Composite Overhead Conductors

CHAPTER 50

Composite Overhead Conductors

BARE OVERHEAD CONDUCTORS

Bare overhead conductors form an integral part of almost every electrical power transmission and distribution network. This chapter describes the construction and principal design aspects of these conductors.

In addition to the power carrying phase conductors, single or double circuits on steel supporting towers are normally protected against lightning by at least one earth wire which also passes fault currents. The conductor design for a given application is based on the cost effective satisfaction of the electrical, mechanical and environmental requirements.

Construction

The majority of conductors are configured in a concentric lay stranded form - that is, a single straight core wire surrounded by one or more layers of helically stranded wires. The direction of twist is reversed in adjacent layers, with the direction of lay of the outermost layer conventionally being right-handed.

Types

In addition to stranded conductors constructed from copper or cadmium copper wire, a variety of constructions based on aluminium are available to give an optimum solution to the line requirements. These are as follows.

AAC

ACSR

All Aluminium Conductor is the lowest cost conductor for a given current rating but its low strength:weight ratio makes it suitable for relatively short span lengths only.

Aluminium Conductor Steel Reinforced has a higher strength:weight ratio than AAC and because of this can be used on longer spans and can withstand more severe weather conditions. It also possesses a higher modulus of elasticity and a lower coefficient of thermal expansion, both of

709

710 Electric Cables Handbook

AAAC

ACAR

AACSR

which enhance its mechanical performance. These properties can be varied by altering the aluminium:steel ratio in the stranding geometry. Higher strength steel can also be used which improves the strength:weight ratio without affecting other properties. The central steel core normally consists of galvanised steel wires but may be of aluminium clad steel wires which give additional conductivity and corrosion protection in exchange for greater cost and some compromise of the mechanical properties. This latter conductor type is abbreviated to ACSAR.

All Aluminium Alloy Conductor is a homogeneous construction of aluminium alloy wires. The alloy is a heat-treated magnesium silicon aluminium type and the resulting conductor offers several advantages over the AAC and ACSR types. This has led to the increasing use of AAAC in modern lines. These conductors have a superior strength:weight ratio, allowing scope for reduced sags, greater support tower spacing or increased current carrying capacity on existing tower structures, lower a.c. resistance leading to lower line losses, absence of galvanic corrosion, less complex installation fittings, and reduced vulnerability to surface damage.

Aluminium Conductor Alloy Reinforced is constructed with a central core of aluminium alloy wires and provides a conductor option between those of AAC and ACSR.

Aluminium Alloy Conductor Steel Reinforced has a high strength:weight ratio at the expense of conductivity and is used where these properties are advantageous such as for earth wire applications.

Although wires used in the conductors are most commonly circular in section, two methods, outlined below, are used to maintain the cross-sectional area of a conductor at the same time as reducing the diameter. This helps to minimise ice and wind loading but requires the sacrificing of some conductor flexibility.

Compacted conductors Conductors stranded from up to two layers of round wires can be manufactured so that at least the outer of these layers is compacted during the stranding process using either a set of shaping rollers or compacting dies.

Segmented conductors These are conductors stranded with at least one layer of specially drawn wires having the appropriate segmental shape.

Materials

Table 50.1 gives the typical characteristics of metallic materials used in the construction of the common conductor types available.

Of the materials listed in table 50. l, aluminium alloy is worthy of further explanation because of the conductivity/strength balance it offers, and its steadily increasing use. The objective in moving to an alloy is to improve the mechanical properties without having too much effect on the conductivity. The alloy most commonly used in this application is

Composite Overhead Conductors

Table 50.1 Characteristics of conductor materials

711

Units Hard Cadmium Hard drawn copper drawn copper aluminium

Aluminium Galvanised Aluminium alloy steel clad steel a

Conductivity %IACS 97 79.2 61 Resistance fl mm2/km 17.71 21.769 28.264 Temperature 0.00381 0 . 0 0 3 1 0.00403

coefficient of resistance per °C

Coetticient of x 10 -6 17 17 23 linear expansion per °C

Linear mass kg/mm 2 km 8.89 8.945 2.703 Ultimate MPa 414 621 160-200

tensile stress Modulus of GPa 125 125 70

elasticity

53 9 20 32.5 192 84.8 0.0036 0.0054 0.0051

23 11.5 12.96

2.7 7.8 6.59 295 1320-1700 1100-1344

70 200 162

a 10% by radius aluminium

an aluminium-magnesium-silicon alloy with the BS EN 573 designation EN AWo6101 or EN AW°6201. The main alloying elements of EN AW-6101A, for instance, are limited to the ranges Mg 0.4-0.9%, Si 0.3-0.7%, Fe <0.4% and Cu <0.5%.

Careful control of the composition and processing of the alloy must be maintained throughout the production process to realise the required combination of strength and conductivity. The alloying elements must first be dissolved into solid solution in the base aluminium by a solution heat treatment which involves raising the temperature of the metal to near its melting point, holding the temperature until the elements are in solution, then rapidly quenching so that the magnesium and silicon remain in solid solution in a metastable state. At this stage the metal (which is conventionally 9.5 mm rod with a comparable strength but significantly reduced conductivity compared with 'electrical grade' aluminium) is normally drawn through a set of dies to its final diameter, a process which increases the strength by 'work hardening'. A further heat treatment or artificial ageing process, at a temperature in the range 150-200°C, accelerates the onset of precipitation of the magnesium and silicon as MgESi. As this precipitation begins to occur the conductivity of the metal increases as the impurities come out of solution and the strength initially increases to a maximum before falling away as Mg2Si recombination occurs.

Typical precipitation heat treatment curves are shown in fig. 50.1. These demonstrate the degree of control over the 9.5mm rod composition, processing, subsequent drawing, and heat treatment of the final wire necessary to achieve the desired end result in terms of tensile strength and conductivity.

A variety of combinations of strength and conductivity are specified, reflecting the user's requirements, but in general terms the range of final properties that can be obtained is illustrated in fig. 50.2. The dashed line represents approximately the best balance between mean tensile strength and conductivity that can be obtained; the limiting values of some of the alloys currently in use in Europe are also shown.


380

370

~ " 360

~ a5o

~ 340

_.e 330 .~ C

I - 320

310

Temp. 165"

• .

55

54 ~" 0 _<

s ~

52 ~ : ~ ,~

51 ~

fi0

300 t t I I I I I 49 0 1 2 3 4 5 6 7 8

Ageing time (hours)

Fig. 50.1 Typical heat treatment curves for aluminium alloy wires

E

380

3 6 0 -

3 4 0 -

320 - AL3 (UK)

I I 55 56

300 I I I 51 52 53 54 57

Conduct iv i ty (%IACS)

Fig. 50.2 Relationship between tensile strength and conductivity of heat-treatable aluminium- magnesium-silicon alloys

Standards

International, European, and UK standards for bare overhead conductors at the time of publication are as follows.

IEC 1089 prEN 50182 BS215:1970

Round wire concentric lay overhead electrical stranded conductors Round wire concentric lay overhead electrical stranded conductors Aluminium conductors and aluminium conductors, steel-reinforced Part 1: Aluminium stranded conductors Part 2: Aluminium conductors, steel-reinforced Note: This standard will be replaced by BS EN 50182 on adoption of the European standard

Composite Overhead Conductors 713

BS 3242:1970

BS 7884:1997

Aluminium alloy stranded conductors for overhead power transmission Note: This standard will be replaced by BSEN50182 on adoption of the European standard Specification for copper and copper-cadmium stranded conductors for overhead electric traction and power transmission systems.

Electrical requirements

The key requirements are for adequate current handling capacity, including under short-circuit fault conditions, and acceptable corona discharge performance.

Current carrying capacity The prime requirement for the phase conductors, to carry current with acceptable power loss or voltage drop, is satisfied by the use of the appropriate cross-sectional area of high conductivity metal such as aluminium, aluminium alloy or copper.

The electrical constraint on an earth wire is its ability to carry safely the maximum expected short circuit current and is normally expressed in kA 2 s for a given allowable conductor temperature rise. This short circuit rating is also known as the 'I2t rating'.

The fault current carrying capacity of a conductor can be determined by consideration of the following:

(a) the maximum temperature reached during current transients; (b) the mechanical degradation of the cable caused by the temperature and duration

of the fault; (c) the amplitude, rate of change and frequency of the electrodynamic forces and

bending moments during the fault.

It is conventional for the first criterion to be used when designing a conductor or earth wire although consideration of the second and third criteria is normally addressed during the later testing stages of the cable's development.

Fault current calculations ~ reflect the conversion of the electrical energy dissipated during a fault event into thermal energy with the result that there is an attendant temperature rise in the conductor. In the simplest method of calculation the resistance, heat capacities, and densities of the cable's constituent materials, and the cable dimensions are assumed to remain constant throughout the fault and temperature rise. More exact methods assume quadratic dependencies of these quantities and there are instances recorded of hybrid calculation methods using mixtures of invariants and linear and quadratic dependencies. In general it is assumed that the metal content in conductors will almost instantly reach a temperature peak, and that the non-metallic parts will be heated subsequently by conduction.

An acceptable level of accuracy whilst not being over-complicated may be achieved by assuming temperature-invariant densities, heat capacities, and dimensions, and a linearly varying resistance with temperature. Thus for the conducting material in the cable we define:

A = cross-sectional area (m 2) w = weight (kg/km) s = specific heat capacity (J/°C/kg)

R~ ---resistance at initial temperature (~/km)


a = temperature coefficient of resistance (°C-~) T = duration of fault current (s) t~ = initial temperature before fault (°C) t2 = maximum allowable final temperature after fault I = fault current (r.m.s. if a.c.) (A)

(oc)

Additionally

R(t) = RIll + a( t - h)] (50.1)

The cable fault current rating (usually expressed in units of kA 2 s), or the 'action integral' of the fault may be shown to be given by:

i2 T = w__~s it2 dt R~ Jt, 1 "F Ot(t - - t l )

ws 1 : - - -- loge[1 + a(t2 - tl)] (50.2)

RI c~

Cables which comprise different conductive materials may be catered for quite simply. The values for the different material parameters can be combined for each of the different materials in the cable. If there are n components in the construction then the following expressions hold:

A = E A i i=l

W = - ~ wi i=l

S = ~ wis i i= I

~ Aioti o~= A

i=1

1 R ~ - - - ~

~ 1

(cross-sectional area) (50.3)

(weight) (50.4)

(heat capacity) (50.5)

(thermal coefficient of resistance) (50.6)

(resistance) (50.7)

European utilities tend to limit the maximum fault current temperature to around 200°C primarily to avoid a reduction in tensile strength of the load-bearing elements. In a typical fault the line will be de-energised in a fraction of a second hence most specifications cite a worst-case fault duration of one second. It is important to realise that the fault carrying capacity of any conductor, and therefore the temperature it reaches during the fault, is primarily dependent on the conductivity of the cable.

Lightning Lightning is an associated problem which tends to affect the outer layer of conductor strands. The peak current which may flow during a lightning strike may be up to 200 kA


although its duration tends to be many orders of magnitude shorter than a system fault. The heating effect tends to be localised near the strike site and damage only rarely occurs to individual strands and then only to those less than about 2.5 mm in diameter. Care should be exercised by the line designer to ensure that the conductor supplied is capable of withstanding the severity and frequency of lightning strikes expected for a given area. 2 If an individual strand of the cable is damaged then a preformed repair element can be applied which should remedy the situation and ensure the continued integrity of the cable.

Corona If the electrical field at the surface of a conductor exceeds the dielectric strength of the air then the air will continuously ionise and recombine, a phenomenon known as corona discharge. The discharge is a source of radio interference, audible noise and power loss.

Corona performance of a line is affected by the line-neutral voltage, the geometry of the line in terms of conductor spacing and the use of bundled conductors for each phase, the overall conductor diameter, the freedom of imperfections on the conductor surface, and the atmospheric conditions (temperature, pressure, humidity, pollution).

Mechanical requirements

Conductors must have the mechanical attributes that allow them to be installed etticiently and, once erected, satisfy given limits covering minimum ground clearances, tensile load and vibration under all foreseeable weather conditions.

When a new line is designed the size and strength of the supporting poles or towers can enter the optimisation equation. However, when refurbishing or up-grading lines, the task of the conductor designer is often to provide conductors which can deliver the required electrical and mechanical performance when installed on existing support structures.

Tensile strength The rated tensile strength (RTS) of a conductor is an estimate of the conductor breaking load and is calculated by summing the strength of the component wires. The strength of each component wire is taken as the product of its nominal area and the minimum tensile stress specified for a wire of the given diameter and material. This tensile stress is the ultimate tensile stress (UTS) for all materials except when steel forms a component of a composite conductor. In this case, the strength of the steel, which can only contribute strength corresponding to an elongation compatible with that of the other material(s) of the conductor, is taken to be the specified stress at 1% extension. The calculation of these ratings is best illustrated by some examples.

Example 1 7/3.30 mm AAAC (UK Code 'Hazel') Minimum UTS of wire after stranding = 295 N/mm 2 RTS = (no. of wires) × (nom. wire area) x 295

= 17.7 kN


Example 2 54/7/3.18 mm ACSR (UK Code 'Zebra') Minimum UTS of aluminium wire---165 N/mm 2 Minimum stress @ 1% extn. of steel = 1100 N/mm 2 A1. s trength--(no, of wires)x (nom. wire a r ea )x 165

= 70.7 kN Steel strength = (no. of wires) x (nom. wire area) × 1100

= 61.2kN R T S = 131.9kN

Vibration Wind blowing across overhead conductors leads to the formation of vortices down- stream of the conductor which in turn cause a number of modes of conductor vibration. In steady, low velocity winds of the order of 1 to 15m/s and with the wind approximately at right angles to the line, so-called aeolian vibration may occur at a frequency in the range 3-100 Hz with an amplitude up to the conductor diameter.

When conductors are arranged in bundles, sub-conductor vibration may be induced at a frequency in the range 0.15-3 Hz with an amplitude up to the individual conductor spacing. This vibration is a function of conductor diameter, conductor spacing, wind velocity and angle.

Where there are very long spans, or when snow or ice accretion has modified the conductor profile, right-angle winds of moderate to high speed may cause aerodynamic lift conditions which can lead to low frequency oscillation of several metres amplitude, known as galloping.

Vibration dampers fitted to the line, either close to the supporting structures or incorporated in the bundle spacers, are used to reduce the threat of metal fatigue at suspension and tension fittings. In order to minimise instability, such as susceptibility to 'galloping', a rule of thumb used by the conductor designer is to limit the number of wires in the outer layer to about 24.

Sags and tensions After a conductor is erected the sag between supporting structures must satisfy given ground clearance limits and the tension must remain within given limits to reduce the effects of vibration and to contain the probability of failure for a range of weather conditions.

The equation below links together the effects of temperature, wind and ice loading in a single model. Clearly, however, there are significantly shortcomings in the base assumptions for this model, most critically, the actual characteristics of wind, and the effective drag coefficient of both bare and iced conductors.

T - W2L2EA -- Ti W~L2EA EAa( t - ti) (50.8) 24T 2 24T~

where T = tension in loaded cable (N) Ti = installation tension (N) W = effective cable weight per unit length (N/m) L = span length (m) E = cable modulus (Pa or N/m 2)


A = cable cross-sectional area (m 2) a = cable thermal expansion coefficient (°C-l) t = temperature (°C) ti = installation temperature (°C)

The effective cable weight is given by the resultant load acting on the cable. Normally this is purely due to the effect of gravity. However, if the conductor is iced we must remember to include any additional weight of ice in order to define Wg~v. I f the cable is being loaded by cross-winds then we must incorporate this loading by using vector addition. Hence

where

I.~:2 ~1/2 W = ( W ~ + . ,~.a~ (50.9)

Wwind = ½ CDpV2D (50.10)

and CD = cable drag coefficient (no units) p = density of air (kg/m 3) v = wind speed (m/s)

D = cable diameter (m)

To a first approximation, the sag of a conductor is given by

WL: S a g - 8T (50.11)

where W = conductor weight per unit length (N/m) L = span length (m) T = horizontal conductor tension (N)

As the tension will be a function of the rated tensile strength of the conductor under given conditions to provide a safety factor, this equation demonstrates that for a given span length, the strength:weight ratio of the conductor will be a key factor in determining the sag.

Once erected, the conductor will be subject to changing conditions. These will range from the maximum operating temperature at one extreme, to the lowest temperature and with the worst conditions of ice and wind loading envisaged, at the other extreme. Using values for the modulus of elasticity and coefficient of linear expansion for the conductor, the resulting sags and tensions between these extremes can be modelled. Such a model is used at the design stage to ensure that safety and vibration requirements are met, and during installation to give the initial sag value corresponding to the temperature at that time.

Creep If a single strand of wire is subjected to a constant tensile load, within its elastic limit, an immediate elastic extension will occur. If the load is maintained the extension will continue, although at a slow and ever decreasing rate. This extension, which is inelastic, is called creep.

A high rate of primary creep gives way to a lower rate of secondary creep, which tends to produce a straight line on a log/log scale. In addition to time, the level of creep of a given wire material is dependent on temperature and stress.


1000

f

10 I I I I I I I I ] I I I i l l l l l I I I ~ I I I I I I

o.1 1 lO 10o

Time (hours)

Fig. 50.3 Typical conductor creep characteristics

I I I I l l l

lOO0

A stranded conductor exhibits the same form of creep, but the primary component is increased by the effect of settlement and bedding down of the wires within the structure. A typical graph of creep against time is shown in fig. 50.3.

As the effect of creep on a conductor in service will be to reduce the tension and ground clearance, allowance must be made for it at the time of erection. Pre-stressing the conductor at a relatively high tension for several hours before adjusting the sag can remove a significant amount of the potential creep, but a more common method is to raise the value of the maximum temperature at the design sag and tension modelling stage by an amount equivalent to the creep expected, ensuring that design clearances will be maintained after creep has occurred.

Environmental requirements

Corrosion An overhead conductor in service is exposed to the corrosive effects of the atmosphere which may eventually lead to a reduction in tensile strength and an increase in electrical resistance. Corrosion is worst in locations where sea-salt aerosol and/or industrial halides are present in the air.

In multi-metal conductors galvanic cells may be formed between differing metals. In ACSR conductors, for example, this causes an initial loss of the zinc coating over the steel wires, and is followed by the rapid corrosion of aluminium once it is in direct electrochemical contact with steel. Homogeneous conductors such as all aluminium alloy (AAAC) avoid this form of corrosion but are subject to the formation of crevice corrosion cells. In the presence of moisture these cells can form in the wire interstices where differentials in aeration can exist.

Filling the interstices of the inner layers with a grease compound with a temperature performance which exceeds that of the conductor in service has been found to give effective protection from the above corrosive effects.


COMPOSITE OVERHEAD CONDUCTORS

The concept of incorporating communication transmission media within overhead electrical conductors is a long-standing one, and started with the introduction of copper wires into earth wires 3 long before the widespread use of optical fibres. However, optical fibres offer the dual benefits of immunity from electrical interference and an almost infinite bandwidth which makes them ideal for this application. 4 An Optical Ground Wire (OPGW) is an electrical and mechanical analogue to a conventional high voltage earth wire, but incorporates a number of optical fibres for communications within it. At present typically 24 or 48 fibres are housed in such cables. On lower voltage lines (rated up to 150 kV) it has proven possible to substitute the same type of cable for a phase conductor (OPPW), which is particularly useful in the case of lines not equipped with earth wires, although this type of system is rendered more complex by the difficulties associated with earthing the optical fibres. Other methods of installing fibre optics on power transmission lines have since been developed 5'6 but the OPGW concept remains the solution of choice for most utilities.

OPGW constructions

Different manufacturers supply OPGW employing a variety of ways of packaging the optical elements. Several of these methods have been discussed earlier, but the peculiarities of the OPGW environment have given rise to a number of preferred designs. The first design variant was patented by BICC 7 in 1977 and was based on a conventional loose tube cable of the time. A fully-filled optical core consisting of a number of polymeric loose tubes stranded around a central supporting member and having a layer of tape and a polyethylene sheath was used. This was run into a C-section aluminium alloy tube. Around this tube one or more layers of aluminium, aluminium alloy, or aluminium-clad steel wires were stranded to form the finished OPGW. Figure 50.4 shows the BICC FIBRAL ® OPGW of this type which has been extensively used by the UK's National Grid Company (NGC).

This type of loose tube OPGW has many desirable features. The fibres are fully protected from longitudinal strain up to the strain margin of the cable, which at a value of around 0.6% exceeds all foreseeable extensions of the earth wire during service. Also, the fairly substantial aluminium alloy tube provides a high degree of lateral crush resistance for when the cable passes over pulleys and is clamped during installation. It is currently possible to house up to 48 fibres in this type of cable structure although in theory the number of fibres may be arbitrarily increased either by changing the number of fibres per tube or the number of loose tubes. However, it then becomes increasingly difficult to keep the cable diameter down to a realistic size whilst still maintaining a sufficient strain margin.

Other manufacturers have chosen to produce cables which offer less direct protection from tensile loadings to the optical fibres. One such solution has been to combine several fibres together into a discrete bundle and house this in a helical slot along an extruded aluminium rod which is located at the centre of the cable. 8 An aluminium alloy tube is welded around this central former and then conductor wires are stranded around in a conventional manner. Usually there are six fibres per bundle, and it is possible to have up to six slots, although there is little reason, other than practicality, preventing further increase in the number of fibres.


polyethylene sheathing . ~ . ~ , ~

Fibres located in oriented I ~ polymer tubes ensuring I / ~ "~_~.__-~

~ ' ~ "-- / Aluminium i C-section tube

lUibPr te~ t4o8 i°n tPet ir nCaalt io n a I Polyester tapes and standards moisture-blocking compound

/

J

Metallic section designed as equivalent to existing earth wire, Design parameters include:

12t Modulus Weight UTS

Fig. 50.4 Loose tube OPGW FIBRAL ®

Generally the fibre bundles are laid tightly into their respective slots yielding negligible strain margin, and so it is important to design the cable such that the maximum anticipated service extension will not cause undue levels of crack growth on the optical fibre surface. In order to be confident that this will be so, the use of fibres which have been tested to a higher than normal p roof level may be required. Optical attenuation penalties arising from lateral pressures produced when the cable is subject to strain may be avoided by using upcoated buffered fibres which ensure that small- scale imperfections in the surface of the central spacer are not transferred to the fibres. Some manufacturers use fibre which has special heat resistive coatings to allow their cables to withstand temperatures of up to 300°C under fault conditions, 9-11 although in these circumstances the effects of such high temperatures on the cable's load bearing elements should be considered.12 Figure 50.5 illustrates a typical example of an OPGW employing fibre bundles.

A compromise in terms of the mechanical protection of fibres in an O P G W is one in which the fibres, or fibre units, are stranded around a central member which incorporates an outer layer of soft-compressible material. 13 In such a cable, when the cable comes under tension the fibres are allowed to move radially inwards by deforming the soft layer beneath them. This type of cable is less widespread than the foregoing options and generally has been limited to cables accommodating relatively few fibres.

More recently a new type of OPGW structure has become available. One of the metallic strands in an otherwise conventional earthwire may be replaced by a hollow metal tube containing up to twelve optical fibres. 14 In an earth wire consisting of a significant number of individual strands the reduction in strength and electrical conductivity caused by the elimination of one of the solid strands is minimal, and this means that the OPGW's properties are very similar to the earth wire which it replaces. Figure 50.6 shows an example of such an OPGW.

Usually it is one of the inner layer of steel strands in an ACSR conductor which is replaced by a steel tube. This means that the tube is stranded around the cable ensuring


minium-clad steel wire

,minium alloy tube

,minium slotted spacer

~dle of optical fibres

Fig. 50.5 Slotted rod OPGW

ayer of aluminium alloy wires

fibres ]el

I stainless steel tube

tyer of aluminium-clad steel wires

Fig. 50.6 Generic fibre in steel tube type OPGW

that the fibres are endowed with an adequate strain margin as in a normal loose tube optical cable. Typically eight or twelve conventional optical fibres may be inserted into a typical 2.5 mm diameter tube during manufacture, and a number of metallic strands may be substituted by such tubes. The outer layers of strands are applied as normal, thereby ensuring that the metal tube is not subjected to the effects of lightning strikes.


Occasionally the central metal tube in an ACSR may be replaced by what may be quite a large tube (up to 5 mm diameter), but in this case an extra complication results f rom having to provide an overlength of fibre in order to ensure a significant strain margin.

Such metal tubes are commonly made by forming a strip of metal (usually about 0.1 mm thick) into a tube shape, and then laser-welding the seam. Fibres and a filling compound are incorporated into the tube during this process, and special precautions are taken to protect the fibres from the localised heating effects introduced by the welding operation.

Design considerations

OPGWs are amongst the most technically demanding optical cables to design. They have to be capable of maintaining optical and electrical operation over a wide range of temperatures, typically from -50°C to +80°C, whilst subject to ever-changing wind or ice loadings. In addition, all cable components have to be capable of surviving the effects of system electrical faults and lightning strikes. Furthermore, because such cables are frequently retro-fitted on to existing power lines it is necessary that they have similar characteristics to the conventional earth wires which they replace, and preferably may be installed without a line outage.

Fault currents

For a given level of fault protection it is possible to manipulate the construction of the cable to yield a desired peak temperature and it is thus incorrect to assert that an OPGW which can withstand a temperature of 300°C is any better than one which can withstand 200°C. It is likely that the cable capable of withstanding a higher temperature may be slightly smaller and may exert lower tower loadings on an overhead line.

The exact thermal behaviour of an OPGW during a fault current event has been the topic of several papers ~5-~7 but until recently the temperature which the fibres experience have only been ascertainable by indirect means. No t surprisingly the precise construction of the OPGW will have a bearing on the temperature experienced by the fibres. For example, in an OPGW employing a dielectric core of stranded polymeric

180 ~ ~ ~ ~ . . . . _ . . . . ! i : : - stranded metal tube 1 60

iiii t i i )~, polymeric core " ~?.=.~ 140100120 ................... . ~ '

~- 6 o

4O

2 o •

0 0 50 100 150 2~ 2~ Time (s)

Fi~. 50.7 Fault current characteristics o£ an OPGW


tubes we might expect the heat from the energised conductor strands to be conducted relatively slowly to the fibres, whilst within a metal tube an essentially unhindered thermal pulse might reasonably be expected.

Recently, direct verification of the fibre temperature along its length has become possible by using the Raman optical sensing technique. Figure 50.7 shows a comparison between conventional measurements of temperature by thermocouples inserted between conductor strands and temperatures measured directly by the optical fibre using the distributed sensing method.

OPGWs are not appreciably more prone to lightning damage than ordinary conductors. ~8 The normal design constraints governing the minimum conductor strand diameter to withstand given lightning severities apply.

Mechanical loads The area of environmental conductor loading is extremely complex and a large body of work on the subject exists. ~9 However, there is no easy to use, universally applicable model of conductor behaviour which can be employed. Consequently, a simple model of an aerial conductor's responses to loadings is usually adopted by most cable companies and this is usually designed to take into account the combined effects of temperature, wind and ice loadings on the cable.

Under normal conditions an OPGW is suspended between towers with a tension such that the sag of its catenary is similar to that of an ordinary overhead conductor. Typically this tension is about 15°,/o of the ultimate strength of the cable and the resultant cable strain is of the order of 0.05%. Changes in temperature of the cable result in changes in its length (calculable using the simple composite thermal expansion coefficient described earlier) and associated fluctuations in cable tension. A temperature increase will extend the cable, leading to an associated tension decrease and an increase in sag which may be significant in the event of a fault current. An almost instantaneous temperature rise of 150°C will result in a correspondingly sudden sagging of the cable (up to 0.3%) which places significant demands on the mechanical integrity of the cable and the fibres contained therein. Severe low temperatures, such as those experienced in arctic climes, impose not only increased tensions on the OPGW, but also place great demands on the fibre coatings and other protective materials in the cable. It should be clear that in the case of loose tube cables the tube filling compounds should allow movement of the fibres at both these extreme low temperatures and during high temperature excursions.

Ice loading is usually modelled by effectively altering the weight of the cable to reflect the weight of ice accreted along its length. The effect of such a loading is to increase both the sag and the tension of the cable. Typically cable specifications drawn up by utilities require that a cable be able to operate under the influence of a specified radial thickness of ice. However, seldom is the density and exact form of the ice accretion taken into account. Only rarely will the accretion have a density approaching the 'textbook' figure for ice of 900 kg/m 3 and it is similarly rare for it to assume a uniform thickness around the cable circumference. Consequently, the weight of a real ice accretion approaching the specified radial loading is usually less than the theoretical value expected and so the cable will actually not experience the anticipated increase in sag and tension.

Similarly inadequate is the way in which wind loadings on overhead conductors are specified. This normally takes the form of a single wind speed figure under the influence


4

3.5

~ 3

~ 2.5 I~

~ 2 ._ - ~

o ~ 1.5 . . J

1

0.5

Conductor load increase due to wind

4.5

o ~,.. ,_-~.,"~'~ . . . . . . . .

o , : 1 2 o9:38 12:oo I , : 2 , 16:, 19:12 21:3 oo:oo

Time

Fig. 50.8 Tension of a wind-loaded aerial cable

of which the cable must be able to operate satisfactorily. Design conservatism ensures that this wind speed is assumed to act uniformly along the length of a complete span and at an angle normal to it. This results in a horizontal force (related via the drag coefficient of the cable to the wind speed) acting at right angles to the plane of the original catenary of the cable. This in turn produces cable 'blow-off ' which assumes the form of a catenary projected in the direction of the wind. Any wind incident on an aerial conductor will result in an extension and increase in tension of the cable. Some typical tension data for a cable exposed to wind loading is shown in fig. 50.8.

In addition to these long-term loading effects there exist additional causes of short- term transient loadings which are harder to model. These include aeolian vibration (high frequency, low amplitude oscillations), galloping (low frequency, high amplitude movements), and sleet jump (the sudden relaxation of tension when ice falls off a conductor). Simulation tests of cables help to ensure that these phenomena can be accommodated safely and some mechanical countermeasures to the oscillations have been developed.

Under the worst case combination of environmental conditions anticipated it would be expected that a conductor or OPGW would experience an extension of up to about 0.5%. 20 Thus a typical design strain margin of 0.6% provides ample protection for an OPGW's fibres under the worst conditions envisaged and provides some defence against phenomena such as long term conductor creep.

Historically in the UK the worst case conditions under which an aerial conductor was expected to operate consisted of a combination of a 22 m/s wind speed and a 12.5 mm radial thickness of ice. To apply this condition universally over the country is clearly unrealistic and so considerable effort has been expended in attempting to generate a probabilistic approach to conductor loading. The current state of the art of this approach is embodied in BS 8100 which constitutes a code of practice for lattice towers and masts. Other standards are applicable in this area, e.g. IEC 826. 2~


Installation and accessories OPGWs may be installed in exactly the same way as conventional earth wires although some designs may require slightly more caution. For instance, designs where the optical fibres are housed in a central tube may be prone to damage if excessive lateral pressure is exerted as the cable is passed under tension over a pulley which is too small. If a cable employs aluminium-clad steel wires in its outer layer of wires then it is prudent to avoid excessive scratching of the strand surfaces which in turn might compromise the integrity of the aluminium layer.

It is especially critical to ensure that precautions are taken to prevent the unwinding of wire strands during installation in designs with a single layer of conductor strands. Torsioning of an underlying fibre optic core due to this effect may unacceptably compromise the strain margin of the cable, and so the conventional preventative means are adopted. In multi-layer cables detorsioning can be built into the design.

Most installations of OPGW are carried out as part of the construction of new transmission lines, or as a planned refurbishment of an existing line, and therefore outages are usually available, allowing normal tension stringing to be carried out. However, if there is no other requirement to de-energise the line than to install the new fibre optic link, then the utility may prefer an installation method that can be carried out live-line. Cradle-block stringing has been developed to allow the replacement of an earth wire with an OPGW whilst maintaining full operation and functionality of the transmission system.

When terminating OPGWs at towers it is usual to use preformed deadend grips which minimise the radial crushing forces on the cable which otherwise may damage the fibre optics housed inside. Further protection is afforded by the use of armour rods between the OPGW and the actual pre-formed deadends.

Standards Despite having been installed for more than 15 years there are still no published standards relating specifically to OPGW, although both IEC and IEEE are nearing the publication of their deliberations. The draft IEEE standard, P1138, concerns itself with the construction of composite fibre optic groundwires for use on electric utility power lines and it is intended to help assure the ground wire functionality and maintenance, whilst preserving the integrity of the fibre optics and optical transmission. Suitable standards for the optical and mechanical characteristics of the cable are cited within it, and a range of tests specific to OPGW are described. 22 The IEC draft specification, 1396, is similar, and draws on a range of IEC standards to cover the optical, electrical and mechanical aspects of OPGW design. Various countries and individual utilities have their own relevant standards, for example the U K National Grid Company's standard NGTS 3.8.9. CIGRE have also taken a keen interest in the development of OPGWs, and other means of installing fibre optics on power transmission networks, and have issued a comprehensive planning guide. 23

Optical fibres on energised conductors (OPPW)

The technology of design and manufacture of OPGW has been further developed to enable fibres to be installed within phase conductors (optical phase wire or OPPW). This may be particularly advantageous on towers or wood poles which do not have


earth wires. In addition, as overhead lines invariably have more than one phase conductor the possibility exists of installing many more fibres than if one OPGW or self-supporting cable were used. Live line installation of such systems is not possible and the added cost incurred for outages and the phase-to-earth transition equipment for the fibre at terminations makes these systems less attractive than those employing OPGW, wrap cable or ADSS cable if other considerations are equal. However such cables do share the advantage with OPGW of providing the optical elements with heavy armouring, thereby providing more resistance to gunshot and other abuses than wrap cables and all-dielectric self-supporting cables. Of all aerial cables the OPPW is likely to

Optical fibre splices ~ ~

Optical joint housing [ " I~P, , .~ I-~ I I Live composite conductor Optical cable ' t - ' ,,-~L'~ ~ ~ ' ~ [ ~ B ~ ~!11

Fluid pressure tank

Stress control dng

Individual optical fibres in polymer tubes

Porcelain housing

Oil filling

IO--~ 1 ~ " - ~ ~ Poepdt eic~la~:21 i:r~ ~ 1 \ \ fitting

~ ~ 1 Optical ground cable

Fig. 50.9 An early method which allows the optical core of an OPPW to pass to ground potential 24


Double distribution tie

Helical dead end / /

Reinforcing rods ~

Optical phase conductor

/ Heat-shrinkable termination

i Aluminium binder

Wedgetap off insulator

HOPE conduit

Fig. 50.10 Two phase to earth transitions employed in the Isle of Man on a 33kV wood pole route 25

run the hottest. This varies from site to site, but careful consideration must be given to the choice of materials in the optical core if the cable is to run at 65°C or above for many years or decades.

In addition to their use for providing telecoms, embedded fibres have been used directly to monitor the temperature of overhead line conductors much as they have for underground cables. Knowledge of temperature allows the conductors to be driven closer to their thermal limits than they would otherwise be thereby allowing the distribution network to be driven closer to its operation limits. Interest in such measurement for overhead lines has principally be confined to urban Japan where population densities are very high. In general there is currently little interest in this application in Europe.

The principal challenge in designing a system based on OPPW is the need to bring the optical fibres to earth potential for connection to other cables, terminal equipment and so forth. The technical complexity of this increases considerably with system voltage. BICC were the first company to develop such systems for their F IBRA L ® cable. 24 In this system an oil-filled porcelain housing was used to effect the transition for systems for up to 400 kV. One such design is shown in fig. 50.9. More modern designs tend to be specifically engineered for a particular system voltage. Figure 50.10 shows a design (necessary for jointing two lengths of cable at ground potential) for a 33 kV wood pole route installed on the Isle of Man. This design used heat-shrink polymeric fittings and is more typical of modern systems. 25 An alternative design using more traditional techniques for a 26 kV line is described by Mercier et al. 26

Joints between the fibres of two lengths of OPPW are made at phase voltage wherever possible. This reduces the cost and complexity of the joints, 27-29 but is only possible on large structures where the joint box can be satisfactorily mounted and supported.


This essentially limits phase-voltage jointing to lattice towers and high voltage networks. In other cases back-to-back phase/earth transitions are made and the cable is jointed underground.

There is clearly a need for the optical phase wire to have similar electrical characteristics to the phase conductor it is replacing. This is readily achieved using combinations of metals and strand sizes available to design engineers. Issues of lightning resistance and fault current ratings are addressed in exactly the same manner as for OPGW.

REFERENCES

(1) Morgan, V. (1991) Thermal Behaviour of Electrical Conductors. John Wiley. (2) Bonicel, J. et al. (1995) 'Lightning strike resistance of OPGW'. Proc. 44th lnt. Wire

and Cable Symp., pp. 800-806. (3) Dagef6rde, H. et al. (1972) 'ASCR-Aluminium steel communication rope'. Proc.

21st Int. Wire and Cable Syrup., pp. 253-268. (4) Maddock, B. et al. (1980) 'Optical fibre communication using overhead trans-

mission lines'. CIGRE paper 35-01. (5) Coulson, A. (1992) 'High power communications'. Electrical Power Engineering,

222-223. (6) Haywood, B. and Knight, I. (1995) 'Aerial fibre optic systems'. Distribution 2000,

Brisbane. (7) GB patent 1598438 filed 13 May 1977. (8) Ghannoum, E. et al. (1995) 'Optical ground wire for Hydro-Quebec's

telecommunication network'. Proc. IEEE/Power Engineering Society Winter Meeting, New York.

(9) Okuyama, S. et al. (1986) 'High heat-resistant optical fiber coated with thermal- cured type silicone and fluorine polymer'. Proc. 35th lnt. Wire and Cable Syrup., pp. 183-188.

(10) Araki, S., Shimomichi, T. and Suzuki, H. (1988) 'A new heat resistant optical fiber with special coating', Proc. 37th lnt. Wire and Cable Symp., pp. 745-750.

(11) Biswas, D. (1990) 'High temperature optical and mechanical properties of polyimide coated fibres'. Proc. 39th Int. Wire and Cable Symp., pp. 722-725.

(12) Ash, D. et al. (1979) 'Conductor systems for overhead lines: some considerations in their selection'. Proc. lEE 126 (4), 333-341.

(13) Sato, T. and Kawasaki, M. (1987) 'Optimum design of composite fiber-optic overhead ground wire (OPGW)'. IEEE/CSEE Joint Conf. on High Voltage Transmission Systems in China, pp. 393-398.

(14) Schneider, J., Schmelter, J. and Herff, R. (1988) 'Optical ground wire design with a minimum of dielectrics'. Proc. 37th lnt. Wire and Cable Symp., pp. 83-92.

(15) Madge, R., Barett, J. and Grad, H. (1989) 'Performance of optical ground wires during fault current tests'. Proc. 1EEE/Power Engineering Society Winter Meting, New York, pp. 1552-1559.

(16) Madge, R., Barett, J. and Maurice, C. (1992) 'Considerations for fault current testing of optical ground wire'. Proc. IEEE/Power Engineering Society Winter Meeting, New York, pp. 1786-1792.


(17) Tanaka, S. et al. (1992) 'Analysis of OPGW short-circuit characteristics'. Sumitomo Electrical Technical Review 33, 147-153.

(18) Carter, C., Baldwin, R. and Jones, C. (1984) 'Lightning simulation tests on power transmission conductors carrying embedded optical communications cable', lEE Conf. on Lightning and Power Systems, IEE Conf. Pub. No. 236, 207-209.

(19) ASCE (1984) Guidelines for Transmission Line Structural Loading. (20) Dey, P. et al. (1982) 'Optical communication using overhead power transmission

lines'. CIGRE Conf. on Large High l~oltage Electric Systems, Paris. (21) Orawski, G. (Sept. 1991) 'Overhead lines - loading and strength: the probabilistic

approach viewed internationally'. Power Engineering Journal, 221-232. (22) Dawson, J. (1991) 'Development of test methods for OPGW'. Wire Journal lnt.,

58-60. (23) Martin, J. (1993) 'Optical fibre planning guide for power utilities'. CIGRE Paper

No. 35-04. (24) Dey, P., Gaylard, B., Holden, G., Taylor, J. E., Smith, P., Carter, C. N.,

Maddock, B. J. and Kent, A. J. (1984) 'Optical communication using overhead power transmission lines'. Elektron, 38-44.

(25) Friday, A., Smart, T. J., Evans, J. and Bevan, J. (1995) 'Design, development and installation of an optical phase conductor (OPPC) on a 33 kV wood pole line'. CIRED 13th lnt. Conf. on Electricity Distribution, 11/1-6.

(26) Mercier, E., Rosset, C. and Reber, H. (1991) 'Phase conductor of an overhead line with optical fibres'.CIRED l l th Int. Conf., paper 3.15.

(27) Amerpohl, U. and Bausch, J. (1992) 'Phasenseile mit LWL'. TEZ 113, 368-371. (28) Amerpohl, U. and Bausch, J. (1993) 'Erstes Phasenseilprojekt mit Lichtwellenlei-

tern in Ostereich - ein Erfahrungsbericht.' OZE 46, 373-377. (29) Znoyek, G. (1993) 'LWL-Phasenseile fur 20kV und 110 kV Freileitungen'. E T Z

114, 636-640.

Composite Overhead Conductors

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