conductor’s breaking strength, consideration must be given to any possible pre stressing of conductors that may result, based on the tension and time involved. It may be noted that the tension at the pulling end will be higher and sometimes exceed the tension at tensioner in the case of long conductor lengths.





9.4. Bottom end Conductor attached to Drum

The bottom end of the conductor is normally fastened to the drum to provide a stationary attachment point during manufacturing. This end attachment shall not be relied as anchor point for the conductor during stringing. The stringing operation shall be stopped before the conductor gets over on the drum and any required back tension shall be transferred from conductor on the drum to some other anchor point in order to withstand the conductor “run out” condition.

9.5. Pay off tension

Pay off brake tension shall be continuously monitored throughout the pull and lowered as the drum empties down. Light and steady back tension, sufficient to prevent over-run in case of a sudden stop, should be maintained on the conductor drums at all times. The tension must also be sufficient to cause the conductor to lie snugly in the first groove of the bull wheel and to prevent slack in the conductor between bull wheels. It may be necessary periodically to loosen the brake on the reel stand as the conductor is payed off. Increase in may cause the conductor to wedge into the underlying layers on the reel. The conductor drums shall be positioned in a way that it will rotate in the same direction as the bull wheels.

Loosening of the stranding that often occurs between the conductor drum and the bull wheels of the tensioner is caused to a great extent by coil memory in the conductor. As the conductor is unwound from the drum and straightens out, the outer strands become loose, a condition that is particularly noticeable in a large diameter conductor and can be best observed at the point at which it leaves the drum. As the conductor enters the bull wheel groove, the pressure of contact tends to push the loose outer strands back toward the drum where the looseness accumulates, leading to the condition commonly known as bird caging . If this condition is not controlled, the strands can become damaged to the extent that the damaged length of conductor must be removed.

The above problem of bird caging can be remedied by allowing enough distance between the conductor drum and tensioner to permit the strand looseness to distribute along the intervening length of conductor and simultaneously maintaining enough back tension on the reel to stretch the core and inner strands to sufficiently tighten the outer strands. It is recommended that the back tension or braking tension of the conductor drum not to exceed 4.5 kN, since drawing down of the conductor into the lower layers on the reel may cause surface damage. For smaller diameters and wooden reels, the back tension should be





considerably less. A possible outcome of excessive brake tension is crushing down of the bottom layer conductor and distorting the underlying conductor strands.

The amount of permissible tension depends on the reel construction. When the conductor has been drawn down to the last few wraps on the reel, the installation procedure should be stopped and any required back tension transferred off the conductor on the reel to some other anchor point. The end conductor attachment in the reel should never be used as a brake or end stop and exposed to a conductor “run-out” condition.

9.6.Stringing sheaves at angles :

The stringing sheaves at significant angle changes shall be supported by means of a sling to allow conductor to roll along in the bottom of the groove. If an angle is suspected to being a potential risk, presence of a crew personnel at the angle structure is recommended during the stringing process to watch and mitigate the undue situations and prevent damage in the conductor. When conductor is pulled at angles, the line tension cause the sheaves to shift towards the sides and conductor to ride on the sheaves. This shift of conductor from center of the sheave creates a torsional moment in the conductor causing it to rotate either clock wise or counter clockwise depending on which side of the sheave the conductor is. To ensure the conductor remain in the middle of the groove of the sheave, a proper support to the sheave at angle is recommended as shown in fig.7.

Fig .6 Damage of conductor due to excessive brake tension





9.7. Maximum Pulling Tension

As per IEEE524 recommendation, the maximum pulling tension applied to the conductor shall not exceed 10% of the conductor rated strength. The recommended pulling tension can be calculated from the following formula which considers the efficiency factor of travelers during tension stringing.

Tmax= T1 ηN

Where T1 is the static tension required to support first span and can be calculated from

T1= WL2 8D

W= weight per unit length of conductor D= average pulling sag during stringing ( not final sag) L= average span length η = travelers efficiency ( and normally taken as 0.98) N = no.of sheaves.

The above formula is depending on many factors and proper allowances are to be taken into account considering the actual field situations. Pulls which include angles and

Fig .7





elevation changes increases the pulling force. The traveler efficiency assumed depends greatly on the size of the sheaves used. In most instances the values arrived with the above formula will have acceptable accuracy. 9.8. Conductor Side Wall Bearing Pressure. Another factor that should be considered when installing overhead conductors is the pressure between the conductor itself and the stringing block lining material. Excessive pressure can damage the conductor by deforming and loosening the aluminum strands. Excessive pressure will also accelerate the wearing away of the stringing sheave groove lining material. This force is referred to as the side wall bearing pressure or the conductor “bearing pressure”. This pressure per unit length between the conductor and the stringing sheave groove is a function of the pulling tension (T) applied to the conductor, the diameter of the stringing sheave at the bottom of the groove (Ds), and the diameter of the conductor itself (Dc). The pressure is independent of the length of radial contact around the sheave. For overhead conductors, the side wall bearing pressure can be expressed by the following equation:

P= 3T Ds x Dc

where

P= side wall bearing pressure

T= conductor tension

Ds= diameter of sheave to bottom of grrove

Dc = overall conductor diameter

As per IEEE 524 the maximum side wall pressure recommended is 3.4 – 4.8 MPa (500-700psi) between conductor and the sheave to minimize the possibility of conductor damage. 9.9.Time in Stringing Sheaves : It is not recommended to leave the conductor in stringing sheaves for long period . As per IEEE524 recommendation, the conductor sagging shall be completed within 24 hours of installation. While the conductor is left in sheaves, the tension shall be kept as low as possible. A low holding tension and working within 24 hours time window, make it unnecessary to apply short term creep correction curves for subsequent sagging. This is more desirable as the short term curves are arrived with assumptions of tension , temperature and time and become less accurate with increase of time. Pre-Tensioning : In some situations pre-stressing may be required prior to final sagging. Pre-tensioning is normally carried out when it is desirable to sag the conductor according





to its final condition. The pre-tension value is specifically calculated for each installation and temperature. Usually the conductor is pulled to 50-60% of the rated strength and held for about an hour. It is necessary to check and confirm the capability of sheaves and supporting hardware to with stand the tension during pre-tensioning phase

10.Sagging

The sagging of conductor can be done using any of the existing methods. There are generally three methods used for setting tensions or sag namely transit method, dynamometer method or stop watch method. The stop watch method is also used to verify tensions ate multiple locations

The stop watch equation is

where t= time for Nth return wave N = number of waves

When sagging the conductor the tension adjustments should be completed within one hour. It is good practice to check the sag at more than one location along the line, selecting level sections with similar span lengths. Bundle conductors should all be sagged at the same time. Once the sagging tension is set, it should not be adjusted again. Appropriate creep compensation values should be considered where the conductor remained in the stringing sheaves for more than the recommended 24 hours before final sagging

Clipping Conductors shall be clipped in within 24 hours after the line is brought into sag. The line tension should never be adjusted again, as creep will have started to take place. IEEE Standard 524 states that the total time for conductors sitting in the sheaves, from initial installation until clipping, should never be more than 72 hours. After sagging finished , the installation of dampers, spacers & spacer-dampers shall also be completed as soon as possible.

11. Hardware : The selection of the hardware is beyond the scope of this document. Customer is requested to consult the fitting manufactures in order to make the appropriate selection and procedure of the hardware.

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References: 1. IEEE Std.524-2003 Guide to the installation of Overhead Transmission Line Conductors 2. KS1883:2010 Electrical Power Transmission and distribution-Overhead power lines-Installation of line conductors (Kenya Bureau of Standards).


Midal Cables Ltd.Bahrain - Kuzhippalathil OHCL Installation (doc... · MIDAL CABLES – BAHRAIN...

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