BY CREANOVA
ENGINEERS…www.creanovaengineers.blogspot.in
A SIMPLE EXPLANATION ABOUT THE
ELECTRICITY
ELECTRICITY SUPPLY
GENERATORS
A
simple AC generator inducing an
emf by changing the area of a coil.
emf by changing the area of a coil.
Faraday's
and Lenz's laws tell us that if one end of a magnet is plunged into and out of
a coil of wire, the induced voltage alternates in direction at the same
frequency as the motion of the magnet. As the magnetic field strength inside
the coil is increased (magnet entering) , the induced voltage in the coil is
directed one way. When the magnetic field strength diminishes (magnet leaving),
the voltage is induced in the opposite direction. The greater the frequency of
the field change, the greater the induced voltage. This is the bases for a
simple generator converting mechanical energy into a simple AC electrical
output.
Rather
than moving the magnet, it is more practical to move the coil. This is best
accomplished by rotating the coil in a staionary magnetic field. The rotating
coil changes the area exposed to the magnetic field and hence the flux through
the coil. From Faraday's law, a change in flux induces an emf or volatge in the
coil. This arrangement (shown on the right) is known as a generator and
it is essentially the opposite of a motor. Whereas a motor converts electrical
energy into mechanical energy, a generator converts mechanical energy into
electrical energy.
An
early generator of the type that Faraday would have
invented. Using the left hand palm rule, electrons flow from
Brush Y to the centre of the disc and subsequently to Brush Y.
invented. Using the left hand palm rule, electrons flow from
Brush Y to the centre of the disc and subsequently to Brush Y.
In
the years of 181 - 1832, Michael Faraday discovered the operating principle of
electromagnetic generators now known as Faraday's law. He also built the first
electromagnetic generator (left), called the Faraday disk, a type of generator
producing a DC curent using a copper disc rotating between the poles of a
horseshoe magnet. It produced a small DC voltage.
This
design was inefficient due to self-cancelling counterflows of current in
regions of the disc that were outside of the influence of the magnetic field.
It was also not possible to add multiple current paths (such as more loops) and
this further reduced the output and efficiency of such a generator.
AC GENERATOR OPERATION AND OUTPUT
A
simple AC generator with a slip-ring commutator. The axle is being
turned anticlockwise by some external source of mechanical energy.
The lower diagram shows the AC output and how it corresponds to
the relative position of the armature.
turned anticlockwise by some external source of mechanical energy.
The lower diagram shows the AC output and how it corresponds to
the relative position of the armature.
The
most basic of generators looks like a motor with almost the same components
(shown right). The axle is turned by a source of mechanical energy and a
current (or emf) is induced in the coil. In the vertical position shown, the
emf is zero because the rate of change of flux is zero at that instant. After
rotating anticlockwise through 90°, the current in the coil increases to a
maximum. After another 90° the emf returns to zero again as the coil is in the
vertical postion again. It is at this point that the current changes direction
because now the flux is passing through the coil in the other direction.
You
should remember that in a DC motor, the current in the coil would also reverse
once every half-cycle, but the split ring commutator ensures that it always
flow in the same direction and this keeps the torque in the same direction. In
this AC generator, there is no source current that needs to flow in the same
direction such as a motor. The purpose of a generator is to produce a current.
Without a split-ring commutator, the current in the coil and the external
circuit reverses once every half-cycle. A slip-ring commutator is used instead
and this produces an AC sinusoidal output.
The
result of increasing the speed of rotation of an AC generator on
its AC output.
its AC output.
You
should also remember from Faraday's law that a change in flux will induce an
emf, but if the change in flux happens more quickly, a larger emf will be
induced. If we apply this principle to the operation of a generator, turning
the generator at a faster speed will (a) increase the size of the emf, and (b)
increase the frequency of the AC output.
The
solid line in the graph on the left indicates the output from a generator
turning with a frequency of 10 Hz (or a period of 0.1 s). If the frequency of
rotation increases to 20 Hz (period of 0.05 s) as shown by the dotted line, the
maximum emf doubles. This is consistent with Faraday's law which says that the
size of the emf is directly proportional to the rate of change of flux. If we
double the frequency of rotation, we double the rate of change of flux and
hence the maximum emf.
Electric
power generating stations provide AC electric power to domestic and industrial
consumers. In a power station, mechanical or heat energy is converted into
electrical energy by means of a turbine connected to a generator. A turbine is
a machine whose shaft is rotated by jets of steam, water or wind directed onto
blades attached to a wheel. The generators used in power stations have a
different structure to those studied so far. A typical generator has an output
of 22 kV. This requires the use of massive, heavy coils which would place huge
forces on bearings if they were required to rotate. To eliminate this problem,
a power station generator has stationary coils mounted on an iron core (stator).
The coils are linked in pairs on opposite sides of the rotor. The rotor is an
electromagnet that spins with a frequency of 50 Hz. This means that electricity
is supplied to consumers as AC with a frequency of 50 HZ. Said another way, the
coils are stationary and the magnets rotate between them at a frequency of 50
cycles per second (Hz).
The turbine and generator in a typical hydro-electric power station. The turbine is moved by water which causes the
rotor of the generator (an electromagnet) to turn which induces an AC current in the stator.
A
rectifier is an electrical device that converts AC to DC. Since electricity is
supplied as AC and most small electrical devices in the home run on DC, a
rectifier is needed to convert operating currents from AC to DC. The rectifier
for a printer or laptop computer, for example, is usually located in a large
box with the transformer attached somewhere along the length of the power
cable.
A
rapidly varying electric current (AC or DC) is unsuitable, however, for most
electrical appliances and uses. Many such applications require a steady almost
constant emf. If your lights were supplied by a 50 Hz AC power supply for
example, you would see them flickering and the current rose and fell. The
emf/current can be smoothed by using three or more coils in a generator, offset
from each other by 120°. Each coil produces single phase AC which is
transmitted over power lines separately from the other two phases. When the
current from all three phases is superimposed, a smother output is produced
(either AC or DC, depending on whether a rectifier has been used). This
three-phase supply is commonly used in industrial and domestic power supply and
uses generators with three sets of stator coils.
DC GENERATORS
DC
generators are sometimes called dynamos and are able to produce a small but
reliable direct current. Such devices use a split-ring commutator so that the
alternating current in the coils is converted to a direct current in the
external circuit. Dynamos are used for small scale DC power production such as
flash lights on a bicycle or a generator that uses petrol (or some other fuel)
to produce electricity when there is no mains electricity supply.
The
output from a DC generator would look the same as that shown above for a single
phase AC generator.
AC VS DC GENERATORS
An
AC generator produces a sinusoidal output voltage through the use of
slip rings. The slip rings rotate with the coil and are connected to the
external circuit with brushes. The use of slip rings in a generator will
produce an alternating current and these types of generators are called AC
generators.
A
simple DC generator produces an direct current through the use of a
split ring commutator. The split ring commutator changes the direction of the
output current every half cycle to ensure that it is always in the same direction
in the external circuit. You should note that the current is still changing
every half cycle in the coil. It is only the current in the external circuit
that is operating in the same direction.
COMPARISON
OF GENERATORS TO ELECTRICAL MOTORS
A
DC electric motor will have the same stucture as a DC generator. The difference
between them, however, is inherent in their function. Generators convert
kinetic energy into electrical energy while motors covert electrical energy
into kinetic energy.
ADVANTAGES
AND DISADVANTAGES OF USING AC AND DC GENERATORS
The
relative advantages and disadvantages of AC and DC generators relate to two
features of their design. Firstly, DC generators use a split-ring commutator,
while AC generators use slip rings. Secondly, DC generators produce an output
current that is induced in the rotor, whereas the roles of the rotor and the
stator can be reversed in an AC generator.
The
commutator of a DC generator consists of a number of metal bars separated by
narrow gaps filled with insulating material. As the brushes remain in contact
with the commutator under spring pressure, they are constantly striking the
leading edge of each successive bar. This wears the brushes and they need to be
replaced regularly. The commutator bars also wear down until the insulating
material between them prevents the brushes from making proper contact with the
bars, reducing the efficiency of the generator. Pieces of metal worn from the
commutator bars can become lodged in the gaps, causing a spark between bars and
reducing the output of the generator. In contrast, the slip rings of an AC
generator have continuous, smooth surfaces, allowing the brushes to remain
continuously in contact with the slip ring surface. Thus the brushes in an AC
generator do not wear as fast as in a DC generator. There is no possibility of
creating an electrical short circuit between segments in an alternator because
the slip rings are already continuous. An AC generator therefore requires less
maintenance and is more reliable than a DC generator. Most commercial
generators are AC generators.
In
a DC generator the current is generated in the rotor and is then drawn from the
windings through the commutator and out via the brushes. The larger the current
required, the heavier the rotor coils must be, placing high demands on bearings
and supporting structures. In addition, drawing large currents through the
commutator-brush connection increases the likelihood of electric sparks forming
as the brush breaks contact with each bar in turn. This reduces the efficiency
of the generator and limits the usefulness of DC generators to relatively low
current applications.
In
an AC generator designed for high current applications, such as in a power
station, the current is produced in the stator windings rather than in the
rotor. The rotor is used to create the field magnetization that induces the AC
current in the stator when the rotor is rotated. It is much easier to draw the
current through a fixed connection in the stator rather than through a commutator
from a moving rotor. Thus AC generators are better suited to high current
demands than DC generators.
An
advantage of a DC generator is that its output can be made smoother by the
arranging many coils in a regular pattern around the armature. The brushes are
arranged to make contact only with the commutator bars corresponding to the
coils producing the greatest emf at a particular time. The result is an output
voltage that “ripples” about a mean value rather than fluctuating between zero
and the maximum twice per revolution. The more coils, the smoother the output
DC voltage ripple. This is an advantage for use with equipment that needs a
steady voltage rather than a sinusoidally varying voltage. This cannot be
achieved with an AC generator without the addition of a rectifying and
smoothing circuit.
An
advantage of AC generators is that they can easily be designed to produce
three-phase electricity by the use of six stator poles and a single
electromagnet rotor. The coils are mounted in opposing pairs spaced evenly
around the stator, and connected in pairs to the three phases of the power
supply. The rotor induces alternating current in successive pole pairs. The
sinusoidally varying voltages are then 120 degrees out of phase with each
other. AC generators are ideal for generating electricity on a large scale for
distribution over a wide area.
EFFECT OF GENERATORS ON SOCIETY AND THE ENVIRONMENT
The
development of AC generators has led to the widespread application of some of
the useful features of AC electricity. AC generators are simpler and cheaper to
build and operate than DC generators. Because AC electricity can easily be
transformed, it can be transmitted cheaply over great distances, allowing a
wide range of primary energy sources to be exploited. This has allowed the
development of extensive, reliable AC electricity networks for domestic and
industrial use throughout much of the world. This in turn has had both positive
and negative effects on society and the environment.
The
affordability of electricity has promoted the development of a wide range of
machines, processes and appliances that depends on electricity. Many tasks that
were once performed by hand are now accomplished with a purpose-built
electrical appliance and most domestic and industrial work requires less
labour. Other new tasks can now be achieved that were formerly impossible, such
as electronic communication. However, this has led to a reduction in the demand
for unskilled labour and an increase in long-term unemployment. The ready availability
of electricity has led to increasing dependency on electricity. Essential
services such as hospitals are forced to have a back-up electricity supply,
“just in case”. Any disruption to supply compromises safety and causes
widespread inconvenience and loss of production. A major electricity failure
can precipitate an economic crisis. The global electricity industry lobby is
very powerful but is not always just. Social values may give way to economic
pressures, especially in developing countries where often the poorest people
lose their livelihood to make way for new energy developments.
AC
power generating plants can be located well away from urban areas, shifting
pollution away from homes and workplaces, thus improving the environment of
cities. However, many environmental effects of the growth in the electricity
industry are negative. Power transmission lines criss-cross the country with a
marked visual impact on the environment, often cutting a swathe through
environmentally sensitive areas. Remote wilderness areas can easily be tapped
for energy resources such as their hydro-electric potential. Air pollution from
thermal power stations burning fossil fuels may be a cause of acid rain. In
addition it contributes to the global increase of atmospheric carbon dioxide
which may be linked to long-term global climate change. Nuclear power stations
leave an environmental legacy of radioactive waste that will last many
thousands of years.
The
effects of the development of AC generators on society and the environment have
been far-reaching. Some effects have changed the way people live, but not
always for the better. Many people now enjoy increased convenience and leisure,
many new industries flourish on new technologies made possible by electricity,
but the dislocation and unemployment experienced by some can be devastating.
Many aspects of the development of electricity have led to environmental
degradation, often in remote areas where the long-term effects are poorly
understood. These effects seem likely to be ongoing, as the compromise between
economic interests and social and environmental values often favours the
economic. We have not yet learned to live with AC electricity in a sustainable
way.
Consider
a pair of coils side by side as shown in the diagram on the right. In (a), one
is connected to a battery and the other is connected to a galvanometer. It is
customary to refer to the coil connected to the power source as the primary
(input) and the other as the secondary (output). As soon as the switch is
closed in the primary and the current passes through its coil, a current is
induced in the secondary coil, even though there is no material connection
between the two coils. Only a brief surge of current occurs in the secondary,
however. Then when the primary switch is opened, a surge of current again
registers in the secondary but in the opposite direction. The explanation is
that the magnetic field that builds up around the primary extends into the
secondary coil. Changes in the magnetic field of the primary are sensed by the
nearby secondary. These changes of magnetic field intensity at the secondary
induce voltage in the secondary according to Faraday's law.
If
we place an iron core inside the primary and secondary coils of the arrangement
(b), the magnetic field within the primary is intensified by the alignment of
the magnetic domains in the iron. The magnetic field is also concentrated in
the core, which extends into the secondary, so the secondary intercepts more of
the field change (flux). The galvanometer will show greater surges of current
when the switch of the primary is opened or closed.
Instead
of opening and closing the switch to produce the changing flux, suppose that
alternating current is used to power the primary. The rate of change of flux in
the primary and hence in the secondary is equal to the frequency of the
alternating current. Now we have a transformer.
Voltages
may be stepped up or stepped down with a transformer. To see how, consider the
simple case shown in the diagram below right.
The
operation of a simple transfomer in stepping up a voltage.
Suppose
the primary consists of one loop connected to a 1 V alternating source.
Consider the symmetrical arrangement of a secondary of one loop that intercepts
all of the changing magnetic fields lines of the primary. Then a voltage of 1 V
is induced in the secondary as in (a). If another loop is wrapped around the
core so that the transformer has two secondaries, (b), it intercepts the
magnetic field change. We see that 1 V also. There is no need to keep both
secondaries separate, for we could join them (c) and still have a total induced
voltage of 1 + 1 = 2 V. This is equivalent to saying that a voltage of 2 V will
be induced in a single secondary that has twice the number of loops as the
primary. If the secondary is wound with three times as many loops as the
primary, three times as much voltage will be induced. Stepped-up voltage may
light a neon sign or operate the picture tube in a television receiver or send
power over long distances.
If
the secondary has fewer turns than the primary, the alternating voltage
produced in the secondary will be lower than that produced in the primary. The
voltage is said to be stepped down. The stepped-down voltage may safely operate
a toy electric train. If the secondary has half as many turns as the primary,
then only half as much voltage is induced in the secondary. This means that
electrical energy can be fed into the primary at a given alternating voltage
and taken from the secondary at a greater or lower alternating voltage,
depending on the relative number of turns in the primary and secondary coil
windings.
Step-up
transformers are commonly used at power stations to increase voltage and reduce
current for long-distance transmission. Step-up transformers are also used in
older style televsision sets with a cathode ray tube to step up the voltage so
that electrons can be accelerated towards the screen from an elecrton gun.
Step-down
transformers are used at substations and at other places in the electricity
grid to convert transmission lines voltages to acceptable levels for domestic
and industrial use. They are also used in computers, radios, CD players and
other small electrical appliances in combination with a rectifier to reduce
household electricity to very low voltages for these electronic components.
The
equation below can be used to calculate the voltage in the primary or secondary
coil based on the ratio between the number of coils in the primary and the
secondary.
Equation - Voltage in a
Transformer
|
|||
Vp
Vs
np
ns
|
voltage
in primary coil
voltage
in secondary coil
number
of turns in primary coil
number
of turns in secondary coil
|
volts
(V)
volts
(V)
no
units (integer)
no
units (integer)
|
CONSERVATION OF ENERGY
Conservation
of
energy in a
transformer.
energy in a
transformer.
It
might seem that you get something for nothing from a transformer that steps up
voltage. Not so, for energy conservation always regulates what can happen. When
voltage is stepped up, current in the secondary is less than in the primary.
The transformer actually transfers energy from one coil to the other. The rate
at which energy is transferred is called power. The power used in the secondary
is provided by the primary. The primary gives no more than the secondary uses,
in accord with the law of conservation of energy. If the slight power losses
due to heating of the core are neglected, then since power is the product of
voltage and current, we can see from the equations in the box on the right that
energy is always conserved.
We
see that if the secondary has more voltage than the primary, it will have less
current than the primary to ensure that whatever energy goes into a
transformer, the same energy must come out.
The
ease with which voltages can be stepped up or down with a transformer is the
principal reason that most electrical power is AC rather than DC.
MAKING TRANSFORMERS MORE EFFICIENT
A
transformer has an iron core to concentrate the magnetic field to achieve the
maximum possible inductive coupling between the primary and secondary coils. As
the changing flux intersects the core, eddy currents are induced in the iron.
Heating occurs because of the rather high resistance of the iron to the eddy
currents. This heat represents a power loss to the electrical system and excessive
heating can damage or destroy the transformer.
Eddy
currents in (a) an ordinary iron core, and (b) a laminated iron
core.
core.
One
of the best ways to overcome difficulties of heating in transformers is to
reduce the size of the eddy currents. Transformer cores are made of laminated
iron, that is, many thin sheets of iron pressed together but separated by thin
insulating layers. This limits the circulation of any eddy currents to the
thickness of one lamina, rather than the whole core, thus reducing the overall
heating effect.
Once
the transformer does get hot it must be cooled to prevent overheating. Several
strategies have been developed to keep transformers cool:
- Heat-sink fins are added to the metal transformer case so that heat dissipation to the environment can occur more quickly over a larger surface area.
- The transformer case may be made of a black material so that the heat produced internally is efficiently radiated to the environment. Most small transformer-rectifier units found around the home are coloured black.
- Pad-mounted transformers at ground level have ventilated cases to allow air to remove heat by convection. They may also have an internal fan to assist air circulation to remove excess heat faster.
- The transformer case may be filled with a non-conducting oil that transports the heat produced in the core to the outside where the heat can be dissipated to the environment. The oil may circulate from hotter to cooler regions by convection alone, or circulation may be assisted by a pump. The case may have design features such as cooling tubes and radiator slats to increase the rate of heat dissipation.
- Large transformers such as at substations are always located in the open or in well-ventilated areas to maximise airflow around them for cooling. These are fitted with a combination of cooling mechanisms including pumps to circulate cooling oil through large radiators, and fans to increase the airflow over the radiators. The fans are often thermostatically controlled and cut in at a specified temperature, usually around 50°C.
TRANSFORMERS IN DOMESTIC APPLIANCES
Electricity
supplied to homes is typically 240 V AC. Many domestic appliances are designed
to run most efficiently at this voltage. Such appliances are connected directly
to the mains supply without the need for a transformer.
Some
appliances contain components that require a transformer because they operate
best at lower voltages than the mains supply. In a microwave oven, for example,
large, energy consuming parts such as the turntable motor and the microwave
transducer may be connected directly to the mains, while the control and
display panel is supplied with low voltages from a step-down transformer in a
built-in power supply unit.
Many
small portable appliances, such as personal CD players and mobile telephones,
have been designed to run on batteries. These require low DC voltages, either
as an alternative to batteries or to recharge the batteries. When the whole
appliance is designed to run at the same low voltage, a step-down transformer-rectifier
may be built into the plug of the power supply lead that connects to the mains
supply. Alternatively, a normal power lead connects the mains to a built-in
power supply unit that contains a step-down transformer and a rectifier.
Appliances
such as television receivers and computer monitors contain cathode ray tubes
that require voltages well above the mains supply, up to around 25 kV, to
accelerate electrons toward the screen. These use a built-in step-up
transformer to provide the necessary voltage. The power supply unit may contain
both a step-up and a step-down transformer.
IMPACT OF TRANSFORMERS ON SOCIETY
The
development of transformers made it possible to transmit electrical energy
efficiently over great distances. This has had a range of impacts on society.
Even
very remote communities now have access to grid-supplied high-voltage
electricity which is stepped down locally by transformers. This has raised
living standards in rural communities through provision of, for instance,
electric lighting, refrigeration and air conditioning, and increased the scope
of rural industries.
Large
cities have been allowed to spread, because electricity is readily available as
an energy source, thanks to transformers. This has led to social dislocation in
urban “deserts”, as people have moved further from family and friends and
workplaces.
Industry
is no longer clustered around power stations or other sources of energy. Power
stations can be in remote locations and high-voltage electrical energy can be
distributed almost anywhere, to be stepped down near the point of use. This has
allowed industries to be decentralised and has facilitated the development of
industrial areas away from residential areas. This has relocated pollution away
from homes, but it means that many people now spend significant time travelling
between home and work.
With
the development of the transformer, people have changed the way they live, as
electricity to every home has become an affordable necessity rather than a
luxury.
Electric
power transmission is the bulk transfer of electrical energy, from generating
power plants to electrical substations located near demand centers and then
between higher voltage substations and industry or homes. Transmission lines,
when interconnected with each other, become transmission networks. These are
typically referred to as "power grids" or just "the electricity
grid".
Electricity
is typically consumed in homes and industry at 240 V and 415 V, respectively.
If there were no transformers, electricity would have to be generated and
distributed at these same voltages. To supply the power demands of even a small
town, the current at these voltages would be very large, leading to large and
costly transmission losses and possible overheating of conductors. If the power
demand were to increase, the number of conductors would need to multiply, to
keep the current per conductor within reasonable limits.
For
a large city there would need to be many power stations spaced every few
kilometres. If different voltages were needed, these would require separate
power stations and separate distribution systems, adding to the network of
cabling required. The result would be an expensive, unsightly, unreliable web
of cables serving consumers only within a limited distance from each power
station.The use of transformers with AC electricity overcomes many of these
problems.
It
is more efficient to generate electricity at high voltages, such as 12 kV, than
at low voltages. Power stations can run efficiently at their design voltage and
different transformers can be used to simply step the voltage up for
transmission or down for local use as required.
It
is much more efficient to use very high voltages, up to 500 kV, for
transmission lines, because at these voltages the currents are relatively small
and transmission line losses are less of a problem(see below). The higher the
voltage, the smaller the line losses, and the greater the distance of
transmission, the more important this saving is. Because the current is smaller
at high voltages, fewer, smaller conductors are necessary for any particular
power load than at lower voltages. High voltages are easily achieved for
economical transmission by the use of step-up transformers.
Electrical
energy is usually consumed at low voltages, but at widely scattered locations.
Transformers are used to progressively step the voltage down from the
transmission lines to the consumer. Major transmission lines in the national
grid typically carry 330 kV. At regional sub-stations, step-down transformers
reduce this to 110 kV for regional distribution. Local sub-station transformers
typically step this down further to 11 kV for distribution along suburban
streets. Pole-mounted transformers step this down again for supply to houses
and factories at 240/415 V. The stepped-down voltage used at each stage of
distribution is chosen to balance the power, and hence the current,
requirements, and therefore also the transmission losses, against the area over
which distribution is required.
Typical values for the voltage at various points in the electricity grid from generator to consumer.
RESISTIVE ENERGY LOSSES
Heat
is generated in transmission lines because of the resistance of the wires. The
resistance per kilometre is small, but the resistance of a long transmission
line is significant. Distances are often great, up to hundreds of kilometres,
because power stations are often located in remote places, close to the primary
energy source such as a major coal field or a system of dams for a hydroelectric
scheme, rarely close to consumers in the city.
The
power loss in transmission lines is given by the relationship: P = VI or P = I2R.
Power loss is proportional to the square of the current. As the resistance of
the conductor is relatively constant, power loss is affected most by the size
of the current. Increasing the current by a factor of two increases the power
loss by a factor of four.
Energy
losses are kept to a minimum by transmitting the electricity at the highest
practicable voltage, with the lowest practicable current. Generally, the
greater the distance, the higher the voltage. Closer to the consumer, voltages
are lower but energy losses are not substantial since distances are shorter and
the current is shared by many separate distribution lines.
The
type of electricity transmitted over long distances is predominantly AC, since
AC can be changed easily to high voltages and correspondingly low currents by
the use of a step-up transformer. With advances in solid state technology it is
becoming easier to step DC voltages up and down, and DC is increasingly being
used for long distance power transmission.
Energy
losses can also be minimised through careful choice of materials and design of
conductors. Transmission lines are typically made of either copper or
aluminium, as these metals have low resistivity, that is, they are good
conductors. Resistance is inversely proportional to the area of cross-section
of the conductor, so the thicker a conductor, the lower the heat losses.
However, heavier conductors require more expensive support structures.
Aluminium has higher resistivity than copper but it is much lighter than
copper, and less susceptible to corrosion. The smaller weight and lower
maintenance costs more than compensate for the larger diameter of aluminium
needed to carry a certain current. Recent experiments with superconducting
materials show some promise for reducing energy losses from high voltage
transmission lines even further in the future.
For
energy losses to be minimised, the transmission voltage must be very high. This
requires high poles or towers and large insulators. These are expensive to
build and maintain and have an adverse effect on the visual environment. Trees
must be kept well clear of high voltage transmission lines to avoid damage to
the lines during storms and to reduce the possibility of a short to earth. This
often requires a wide corridor to be cleared, sometimes through environmentally
sensitive areas.
TRANSMISSION LINES AND TOWERS
High
voltage transmission lines (above)
and ceramic insulators.
and ceramic insulators.
High
voltage transmission lines are kept away from their supporting structures by
chain insulators to reduce the likelihood of a discharge between the conductor
and the support structure. Insulator chains can be up to around 2 metres in
length: generally, the higher the voltage, the longer the chain.
Insulators
are constructed either of ceramic segments joined together with metal links or
of rubber discs with a fibre glass core. Their design reduces the possibility
of charge leaking through the insulators themselves. The metal links in ceramic
insulators are isolated from each other, and the fibreglass is a non-conductor,
so there is no continuity of conduction. The insulator segments are designed to
shed water and prevent dust from building up, as either moisture or dust can
make a conductive path across the surface of the insulator. The disc-like shape
of the segments, whether ceramic or rubber, ensures a long pathway for any
spark discharge across the insulator.
Transmission
lines and supporting structures have a number of protective features associated
with their design. In the event of a transmission tower being struck by
lightning, the metal tower itself acts as a conductor to take the charge to the
ground. The towers are well earthed, with a large surface area of metal buried
in the ground, enabling the charge from any lightning strike to dissipate
harmlessly in the earth. Towers are widely spaced to ensure that, should one
tower be struck, the adjacent towers suffer no damage from the lightning
strike.
Not
all the wires on a transmission tower carry the electric current. The uppermost
wires are called shield conductors, as they are designed to reduce the chance
of a lightning strike to the transmission wires. Shield conductors are
connected directly to the transmission towers without the use of insulators so
that they can conduct charge between the clouds and the earth as it builds up,
to neutralise the charge distribution. If the shield conductors are struck
directly by lightning the current is conducted safely to earth.
In
the late nineteenth century, Edison favoured generating and supplying direct
current (DC) electricity while Westinghouse promoted the use of alternating
current (AC) electricity.
Edison
had the initial advantage that the technology for generating DC was well
established and DC worked well over short distances. However, DC could only be
generated and distributed at the voltages at which it was used by consumers. This
meant that currents in conductors were large, leading to huge and expensive
energy losses over distances of more than one or two kilometres. To supply a
large city required many power stations throughout the city and an unattractive
proliferation of wires to carry the required current.
The
great advantage of AC was that, through the use of transformers , the voltage
could be stepped up or down as required. This meant that AC could be generated
at moderately low voltages, stepped up to high voltages for transmission over
great distances and stepped down again to lower voltages for consumers. The
higher voltage meant that AC could be transmitted over greater distances than
DC, with smaller energy losses. Power stations could be fewer and further apart
and conductors could be lighter.
The
economic advantages of AC, including the smaller energy losses and the economy
of scale in needing fewer power stations further apart, along with the
unattractive web of wires required for DC, supported Westinghouse's solution to
the supply of electricity over Edison's. AC received a boost in popularity with
Tesla's invention of the induction motor which operates only on AC.
Competition
was not always open and fair. Edison had a vested interest in DC as he owned
hundreds of DC power stations and all of his many electrical inventions to that
time ran on DC. Edison attempted to prove that AC was very dangerous by
electrocuting animals on stage and convincing authorities to use AC for the
first electric chair. He resorted to legal tactics in an attempt to have AC
banned and to prevent its use with his inventions. Edison seems to have
unreasonably shunned AC electricity. AC eventually came to be the dominant form
in which electricity is generated world-wide.
But
DC has the advantage of not causing losses through electromagnetic radiation or
magnetic induction. With solid-state switching it is now relatively simple to
change between DC and AC at high or low voltages. High voltage DC transmission
is now practicable. Scientists are striving to develop super-conducting wires
for power transmission. If they do, DC could become the preferred current for
long distance transmission. There is already a 500 kV DC submarine transmission
line carrying 2800 MW over 50 km between the two islands of Shikoku and Kansai
in Japan.
BY CREANOVA
ENGINEERS…www.creanovaengineers.blogspot.in
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