9.3.D - Electricity Supply


Generators operate using Faraday's law of electromagnetic induction and convert mechanical energy to electrical energy. They are the workhorses of the electricty grid, producing billions of joules of electrical energy for domestic and industrial use.

Transformers are used in the electricy grid to change the voltage at various points in the network. Transformers also operate on the principle of electromagnetic induction and Faraday's law. Transformers can be used because AC (not DC) electricity is generated and transmitted through the grid. Transmitting electricity over long distances at high voltages keeps energy losses to a minium and ensures that electricity is affordable and that its generation is kept away from urban centres.

In this topic students:

  • Describe and explain the operation of AC and DC generators and their outputs,
  • Describe and explain the operation of transformers, and
  • Describe and explain how electricity is transmitted over long distances from generation to consumer and how energy losses are minimised.


Generators

A simple AC generator inducing an
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.

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.

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.

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.


The the DC output from a rectifier or DC generator with
a split-ring commutator.

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 as shown below.



The DC output from a three-phase AC generator when a rectifier is ised to convert AC to DC

 

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.

Transformers

The simplest transformers showing the
characteristic primary and secondary
coils.

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.

A typical transformer showing the magnetic flux being
transferred from the primary to the secondary via a soft
iron core.

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)

You should use this equation when determining the primary or secondary voltage based on the ratio of the number of turns or the ratio of turns in the primary and secondary to produce the desired voltage output in the secondary.

 

Conservation of Energy

Conservation of
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.

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.

 

Electricity Transmission

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 (below).

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.

 

Edison v Westinghouse

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.

Check out this website which is an excellent summary of the conflict between the two electricity pioneers and the reasons for it.