Design overhead distribution systems
Design overhead distribution systems
Design overhead distribution systems
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Electrical, Electronics<br />
Engineering Department<br />
<strong>Design</strong> <strong>overhead</strong><br />
<strong>distribution</strong> <strong>systems</strong><br />
UETTDRDS05B<br />
(Vol 1 of 2)<br />
Version No 1 2 3 4 5<br />
Date 10/2005 06/2009 05/2010<br />
Refer to: DMcR DK DK<br />
Chisholm Institute of TAFE<br />
Dandenong Campus<br />
Stud Road<br />
DANDENONG 3175<br />
Tel: +61 3 9212 5200<br />
Fax: +61 3 9212 5232
DESIGN OVERHEAD DISTRIBUTION SYSTEMS<br />
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2
DESIGN OVERHEAD DISTRIBUTION SYSTEMS<br />
Contents<br />
Electric power transmission ....................................................................................................... 5<br />
Overhead transmission .......................................................................................................... 7<br />
Structures .............................................................................................................................. 8<br />
Conductors ............................................................................................................................ 9<br />
Train power .......................................................................................................................... 10<br />
Further applications.............................................................................................................. 10<br />
Usage of area under <strong>overhead</strong> power lines .......................................................................... 11<br />
Electricity Power Supply Systems ............................................................................................ 12<br />
Distribution Systems and Voltage Levels ............................................................................. 12<br />
Types of <strong>distribution</strong> feeder <strong>systems</strong> .................................................................................... 14<br />
Revision exercise 1 ................................................................................................................. 17<br />
Overhead and Underground Power Systems ....................................................................... 18<br />
Overhead Insulated Conductors ........................................................................................... 19<br />
Revision exercise 2 ................................................................................................................. 19<br />
Three Phase, Single Phase and Single Wire Earth Return Systems ........................................ 20<br />
Revision Exercise 3 ................................................................................................................. 26<br />
Revision Exercise 4 ................................................................................................................. 28<br />
Revision exercise 5 ................................................................................................................. 31<br />
Terminology ......................................................................................................................... 33<br />
Revision exercise 6 ................................................................................................................. 34<br />
Revision exercise 7 ................................................................................................................. 39<br />
Equipment used in the control of voltage ................................................................................. 40<br />
Types of voltage regulators .................................................................................................. 42<br />
Revision exercise 8 ................................................................................................................. 47<br />
Revision exercise 9 ................................................................................................................. 51<br />
Wires and Cables .................................................................................................................... 53<br />
3
DESIGN OVERHEAD DISTRIBUTION SYSTEMS<br />
MODULE TITLE <strong>Design</strong> <strong>overhead</strong> <strong>distribution</strong> <strong>systems</strong><br />
Nominal Duration 40 hours<br />
Module Code or Number UETTDRDS05A<br />
T2.2.2 Transmission, <strong>distribution</strong> and rail power <strong>systems</strong><br />
T2.2.4 Powerline <strong>distribution</strong> installation<br />
T2.4.6 High voltage SWER system<br />
List of essential resources<br />
- Voltage regulation relay - typical working sample<br />
- Line drop compensator<br />
- Phantom line circuit - simulating a typical transmission or <strong>distribution</strong> feeder<br />
- Sample VT and CT<br />
- Local <strong>distribution</strong> system diagrams - with fault levels indicated<br />
- Low voltage AC supply<br />
- Extra low voltage AC supply<br />
- Low voltage DC supply<br />
Items such as the voltage regulation relay, line drop compensator and VT may be difficult to<br />
obtain, student exposure to this equipment may be obtained with the aid of industrial visits.<br />
4
DESIGN OVERHEAD DISTRIBUTION SYSTEMS<br />
Electric power transmission<br />
Electric power transmission is the bulk transfer of electrical energy, a process in the delivery of<br />
electricity to consumers.<br />
A power transmission network typically connects power plants to multiple substations near a<br />
populated area.<br />
The wiring from substations to customers is referred to as electricity <strong>distribution</strong>, following the<br />
historic business model separating the wholesale electricity transmission business from<br />
distributors who deliver the electricity to the homes.<br />
Electric power transmission allows distant energy sources (such as hydroelectric power plants) to<br />
be connected to consumers in population centres, and may allow exploitation of low-grade fuel<br />
resources such as coal that would otherwise be too costly to transport to generating facilities.<br />
5
DESIGN OVERHEAD DISTRIBUTION SYSTEMS<br />
Transmission lines<br />
Usually transmission lines use three phase alternating current (AC). Single phase AC current is<br />
sometimes used in a railway electrification system. High-voltage direct current <strong>systems</strong> are used<br />
for long distance transmission, or some undersea cables, or for connecting two different ac<br />
networks.<br />
Electricity is transmitted at high voltages (110 kV or above) to reduce the energy lost in<br />
transmission.<br />
Power is usually transmitted as alternating current through <strong>overhead</strong> power lines. Underground<br />
power transmission is used only in densely populated areas because of its higher cost of<br />
installation and maintenance when compared with <strong>overhead</strong> wires, and the difficulty of voltage<br />
control on long cables.<br />
A power transmission network is referred to as a "grid". Multiple redundant lines between points<br />
on the network are provided so that power can be routed from any power plant to any load center,<br />
through a variety of routes, based on the economics of the transmission path and the cost of<br />
power.<br />
Much analysis is done by transmission companies to determine the maximum reliable capacity of<br />
each line, which, due to system stability considerations, may be less than the physical or thermal<br />
limit of the line.<br />
Deregulation of electricity companies in many countries has led to renewed interest in reliable<br />
economic design of transmission networks.<br />
A power station (also referred to as a generating station, power plant, or powerhouse) is an<br />
industrial facility for the generation of electric power.<br />
Power plant is also used to refer to the engine in ships, aircraft and other large vehicles. Some<br />
prefer to use the term energy centre because it more accurately describes what the plants do,<br />
which is the conversion of other forms of energy, like chemical energy, gravitational potential<br />
energy or heat energy into electrical energy. However, power plant is the most common term in<br />
the U.S., while elsewhere power station and power plant are both widely used, power station<br />
prevailing in many Commonwealth countries and especially in the United Kingdom.<br />
At the center of nearly all power stations is a generator, a rotating machine that converts<br />
mechanical energy into electrical energy by creating relative motion between a magnetic field and<br />
a conductor.<br />
The energy source harnessed to turn the generator varies widely. It depends chiefly on which<br />
fuels are easily available and on the types of technology that the<br />
power company has access to.<br />
Hydroelectric Station Steam Electric Station<br />
6
DESIGN OVERHEAD DISTRIBUTION SYSTEMS<br />
Overhead transmission<br />
Overhead conductors are not covered by insulation.<br />
The conductor material is nearly always an aluminium alloy, made into several strands and<br />
possibly reinforced with steel strands.<br />
Copper was sometimes used for <strong>overhead</strong> transmission but aluminium is lower in weight for<br />
equivalent performance, and much lower in cost.<br />
Overhead conductors are a commodity supplied by several companies worldwide. Improved<br />
conductor material and shapes are regularly used to allow increased capacity and modernize<br />
transmission circuits.<br />
Conductor sizes range from 12 mm² (#6 American wire gauge) to 750 mm² (1,590,000 circular<br />
mils area), with varying resistance and current-carrying capacity.<br />
Thicker wires would lead to a relatively small increase in capacity due to the skin effect, that<br />
causes most of the current to flow close to the surface of the wire.<br />
Today, transmission-level voltages are usually considered to be 110 kV and above.<br />
Lower voltages such as 66 kV and 33 kV are usually considered sub-transmission voltages but<br />
are occasionally used on long lines with light loads.<br />
Voltages less than 33 kV are usually used for <strong>distribution</strong>.<br />
Voltages above 230 kV are considered extra high voltage and require different designs compared<br />
to equipment used at lower voltages.<br />
Since <strong>overhead</strong> transmission lines are uninsulated, design of these lines requires minimum<br />
clearances to be observed to maintain safety.<br />
Adverse weather conditions of high wind and low temperatures can lead to power outages: wind<br />
speeds as low as 23 knots (43 km/h) can permit conductors to encroach operating clearances,<br />
resulting in a flashover and loss of supply.<br />
Oscillatory motion of the physical line can be termed gallop or flutter depending on the frequency<br />
and amplitude of oscillation.<br />
An <strong>overhead</strong> power line is an electric power transmission line suspended by towers or poles.<br />
Since most of the insulation is provided by air, <strong>overhead</strong> power lines are generally the lowest-cost<br />
method of transmission for large quantities of electric power.<br />
Towers for support of the lines are made of wood (as-grown or laminated), steel (either lattice<br />
structures or tubular poles), concrete, aluminum, and occasionally reinforced plastics. The bare<br />
wire conductors on the line are generally made of aluminum (either plain or reinforced with steel<br />
or sometimes composite materials), though some copper wires are used in medium-voltage<br />
<strong>distribution</strong> and low-voltage connections to customer premises.<br />
The invention of the strain insulator was a critical factor in allowing higher voltages to be used. At<br />
the end of the 19th century, the limited electrical strength of telegraph-style pin insulators limited<br />
the voltage to no more than 69,000 volts.<br />
Today <strong>overhead</strong> lines are routinely operated at voltages exceeding 765,000 volts between<br />
conductors, with even higher voltages possible in some cases.<br />
Overhead power transmission lines are classified in the electrical power industry by the range of<br />
voltages:<br />
Low voltage – less than 1000 volts, used for connection between a residential or small<br />
commercial customer and the utility.<br />
Medium Voltage (Distribution) – between 1000 volts (1 kV) and to about 33 kV, used for<br />
<strong>distribution</strong> in urban and rural areas.<br />
High Voltage (Subtransmission if 33-115kV and transmission if 115kV+) – between 33 kV and<br />
about 230 kV, used for sub-transmission and transmission of bulk quantities of electric power and<br />
connection to very large consumers.<br />
7
DESIGN OVERHEAD DISTRIBUTION SYSTEMS<br />
Extra High Voltage (Transmission) – over 230 kV, up to about 800 kV, used for long distance,<br />
very high power transmission.<br />
Ultra High Voltage – higher than 800 kV.<br />
Lines classified as "High voltage" are quite hazardous. Direct contact with (touching) energized<br />
conductors still present a risk of electrocution.<br />
A major goal of <strong>overhead</strong> power line design is to maintain adequate clearance between energized<br />
conductors and the ground so as to prevent dangerous contact with the line.<br />
This is extremely dependent on the voltage the line is running at.<br />
Structures<br />
Structures for <strong>overhead</strong> lines take a variety of shapes depending on the type of line. Structures<br />
may be as simple as wood poles directly set in the earth, carrying one or more cross-arm beams<br />
to support conductors, or "armless" construction with conductors supported on insulators attached<br />
to the side of the pole.<br />
Tubular steel poles are typically used in urban areas. High-voltage lines are often carried on<br />
lattice-type steel towers or pylons. For remote areas, aluminium towers may be placed by<br />
helicopters.<br />
Concrete poles have also been used. Poles made of reinforced plastics are also available, but<br />
their high cost restricts application.<br />
Each structure must be designed for the loads imposed on it by the conductors.<br />
A large transmission line project may have several types of towers, with "tangent" ("suspension"<br />
or "line" towers, UK) towers intended for most positions and more heavily constructed towers<br />
used for turning the line through an angle, dead-ending (terminating) a line, or for important river<br />
or road crossings.<br />
Depending on the design criteria for a particular line, semi-flexible type structures may rely on the<br />
weight of the conductors to be balanced on both sides of each tower.<br />
More rigid structures may be intended to remain standing even if one or more conductors is<br />
broken.<br />
Such structures may be installed at intervals in power lines to limit the scale of cascading tower<br />
failures.<br />
Foundations for tower structures may be large and costly, particularly if the ground conditions are<br />
poor, such as in wetlands.<br />
Each structure may be considerably strengthened by the use of guy wires to resist some of the<br />
forces due to the conductors.<br />
Power lines and supporting structures can be a form of visual pollution. In some cases the lines<br />
are buried to avoid this, but this is more expensive and therefore not usual.<br />
Insulators<br />
Insulators must support the conductors and withstand both the normal operating voltage and<br />
surges due to switching and lightning. Insulators are broadly classified as either pin-type, which<br />
support the conductor above the structure, or suspension type, where the conductor hangs below<br />
the structure. Up to about 33 kV (69 kV in North America) both types are commonly used.<br />
At higher voltages only suspension-type insulators are common for <strong>overhead</strong> conductors.<br />
Insulators are usually made of wet-process porcelain or toughened glass, with increasing use of<br />
glass-reinforced polymer insulators.<br />
Suspension insulators are made of multiple units, with the number of unit insulator disks<br />
increasing at higher voltages.<br />
The number of disks is chosen based on line voltage, lightning withstand requirement, altitude,<br />
and environmental factors such as fog, pollution, or salt spray. Longer insulators, with longer<br />
creepage distance for leakage current, are required in these cases. Strain insulators must be<br />
strong enough mechanically to support the full weight of the span of conductor, as well as loads<br />
due to ice accumulation, and wind.<br />
Porcelain insulators may have a semi-conductive glaze finish, so that a small current (a few<br />
milliamperes) passes through the insulator.<br />
8
DESIGN OVERHEAD DISTRIBUTION SYSTEMS<br />
This warms the surface slightly and reduces the effect of fog and dirt accumulation. The<br />
semiconducting glaze also insures a more even <strong>distribution</strong> of voltage along the length of the<br />
chain of insulator units.<br />
Insulators for very high voltages, exceeding 200 kV, may have grading rings installed at their<br />
terminals.<br />
This improves the electric field <strong>distribution</strong> around the insulator and makes it more resistant to<br />
flash-over during voltage surges.<br />
Conductors<br />
Aluminum conductors reinforced with steel (known as ACSR) are primarily used for medium and<br />
high voltage lines and may also be used for <strong>overhead</strong> services to individual customers. Aluminum<br />
conductors are used as it has the advantage of better resistivity/weight than copper, as well as<br />
being cheaper. Some copper cable is still used, especially at lower voltages and for grounding.<br />
While larger conductors may lose less energy due to lower electrical resistance, they are more<br />
costly than smaller conductors.<br />
An optimization rule called Kelvin's Law states that the optimum size of conductor for a line is<br />
found when the cost of the energy wasted in the conductor is equal to the annual interest paid on<br />
that portion of the line construction cost due to the size of the conductors.<br />
The optimization problem is made more complex due to additional factors such as varying annual<br />
load, varying cost of installation, and by the fact that only definite discrete sizes of cable are<br />
commonly made.<br />
Since a conductor is a flexible object with uniform weight per unit length, the geometric shape of a<br />
conductor strung on towers approximates that of a catenary.<br />
The sag of the conductor (vertical distance between the highest and lowest point of the curve)<br />
varies depending on the temperature.<br />
A minimum <strong>overhead</strong> clearance must be maintained for safety.<br />
Since the temperature of the conductor increases with increasing heat produced by the current<br />
through it, it is sometimes possible to increase the power handling capacity (uprate) by changing<br />
the conductors for a type with a lower coefficient of thermal expansion or a higher allowable<br />
operating temperature.<br />
Bundled conductors are used for voltages over 200 kV to avoid corona losses and audible noise.<br />
Bundle conductors consist of several conductor cables connected by non-conducting spacers.<br />
For 220 kV lines, two-conductor bundles are usually used, for 380 kV lines usually three or even<br />
four. American Electric Power is building 765 kV lines using six conductors per phase in a bundle.<br />
Spacers must resist the forces due to wind, and magnetic forces during a short-circuit.<br />
Overhead power lines are often equipped with a ground conductor (shield wire or <strong>overhead</strong> earth<br />
wire).<br />
A ground conductor is a conductor that is usually grounded (earthed) at the top of the supporting<br />
structure to minimise the likelihood of direct lightning strikes to the phase conductors.<br />
The ground wire is also a parallel path with the earth for fault currents in earthed neutral circuits.<br />
Very high-voltage transmission lines may have two ground conductors.<br />
These are either at the outermost ends of the highest cross beam, at two V-shaped mast points,<br />
or at a separate cross arm.<br />
Older lines may use surge arrestors every few spans in place of a shield wire, this configuration is<br />
typically found in the more rural areas of the United States.<br />
By protecting the line from lightning, the design of apparatus in substations is simplified due to<br />
lower stress on insulation.<br />
Shield wires on transmission lines may include optical fibers (OPGW), used for communication<br />
and control of the power system.<br />
Medium-voltage <strong>distribution</strong> lines may have the grounded conductor strung below the phase<br />
conductors to provide some measure of protection against tall vehicles or equipment touching the<br />
energized line, as well as to provide a neutral line in Wye wired <strong>systems</strong>.<br />
9
DESIGN OVERHEAD DISTRIBUTION SYSTEMS<br />
While <strong>overhead</strong> lines are usually bare conductors, rarely <strong>overhead</strong> insulated cables are used,<br />
usually for short distances (less than a kilometer). Insulated cables can be directly fastened to<br />
structures without insulating supports.<br />
An <strong>overhead</strong> line with bare conductors insulated by air is typically less costly than a cable with<br />
insulated conductors.<br />
A more common approach is "covered" line wire. It is treated as bare cable, but often is safer for<br />
wildlife, as the insulation on the cables increases the likelihood of a large wing-span raptor to<br />
survive a brush with the lines, and reduces the overall danger of the lines slightly. These types of<br />
lines are often seen in the eastern United States and in heavily wooded areas, where tree-line<br />
contact is likely.<br />
The only pitfall is cost, as insulated wire is often costlier than its bare counterpart.<br />
Many utility companies implement covered line wire as jumper material where the wires are often<br />
closer to each other on the pole, such as an underground riser/Pothead, and on reclosers, cutouts<br />
and the like.<br />
Aerial bundled cable<br />
Low voltage <strong>overhead</strong> lines may use either bare conductors carried<br />
on glass or ceramic insulators or an aerial bundled cable system.<br />
The number of conductors may be anywhere between four (three<br />
phase plus a combined earth/neutral conductor - a TN-C earthing<br />
system) up to as many as six (three phase conductors,<br />
separate neutral and earth plus street lighting supplied by a<br />
common switch).<br />
Train power<br />
Overhead lines or <strong>overhead</strong> wires are used to transmit electrical energy to trams, trolleybuses or<br />
trains.<br />
Overhead line is designed on the principle of one or more <strong>overhead</strong> wires situated over rail tracks.<br />
Feeder stations at regular intervals along the <strong>overhead</strong> line supply power from the high voltage<br />
grid. For some cases low-frequency AC is used, and distributed by a special traction current<br />
network.<br />
Further applications<br />
Overhead lines are also occasionally used to supply transmitting antennas, especially for efficient<br />
transmission of long, medium and short waves.<br />
For this purpose a staggered array line is often used.<br />
Along a staggered array line the conductor cables for the supply of the earth net of the<br />
transmitting antenna are attached on the exterior of a ring, while the conductor inside the ring, is<br />
fastened to insulators leading to the high voltage standing feeder of the antenna.<br />
10
DESIGN OVERHEAD DISTRIBUTION SYSTEMS<br />
Usage of area under <strong>overhead</strong> power lines<br />
Use of the area below an <strong>overhead</strong> line is restricted because objects must not come too close to<br />
the energized conductors. Overhead lines and structures may shed ice, creating a hazard.<br />
Radio reception can be impaired under a power line, due both to shielding of a receiver antenna<br />
by the <strong>overhead</strong> conductors, and by partial discharge at insulators and sharp points of the<br />
conductors which creates radio noise.<br />
In the area surrounding <strong>overhead</strong> lines it is dangerous to risk interference; e.g. flying kites or<br />
balloons, using ladders or operating machinery.<br />
Overhead <strong>distribution</strong> and transmission lines near airfields are often marked on maps, and the<br />
lines themselves marked with conspicuous plastic reflectors, to warn pilots of the presence of<br />
conductors.<br />
Construction of <strong>overhead</strong> power lines, especially in wilderness areas, may have significant<br />
environmental effects.<br />
Environmental studies for such projects may consider the effect of brush clearing, changed<br />
migration routes for migratory animals, possible access by predators and humans along<br />
transmission corridors, disturbances of fish habitat at stream crossings, and other effects.<br />
11
DESIGN OVERHEAD DISTRIBUTION SYSTEMS<br />
Electricity Power Supply Systems<br />
The generation and supply of electrical energy to customers is an important part of the community<br />
in which we live to-day.<br />
Within Australia, this is generally separated into the sections of generation, transmission (with<br />
subtransmission) and <strong>distribution</strong>. Figure 1 shows a simple sketch of a system to help you<br />
visualise these various sections.<br />
In most states, the management and control of generation transmission and <strong>distribution</strong> are being<br />
separated as a matter of governmental policy, to provide competition between various generating<br />
authorities and supply authorities and to improve service.<br />
As shown in Figure 1, the extent of voltages falling within the responsibility of supply authorities is<br />
generally 132kV and below, with some 66kV in rural areas.<br />
Another aspect of the supply of electrical energy to customers is the increase in supplementary<br />
generation, sometimes by individual customers.<br />
This can be either local generation or cogeneration, both of which are operated in conjunction<br />
with the supply from the transmission system or the supply authority.<br />
The many features of cogeneration (which includes heat recovery from spent fuel) are covered in<br />
another subject.<br />
Distribution Systems and Voltage Levels<br />
Various Voltage Levels in Distribution<br />
In older <strong>systems</strong> the supply authority collects the bulk energy at 66 kV or less from the<br />
transmission substation.<br />
As indicated in Figure 1, there are specific voltage values used in the <strong>distribution</strong> of electrical<br />
power.<br />
These voltage values, which are all ‘line to line’ values are 66kV, 22kV, 11kV, 6.6kV and<br />
400/230V.<br />
Some of these values are rarely used in public <strong>distribution</strong> networks but are common in private<br />
networks in large industrial sites (eg 3.3kV, 6.6kV).<br />
Considered in its most simple form, electric power <strong>distribution</strong> consists of two main elements - (i)<br />
the retail function, or buying and selling of electrical energy and (ii) the <strong>distribution</strong> function, which<br />
is the transport and dissemination of the power.<br />
These functions have been recognised as being quite separate and financially separated from<br />
one another in the new competitive electricity market which has been set up in Australia in recent<br />
years.<br />
As electricity is not ‘stored’, many aspects of <strong>distribution</strong> are influenced by minimising the costs<br />
associated with the (instantaneous) ‘buy and sell’ operation.<br />
The choice of voltage to be used on any particular section in the <strong>distribution</strong> system will be<br />
influenced, among other factors, by:<br />
• decisions associated with voltage drops resulting from large current loads<br />
• capital cost of transformers used to change voltage levels<br />
• capital costs of construction of <strong>distribution</strong> lines and associated switchgear to operate at<br />
the chosen voltage<br />
• environmental aspects of the system installation.<br />
12
DESIGN OVERHEAD DISTRIBUTION SYSTEMS<br />
G Y<br />
TRANSMISSION<br />
500 or 330 kV<br />
Y Y Y Y Y<br />
Terminal<br />
station<br />
transformer<br />
with on-load<br />
tap changer<br />
SUBTRANSMISSION<br />
66 kV<br />
Zone<br />
substation<br />
transformer<br />
with on-load<br />
tap changer<br />
DISTRIBUTION<br />
22, 11 or 6.6 kV<br />
Figure 1. A simple power system<br />
13<br />
Y<br />
R<br />
Distribution transformer with<br />
off-load tap changer<br />
400 / 230 V<br />
<strong>distribution</strong><br />
Regulator<br />
to boost<br />
<strong>distribution</strong><br />
HV where<br />
required<br />
22, 11 or 6.6 kV<br />
<strong>distribution</strong>
DESIGN OVERHEAD DISTRIBUTION SYSTEMS<br />
For any given electrical load (in kVA), the higher the load voltage the lower will be the resultant<br />
current required.<br />
As it is the current flowing in the supply cabling which creates the voltage drop and the heating<br />
(I2R) loss, to minimise losses we try to keep the values of line current as low as possible. In this<br />
way the necessary <strong>distribution</strong> line voltage level can be determined, along with the resultant cost<br />
of constructing the line.<br />
This explains why bulk generation and transmission, where large quantities of power are<br />
produced in big ‘base-load’ generation stations and transmitted to major load centres, is done at<br />
very high voltages.<br />
This also explains why, with the exception of the low voltage <strong>distribution</strong> network, where voltages<br />
are set at 230 volts single phase/400 volts three phase, the highest voltages practical are used<br />
throughout all <strong>distribution</strong>, subtransmission and transmission <strong>systems</strong>.<br />
Because of the large cost differential between having electricity cables (conductors) <strong>overhead</strong>,<br />
suspended on insulators fixed to poles, or underground, in heavily insulated cables, many<br />
<strong>distribution</strong> <strong>systems</strong> are <strong>overhead</strong>, this being the cheaper system.<br />
However, current design tends toward placing <strong>distribution</strong> assets underground for both aesthetic<br />
and practical reasons – communities prefer a “cleaner” skyline and underground assets are less<br />
likely to be damaged (car accidents, building site cranes, etc).<br />
Types of <strong>distribution</strong> feeder <strong>systems</strong><br />
Radial<br />
Many <strong>distribution</strong> <strong>systems</strong> operate using a ‘radial feeder’ system. A typical radial feeder system is<br />
shown schematically in Figure 2.<br />
CB<br />
Zone substation 22, 11 or 6.6 kV bus<br />
CB CB CB<br />
CB<br />
22, 11 or 6.6 kV radial feeders<br />
Figure 2. Radial feeder system<br />
Radial feeders are the simplest and least expensive, both to construct and for their protection<br />
system.<br />
This advantage however is offset by the difficulty of maintaining supply in the event of a fault<br />
occurring in the feeder.<br />
A fault would result in the loss of supply to a number of customers until the fault is located and<br />
cleared.<br />
The next level of reliability is given by a ‘parallel feeder’ system.<br />
14<br />
Circuit<br />
breakers
DESIGN OVERHEAD DISTRIBUTION SYSTEMS<br />
Parallel feeders<br />
A greater level of reliability at a higher cost is achieved with a parallel feeder. A typical parallel<br />
feeder system is shown schematically in Figure 3.<br />
In the event of a line fault only one of the feeder sets of cables will be affected, thus allowing the<br />
remaining parallel feeder to continue to supply the load.<br />
To improve the reliability factor it may be possible to have the separate sets of cables follow<br />
different routes.<br />
In this case the capital cost is double that of a radial feeder but there is a greater reliability factor<br />
for the line.<br />
This may be justified if the load is higher, more customers are being supplied, or there are loads<br />
such as hospitals which require high levels of reliability.<br />
Parallel feeders are more common in urban areas or for feeders to large single customers, where<br />
load shedding in an emergency may be possible.<br />
Typically, a<br />
large<br />
industrial<br />
customer<br />
CB Subtransmission 66 kV bus<br />
CB<br />
Y<br />
Y<br />
CB<br />
66/22, 11 or 6.6 kV transformers<br />
Zone substation 22, 11 or 6.6 kV bus<br />
CB CB CB<br />
CB<br />
22, 11 or 6.6 kV parallel feeders usually<br />
routed from substation to load centre using<br />
different routes.<br />
Figure 3. Parallel feeder system<br />
15<br />
Y<br />
Y<br />
CB<br />
Circuit<br />
breakers
DESIGN OVERHEAD DISTRIBUTION SYSTEMS<br />
Ring main<br />
A similar level of system reliability to that of the parallel arrangement can be achieved by using<br />
‘ring main’ feeders.<br />
This usually results from the growth of load supplied by a parallel feeder where the cabling has<br />
been installed along different routes.<br />
These are most common in urban and industrial environments.<br />
Whilst the start and finish ends of the ring are at the same location, power is delivered by both<br />
pathways of the ring into substations located around the ring.<br />
Should a fault occur on a feeder cable at any point around the ring the faulty section may be<br />
isolated by the operation of the protecting circuit breakers, at the same time maintaining supply to<br />
all substations on the ring.<br />
In typical urban/suburban ring-main arrangements, the open ring is operated manually and loss of<br />
supply restored by manual switching.<br />
Current practice is to use ‘<strong>distribution</strong> automation’, where operation and supply restoration in the<br />
feeder rings is done automatically by centrally-controlled supervisory <strong>systems</strong>.<br />
This gives the advantages of ring main <strong>systems</strong> as line voltage drops are reduced at the various<br />
load Substations there is a ‘firm’ supply (ie an alternative path is available if the primary one fails)<br />
to each load substation.<br />
Zone<br />
Substations<br />
CB CB<br />
CB CB CB CB CB CB CB CB<br />
CB CB CB CB<br />
CB CB CB CB<br />
Figure 4. A ring main feeder system<br />
Meshed <strong>systems</strong><br />
In transmission and sub-transmission <strong>systems</strong>, usually parallel, ring or interconnected (‘mesh’)<br />
<strong>systems</strong> are used.<br />
This ensures that alternative supply can be made to customers in the event of failure of a<br />
transmission line or element.<br />
The extra expense can be justified because of the much greater load and number of customers<br />
that are affected by failure of lines at transmission or sub-transmission levels. The general rule is<br />
that where large loads or numbers of customers are involved, then some form of standby, in the<br />
form of deliberate redundancy, is built into the network design, through the use of parallel,<br />
meshed or ring type feeders.<br />
Only in outer rural areas would one consider using only radial supply at a sub-transmission level.<br />
On the other hand, simple radial supply is almost universally used for low voltage (400V) feeders,<br />
even in urban areas, because they supply relatively few customers.<br />
16<br />
Terminal Station
DESIGN OVERHEAD DISTRIBUTION SYSTEMS<br />
Revision exercise 1<br />
1. List the voltage levels used in the various stages of transmission and <strong>distribution</strong> in<br />
electric power <strong>systems</strong>.<br />
2. List the main sections of a power system, starting with the generators and ending with<br />
the customer’s load?<br />
3. State where one would use each of the following types of feeders. Give reasons in<br />
each case.<br />
a. Radial feeders<br />
b. Parallel feeders<br />
c. Ring feeders<br />
d. Meshed feeders<br />
4. How can the cost of the higher-level feeder <strong>systems</strong> be justified?<br />
17
DESIGN OVERHEAD DISTRIBUTION SYSTEMS<br />
Overhead and Underground Power Systems<br />
As stated previously, initial <strong>overhead</strong> line construction is less expensive than underground cabling<br />
for the same kVA load. In rural or semi-rural areas, the sheer cost of underground cabling would<br />
make it impossible for customers to be able to afford the cost of supply.<br />
The down side is that <strong>overhead</strong> lines operate under continual mechanical stress with exposure to<br />
varying climatic conditions.<br />
This results in progressive deterioration in time as a result of corrosion, mechanical wear and<br />
fatigue, timber rot, etc.<br />
All components must be periodically inspected and replaced as required. They are exposed to<br />
environmental impacts such as storms, lightning, wind-blown debris and traffic impact (of poles)<br />
which means <strong>overhead</strong> <strong>systems</strong> are rarely as reliable as underground ones.<br />
The greater spacing of <strong>overhead</strong> line conductors generally results in higher system inductance<br />
than for a cable system.<br />
This means an <strong>overhead</strong> line has a greater voltage drop than an underground cable of equal<br />
current-carrying capacity and hence cannot supply power over as long a distance as the<br />
underground equivalent, particularly for lower voltage <strong>distribution</strong> <strong>systems</strong>.<br />
Even though poles are considered unsightly in urban locations, the capacity of an <strong>overhead</strong><br />
feeder can be readily increased by replacing it with larger conductors and/or increasing the<br />
voltage insulation/operating level.<br />
This flexibility is one big advantage of <strong>overhead</strong> <strong>systems</strong>.<br />
The reliability of underground <strong>distribution</strong> is greater than for <strong>overhead</strong> <strong>systems</strong> because of the<br />
lower number of fault interruptions, as discussed above.<br />
However, when interruptions do occur they are generally of much longer duration than those<br />
associated with <strong>overhead</strong> lines.<br />
Underground cables have comparatively higher capacitance, and under light load conditions can<br />
affect system power factor.<br />
In some cases, high charging currents and high transient voltages may accompany switching of<br />
cable <strong>systems</strong> that are open-circuited (especially at the higher <strong>distribution</strong> voltages, eg 22kV).<br />
This light-load performance can limit total underground feeder length.<br />
On the other hand, this does mean they can supply heavy load, especially when it is inductive in<br />
nature, over longer distances than <strong>overhead</strong> lines.<br />
The cost of underground cabling includes:<br />
• the cost of trenching (and sometimes compensation for private amenity) coping with<br />
obstructions from other utilities such as gas, water, sewerage / drainage, telephone, etc<br />
• the reinstatement of footpaths and road surfaces.<br />
• The subsequent installation of another underground cable along the same path for system<br />
reinforcement by a parallel or ring feeder, generally only exacerbates these difficulties.<br />
18
DESIGN OVERHEAD DISTRIBUTION SYSTEMS<br />
Overhead Insulated Conductors<br />
A more recent development has been the use of <strong>overhead</strong> conductors which are insulated.<br />
These conductors are more expensive than bare <strong>overhead</strong> but cheaper than full underground, as<br />
full cable outer mechanical protection is not needed, nor is expensive trenching and restoration.<br />
Being insulated provides some protection against wind-blown debris short-circuiting conductors<br />
together or conductor clashing.<br />
There are two basic types:<br />
1. Aerial bundled cable, or ABC for short, consisting of fully insulated (ie safe to touch)<br />
conductors tightly wrapped around a mechanical bearer wire (usually steel).<br />
2. Covered conductors, consisting of ‘partly insulated’ conductors mounted on insulators<br />
similar to bare <strong>overhead</strong> lines. These are cheaper than ABC and offer most of the<br />
protection features against wind-blown debris as does ABC but are not touch safe.<br />
ABC in particular is heavy and unsightly requiring more poles than bare-wire <strong>overhead</strong>, but trees<br />
can grow around it and hide it almost completely from view in due course. Covered conductor can<br />
also tolerate trees near it, but not to the same extent as ABC. It is less heavy and less unsightly<br />
when not shielded by trees.<br />
Revision exercise 2<br />
1. What are the three main types of conductor/cable <strong>systems</strong> used in <strong>distribution</strong>?<br />
2. What are the advantages and disadvantages of underground <strong>distribution</strong> cable<br />
networks?<br />
3. Where would one use aerial insulated conductor <strong>systems</strong> instead of either bare wire<br />
<strong>overhead</strong> or underground <strong>systems</strong>?<br />
19
DESIGN OVERHEAD DISTRIBUTION SYSTEMS<br />
Three Phase, Single Phase and Single Wire Earth Return Systems<br />
By far the majority of customers have electricity supplied to their homes, offices, factories, etc. as<br />
a three phase and/or single phase supply at voltages of 400 volts and/or 230 volts.<br />
Most houses are supplied at 230 volt single phase, as this is cheapest for light loads, involving<br />
only two wires.<br />
Medium sized ‘business’ customers (ie, factories, shops) will normally be supplied at 400 volt<br />
three-phase.<br />
This involves running four wires into the customer and is still cost effective in urban areas<br />
because the street supply is always three phase.<br />
The low voltage consists of three 230 Volt active phases and one ‘neutral’.<br />
The neutrals are then connected to the customers load using the MEN system of earthing.<br />
This provides a low resistance connection to earth providing protection to both the customers and<br />
the supply system in the event of faults.<br />
Large customers cannot be supplied economically at 400/230 volts because of the high cost of<br />
providing all the cabling that would be required.<br />
nstead, such customers (eg mines, large industrial sites, large commercial centres) are usually<br />
supplied at a high voltage instead of the conventional 400/230 Volts.<br />
In this case the customer would usually own (or lease) the power transformer or transformers<br />
needed to reduce the high voltage supply, and in return the customer would have a lower tariff for<br />
energy consumed.<br />
High voltage single phase <strong>systems</strong> are commonplace in rural areas, where the cost of supplying<br />
customers with relatively small loads at remote locations is a major problem. In such areas, the<br />
high voltage (usually 22kV or 11kV) feeder has only two wires and substations only supply one<br />
customer or a small group of customers close together, eg a farm and its associated outbuildings).<br />
There is no real low voltage network, other than single phase supply direct to the farm buildings.<br />
In such situations, the method used is to arrange the 22/11 kV <strong>overhead</strong> line to pass as close as<br />
practical to most of the customers.<br />
At the nearest pole to a customer a double wound single phase pole mounted transformer is<br />
connected with the primary across the two high voltage wires and the secondary providing<br />
nominally 230 volts, as a single phase to neutral supply.<br />
Single wire earth return (SWER) or single wire ground return is a single-wire transmission line for<br />
supplying single-phase electrical power from an electrical grid to remote areas at low cost.<br />
It is principally used for rural electrification, but also finds use for larger isolated loads such as<br />
water pumps, and light rail.<br />
Single wire earth return is also used for HVDC over submarine power cables.<br />
20
DESIGN OVERHEAD DISTRIBUTION SYSTEMS<br />
Description<br />
SWER is a choice for a <strong>distribution</strong> system when conventional return current wiring would cost<br />
more than SWER’s isolation transformers and small power losses.<br />
Power engineers experienced with both SWER and conventional power lines rate SWER as<br />
equally safe, more reliable, less costly, but with slightly lower efficiency than conventional lines.<br />
Some single wire transmission proposals have used radio-frequency AC.<br />
SWER uses conventional 50Hz or 60Hz AC power, and is therefore much less expensive.<br />
Conventional low frequency 50Hz or 60Hz power is supplied to the SWER line by an isolating<br />
transformer of up to 300 kVA.<br />
This transformer isolates the grid from ground or earth, and changes the grid voltage (typically 22<br />
kilovolts line to line) to the SWER voltage (typically 12.7 or 19.1 kilovolts line to earth).<br />
SWER line<br />
21
DESIGN OVERHEAD DISTRIBUTION SYSTEMS<br />
The SWER line is a single conductor that may stretch for tens or even hundreds of kilometres,<br />
visiting a number of termination points.<br />
At each termination point, such as a customer's premises, current flows from the line, through the<br />
primary coil of a step-down transformer, to earth through an earth stake.<br />
From the earth stake, the current eventually finds its way back to the main step-down transformer<br />
at the head of the line, completing the circuit.<br />
SWER is therefore a practical example of a phantom loop.<br />
SWER burns up grounding poles or may fail to reset breakers in areas with high resistance soil.<br />
In Australia, locations with very dry soils need the grounding poles to be extra deep.<br />
Experience in Alaska shows that SWER needs to be grounded below permafrost, which is highresistance.<br />
The secondary winding of the local transformer will supply the customer with either single ended<br />
single phase (N-0) or split phase (N-0-N) power in the region’s standard appliance voltages, with<br />
the 0 volt line connected to a safety earth that does not normally carry an operating current.<br />
A large SWER line may feed as many as 80 <strong>distribution</strong> transformers.<br />
The transformers are usually rated at 5 kVA, 10 kVA and 25 kVA. The load densities are usually<br />
below 0.5 kVA per kilometer (0.8 kVA per mile) of line.<br />
Any single customer’s maximum demand will typically be less than 3.5 kVA, but larger loads up to<br />
the capacity of the <strong>distribution</strong> transformer can also be supplied.<br />
Some SWER <strong>systems</strong> in the USA are conventional <strong>distribution</strong> feeders that were built without a<br />
continuous neutral (some of which were obsoleted transmission lines that were refitted for rural<br />
<strong>distribution</strong> service).<br />
The substation feeding such lines has a grounding rod on each pole within the substation; then on<br />
each branch from the line, the span between the pole next to and the pole carrying the<br />
transformer would have a grounded conductor (giving each transformer two grounding points for<br />
safety reasons).<br />
22
DESIGN OVERHEAD DISTRIBUTION SYSTEMS<br />
History<br />
At the end of the 19th century, Nikola Tesla demonstrated that only a single wire was necessary<br />
for power <strong>systems</strong>, with no need for a wired return conductor (using the Earth instead).<br />
Lloyd Mandeno fully developed SWER in New Zealand around 1925 for rural electrification.<br />
Although he termed it “Earth Working Single Wire Line” it was often called “Mandeno’s<br />
Clothesline”. More than 200,000 kilometres have now been installed in Australia and New<br />
Zealand.<br />
It is considered safe, reliable and low cost, provided that safety features and earthing are correctly<br />
installed.<br />
The Australian standards are widely used and cited. It has been applied in Saskatchewan of<br />
Canada, Brazil, Africa, portions of the United States' Upper Midwest, and SWER interties have<br />
been proposed for Alaska and prototyped.<br />
Characteristics<br />
Safety<br />
SWER violates common wisdom about electrical safety, because it lacks a traditional metallic<br />
return to a neutral shared by the generator.<br />
SWER’s safety is instead assured because transformers isolate the ground from both the<br />
generator and user.<br />
However, certain groups claim that stray voltages from SWER can injure livestock.<br />
Grounding is critical because of the significant currents on the order of 8 amperes that flow<br />
through the ground near the earth points, so a good-quality earth connection is needed to prevent<br />
risk of electric shock due to earth potential rise near this point.<br />
Separate grounds for power and safety are also used.<br />
Duplication of the ground points assures that the system is still safe if either of the grounds is<br />
damaged.<br />
A good earth connection is normally a 6 m stake of copper-clad steel driven vertically into the<br />
ground, and bonded to the transformer earth and tank.<br />
A good ground resistance is 5–10 ohms.<br />
SWER <strong>systems</strong> are designed to limit the voltage in the earth to 20 volts per meter to avoid<br />
shocking people and animals that might be in the area.<br />
Other standard features include automatic reclosing circuit breakers (reclosers).<br />
Most faults (over-current) are transient.<br />
Since the network is rural, most of these faults will be cleared by the recloser.<br />
Each service site needs a rewirable drop out fuse for protection and switching of the transformer.<br />
The transformer secondary should also be protected by a standard high-rupture capacity (HRC)<br />
fuse or low voltage circuit breaker.<br />
A surge arrestor (spark gap) on the high voltage side is common, especially in lightning-prone<br />
areas.<br />
Bare-wire or ground-return telecommunications can be compromised by the ground-return current<br />
if the grounding area is closer than 100 m or sinks more than 10 A of current.<br />
Modern radio, optic fibre channels and cell phone <strong>systems</strong> are unaffected.<br />
23
DESIGN OVERHEAD DISTRIBUTION SYSTEMS<br />
Cost advantage<br />
SWER’s main advantage is its low cost.<br />
It is often used in sparsely populated areas where the cost of building an isolated <strong>distribution</strong> line<br />
cannot be justified.<br />
Capital costs are roughly 50% of an equivalent two-wire single-phase line.<br />
They can be 70% less than 3-wire three-phase <strong>systems</strong>.<br />
Maintenance costs are roughly 50% of an equivalent line.<br />
SWER also reduces the largest cost of a <strong>distribution</strong> network, the number of poles.<br />
Conventional 2-wire or 3-wire <strong>distribution</strong> lines have a higher power transfer capacity, but can<br />
require seven poles per kilometre, with spans of 100 m to 150 m. SWER’s high line voltage and<br />
low current permits the use of low-cost galvanized steel wire.<br />
Steel’s greater strength permits spans of 400 m or more, reducing the number of poles to 2.5 per<br />
kilometre.<br />
Reinforced concrete poles have been traditionally used in SWER lines because of their low cost,<br />
low maintenance, and resistance to water damage, termites and fungus.<br />
Local labor can produce them in most areas, further lowering costs.<br />
If the cable contains optic fibre, or carries telephone service, this can further amortize the capital<br />
costs.<br />
Reliability strengths<br />
SWER can be used in a grid or loop, but is usually arranged in a linear or radial layout to save<br />
costs. In the customary linear form, a single-point failure in a SWER line causes all customers<br />
further down the line to lose power.<br />
However, since it has fewer components in the field, SWER has less to fail. For example, since<br />
there is only one line, winds can’t cause lines to clash, removing a source of damage, as well as a<br />
source of rural brush fires.<br />
Since the line can't clash in the wind, and the bulk of the transmission line has low resistance<br />
attachments to earth, excessive ground currents from shorts and geomagnetic storms are far<br />
more rare than in conventional metallic-return <strong>systems</strong>.<br />
So, SWER has fewer ground-fault circuit-breaker openings to interrupt service.<br />
Power quality weakness<br />
SWER lines tend to be long, with high impedance, so the voltage drop along the line is often a<br />
problem, causing poor regulation.<br />
Variations in demand cause variation in the delivered voltage.<br />
To combat this, some installations have automatic variable transformers at the customer site to<br />
keep the received voltage within legal specifications.<br />
When used with distributed generation, SWER is substantially more efficient than when it is<br />
operated as a single-ended system.<br />
For example, some rural installations can offset line losses and charging currents with local solar<br />
power, wind power, small hydro or other local generation.<br />
This can be an excellent value for the electrical distributor, because it reduces the need for more<br />
lines.[<br />
After some years of experience, the inventor advocated a capacitor in series with the ground of<br />
the main isolation transformer to counteract the inductive reactance of the transformers, wire and<br />
earth return path.<br />
The plan was to improve the power factor, reduce losses and improve voltage performance due to<br />
reactive power flow.<br />
Though theoretically sound, this is not standard practice.<br />
24
DESIGN OVERHEAD DISTRIBUTION SYSTEMS<br />
Networks and circuits<br />
As demand grows, a well-designed SWER line can be substantially upgraded without new poles.<br />
The first step may be to replace the steel wire with more expensive copper-clad or aluminium-clad<br />
steel wire.<br />
It may be possible to increase the voltage.<br />
Some distant SWER lines now operate at voltages as high as 35 kV.<br />
Normally this requires changing the insulators and transformers, but no new poles are needed.<br />
If more capacity is needed, a second SWER line can be run on the same poles to provide two<br />
SWER lines 180 degrees out of phase.<br />
This requires more insulators and wire, but doubles the power without doubling the poles.<br />
Many standard SWER poles have several bolt holes to support this upgrade.<br />
This configuration causes most ground currents to cancel, reducing shock hazards and<br />
interference with communication wirelines.<br />
Two phase service is also possible with a two-wire upgrade:<br />
Though less reliable, it is more efficient.<br />
As more power is needed the lines can be upgraded to match the load, from single wire SWER to<br />
two wire, single phase and finally to three wire, three phase.<br />
This ensures a more efficient use of capital and makes the initial installation more affordable.<br />
Customer equipment installed before these upgrades will all be single phase, and can be reused<br />
after the upgrade.<br />
If moderate amounts of three-phase are needed, it can be economically synthesized from twophase<br />
with on-site equipment.<br />
Regulatory issues<br />
Many national electrical regulations (notably the U.S.) require a metallic return line from the load<br />
to the generator.<br />
In these jurisdictions, each SWER line must be approved by exception.<br />
Use in interties<br />
In 1981 a high-power 8.5 mile prototype SWER intertie was successfully installed from a coal<br />
plant in Bethel to Napakiak in Alaska, United States.<br />
It operates at 80 kV, and has special lightweight fiberglass poles forming an A-frame.<br />
The poles can be carried on lightweight snow machines, and most poles can be installed with<br />
hand tools on permafrost without extensive digging.<br />
Erection of “anchoring” poles still required heavy machinery, but the cost savings were dramatic.<br />
The phase conductor also carries a bundle of optical fibres within the steel armour wire, so the<br />
system supplies telecommunications as well as power.<br />
Researchers at the University of Alaska Fairbanks, United States estimate that a network of such<br />
interties, combined with coastal wind turbines, could substantially reduce Alaska’s dependence on<br />
increasingly expensive diesel fuel for power generation.<br />
Alaska’s state economic energy screening survey advocated further study of this option to use<br />
more of the state’s underutilized power sources.<br />
Use by developing nations<br />
At present, certain developing nations have adopted SWER-<strong>systems</strong> as their mains electricity<br />
<strong>systems</strong>. Notably Laos and South Africa.<br />
25
DESIGN OVERHEAD DISTRIBUTION SYSTEMS<br />
Use for HVDC <strong>systems</strong><br />
Many HVDC <strong>systems</strong> using submarine power cables are (or were until their expansion to bipolar<br />
schemes) single wire earth return <strong>systems</strong>.<br />
To avoid electrochemical corrosion, the ground electrodes of such <strong>systems</strong> are situated apart<br />
from the converter stations and not near the transmission cable.<br />
The electrodes can be situated in the sea or on land.<br />
Bare copper wires can be used for cathodes, and graphite rods buried in the ground, or titanium<br />
grids in the sea are used for anodes.<br />
To avoid electrochemical corrosion (and passivation of titanium surfaces) the current density at<br />
the surface of the electrodes may be only small and therefore large electrodes are required.<br />
The advantage of such schemes is eliminating the cost of a second conductor, since saltwater is<br />
an excellent conductor.<br />
Some ecologists claim bad influences of electrochemical reactions, but they do not occur on very<br />
large underwater electrodes.<br />
Revision Exercise 3<br />
1. Where would you expect three phase and single phase <strong>systems</strong> to be used? Why?<br />
2. How is supply provided to very large customers?<br />
26
DESIGN OVERHEAD DISTRIBUTION SYSTEMS<br />
Supply Quality<br />
It is the aim of a well-designed electricity system to provide voltage to customers that is stable,<br />
stays within limits and is free from random momentary variations and other disturbing waveforms.<br />
The term ‘supply quality’ or ‘quality of supply’ has come to be a collective term which includes<br />
electrical power parameters such as:<br />
• voltage variation outside of standards<br />
• voltage sags and swells<br />
• repeated fluctuations<br />
• impulses (spikes)<br />
• momentary interruptions (sudden dips)<br />
• frequency variations and waveform distortion (harmonics).<br />
As well as the problems caused by such variations with computing equipment, variable speed<br />
drive controllers and other sensitive electronic controls, there can also be interruptions to<br />
customers resulting in loss of production or amenity.<br />
There is also a need to ensure the robustness of a supply, which includes its ability to continue to<br />
meet power demands under excessive loads, operation of system protection equipment, etc. The<br />
result is that greater care has to be given to what, in the past, may have been considered ‘risk<br />
factors’ that the customer was once expected to simply put up with as a part of his electricity<br />
supply.<br />
Some of the more established factors considered in supply quality are now listed and discussed.<br />
Voltage and frequency Stability<br />
Standards and legislation in the various countries set out the values and acceptable tolerances on<br />
voltage and frequency values allowable from a distributor.<br />
If frequency varies even by small amounts from the nominal 50 hertz (cycles per second) this will<br />
cause problems for generation equipment and to customers.<br />
Once system frequency falls as low as only 49.5 Hz the system usually is on the verge of<br />
collapse.<br />
From a customer’s perspective, voltages which are too high will severely reduce the life of<br />
incandescent lamps and possibly damage sensitive equipment, whilst voltage values which are<br />
too low will significantly increase the current drawn (relative to the rated current and voltage) for<br />
any motor driven loads, resulting in possible thermal damage to motors and reduce the<br />
starting/pull-out torques of motors.<br />
For these reasons considerable care needs to be taken to ensure that voltage values are held<br />
stable during the daily cycle of load changes.<br />
The maximum variation in voltage (ie difference between highest and lowest voltage) is referred<br />
to as ‘voltage spread’, ‘variation’ or ‘bandwidth’.<br />
This is usually expressed as a percentage of the nominal voltage.<br />
In Australia, standards set the maximum voltage regulation for 230 volt <strong>systems</strong> to be 230 volts<br />
plus 10% or minus 14%, ie a maximum of 253 volts and a minimum of 198 volts for all except<br />
emergency conditions.<br />
Voltage Fluctuations<br />
Rapidly varying loads, ie those where the current drawn from the power system goes up and<br />
down in magnitude frequently, cause voltage drops in the system which is also rapidly varying<br />
and consequently other customers are subjected to them.<br />
This is an additional problem on top of the slow variations in voltage throughout the day discussed<br />
above.<br />
Loads of this nature include electric motors which start up, run and shut down (or go from light to<br />
full load and back again) frequently, eg rolling mills, metal shredders, as well as arc welders and<br />
the like.<br />
The voltage drop can be seen by customers as a dip in the output of lights; if it occurs frequently it<br />
causes a flickering of the lights which can be extremely annoying.<br />
27
DESIGN OVERHEAD DISTRIBUTION SYSTEMS<br />
The more rapid the flicker, the worse the visual distress that is caused.<br />
There are standards which limit the amount and frequency of such voltage fluctuations on the<br />
system.<br />
Momentary Interruptions<br />
Another problem somewhat akin to voltage fluctuations is momentary interruptions to supply.<br />
These can occur for a number of reasons, usually when transient faults on the system may cause<br />
a large but very brief voltage dip on the system, or when the primary supply may fail and the<br />
system will switch across to a standby supply, eg on a ‘ring’ system.<br />
The effect is sometimes low or no supply from less than a second to maybe several seconds.<br />
Normally this will be of minor inconvenience, often hardly noticed by most customers.<br />
It does cause problems for voltage sensitive equipment such as computers, digital clocks which<br />
often have to be reset and to ‘undervoltage’ relays on large electric motors, which often will trip<br />
the motor out (on the mistaken assumption that the voltage will stay low and the motor overheat).<br />
In the past momentary interruptions rarely caused problems but with increasing numbers of<br />
voltage sensitive devices, especially electronic and digital ones, it is becoming a major issue for<br />
electricity distributors.<br />
Revision Exercise 4<br />
1. What are the main areas of concern under the broad definition of ‘supply quality’?<br />
2. What problems can be caused by the following power supply variations?<br />
a) Excessive voltage<br />
b) Too low a voltage<br />
28
DESIGN OVERHEAD DISTRIBUTION SYSTEMS<br />
3. Why are momentary dips on the system more of a problem now than they used to be?<br />
Power Factor Considerations<br />
As most electrical loads are inductive, and <strong>distribution</strong> lines, when loaded, are also inductive, a<br />
heavily loaded <strong>distribution</strong> system usually appears to be an inductive impedance and operates at<br />
a ‘lagging’ power factor.<br />
This means that the current sine wave lags somewhat behind the voltage sine wave.<br />
Depending on the line construction and its length, a lightly-loaded or open circuit <strong>overhead</strong> line<br />
may appear to be a capacitive load.<br />
This means the current now leads the voltage and reduces voltage drop, but causes other<br />
complications including sometimes voltages which are too high at the receiving end.<br />
The component (ie proportion) of the current in phase with the voltage sine wave represents the<br />
actual energy supplied by the system.<br />
The rest of the current doesn’t supply useful energy to the customer and causes unwanted<br />
voltage drops and losses in the <strong>distribution</strong> system.<br />
As the customers typically pay for energy in kilowatt-hours, to make best use of the current<br />
carrying capacity of a line we endeavour to minimise the reactive current component carried in the<br />
system. (This is determined by the amount of inductance and capacitance in the system.)<br />
This can be achieved in a number of ways, the most popular of which is capacitor power factor<br />
correction at substations throughout the system.<br />
This consists of adding shunt (ie from each phase to ground) capacitors, which increases the<br />
capacitance at selected points to directly counteract the inductance in the system, or the<br />
customers’ loads.<br />
Waveform Distortion and Harmonics<br />
Ideally, a power system should have voltages and currents which are pure sine-waves.<br />
The ever increasing use of fluorescent lighting and electronic light dimmers, industrial rectifiers,<br />
variable speed motor drive equipment and computers (with switched mode power supplies)<br />
results in severe distortion of the sinusoidal waveform of the voltage in the <strong>distribution</strong> system.<br />
As this electronic equipment is usually connected by the customer the distributor has little control<br />
over the extent of the equipment connected but must, on the other hand, ensure the supply<br />
complies with the relevant requirements.<br />
The analysis of the effect of waveform distortion is usually done by considering the magnitude of<br />
the various harmonic components of the voltage which are present to create this distort it in<br />
Instruments are available to measure the relative magnitudes of the harmonics present.<br />
Harmonics are classified as ‘odd’ or ‘even’, depending on the numeric factor by which their<br />
frequency varies from the fundamental frequency (of 50 hertz).<br />
29
DESIGN OVERHEAD DISTRIBUTION SYSTEMS<br />
Harmonics are also classified into groups designated as ‘positive’, ‘negative’ and ‘zero’ sequence<br />
values or components.<br />
Frequency Harmonic<br />
(Hertz) Identification Odd / Even Sequence<br />
50 Fundamental Positive<br />
100 2 ND Even Negative<br />
150 3 RD Odd Zero<br />
200 4 TH Even Positive<br />
250 5 TH Odd Negative<br />
300 6 TH Even Zero<br />
350 7 TH Odd Positive<br />
400 8 TH Even Negative<br />
450 9 TH Odd Zero<br />
500 etc Even<br />
Note: The fundamental can be thought of as an odd harmonic. A DC component,<br />
if present, is essentially a “zero” harmonic (at 0 hz) and is zero sequence,<br />
flowing down all phases to ground. This would fit into the overall pattern.<br />
Table 1. Sequencing and Frequencies of Fundamental and harmonic Currents<br />
(based on 50 Hz system)<br />
Whenever a neutral/earth connection is made, any ‘zero’ sequence components of current<br />
(resulting from corresponding voltages in the system) will flow to the ‘ground’.<br />
If these values of current are significant (i.e. greater than 15% of the values of line currents) there<br />
can be a measurable voltage drop created in the neutral/earth connection.<br />
The effect of this voltage drop is to ‘move’ the neutral (star point) voltage, and this can result in<br />
excessively high or low voltage values being applied to any connected equipment.<br />
Other problems which result from harmonic distortion are manifest at or near the ‘earth stake’<br />
location, where it is possible for dangerously high voltage values to result on the surface of the<br />
soil, or heating of the soil can occur, due to high earth current density.<br />
Harmonics, especially the ‘negative sequence’ ones, cause overheating problems in electric<br />
motors and harmonics of all types interfere with electronic and computer-based equipment.<br />
30
DESIGN OVERHEAD DISTRIBUTION SYSTEMS<br />
Revision exercise 5<br />
1. Describe what power factor means and how it can be controlled.<br />
2. List the main problems caused by harmonics in a power system.<br />
31
DESIGN OVERHEAD DISTRIBUTION SYSTEMS<br />
Voltage Regulation<br />
Voltage variation is the variation in the magnitude of the voltage value between its highest and<br />
lowest levels that occurs as the connected load changes between maximum and minimum.<br />
Voltage regulation, in an item of plant such as a transformer, or down a feeder, is the drop in<br />
voltage which occurs from the supply end to the connected load.<br />
Both are usually expressed as a ratio or percentage of the load voltage. For an <strong>overhead</strong> line it<br />
can be defined mathematically as<br />
Voltage Regulation<br />
ES − E<br />
=<br />
E<br />
where: ES is the voltage at the supply end, and<br />
ER is the voltage at the load.<br />
R<br />
R<br />
The variation of voltage at a consumer’s supply point should be kept to a minimum for the<br />
following reasons:<br />
1. When incandescent lamps are connected their performance is very sensitive to variations<br />
in the supply voltage. A five per cent reduction below rated voltage results in a decrease<br />
between fifteen and twenty per cent of the light output; a five percent increase above rated<br />
voltage shortens the ‘life’ of the lamp by up to 40%.<br />
2. Variations from rated voltage reduce the effectiveness of induction motors and<br />
transformers. When operating above rated voltage, the iron in the magnetic circuit of the<br />
motor/transformer will be closer to saturation or over saturated resulting in a larger<br />
magnetising current and greater heating of the motor/transformer. If the voltage is below<br />
the rated value, the starting torque and pull-out torque are considerably reduced and<br />
heavier currents have to be drawn which can overheat motors.<br />
3. Depending on the internal circuitry, electronic control equipment, computers and television<br />
equipment may not function adequately with wide variations in supply voltage.<br />
There is also an effect on the <strong>distribution</strong> system as operating above rated voltage causes<br />
excessive heating in <strong>distribution</strong> transformers and results in waveform distortion due to saturation<br />
of the iron, causing the introduction of third harmonic values of voltage.<br />
These also can cause heating of earth stakes and local earth potential rise problems, if they are<br />
very high.<br />
This variation in the voltage at the connected load between its highest to its lowest value is often<br />
referred to as ‘bandwidth’.<br />
Limits and standards<br />
There are accepted standards which dictate maximum voltage variations in a public electricity<br />
supply system. The international standard recently adopted in Australia is based on 230 volts<br />
rather than 240 volts, to enable standardisation of appliances, motors and the like. It specifies 230<br />
volts plus 10% minus 14% as the maximum permissible regulation.<br />
The voltages will however apply to ‘utilisation voltages’ rather than the voltage at ‘customer’s<br />
terminals’ as does the existing standard.<br />
Its important at this stage to fully understand the terminology used and these are explained in the<br />
following section.<br />
32
DESIGN OVERHEAD DISTRIBUTION SYSTEMS<br />
Terminology<br />
Distribution system<br />
This essentially, is the feeders and substations and their ancillary equipment - poles, switches,<br />
circuit breakers, fuses plus the support equipment such as supervisory control and protection<br />
<strong>systems</strong> which makes up the public supply network.<br />
The ‘public’ network is one which is ‘shared’, ie supplies at least two or more customers over the<br />
one set of assets.<br />
A ‘private’ system is one which feeds just the one customer, even when the customer is very large<br />
and may have a private network which contains many local substations and feeders.<br />
The <strong>distribution</strong> system also means that part of the public network which operates at voltages<br />
below that defined as ‘transmission’ and typically takes voltages at medium to high levels and<br />
reduces these down for use by customers.<br />
The role and design of other ancillary equipment is covered in other courses and will not be<br />
discussed further in this module; however it is equally important to the function of the <strong>distribution</strong><br />
system as are the feeders discussed here.<br />
Customer’s terminals<br />
This is the connection point between the public <strong>distribution</strong> system and the take-off to supply<br />
individual customers.<br />
Typically for residential customers, it is the ‘barge board’ on the front of the house. For a farm it is<br />
typically the first low voltage pole just inside the property of the customer, where that take-off<br />
supplies just the one customer only.<br />
Customer voltage<br />
This is the voltage at the ‘customer’s terminals’, as defined above.<br />
As discussed previously, the current Australian standards for maximum variation in voltage refer<br />
to the voltage at the customers’ terminals.<br />
Base voltage<br />
This is the nominal voltage of the <strong>distribution</strong> system.<br />
For low voltage <strong>systems</strong> in Australia, this is almost universally 240 volts single phase/415 volts<br />
three phase.<br />
There is a separate base voltage for each voltage layer in the <strong>distribution</strong> system, eg 11 kV, 66<br />
kV, 132 kV etc.<br />
Utilisation voltage<br />
This is the voltage at the ‘point of utilisation’ i.e. the power point inside the customer’s premises. It<br />
is a more realistic representation of voltage conditions that appliances will be subject to in their<br />
typical use than is ‘customer voltage’.<br />
Under heavy loading conditions, it will be lower than the voltage at the customer’s terminals, due<br />
to the voltage drop in the customer’s wiring which runs between the customer terminals and the<br />
actual power point.<br />
The proposed 230 volts standard will recognise this by using utilisation voltage in its definition of<br />
limits rather than customer voltage.<br />
33
DESIGN OVERHEAD DISTRIBUTION SYSTEMS<br />
Revision exercise 6<br />
1. What is the definition of:<br />
Customers’ terminals<br />
Base voltage<br />
Customer voltage<br />
Utilisation voltage<br />
2. With regard to voltage variation, what is the current 400/230 volt supply standard in<br />
Australia?<br />
3. With regard to voltage variation, what is the current low voltage supply standard in<br />
China?<br />
34
DESIGN OVERHEAD DISTRIBUTION SYSTEMS<br />
Causes of voltage variation<br />
Voltage variations on the <strong>distribution</strong> system may result from one or more of the following causes:<br />
1. The voltage at the source (from the transmission system or generator) may not be<br />
controlled, or is controlled only within certain limits.<br />
2. The voltage at the secondary of a transformer varies as the load changes. With a large<br />
load current the voltage drop due to the transformer windings needs to be subtracted<br />
from the (open circuit) voltage of the secondary terminals (This is the voltage<br />
regulation of the transformer). As the load current decreases the terminal voltage<br />
increases, being at its greatest when there is no load current from the transformer.<br />
3. The voltage drop in the <strong>distribution</strong> lines when delivering current, due to the inherent<br />
resistance and reactance of the <strong>overhead</strong> lines.<br />
Variation of resistance of conductors<br />
The value of resistance for conductors is usually stated for an ambient temperature of 20°C,<br />
however it is not unusual for an <strong>overhead</strong> line to have conductors at temperatures up to 65°C or<br />
more in summer time with large current loads. Due to this rise in conductor temperature it is<br />
possible to have the resistance of the conductor increase by up to 20% or more. The method of<br />
calculating this increase is as follows:<br />
Starting from a value of (d.c.) resistance at 20°C, the (d.c.) resistance of the conductor at any<br />
other temperature 8°C can be calculated from:<br />
Rθ = R20[1 + α2O(θ – 20)]<br />
where: Rθ = d.c. resistance at 8°C<br />
R20 = d.c. resistance at 20°C<br />
α20 = temperature coefficient of resistance at 20°C.<br />
For copper, α20 = 0.00393 per deg C<br />
For aluminium, α20 = 0.00403 per deg C.<br />
This value then needs to be corrected to allow for the effect of a.c. voltages being applied to the<br />
conductors and the resultant slight variations in current density in the conductor, the effect of steel<br />
cores (if present) and any insulation (as in Aerial Bundled Cable or ABC). The ‘correcting value’<br />
(R’) is found by measurement (empirically) and added to the previously calculated value of Rθ, so<br />
that the effective a.c. resistance Rθ2 is given by:<br />
Rθ2 = Rθ + R’<br />
Typically this adds about 10 to 15% to the DC resistance.<br />
35
DESIGN OVERHEAD DISTRIBUTION SYSTEMS<br />
Inductance of <strong>overhead</strong> conductors<br />
Inductance of feeders is caused by the magnetic fields around the currents (‘magnetic flux’)<br />
interacting with the conductors and inducing a reverse voltage or ‘back emf’ which opposes the<br />
current flow. This is manifest as a voltage drop in the conductors. The inductance of a nonmagnetic<br />
conductor at 50 hertz, relative to another similar conductor can be calculated from the<br />
equation:<br />
XL = 0.0156 + 0.1447 log10<br />
d<br />
milli-Henries per km<br />
r<br />
where: d = distance between conductors in metres<br />
r = radius of the conductor in metres<br />
In practice it is easier and more convenient to use pre-calculated tables for determining line<br />
reactance in ohms rather than individual calculations, or use computers and software packages<br />
such as spreadsheets.<br />
Because conductor spacings for <strong>overhead</strong> lines are necessarily large, there is not a great deal of<br />
variation in <strong>overhead</strong> line reactance values for various conductors.<br />
Schedules showing some selected values are given in Table 1 (straight aluminium or copper<br />
conductors) and Table 2 (steel-cored aluminium conductors).<br />
Inductive Reactance ohms/km at 50 Hz and 1 metre spacings<br />
Conductor Reactance Conductor Reactance<br />
7 / 1.25 0.414 7 / 4.50 0.334<br />
7 / 1.75 0.393 7 / 4.75 0.330<br />
7 / 2.00 0.384 19 / 1.75 0.359<br />
7 / 2.25 0.377 19 / 2.00 0.350<br />
7 / 2.50 0.371 19 / 2.75 0.330<br />
7 / 2.75 0.365 19 / 3.00 0.325<br />
7 / 3.00 0.359 19 / 3.25 0.320<br />
7 / 3.50 0.350 19 / 3.75 0.311<br />
7 / 3.75 0.345 19 / 4.75 0.296<br />
Table 2. Non-Ferrous Conductors<br />
Inductive Reactance ohms/km at 50 Hz and 1 metre spacing<br />
Conductor Reactance Conductor Reactance<br />
6 / 1 / 1.25 0.372 30 / 7 / 2.50 0.315<br />
6 / 1 / 3.00 0.362 30 / 7 / 3.00 0.304<br />
6 / 1 / 3.75 0.346 30 / 7 / 3.50 0.294<br />
Table 3. Steel-cored aluminium conductors (ACSR/GZ and ACSR/AZ)<br />
36
DESIGN OVERHEAD DISTRIBUTION SYSTEMS<br />
Separation mm<br />
0 25 50 75 100 125 150 175 200 225<br />
250 –0.087 –0.081 –0.076 –0.071 –0.066 –0.062 –0.058 –0.054 –0.050 –-0.047<br />
500 –0.044 –0.040 –0.038 –0.035 –0.032 –0.030 –0.027 –0.025 –0.022 –0.020<br />
750 –-0.018 –0.016 –0.014 –0.012 –0.010 –0.008 –0.007 –0.005 –0.003 –0.002<br />
1000 0 0.002 0.003 0.005 0.006 0.007 0.009 0.010 0.011 0.013<br />
Table 4. Correction factors<br />
Note the use of 1 metre as a standard spacing in the tables. In Table 3, correction values are<br />
shown which can be applied if the conductor spacing is not 1 metre.<br />
For example if the spacing was 525 mm, looking across the second row labelled ‘500’ to the<br />
column ‘25’, we see that the correction factor is –0.040.<br />
You can see that the reactance per unit length of an <strong>overhead</strong> line does not vary greatly with the<br />
spacing regardless of the conductor size.<br />
For instance, with a 19/2.75 (being a 112.9 square millimetre conductor) the reactance varies only<br />
between 0.286 ohms per km (0.330 - 0.044 = 0.286) at 0.5 metre spacing and 0.343 ohms per km<br />
(0.330 + 0.013 = 0.343) at 1.25 metre spacing.<br />
From a practical aspect, therefore, there is no appreciable error resulting if we assume a constant<br />
reactance value for any particular conductor size on the assumption that spacings for the line<br />
voltage will stay within a specific range.<br />
The graph in Figure 5 illustrates the relationship between resistance, reactance and impedance<br />
for conductors with various values of cross sectional area.<br />
From this we can see why there is little advantage to be gained by using conductors larger than<br />
112.9 square millimetres (19/2.75) to reduce voltage drop because of the impact of the line<br />
reactance.<br />
Underground cables also have inductance but as the conductors are much closer together than<br />
their <strong>overhead</strong> counterparts (because they are wrapped in insulation) their inductance is also<br />
somewhat lower.<br />
This means a smaller voltage drop for the same load and hence underground cables can transmit<br />
power further than a same-sized <strong>overhead</strong> line.<br />
ABC <strong>overhead</strong> conductors, because they are insulated, are also spaced closer together than bare<br />
conductors and likewise have a somewhat lower inductance, similar to that of underground<br />
cables.<br />
Covered conductors, with spacing between that of bare-wire <strong>overhead</strong> and underground cables,<br />
have a slightly lower inductance than bare-wire <strong>overhead</strong> but not as low as underground.<br />
Transformers at all voltages have inductance, which is caused by some of the magnetic field<br />
produced by the primary coils not linking completely with the secondary coil.<br />
This stray field, called ‘leakage flux’, has its own inductance which causes a voltage drop in the<br />
transformer.<br />
37
DESIGN OVERHEAD DISTRIBUTION SYSTEMS<br />
Ohms per km<br />
1.1<br />
1.0<br />
0.9<br />
0.8<br />
0.7<br />
0.6<br />
0.5<br />
0.4<br />
0.3<br />
0.2<br />
0.1<br />
Impedance Z<br />
0<br />
0 25 50 75 100 125<br />
Cross section – sq mm<br />
REACTANCE BASED ON 3 PHASE<br />
EQUALLY SPACED FLAT CONSTRUCTION<br />
WITH CONDUCTORS 775 MM APART<br />
Figure 5. Relationship between resistance, reactance and impedance for conductors of differing<br />
cross-sectional areas<br />
38<br />
Reactance X<br />
Resistance R<br />
150<br />
175<br />
200
DESIGN OVERHEAD DISTRIBUTION SYSTEMS<br />
Shunt capacitance<br />
Underground cables and long lines at high voltages also have shunt capacitance between<br />
conductors and between conductors and ground.<br />
This is because conductors running parallel to one another behave a little like the plates on a<br />
capacitor.<br />
Capacitance helps to counteract the effect of inductance in feeders, reducing the voltage drop at<br />
full load. it may cause problems at light load, causing the voltage to rise along the line to<br />
unacceptably high levels.<br />
Capacitance is rarely an issue for <strong>distribution</strong> lines and typically is only considered for long<br />
transmission lines at voltages of 132kV and above.<br />
Revision exercise 7<br />
1. What three factors combine to determine the value of the impedance of<br />
<strong>distribution</strong>/transmission lines?<br />
2. What are the factors that influence the inductance of <strong>overhead</strong> lines? Why is<br />
inductance a problem?<br />
39
DESIGN OVERHEAD DISTRIBUTION SYSTEMS<br />
Equipment used in the control of voltage<br />
Introduction<br />
There are three general methods used to control the voltage at the end of a <strong>distribution</strong> feeder.<br />
They are by using:<br />
• control equipment to vary the voltage at the supply end of the feeder<br />
• control equipment at the load end which can vary the voltage<br />
• controlling the current in the line by changing the power factor.<br />
At the transmission source, voltage is controlled by the voltage regulators on the generators.<br />
Voltage control equipment connected at either the supply end or the load end of the feeder would<br />
include off-load tap changing transformers, on-load tap-changing transformers, booster<br />
transformers, moving coil regulators, induction regulators.<br />
Current controlling devices intended to control the power factor are either static or rotary<br />
capacitors.<br />
Rotary capacitors are rarely if ever used in modem power <strong>systems</strong> and will not be discussed.<br />
Voltage changing equipment<br />
Tap-changing transformers are constructed so that the output voltage can be adjusted by means<br />
of a switch to increase or decrease the voltage.<br />
Details of how this is done are given in the subject Power Transformers.<br />
The switches can be designed to either carry no current at the time the voltage value is changed<br />
(off-load tap-changer) or to carry the full rated current (on-load tap-changer).<br />
Usually the voltage is changed in increments of the rated voltage – typically 2.5% for <strong>distribution</strong><br />
(22/11 kV to 400 volt) transformers but finer, say 1.25 – 1.5% for transformers in transmission<br />
substations- with a full range of adjustment up to ±10% of the rated output voltage.<br />
This means that for an 11 kV line the voltage at the supply end can be between 9.9 kV and<br />
12.1kV.<br />
On-load tap-changers ensure there is no interruption to the electricity supply during the change of<br />
voltage value, and as a consequence are preferred, even though they are much more expensive.<br />
When off-load tap-changers are installed the electricity supply must be disconnected for the<br />
duration of time required to change the voltage setting.<br />
Typically, zone and transmission substation transformers are fitted with on-load tap changers<br />
because of the very high numbers of customers which would be affected if they had to be<br />
disconnected each time a tap change had to be made.<br />
The essential elements of the load and the compensating circuits used for the automatic control of<br />
an on-load tap- changer are shown in Figure 6.<br />
Essentially it consists of a voltage sensing relay, which will actuate the tap-changer motor to<br />
move the tap position automatically up or down as the voltage varies away from the set desired<br />
voltage level.<br />
This set level is usually referred to as the ‘float voltage’ of the transformer or substation.<br />
40
DESIGN OVERHEAD DISTRIBUTION SYSTEMS<br />
Figure 6(a). Load and Control Circuits<br />
Main Circuit Control Circuit<br />
ES Sending end voltage eT Voltage transformer output voltage<br />
EZ Line voltage drop eC Compensator voltage drop<br />
ER Receiving end voltage eV Regulating relay voltage<br />
RL Line resistance RC Compensator resistance<br />
XL Line reactance XC Compensator reactance<br />
IL Load current iC C.T. secondary current<br />
Figure 6(b). Phasor diagrams of load and control circuits<br />
The voltage relay senses both transformer output voltage, plus a compensating voltage which<br />
reflects the drop expected in the feeder, as below.<br />
To understand how the system works, firstly consider the simplest case, where the transformer<br />
output voltage drives the relay.<br />
The output of the transformer is measured by the voltage transformer. If the output voltage falls<br />
outside the set level (‘float voltage’) due to say increasing load, the voltage regulating relay will<br />
activate the tap-changer and change one tap position on the transformer, to raise the voltage and<br />
bring the output voltage back to its desired level. Conversely, as load falls off, the output voltage<br />
will start to rise and the voltage regulating relay will cause the transformer to change one tap back<br />
to lower the voltage and again bring it back to the desired level.<br />
We can also compensate for the drop in the feeders going out from the substation by circulating<br />
the output of the current transformer through adjustable resistance and reactance values (which<br />
are set to reflect the resistance and reactance values of the feeder) in the voltage sensing circuit.<br />
The drop in the model impedance ZC in the voltage regulating relay should be able to reflect the<br />
drop in voltage in the feeder, if properly set up.<br />
41
DESIGN OVERHEAD DISTRIBUTION SYSTEMS<br />
The voltage sensing relay would now cause the transformer to change taps in response to<br />
variations in the voltage at the load at the end of the feeder, rather than just at the terminals of the<br />
transformers at the substation.<br />
When the load is increasing, this would mean taps would change earlier than by sensing<br />
transformer output voltage alone and the transformer output voltage would be higher, but at the<br />
load end of the feeder the voltage would be kept at the desired level.<br />
This can be seen in the phasor diagram in Figure 6.<br />
The voltage transformer output et is a reflection of ES, the zone transformer terminal voltage.<br />
By subtracting from the voltage phasor et a voltage phasor ez which is proportional to the line<br />
voltage drop EZ, the resultant voltage ev (which controls the driving mechanism of the tapchanger)<br />
will represent the load voltage ER for all conditions.<br />
This compensation for the voltage drop in the line is called ‘line drop compensation’ (‘LDC’). It is<br />
usually set as a percent boost in voltage at a certain value of transformer load.<br />
In summary, if the LDC is zero, the transformer voltage regulation relay will change taps purely on<br />
transformer terminal voltage.<br />
When the LDC is set at some positive value, the transformer voltage regulation relay will change<br />
taps based on the transformer terminal voltage less the line drop voltage value.<br />
Types of voltage regulators<br />
Regulators<br />
The simplest and most commonly used method of boosting voltage on <strong>distribution</strong> lines by far,<br />
where capacity is not an issue, but where voltage variation is excessive (eg rural feeders) is via<br />
an auto transformer, usually simply (but not accurately) referred to as a ‘voltage regulator’<br />
(because as we will discuss below, there are many types of regulators).<br />
An auto transformer has one common coil instead of separate primary and secondary coils as<br />
with traditional transformers.<br />
Output voltage can be boosted by having more turns on the output tap or reduced (‘bucked’) by<br />
having less turns on the output tap position, as shown in Figure 7.<br />
Figure 7. Voltage Regulator (Auto-transformer)<br />
42
DESIGN OVERHEAD DISTRIBUTION SYSTEMS<br />
Taps are automatically changed by an on-load tap changer described above.<br />
Another device for controlling voltage which may be used by itself or in conjunction with a<br />
transformer is a regulator, of which there are two types:<br />
• induction regulators<br />
• moving coil regulators.<br />
An induction regulator consists of a stator and rotor, and is constructed in a similar manner to a<br />
wound rotor induction motor with flexible connections coming from the rotor which does not rotate.<br />
The angular position of the (stationary) shaft relative to the stator casing is controllable by means<br />
of a manually or motor driven geared wheel.<br />
One winding (the stator) is shunt connected across the lines which need their voltage to be<br />
controlled, whilst the other winding (the rotor) is connected in series with the load or <strong>overhead</strong><br />
line.<br />
Depending on the relative angular positions of the stator and the rotor, the shunt winding induces<br />
a voltage (v1) into the series winding, where the induced voltage may be in phase with the system<br />
voltage or may be up to 1800 out of phase.<br />
The result is that the output voltage can be varied in magnitude between the range:<br />
(V + v1) to (V – v1)<br />
where: V is the input voltage<br />
v1 is the injected series voltage<br />
The normal three phase arrangement has the disadvantage that it introduces a phase shift<br />
between the input and output voltages at all values except full boost and full buck.<br />
This is of no consequence when used on an individual supply, but precludes its use on<br />
interconnected networks.<br />
A moving coil regulator is constructed with two pairs of closely coupled shunt and series coils<br />
A1 - S1 and A2 – S2 respectively as shown in Figure 8.<br />
Figure 8. Circuitry of a moving-coil regulator<br />
43
DESIGN OVERHEAD DISTRIBUTION SYSTEMS<br />
The four coils are mounted on a common magnetic circuit, and a moving coil M is placed over the<br />
top of them.<br />
The moving coil M is short circuited onto itself, and at its limits of travel surrounds one or other of<br />
the pairs of fixed coils.<br />
The shunt coils A1 and A2 are connected with their voltage polarity additive and the series coils S1<br />
and S2 have their voltages in opposition.<br />
The mutual inductance of the short circuited coil M when in the top position, reduces the voltage<br />
across A1 to a minimum and increases that across A2 to a maximum.<br />
In this case, the voltage induced into S1 is a minimum and that in S2 is a maximum.<br />
The range of control on the output voltage depends on the ratios of S2:A2 and S1:A1.<br />
Boosters<br />
Another less common technique for making small adjustments to line voltages uses line booster<br />
transformers.<br />
There are two type of arrangements:<br />
• in phase booster transformers<br />
• quadrature booster transformers.<br />
An in phase regulating booster transformer is used to inject a variable voltage into a line circuit for<br />
voltage regulating purposes.<br />
This equipment would be used where it is desirable to obtain additional voltage control on the<br />
lines when loaded, and there is no wish to purchase a new transformer.<br />
A typical winding arrangement for an in phase booster is shown in Figure 9.<br />
The active conductors of the three phase system are indicated by AA’, BB’, CC’ respectively, and<br />
the relevant voltage levels are shown on the phasor diagram.<br />
Figure 9. Winding arrangement of a separate in-phase voltage regulating booster transformer<br />
44
DESIGN OVERHEAD DISTRIBUTION SYSTEMS<br />
Three series transformers ‘a’ have their secondary windings ‘b’ connected into the lines A-A’,<br />
B-B’, C-C’.<br />
The primary windings of these transformers ‘c’ are excited from the variable outputs of the three<br />
phase transformer ‘e’ whose primary windings are connected across the line ABC in star<br />
configuration.<br />
Variation of the tap-changer ‘x’ between the terminals ‘d to f’ will change the voltage injected into<br />
the line A-A’, B-B’, C-C’ through the transformers ‘a’.<br />
Quadrature boosters, or phase angle control units inject a voltage having a major component at<br />
90 0 electrical to the existing line voltage.<br />
This is achieved by combining voltages from different phases instead of the same phase. A<br />
general method of interconnection is shown in Figure 10.<br />
They are essentially a variation of the in-phase booster described above.<br />
Figure 10. Winding arrangement of phase-angle displacement transformer – quadrature booster<br />
By moving the tap-changer mechanism ‘x’ from terminal ‘g’ to ‘f’ the line voltage will be increased<br />
(‘boost’) and when going from ‘g’ to ‘d’ the line voltage will be reduced (‘buck’).<br />
Phase angle control equipment may be required when two circuits of different impedances<br />
carrying variable loads are connected at two points on the system.<br />
Starting at the point where the lines have their ends connected together and with the other ends<br />
of the lines disconnected, the different line impedances means that there will be a phase<br />
difference between the two voltages at the other ends of the lines when they each carry a current.<br />
This phase displacement will vary as the loads on the two feeder lines vary. When the two feeder<br />
lines are connected into the system the difference in voltages due to the phase displacement at<br />
their ends will cause a circulating current to flow.<br />
45
DESIGN OVERHEAD DISTRIBUTION SYSTEMS<br />
When a quadrature booster is used at the end of one of these lines it is possible to change the<br />
<strong>distribution</strong> of current in the feeders and minimise any circulating currents.<br />
Power factor correction<br />
While voltage control by means of tap changing transformers is the usual method in <strong>distribution</strong><br />
<strong>systems</strong>, power factor correcting capacitors can also have an effect on regulating voltages.<br />
The phasor diagram Figure 11 illustrates the effects on voltage regulation by adding capacitors to<br />
a load and so changing the power factor.<br />
The values of voltage supplied without capacitors connected is shown in full lines (ES) and with<br />
capacitors, which reduces the angle of current lag from Φ to Φ1, in dashed lines (ES1).<br />
Note how ES1 is smaller than ES, ie the voltage regulation is less.<br />
Figure 11. Voltage phasor diagram<br />
For the voltages before the capacitors are connected:<br />
OI = load current at uncorrected phase angle<br />
OER = receiving voltage or load voltage<br />
ERES = line voltage drop due to line current I<br />
OES = sending end voltage<br />
When the capacitors are connected the ‘in phase’ component of load current I remains the same,<br />
but the quadrature component is reduced resulting in a new load current I. Assuming that the load<br />
voltage ER remains constant, then:<br />
OI1 = load current at corrected phase angle<br />
OER = receiving voltage or load voltage<br />
ERES1 = line voltage drop due to line current I1<br />
OES1 = new sending end voltage<br />
It can be seen that the phasor OES1 is less than OES, and so a lower voltage is required at the<br />
sending end to keep the load voltage constant. Normal practice is to keep the sending end<br />
voltage constant and to switch capacitors in and out at the receiving end to adjust the receiving<br />
end voltage.<br />
46
DESIGN OVERHEAD DISTRIBUTION SYSTEMS<br />
Revision exercise 8<br />
1. Name the devices which control voltages in transformers.<br />
2. Describe briefly what is meant by ‘line drop compensation’. How does it work?<br />
3. List different types of voltage regulators.<br />
4. Explain what is meant by power factor correction and how it assists the <strong>distribution</strong><br />
system.<br />
47
DESIGN OVERHEAD DISTRIBUTION SYSTEMS<br />
Voltage profiles<br />
Voltage profile charts are useful for studying patterns and locating causes or reasons for<br />
abnormal voltage conditions.<br />
Put simply, a voltage profile is a chart which shows how the voltage varies as you travel out from<br />
the source of supply to the customers.<br />
To obtain the complete variation of voltage, one would need to draw two profiles—one for full load<br />
conditions, giving minimum volts at the customer, and one for no-load or light load, giving<br />
maximum volts at the customer.<br />
The chart will show all the voltage drops in the various elements which make up the <strong>distribution</strong><br />
feeder, eg:<br />
a) The supply voltage at the zone substation<br />
b) The voltage drop in the 11/22 kV feeder<br />
c) The voltage drop in the <strong>distribution</strong> transformer<br />
d) The voltage drop in the low voltage feeder running from the <strong>distribution</strong> transformer to the<br />
take-off point to the customer’s terminals<br />
e) The voltage drop in the customer’s service line, ie from the take-off point to the ‘customer’s<br />
terminals’.<br />
To make a chart, a common or reference voltage level is selected for the system and the circuit<br />
constants of resistance, reactance, etc. are converted to their effective values at this voltage by<br />
means of the formula:<br />
⎛ E<br />
⎞<br />
R<br />
⎠<br />
2<br />
R2 E ⎟<br />
1<br />
⎟ = ⎜<br />
⎝<br />
1<br />
where: R2 = resistance value effective at reference voltage E2<br />
R1 = resistance value effective at operating voltage E1<br />
48
DESIGN OVERHEAD DISTRIBUTION SYSTEMS<br />
Some practical examples<br />
1. Figure 12 shows a simple <strong>distribution</strong> system for a single consumer represented by a<br />
single line diagram with the voltage profile for full load conditions using a 415 volts base.<br />
Figure 12. Simplified single line diagram of a <strong>distribution</strong> system with voltage profile for full-load<br />
conditions.<br />
Here, we see the components of the voltage drop as we go outwards from the substation down<br />
the feeder to the customer.<br />
2. If we now consider a <strong>distribution</strong> system for the supply of four consumers a simplified<br />
single line diagram could be as shown in Figure 13.<br />
Figure 13. Simplified single line diagram of a <strong>distribution</strong> system for the supply of four customers.<br />
In this situation we are seeking to keep the voltage to all customers within the range ±6% of the<br />
nominal voltage.<br />
Of the four customers, A and B are on <strong>distribution</strong> transformer T1 close to the zone substation,<br />
whilst C and D are on <strong>distribution</strong> transformer T2 remote from the zone substation. Customers A<br />
and C are close to their respective transformers and customers B and D are remote from their<br />
respective transformers.<br />
49
DESIGN OVERHEAD DISTRIBUTION SYSTEMS<br />
The V values indicated on Figure 13 show the percentage impedance voltage drop per unit for<br />
each customer’s full load current.<br />
For example, if all customers take full load currents the voltage drop on feeder A = 1%, the<br />
voltage drop in T1 = 4% because of the two units of full load current, and the voltage drop in the<br />
high voltage feeder = 4% because of the two units of full load current taken by C and D. The light<br />
load conditions are taken as being 25% of full load current.<br />
Figure 14. Voltage profile for light and heavy loads - zone transformer with on load tap changer<br />
Figure 15. Voltage profile for light and heavy loads - zone transformer with on load tap changer,<br />
<strong>distribution</strong> transformer with off load tap changer<br />
50
DESIGN OVERHEAD DISTRIBUTION SYSTEMS<br />
Considering Figure 14, this indicates the voltage variations resulting from where the zone<br />
substation tap changer can vary the voltage but the <strong>distribution</strong> transformers are of fixed ratio.<br />
This is the typical arrangement in most power <strong>distribution</strong> <strong>systems</strong>.<br />
For a light load the zone transformer tap changer is set at 100%, and for full load the zone<br />
transformer might be adjusted to give 10% boost in voltage to compensate for the voltage drop in<br />
the feeder, ie the LDC boost is set at 10%.<br />
At full load with 10% voltage boost, the voltage at D is –6% and the voltage at A is +5%. For light<br />
load, with the zone transformer at 100%, the voltage at D is –4% and at the voltage at A is –1.5%.<br />
3. Considering Figure 15, this indicates the voltages resulting from a situation where the<br />
zone substation tap changer can vary the voltage and the <strong>distribution</strong> transformer T2 can<br />
provide a 2.5% boost using an off load tap changer. Again, for a light load the zone<br />
transformer tap changer is set at 100%, and for full load the zone transformer is adjusted<br />
to give 10% boost.<br />
At full load with 10% voltage boost, the voltage at D is –3.5% and the voltage at A is +5%. For<br />
light load, with the zone transformer at 100%, the voltage at D is –1.5% and at the voltage at A is<br />
–1%.<br />
Diagrams of this type show fairly simply the effects of changing the tap settings on zone and<br />
<strong>distribution</strong> transformers. In contemporary practice, the voltage drop calculations are done quickly<br />
and easily on computers.<br />
Revision exercise 9<br />
1. Describe briefly what a ‘voltage profile’ is. Where might you use one?<br />
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DESIGN OVERHEAD DISTRIBUTION SYSTEMS<br />
2. What is meant by the voltage regulation of a feeder? Express this as an equation.<br />
3. Name the causes of voltage regulation in a <strong>distribution</strong> feeder.<br />
4. What is meant by the inductance of <strong>overhead</strong> lines? What factors influence the inductance<br />
of a line?<br />
5. Name a major advantage that underground cables and ABC <strong>systems</strong> have in terms of<br />
inductance.<br />
52
DESIGN OVERHEAD DISTRIBUTION SYSTEMS<br />
Wires and Cables<br />
The qualities desired in electric-service wires, in so far as the construction is concerned, are<br />
mechanical strength, tenacity, and ability to resist corrosion or other deterioration. In ordinary<br />
practice the breaking strength required for a wire of a given span will depend entirely on the sag,<br />
because increasing the sag will reduce the wire tension approximately in proportion to the sag.<br />
Practical considerations, however, indicate a rather indefinite minimum, below which it is<br />
undesirable to go. Any surface injury, such as local pitting by arcs and nicks caused by careless<br />
handling, or any weakness in the material due to errors in manufacture, will have relatively greater<br />
effect on a small wire than on a large one. Moreover, such faults are more serious in solid wires<br />
than in stranded cables and in hard-drawn wire than in soft-drawn wire. In stranded cables, an<br />
injury to a single strand affects only a fractional part of the entire section; in hard-drawn wire the<br />
surface, or skin material, has approximately twice the unit strength of the interior mass, so that an<br />
injury will have a relatively greater effect on hard-drawn wire.<br />
Fortunately, however, the process of wire drawing insures a large amount of work per unit of<br />
mass, so that the finished product is a very homogeneous and trustworthy material. This quality,<br />
combined with the reduction in stress resulting from any increase in sag caused by stretching,<br />
explains the comparative immunity from mechanical failures.<br />
Copper<br />
The good qualities of copper wire are a matter of common knowledge, and as stated previously,<br />
its manufacture and method of use combine to make it almost unique as a material of<br />
construction. It is fairly immune from corrosive action, as ordinarily used in transmission-line work,<br />
although it is not absolutely indestructible. The principal sources of injury to copper are due to its<br />
softness and low melting point. The former renders it liable to nicks or broken strands in stringing<br />
and clamping, and the latter to burning by arcs.<br />
Unless the voltage is such that insulated or weatherproof wire affords some real protection, there<br />
is no logical structural reason for using it. Otherwise, it merely serves as an additional load and<br />
offers a greater diameter for sleet deposits, besides deteriorating far in advance of the rest of the<br />
construction and frequently hanging in unsightly streamers.<br />
Since the sheen of freshly strung copper is greatly lessened after exposure, it becomes<br />
problematical whether the attention of the casual observer would in reality be attracted more by<br />
the copper or by the size and spacing of insulators which cannot be disguised.<br />
Copper Covered<br />
A comparatively recent development in transmission-line wires is the use of a steel wire covered<br />
with copper. This product is produced by drawing out an ingot of steel which has been previously<br />
encased in a copper covering.<br />
The thickness of the shell of copper may be varied within wide limits, the usual commercial<br />
proportions being an amount of copper that, combined with the lower conductivity of the steel<br />
core, produces a wire having either 30 or 40 percent of the conductivity of a copper wire of the<br />
combined gage. Thirty percent copper-covered wire is about 5 percent stronger than 40 percent<br />
wire of the same gage. The thickness of the shell of copper is quite small, depending in part on<br />
the size of the wire.<br />
Such wire should, therefore, be handled at least as carefully as copper wire. Since the thickness<br />
of the copper decreases with the size of the wires in the cable, it is preferable, at least for<br />
<strong>overhead</strong> ground wires, to use the 40 percent grade or else to use cables of few strands.<br />
53
DESIGN OVERHEAD DISTRIBUTION SYSTEMS<br />
The steel from which the wire is drawn is a high-carbon steel having an ultimate strength of about<br />
90,000 lb. per square inch, and a correspondingly high elastic limit. During the process of copper<br />
coating and wire drawing, there is an annealing effect followed by hardening, the net result being<br />
to produce a wire having an ultimate strength not greatly below that of the original ingot material.<br />
In general, and with the grade of steel commonly used by manufacturing companies, the ultimate<br />
strength of copper-covered wire is from 20 percent to 40 percent greater than that of the<br />
corresponding sizes of hard-drawn copper.<br />
The principal uses of this material in transmission work are for <strong>overhead</strong> ground wires, telephone<br />
wires, and the power wires of the lighter and lower capacity lines, where little future growth of<br />
business may be expected.<br />
Aluminium<br />
Aluminium wire, as now used for power-line purposes, is usually employed in the form of<br />
stranded cables, and when so used is no longer subject to some of the troubles incident to the<br />
earlier installations. As a material, it is quite different from copper, although used for similar<br />
purposes. Therefore, in making price comparisons, it is necessary to consider not only the price<br />
per mile per unit of electrical rating, but also the changes in the general construction of the line.<br />
The conductivity of aluminium is about 60 percent, based on the Matthiesen standard for copper,<br />
making aluminium cables about 1.5 times the area and 1.25 times the diameter of copper cables<br />
having equal conductivity. As the specific weight of aluminium is about 0.33 that of copper, the<br />
weight of aluminium cable will be about 0.5 times that of copper cable having the same<br />
conductivity.<br />
The strength of aluminium is about 0.8 that of soft copper and 0.4 the strength of hard copper.<br />
The net result of these differences is best shown by a concrete example. Thus, a No.00<br />
aluminium cable has about the same conductivity as a No.1 stranded hard-drawn copper cable,<br />
and their other characteristics for a 400 foot span are as follows:<br />
Since the coefficient of expansion of aluminium is considerably higher than that of copper,<br />
aluminium cables are more affected by temperature changes. Relatively greater sags will<br />
therefore occur in hot weather, while at low temperatures there is a greater increase in tension<br />
due to the contraction of the material with its resultant decrease in sag. In addition, the lighter<br />
weight of aluminium renders it more liable to local displacement by wind pressure. It is necessary,<br />
as a result of these differences in the material, to provide greater pin separation and <strong>overhead</strong><br />
clearance for aluminium conductors. From a construction viewpoint, however, the lighter weight of<br />
54
DESIGN OVERHEAD DISTRIBUTION SYSTEMS<br />
aluminium does not possess any particular merit, except possibly greater ease of handling the<br />
reels and pulling out wire in stringing.<br />
The saving in dead load on the supports is negligible and is more than offset by the greater sag<br />
and separation required. Furthermore, the increased diameter imposes a greater wind load. The<br />
choice between aluminium and copper will depend on the relative cost of the two materials and<br />
their accompanying construction, together with the allowances which can be made for the scrap<br />
values of the two installations.<br />
Steel<br />
Steel cable can be obtained of almost any desired unit breaking strength, the commercial grades<br />
ranging from the low grade steel of guy wire, which has an ultimate strength of 60,000 lb. per<br />
square inch, to steels of 200,000 lb. or more.<br />
For transmission-line purposes steel cable is used chiefly for <strong>overhead</strong> ground wires or as power<br />
wires for very long spans. The occasional use of steel messengers for telephone or insulated<br />
cables does not involve any considerable quantity of such material employed on typical<br />
transmission lines.<br />
Steel cables for line work should always be galvanized, and should be larger than is actually<br />
required for strength. The galvanizing of cable is by no means as permanent a protection as the<br />
hot-dip, unwiped process applied to structural steel; therefore some allowance should be made<br />
for future corrosion.<br />
Despite the temptation to use small-gage cables made of the higher grade steels, on account of<br />
their greater strength, it is generally preferable to adhere to medium grades such as the Siemens-<br />
Martin. Guy-strand steel cable is the lowest commercial grade and its quality is relatively much<br />
lower than that of any of the higher grades, Large diameter cables, particularly of high-grade<br />
steel, are rather difficult to handle as they are very stiff. It should be noted that all cables of great<br />
strength require special clamping attachments for dead ending, the ordinary quota of clips and<br />
clamps being inadequate to transmit the tension.<br />
Telephone Wire<br />
Supporting a telephone circuit on long-span transmission-line structures introduces a difficulty, in<br />
that the small wires which are sufficient for telephone service do not have the mechanical strength<br />
to carry safely in long spans.<br />
It is almost impossible to string any ordinary telephone wires so that they will be reasonably<br />
secure on long-span lines. There have arisen, therefore, two general methods of procedure; one<br />
is to use larger and stronger wires; and the other to contemplate failure in the telephone circuit as<br />
a necessary evil.<br />
Solid steel wire No.6 BWG, sometimes called river crossing wire, has an ultimate strength about<br />
equal to that of No.00 stranded hard-drawn copper and can, therefore, be strung equally well in<br />
long-span lines.<br />
Catenary<br />
If two ends of an imaginary wire having perfect flexibility and uniformity of material but no ductility<br />
were supported at two points in the same horizontal plane the wire would take the shape of a<br />
curve known as the catenary. For all practical purposes it may be assumed that actual wires and<br />
cables possess the same characteristics. The curve of the wires between the supports is<br />
therefore known, if the span and sag are known. However, as the equation of the catenary is<br />
rather complicated, while that of the parabola, which closely resembles it, is simple, the latter is<br />
usually employed instead; thus,<br />
y²= ax<br />
"a" being an constant found by substituting the known value of sc for the point on the curve at the<br />
support, ie.,<br />
a = in which S is the length of the span and d is the sag<br />
55
DESIGN OVERHEAD DISTRIBUTION SYSTEMS<br />
Assume a uniform load on each lineal foot of the span, and imagine half of the span removed and<br />
the wire held in place by the tension T at the middle of the span. Then considering the moments<br />
about the remaining support we have the weight of the half span multiplied by its lever arm which<br />
is one-quarter of the span, equals the balancing force T multiplied by its lever arm d.<br />
Since the total weight W = the weight per foot, X the half span or W = we have<br />
X = Td<br />
Therefore = Td<br />
and T =<br />
or, the tension in the wire equals the weight per foot times the span squared divided by eight<br />
times the sag.<br />
It is necessary, however, to take into consideration the effect of a change in length of the wire due<br />
to temperature and loading, and a simple arrangement of formula in which this is done is given<br />
below. The following mathematical treatment is not new, but the writer has found the arrangement<br />
convenient:<br />
S = span, in feet.<br />
d = sag, in feet.<br />
W = load per lineal foot in plane of wire.<br />
A = area of wire, in square inches. .<br />
E = modulus of elasticity.<br />
0 = coefficient of expansion.<br />
t = change of temperature, in degrees.<br />
e = elongation or change of length, within elastic limit.<br />
L₀ = length, in feet, of imaginary wire (W = 0) at normal temperature.<br />
Loc = length, in feet, of imaginary wire, cold (t°F. below normal temperature).<br />
= length, in feet, of imaginary wire, hot (t°F. above normal temperature).<br />
Loh<br />
Index to Subscripts. -<br />
No subscript = normal conditions.<br />
c = cold: t°F. below normal + dead load.<br />
i = cold: ice load + dead load.<br />
cw = cold: wind load + dead load.<br />
iw = cold: ice + wind + dead load.<br />
h = hot: t°F. above normal + dead load.<br />
W iw is the resultant of the vertical dead + ice loads and the horizontal wind load.<br />
W cw is the resultant of the vertical dead load and the horizontal wind load.<br />
56
DESIGN OVERHEAD DISTRIBUTION SYSTEMS<br />
Stresses<br />
Substitute normal values in Eqs. 1, 2, 3, and 4. Assume values of Th, Tiw, Tc, Ti, or Tcw, such that<br />
Eqs. 5 and 6 will give identical values of dh, diw, etc. The tension that will give the same sag by<br />
Eqs. 5 and 6 (independently) is the tension resulting from that sag and the given loading.<br />
T = (1)<br />
L = S (2)<br />
e = (3)<br />
Lo = L – e (4)<br />
(t°. above normal, with dead load.)<br />
Loh = Lo (1 + 0 th) en = Lh = Loh + en<br />
dh = 0.612 (5)<br />
dh = (6)<br />
(t°. below normal, with dead + ice + wind loads.)<br />
Loc = Lo(1 – 0tc) eiw = Liw = Loc + eiw<br />
diw = 0.612 (5)<br />
diw = (6)<br />
(t°F. below normal, with dead load.)<br />
Loc = Lo(1 – 0tc) ec = Lc = Loc + ec<br />
dc = 0.612 (5)<br />
dc = (6)<br />
(t°F. below normal, with dead + ice loads.)<br />
Loc = Lo(1 – 0tc) ei = Li = Loc + ei<br />
di = 0.612 (5)<br />
di = (6)<br />
(t°F. below normal, with dead + wind loads.)<br />
Loc = Lo (1 – 0tc) ecw = Lcw = Loc + ecw<br />
dcw = 0.612 (5)<br />
dcw = (6)<br />
In Tables 1 to 8 are given the physical properties and the wind and ice loads for various wire<br />
gages.<br />
57
DESIGN OVERHEAD DISTRIBUTION SYSTEMS<br />
Table 1 – Properties of Wire Material<br />
It has been urged by some that using the 0.5 inch, ice and 8-lb. wind load with the parabolic<br />
formula for computing the stress in the wire docs not give results which accord with experience,<br />
since actual spans erected with less than the specified sags have not failed in service.<br />
On the other hand, it is sometimes claimed that allowing maximum stresses near the elastic limit<br />
is dangerous. The facts of the matter are:<br />
First, the true catenary formula is scientifically and mathematically correct within the elastic limit.<br />
Second, the parabolic formula, ordinarily used for simplicity, gives results which in the vast<br />
majority of cases are closer to the exact values than the actual wire stringing will be to the<br />
specified stringing.<br />
Third, the material of a wire catenary is more uniform in section, strength, and other<br />
characteristics than that of any other engineering structure; therefore the error of design is<br />
correspondingly less.<br />
Fourth, the stretch of a ductile material, such as copper, permits the sag to increase and the<br />
stress to decrease and, within limits, does not perceptibly decrease the cross-section at any point.<br />
Therefore, a loaded span stretches enough to relieve the stress and docs not fail unless the load<br />
is very excessive.<br />
Fifth, the specified maximum loading is an emergency loading and is not a general or frequent<br />
occurrence on any span or line.<br />
¹ The elastic limit used is in reality the yield point, or point of appreciable extension, as this value seems more<br />
applicable to wire stringing than that obtained by accurate laboratory tests.<br />
The facts of the matter are that the spans in service have either not been subjected to loads in<br />
excess of those required by the catenary formula to develop their elastic limit, or the wire has<br />
stretched and the sag has increased.<br />
Possibly there are two other reasons for seemingly overstressed lines giving satisfactory service.<br />
One is that the poles have bent or the wires slipped through the ties, thus temporarily increasing<br />
the sag.<br />
Second, the wires may have become tempered or hardened by tension, possibly by atmospheric<br />
changes or other action, and by becoming harder have been able to sustain a greater load.<br />
58
DESIGN OVERHEAD DISTRIBUTION SYSTEMS<br />
59<br />
Figure 1 Sags and Tension of No. 1 H.D.<br />
Copper<br />
Figure 2 Sags and tensions of No. 0<br />
H.D.Copper
DESIGN OVERHEAD DISTRIBUTION SYSTEMS<br />
Figure 3 Sags and Tensions on No. 00 H.D. Copper<br />
60
DESIGN OVERHEAD DISTRIBUTION SYSTEMS<br />
Table 2 Properties of Bare Stranded Copper Cable<br />
61
DESIGN OVERHEAD DISTRIBUTION SYSTEMS<br />
Table3 properties of solid copper<br />
62
DESIGN OVERHEAD DISTRIBUTION SYSTEMS<br />
Table 4 Properties of Stranded Aluminium Cable<br />
63
DESIGN OVERHEAD DISTRIBUTION SYSTEMS<br />
Table 5 Table 6<br />
64
DESIGN OVERHEAD DISTRIBUTION SYSTEMS<br />
Figure 4 Normal Sags, copper wires and cables Figure 5 Normal Sags, Aluminium Cables<br />
Table 7 Properties of Wire Material<br />
65