Motors and generators

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1 Gill Sans Bold Physics HSC Course Stage 6 Motors and generators Part 4: Transmission IncorporatingOctober2002 AMENDMENTS

2 Number: Title: Motors and generators This publication is copyright New South Wales Department of Education and Training (DET), however it may contain material from other sources which is not owned by DET. We would like to acknowledge the following people and organisations whose material has been used: Extracts from Stage 6 Physics Syllabus Board of Studies, NSW 2002 Part 1, 2, 3 pp 2,3, Pt 4 p 2, Pt 5 pp 2, 3, Pt 6 p 2 COMMONWEALTH OF AUSTRALIA Copyright Regulations 1969 WARNING This material has been reproduced and communicated to you on behalf of the New South Wales Department of Education and Training (Centre for Learning Innovation) pursuant to Part VB of the Copyright Act 1968 (the Act). The material in this communication may be subject to copyright under the Act. Any further reproduction or communication of this material by you may be the subject of copyright protection under the Act. CLI Project Team acknowledgement: Writer: Editor: Illustrator: Richard Morante Julie Haeusler Tom Brown All reasonable efforts have been made to obtain copyright permissions. All claims will be settled in good faith. Published by Centre for Learning Innovation (CLI) 51 Wentworth Rd Strathfield NSW 2135 Copyright of this material is reserved to the Crown in the right of the State of New South Wales. Reproduction or transmittal in whole, or in part, other than in accordance with provisions of the Copyright Act, is prohibited without the written authority of the Centre for Learning Innovation (CLI). State of New South Wales, Department of Education and Training 2006.

3 Gill Sans Bold Contents Introduction... 2 Edison versus Westinghouse... 3 Transmission... 5 History of transmission...6 Transmission in NSW...7 Transmission lines...8 Summary Suggested answers Exercises Part Part 4: Transmission 1

4 Introduction Electricity is generated at specific locations in NSW. The transmission of that electricity to where it is consumed is an enterprise that requires huge infrastructure investment. The energy losses from transmission are significant and must be kept to a minimum. The monitoring of the grid to ensure regular and reliable energy supply is essential. You are probably familiar with the sight of high voltage transmission lines stretching across the country side. These arteries carry the energy that powers modern society. They appear so common these days that they hardly rate a mention and blend into the natural landscape yet they are an example of the high technology modern society demands to supply its thirst for energy in the useable form you know as electricity. At the end of Part 4, you will have had an opportunity to: analyse secondary information on the competition between Westinghouse and Edison to supply electricity to cities gather and analyse information to identify how transmission lines are: insulated from supporting structures protected from lightning strikes. Extract from Physics Stage 6 Syllabus Board of Studies NSW, amended November The most up to date version can be found on the Board's website at 2 Motors and generators

5 Gill Sans Bold Edison versus Westinghouse In the early days of the electricity industry the competition to supply electricity to cities was intense. It was really a battle of DC versus AC. Thomas Edison proposed that DC was the superior system for the transmission and generation of electricity. George Westinghouse and his partners proposed that electricity should be generated as AC and transmitted as AC. The industry was split. Both the opponents in this battle had the respective advantages to tout for their system. Edison claimed DC was better because DC was safer for the consumer. If you received a shock from the domestic supply of DC by gripping the wire you could simply open the hand and remove it. The same cannot be said of an equivalent voltage AC shock that would tend to paralyse the hand and force you too continue to hold the wire. Edison was said to have made the following statement about AC electricity; "Just as certain as death, [George] Westinghouse will kill a customer within six months after he puts in a system of any size." Quote from Blow, Michael. Men of Science and Invention. New York, American Heritage Publishing Co., Inc., p. 95. Edison had a vision of the establishment of DC generators all over cities transmitting the electricity short distances to where it was required in small local networks. In many instances the idea was that each building would have its own electricity generator. Under these conditions the transmission energy loss problem due to electrical resistance when using a DC system was not such a problem. DC when used under these conditions was at an advantage in the efficiency of generation at the time. If you didn t have to transmit the electricity too far it was a good system. Edison had some other advantages over Westinghouse in the early competition for domination of the electricity industry. These were largely due to his head start. His Pearl Street power station near Wall Street New York was up and running in 1882, and the banker s backing was more easily obtained for small local stations initially. Power stations cost money to build. Large power station as such as those preferred by Part 4: Transmission 3

6 the generators of AC such as Westinghouse required a large capital investment on which it took a number of years to provide a return. The small DC generators used locally were cheaper per unit, represented less risk to the banks and financiers and had their product market guaranteed. They didn t need to look for customers at a distance. The truth of the matter in the end was that AC won out over DC because it was a better system. Its use was able to drive down the cost of electricity supply. Part of the problem with the small DC generation systems was that you needed to have the capacity to run the generator all day but demand was only peaking at certain times. Your capacity had to match the peak demand. The location of customers able to use the electrical generating capacity outside of peak demand periods was easier for large power generating systems with broad distribution networks. More efficient use of expensive generating capacity by AC generators meant that costs for electricity could be reduced. Cheaper electricity meant more access to consumers as they switched on to the cleaner alternative electrical energy from alternatives such as gas lighting. The whole thing then became self perpetuating. Today electrical hot water is often off peak. The electrical supply companies sell cheap electricity to heat water in periods of low electricity demand. That way their load capacity is more evenly utilised and they do not have vastly excessive capacity under utilised for much of the day. History now tells us now that the AC system of George Westinghouse won the day. In many respects AC wasn t as safe as the DC system but it did have the advantage that it could be transmitted long distances efficiently. The power plant could also be established close to the energy source and the electricity moved into the city along transmission wires. This was pivotal to the success of the Niagara Falls hydro electric power plant built by Westinghouse in This decentralisation meant that the power plant could be built on relatively cheap ground and that the generation and accompanying pollution (if not a hydro electric station) was on the outskirts of the cities or in the surrounding countryside. To sum it up, the battle for domination of the electricity industry and control of the lucrative city market was lost by Edison in the short term but won by both Westinghouse and Edison s General Electric Corporation. The two companies started by these men still dominate the generator industry today. It was an economic and political battle. The winner was a technological society! To see sites that describe some of the battle between Edison and Westinghouse to provide electricity to cities see links on the physics websites page at: 4 Motors and generators

7 Gill Sans Bold Transmission A high voltage transmission tower. (Photo: Ric Morante. ) Part 4: Transmission 5

8 History of transmission In 1876, the California Electric Company of San Francisco opened. Its purpose was the selling of electricity. The company s market was small in terms of the number of customers served but it was still the first electrical transmission system. In 1881, Lucien Gualard and John Gibbs patented an AC transmission system in England. In 1882, Thomas Edison opened the Pearl Street Station in New York. This system provided DC electricity to around one half of a square kilometre. In 1885, George Westinghouse and William Stanley developed the transformer to a high level of efficiency. In 1886, Stanley demonstrated the advantages of AC transmission. In this demonstration he used a transformer to step up a generated voltage to 3000 V for transmission. Then, after transmitting it around 1200 m he used a second transformer to step down the voltage to 500 V. This was a much more highly efficient means of transmitting electricity than with the competing DC system where the energy lost during transmission was much greater. In 1887, Nicola Tesla patented the polyphase (three phase) AC system of electricity generation and the motors that could use AC electricity. In 1890, the first commercial AC transmission power line was in use between Willamette Falls and Portland, Oregon. This was a distance of 21 km. From this time on, AC transmission began to dominate the electricity transmission business. The acceptance of the use of the AC electricity system as the dominant means of transmitting electrical energy was not smooth. The battle has been termed the transmission wars. The proponents of the DC system were the Edison Electric Company. The proponents of the AC system were Tesla and his business partner George Westinghouse. The battle for dominance was not clean. It had all the intrigue of an adventure story. To access information that give insights into this battle of the currents go to links on the physics websites page at: 6 Motors and generators

9 Gill Sans Bold A power plant produces power that has to be transmitted to a city 12 km away. If they transmit the electricity at V, and the transmission wire has a resistance of 1 Ωkm 1, how much power is lost in transmitting 20 MW (megawatts) of electricity to the city? Check your answer. Do Exercises 4.1 and 4.2 now. Transmission in NSW The population base in NSW and hence the greatest demand for electric power is in the eastern coastal part of the state. The transmission system in NSW links the main power stations and those of the Snowy Mountains hydroelectric scheme through more than km of network. This network also extends into the power stations that feed into Victoria and form part of the national grid. The transmission system has developed as the demand for electricity has increased. In 1950 the only high voltage transmission line in operation in NSW was a 132 kv line linking power stations at Port Kembla near Wollongong and Burrinjuck Dam near Yass. The 132 kv network was expanded as demand increased for power and large decentralised power stations were built closer to energy supplies of coal in NSW. In 1959 a 330 kv line was built from the new Snowy Mountains Hydroelectric Scheme to Yass. This network of 330 kv lines has expanded to provide electricity from power stations scattered around NSW to the major centres of demand. In 1979 far western NSW towns such as Broken Hill were connected into the 220 kv power grid of Victoria. In 1984 a 500 kv line was established between Eraring Power Station on Lake Macquarie and Kemps Creek substation in south western Sydney. A second 500 kv line was commissioned in 1986 between Bayswater and Mount Piper power stations. The longer the distances travelled by the electricity between generation and use, and the greater the demand for electricity the higher the voltages required for efficient transmission become. Part 4: Transmission 7

A 330 kv line can carry ten times the electrical energy of a 132 kv line. As demand for electricity rises in the urban centres higher voltage lines must be constructed.

Transmission lines You are familiar with transmission lines.

10 A 330 kv line can carry ten times the electrical energy of a 132 kv line. As demand for electricity rises in the urban centres higher voltage lines must be constructed. Listen to the tape called Substations and transformers or alternatively download and listen to the tape on steaming audio from the physics website page at: Transmission lines You are familiar with transmission lines. Most neighbourhoods still have transmission lines above ground though there is a tendency in newer suburbs to place the electrical power lines below the ground. The electricity lines you are probably most familiar with are not really transmission lines at all they are distribution lines. These distribution lines carry electricity from a substation to your home. A distribution line attached to a power pole is shown in the photograph below. These lines often carry electricity at around to V. Electricity is transformed to domestic supply voltage at 240 V. shield conductor earth from the shield conductor leading to the ground A power distribution pole. (Photo: Ric Morante.) 8 Motors and generators

Gill Sans Bold Transmission lines are the giant towers carrying high voltages (of the order of 300 000 to 500 000 V) across country from power stations to the substations that supply electricity to

11 Gill Sans Bold Transmission lines are the giant towers carrying high voltages (of the order of to V) across country from power stations to the substations that supply electricity to consumers. A set of transmission lines on a pole is shown in the figure following. A high voltage transmission pole. This particular pole is carrying 33 kv. (Photo: Ric Morante.) 1 Look at the photo above. What is the transmission tower made from? Like most transmission towers the tower shown in the photograph above is made from steel. This material is conductive. As a consequence, this presents a problem for the transmission tower during electrical storms. Notice that the transmission tower is the highest thing in the area. In fact, the area around high voltage transmission lines is a buffer zone where construction is not allowed. The higher the voltage carried by the transmission lines the larger the buffer zone. The tower is therefore a high conductor exposed to lightning strike. 2 Look again at the photo of the transmission tower. Is there any evidence of active lightning protection such as the installation of a lightning rod system around the tower? Most transmission towers have a similar arrangement to that shown in the photograph on the previous page. Part 4: Transmission 9

Each of the three separate bundled wires shown on each side of the tower in the photo above carry a single phase of the three phase AC transmission.

12 Each of the three separate bundled wires shown on each side of the tower in the photo above carry a single phase of the three phase AC transmission. Commercial generators produce three phase current because that is the most efficient generator design. Each of these wire pairings that carries a single phase of the electric current is separated from the tower by a set of insulators bundled together known as an insulator chain. The insulator chain looks like a set of plates stacked on top of each other. These plates are usually a rubber or ceramic material. ceramic insulators that can be connected to form an insulator chain note the metal hooks are not in contact with other metal parts to ensure insulation A variety of ceramic transmission line insulators. There is no direct contact between the metal parts of the insulators. (Photo: Ric Morante.) 10 Motors and generators

Gill Sans Bold Chain insulators for high voltage transmission lines. These insulators are rubber with a fibre glass core running through them.

The insulator chains are shaped like stacked saucers to ensure that dust build up on the insulator doesn t occur and increase the conductivity and to ensure a long pathway for the electricity in case

13 Gill Sans Bold Chain insulators for high voltage transmission lines. These insulators are rubber with a fibre glass core running through them. These particular insulator chains are around 2 m in length. The insulator chains are shaped like stacked saucers to ensure that dust build up on the insulator doesn t occur and increase the conductivity and to ensure a long pathway for the electricity in case of spark discharge or arcing. (Photo: Ric Morante.) An electricity substation. Note the extensive use of insulator chains where the transmission lines come into contact near metal supports. Their plate like shapes are designed to prevent electricity arcing over them. (Photo: Ric Morante.) The photograph below shows a high voltage transmission tower with the lightning protection features labelled. The top pair of wires on the transmission tower are called the shield conductors. Notice that they are Part 4: Transmission 11

the highest pair of wires and that they do not have an insulator chain. The shield conductors are connected directly to the metal transmission tower.

14 the highest pair of wires and that they do not have an insulator chain. The shield conductors are connected directly to the metal transmission tower. They act like lightning conductors to prevent the current transmitting layers from being struck by lightning. The shield conductors protect the transmission wires beneath them from lightning strike. shield conductor pair of electrical conductors phase 1 conductor insulation chain phase 2 conductor phase 3 conductor High voltage transmission tower. (Photo: Ric Morante.) 3 Identify any structures you can see on the photo that may be designed to protect the transmission lines in event of a lightning strike hitting the tower? 4 How do you think the structures you identified might work? 12 Motors and generators

15 Gill Sans Bold 5 Lightning strikes occur most frequently on objects that are relatively high above the ground compared to their surroundings. One strategy to protect transmission lines from lightning strike is to place the transmission lines underground. Why do you think this form of protection isn t used more often? Check your answers. Transmission lines have a certain amount of passive protection from lightning strikes. The towers themselves can act as conductors to take any excess charge to the ground. To facilitate the discharge of the excess current from the lightning strike, the towers are well earthed with a large surface area of metal buried to enable the rapid dissipation of the charge into the ground. In other words, the base of the tower has low resistance. In addition, the tower is isolated from the adjoining towers by a minimum distance of around 150 to 200 m. This means that should one tower be struck any adjacent tower should suffer no effects of the lightning strike due to the significant distance between towers allowing the current to dissipate before reaching the adjacent towers. The top wires (shield conductors) are connected to the transmission towers directly. Should the shield conductors be struck by lightning, the lightning will be conducted through the towers and earth wires attached to the tower to earth. Part 4: Transmission 13

16 Summary The following questions are designed to make you think about the learning in this part. Complete your answers to form a summary of the learning you should have done in this part. Why are there energy losses that occur as energy is fed through transmission lines from the generator to the consumer? What are some of the claimed physiological effects on humans living near high voltage power lines? Transmission lines are protected from lightning strikes by: 14 Motors and generators

17 Suggested answers History of transmission If the power plant is to deliver 20 x 10 6 W of power at 100,000 V, then the current required is: I = P delivered /V = W V = 200 A. With a resistance of 2 ohms, this means that the power lost is: P lost = I 2 R d = (200 A) 2 (1 Ωkm 1 ) 12 km = W. Transmission lines 1 The transmission tower is made from some metal. It is probably steel. 2 None that is visible. 3 The two shield conductors and the insulator chains. 4 The shield conductors are the highest wires on the towers and span the gap between the towers. Since lightning tends to strike the higher points first, then that point will be struck first and will carry the charge through the tower to earth. This protects the wires below from lightning strike. The insulator chain isolates the conducting transmission cable from the tower. The plate like insulators are designed to stop any flashing of the charge when the lightning strikes occurring from the tower to the transmission lines. 5 The cost of making an underground system for the transmission of electricity is much higher initially. After construction the maintenance of an underground system is also much more expensive. Part 4: Transmission 15

18 16 Motors and generators

19 Exercises Part 4 Exercises 4.1 to 4.3 Name: Exercise 4.1 Explain why AC electricity eventually became the preferred mechanism to produce and distribute electricity on a commercial scale. Exercise 4.2 In terms of the history of the development of the electricity industry, identify the developments in technology that led to electricity eventually being transmitted as AC current. Part 4: Transmission 17

20 Exercise 4.3 Look at the power pole shown in the photograph below. Label clearly on the photograph in the indicated areas 1 4 the names of the features that insulate the transmission lines from the pole and protect the transmission lines from lightning strike. Explain how all of these features work Motors and generators

Motors and Generators competition between Westinghouse and Edison to supply electricity to cities

Motors and Generators competition between Westinghouse and Edison to supply electricity to cities Motors and Generators Question 1 Analyse secondary information on the competition between Westinghouse and Edison to supply electricity to cities (3. Generators are used to provide large scale power production,