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A helicopter is handy for inspecting high voltage power lines - provided one doesn't get too close!' These are 330kV transm~ssion lines from Vales Point in NSW. 94 SILICON CHIP The Story Of Electrical Energy, Pt.4 The huge quantities of electrical energy generated in modern power stations must be carried to the users, homes and industry. In this episode, we look at modern high voltage transmission lines, their design, construction and the materials used. By BRYAN MAHER Though city dwellers take electricity for granted, the recent storms and floods around Australia have served to remind us of our dependence on light and power. To live, work and travel, we all need electric power. In previous chapters, we looked at the outstanding engineering effort needed to generate the electricity supply. Now we look at how that power gets to the people. Not surprisingly the longest power transmission lines are found in the largest states. Furthermore, heavy industry needs heavy electricity systems. Queensland, the largest eastern state, has over 4000 kilometres of 275kV lines, running almost the length of the east coast. The total length of all high and low voltage circuits is almost 170,000 kilometres. Gladstone power station, the ma- . jar source of Queensland's electricity in the early 1980s, is approximately 500km from Brisbane. Therefore, a number of 275kV lines carry 1000 megawatts (1 Gigawatt) or more of power over tp.is long distance to Southpine substation just north of Brisbane. From there, 275kV lines run to Swanbank (near Ipswich), Belmont and Mudgereeba (Gold Coast) substations. At these main substations, the voltage is transformed down to 110kV for distribution to 17 smaller substations in the Brisbane area and 8 on the Gold Coast. In areas north of the Sunshine Coast, 132kV is the intermediate voltage used. At each substation, power is again transformed down to 33kV or 11kV. Finally, the 11kV system distributes the power to hundreds of small street substations. It is here that the final stepdown to 415/240V 3-phase street mains takes place to feed your home and local industries. Many large buildings and industries are supplied at high voltage, often 1 lkV. A few very large industrial plants, such as aluminium smelters, purchase power at 132kV. Similar situations exist in all Australian states, cities and towns .. The first 330kV line This is what high voltage switches look like. These are 11kV 400-amp air break switches, in the open position. Notice the double current carrying blades and the steel spring arc horn set at 45°. This breaks the current last to stop arcing to the main blade. The backbone of the NSW system is the 5000km network of 330kV lines running the length of the state. The first line of this voltage in Australia was constructed from the Snowy Mountains Hydro power stations to Yass substation in 1960. Canberra also receives power in this way. OCT0BER1990 95 These 330kV transmission lines south of Armidale are very unusual because they use wooden poles. Notice the "dog bones" on the cables, near the insulators. These are vibration dampers, used on all transmission lines with conductors of more than 20mm in diameter. Weighing a kilogram or more, they damp vibrations of 7 to 16Hz. The 330kV system was extended to the Riverina and Victoria in the south, to Wellington in the west, to the Wollongong-Sydney-NewcastleHunter Valley complex and as far north as Armidale. Extensions to Coffs Harbour and Lismore are in progress. An intermediate system of 8600km of 132kV lines distributes electricity to area substations over the whole state. 500kV line Sydney's metropolitan area consumes 48 % of all energy generated by the NSW Electricity Commission. The heaviest transmission system in Australia is the 500kV twin line from Eraring power station to 96 SILICON CHIP Kemps Creek, Sydney. This same voltage is also used in Victoria. Also in NSW is the 224km 500kV circuit from Bayswater in the Hunter Valley to the new Mt Piper power station [between Wallerawang and Mudgee). This line is temporarily energised at 330kV until Mt Piper is completed and on line. Why all these different voltages? One of Australia's earliest major interconnectors was the 66kV line built in 1942 to link Hamilton [Newcastle) and St Leonards substation in Sydney. It was constructed using 19-strand 10-ga uge cadmium copper for the longest span across the Hawkesbury river and 37-strand 12 gauge hard drawn copper elsewhere. That line, capable of carrying 200 amps, was built as a strategic link between power stations during World War 2. At the time, small towns in between Newcastle and Sydney were without electricity even though this line ran along their main streets. In 1943, the citizens of Wyee petitioned for a substation to serve their area. Unfortunately, a 66kV transformer to serve a very small load would have been hopelessly uneconomic, so their request was denied. Today an 1 lkV line from a different source and a small substation supplies Wyee. So most of the power is distributed via llkV lines in the suburbs. But why must transmission lines go as high as half a million volts? Indeed, higher voltages still are found overseas. In England, Europe and the USA, voltages up to 1.2 million volts are used. And modern research is always pushing the limit higher! Why? Consider a simple system of one source [a large power station) and one load [a big city), separated by great distance. Here we have a problem. Naturally, there will be power losses due to resistance in the transmission lines, but being an AC system, there will be inductive and capacitive losses too. The inductance losses exceed those caused by resistance in most high voltage power lines. The inductive reactance may be from 8 to 23 times greater than the value of resistance. Typically, with conductors ranging from 25mm to 50mm in diameter, inductive reactance ranges [overseas) from 0.2 to 0.5 ohms/km, while the resistance is .01 to .075 ohms/km. Hypothetical design Suppose we propose a 10kV line to supply 3GW to our hypothetical city from its remote power station. That implies a line current of 200,000A. Wow! A little arithmetic soon shows that even with the very low resistive component of 0.02 ohms/km, such a current flowing would produce a voltage drop of 4kV/km along the line. Clearly, this is an impossible situation; and the inductive effects are greater still! Obviously, we must either install conductors which are a few thousand times thicker or use a very much smaller current. The former choice would be ridiculously uneconomic in lines of great length. Therefore, we are forced to use smaller current. How? Just transform the supply up to say 50 times higher voltage, say 500kV and hey presto! The current will then be 50 times less, at 4000A. The voltage loss along the line due to the resistance will now be only B0V/km, and the inductive drop will also be down to manageable values. Now we see why very high voltages must be used. Indeed, over long distances, the power that can be successfully transmitted depends roughly on the square of the line voltage. Insulators Very high voltages bring their own problems though. For starters, longer strings of insulators must be Not all transmission towers are massive structures as this photo of Swedish 400kV lines shows. Using guyed towers, the structures are quite light. Note the use bundles of three conductors for each phase. This technique reduces inductance & corona losses by simulating a conductor with a much larger diameter. used, increasing the cost and the weight suspended from the towers. Up to 132kV, we might see either solid standoff insulators, or strings of multiple suspension disc units linked together. Either steel towers or wooden or reinforced concrete poles are used to support lines up to 132kV. Above this voltage we usually see only steel towers. In a few places though, timber poles support 330kV lines; eg, on the northern NSW tablelands just south of Armidale. Fewer insulators are needed on a line if less towers are used, so longer spans save dollars. But longer spans require bigger and more expensive towers; balancing the costs is a finely tuned exercise. Conductor materials After more than 50 years of use, the traditional hard drawn copper and cadmium-copper cables have lost favour with power line designers because of their cost, relatively low strength and high weight. Today, cables usually use pure aluminium for short spans which are under low mechanical tension. More sophisticated materials are used for long spans to withstand greater strain. Most high voltage lines are strung very tight, up to 25 % of the ultimate tensile strength. This minimises conductor sag and swing and thus longer spans and less towers can be used. High tensile strength is necessary to take the continuous strain of conductor weight, plus the considerable wind forces sometimes experienced. Between Newcastle and Sydney, the strongest winds expected are the short powerful gusts associated with thunderstorms. These may gust to over 170km/h at ground level, and 200km/h at conductor height. OCT0BER1990 97 AAAC conductor This 330kV transformer is installed at Wellington Substation in the central west of NSW. It steps the 330kV down to 132kV for regional distribution. Many cables are now constructed using ACSR, which stands for Aluminium Conductor Steel Reinforced. This employs many strands of aluminium surrounding a stranded core of galvanised steel. The current flows mainly in the aluminium while the steel core gives strength. One common size is seven strands of steel (2 layers) encased by 54 of aluminium (3 layers] - a total of 61 made up in concentric rings. The relation between layers and number of strands is given in the accompanying table. The layers are skewed slightly to hold the cable together, as in rope. Consecutive layers skew in opposite directions for stability. This skew, called the lay of the cable, increases strand length and hence the resistance per km by a small amount. Sometimes the steel core uses 19 98 SILICON CHIP strands of a smaller gauge wire, for greater flexibility. Another type of conductor is ACAR, or Aluminium Conductor Alloy Reinforced. In this, the aluminium conduction strands surround a stranded core of aluminium alloy. The manufacturers of this cable claim lighter weight, less corrosion and easier splicing. CABLE CONSTRUCTION Layer No. 1 2 3 4 5 6 7 8 Number Of Strands 1 6 12 18 24 30 36 42 Total Strands 1 7 19 37 61 91 127 169 (Note: all strands equal diameter) The ABB company and their associate Elektrokoppar of Helsingborg, Sweden, manufacture AAAC, an all-alloy cable which goes by the trade name DuctaLex. This alloy is 59% as conductive as copper but transmission losses are lower because there is no steel core. The alloy used is based on aluminium/magnesium/silicon and is claimed to give low weight, high Young's modulus of elasticity (up to 67kN/mm2), low creep (400ppm), high surface hardness and corrosion resistance, and reliable jointing. The first commercial use of this alloy in a power line was in a 400kV Swedish system built in 1977. Because of eventual metal fatigue, transmission lines may have a working life of 30 years. If then dismantled for scrap metal value, DuctaLex cable can go to the melting pot whole. However, scrap ACSR must first be separated out into its steel and aluminium components, and this reduces its value. Fully 80% of Swedish power lines now use alloy cables. In Australia, Alco makes a cable with similar properties. The small additives (0.7% Mg and 0.6% Si) give great strength to this aluminium alloy. Another alloy of aluminium/copper/magnesium is also used. Eraring to Kemps Creek From Eraring power station on Lake Macquarie, near Newcastle, twin 500kV lines run for 143km to Kemps Creek substation in Sydney's western suburbs. Here the voltage is transformed down to 330kV to be fed into the state grid at Sydney-north, Sydney-south, Avon and Dapto substations. Between Eraring and Kemps Creek substation, the 500kV lines run inland, crossing the remote Hawkesbury regions, then the western railway at Werrington station. From there, it follows South Creek, Mamre Road and Kemps Creek. Carried on galvanised steel towers, typically 55-metres high, the cable spans vary in length ac- Tough enough to take it, wherever you take it These 132kV 3-phase transmission towers distribute electricity to substations in the Canberra district. Notice how each phase consists of a 2-cable bundle to reduce line inductance & corona discharge. cording to the terrain but are commonly 400 metres to 900 metres. Much longer spans, up to 1.5km long, are sometimes used in isolated areas where the towers are located on mountain tops. The twin line construction used on the Eraring-Kemps Creek system has two complete 3-phase lines per tower. Compared to the alternative method (two separate sets of towers, each carrying one trio of 3-phase lines), the twin construction method uses less towers and occupies a much narrower land corridor. But twin lines are also less isolated from each other inductively and capacitively. Again, the design is a careful compromise, as we shall see. Conductors Fluke 80 Series multimeters come in a rugged, water and dust resistant case. They can handle up to 1000 VAC (RMS)/DC on any terminal and have an "input alert"™ warning if test leads are in the current jacks and a non-current function is selected . They are shielded against electromagnetic interference and are protected against shock and vibration. Each Fluke 80 Series multimeter comes in a rugged protective holster, with a unique "Flex-Stand"™, which bends and holds to any shape, allowing the multimeter to be stood or hung almost anywhere. To find out just how tough a Fluke 80 Series multimeter can be ... call your local Fluke distributor today. FLUKE AND PHILIPS - THE T & M ALLIANCE PHILIPS Transmission towers carry the OCT0BER1990 99 THIS TESTER CAN PAY FOR ITSELF IN LESS THAN ONE DAY SIMM I SIP MEMORY MODULE and DRAM TESTER * All chips are tested simultaneously * Tests 64K x 8/9, 256K x 8/9, 1M x 8/9, 4M x 8/9 and 16M x 8/9 bits. * Stand alone and portable - no need for a corn puter interface * User friendly LCD interface shows clear instructions and results * Zero insertion-force sockets for fast and easy operation * AC adaptor included * Expansion slot for add on products * Current measurement terminals * Two programmable voltage sources * Automatic current limiters provide full protection for your modules * High speed 16 Bit processor generates complex test algorithms $1499 plus TAX BONUS - SINGLE CHIP ADAPTER FOR TESTING 64K x 1, 256K x 1, IM x 1, and 4M x 1 DRAM chips power cables in groups of three, one cable for each phase. But look closer and you will see that each phase consists of a bundle of four parallel cables. Each of these four cables is typically 29.4mm in diameter, of ACSR. When viewed from ground level, it is hard to realise that the distance between the four cables in the bundle is actually quite large. The bundle measures 650mm diagonally. Why is this bundled conductor arrangement used? Why not use a larger single conductor for each phase? Thereby unfolds a story, taking us back to fundamentals . The laws of nature limit all dynamic systems to a maximum of three fundamental effects. One is an energy loss, while the other two imply energy storage. In electrical installations, the energy loss mechanism is the resistance R, while the two energy storage properties involve inductance L and capacitance C. As we have already seen, the resistance of the conductors causes power loss which is dissipated as heat. The Eraring-Kemps Creek twin lines are rated at an absolute maximum continuous current of 4000A. This is limited by the allowable conductor temperature of 120°C. The resistance is determined by the total cross sectional area of all the parallel conductors but not by their physical arrangement, so has nothing to do with the bundling arrangement (except for a secondary ventilation effect). Reason for bundling PACIFIC MICROELETRONICS PTY LTD 'CENTRAL PARK' UNIT A20, 4 CENTRAL AVENUE THORNLEIGH, NSW, 2120 Telephone: Fax: (02) 481 0065 (02) 484 4460 Australian Representative FUJITSU MICROELECTRONICS PACIFIC ASIA LTD. 100 6) FUJITSU SILICON CHIP To find the reason for the bundled construction we must look to the other two fundamentals, inductance and capacitance. Keeping in mind that the inductive voltage drop is proportional to the product of current, frequency and inductance, how can we minimise this voltage drop per km? We are stuck with a fixed frequency of 50Hz, so we must make efforts to reduce the value of inductance. The inductance of a pair of phase conductors is proportional to log(d/r) where dis the distance between phases and r the radius of the This is now a very common sight in suburban streets - 11kV 3-phase lines on top and 41 5V/240V lines below to feed homes & businesses. phase conductor. For three phases and for twin lines, the equation is a little more complex but the factor log(d/r) persists. Therefore, one way to reduce line inductance is to increase the effective conductor radius, (the geometric mean radius or GMR). Effective conductor radius A bundle of many conductors physically spaced around the circumference of a circle would be an ideal way to increase apparent conductor radius r . As this would have to be done for each phase, it would be so expensive that more economical approximations must be used. Here's where cable bundling comes in. The bundle arrangement used on each phase of the Eraring-Kemps Creek lines, with four conductors held apart on 650mm-diagonal square spacers, is an approximation to a 320mm radius conductor. Hence line inductance is greatly reduced compared to a single heavier conductor. Even with the bundling of conductors, on the Eraring-Kemps Creek line, the inductive reactance is still 15 times greater than the resistance. It would be much worse if bundling were not used. Other lower voltage transmission lines can be seen with bundles of two or three cables per phase. The same principles for reducing losses still apply. Corona On a high voltage line in midspan, the only insulation is the air. Normal atmosphere breaks down and ceases to be an insulator if the voltage gradient near a conductor exceeds 30kV/cm at the peak of the sine wave. The resultant electric discharge into the air is called corona. Unfortunately, the voltage gradient tends to concentrate near each conductor, the voltage stress being approximately proportional to V/(r x log(d/r)). Here V is the voltage from phase to ground, r the conductor radius and d the spacing between phases. Because the use of bundled parallel conductors per phase effectively increases the apparent conductor radius, the voltage stress around the wires is reduced by about 30% (bundle of four compared to a single conductor). The result is much less corona discharge. Marlinised conductors Some lines in use overseas use aluminium cables with fibrous nonconducting material or air spaces interspersed between conduction strands. This simply increases the conductor radius to reduce corona. Corona discharge does result in a real power loss. Typical values in fine weather are up to 5kW/km for a line in the 600kV range using single 40mm diameter conductors per phase. During fogs and rain, this loss increases in proportion to: (a) rain intensity (mm/hr); (b) the voltage; and (c) the fifth power of the voltage gradient on raindrops adhering to the conductor underside. Typical values are 135kW per kilometre for rain at 12mm/h for single conductor lines. Bundling reduces this loss considerably as the effectively larger conductor radius r reduces the voltage gradient. Although they can be quite high, corona discharge losses are not significant economically, considering that such a line may be carrying a few gigawatts to the city at the far end. However, corona discharge, which appears as a pretty blue glow at night, is a strong source of radio frequency interference (RFI). Therefore, for the sake of nearby radio and TV reception and telephones, it is vital that step be taken to minimise corona discharge. Catenary curve Any aerial conductor hangs in the shape of a curve called a catenary. This curve is described by the Gosh function, (quite different from the segments of a circle, elipse or parabola, etc). The shape of any power line suspended from points of equal height is given by the following formula: y = (Th/w)(Cosh(wx/Th) - 1) where: y = height of any point on the conductor x = distance of that point from midspan Th = horizontal component of tension w = conductor weight per unit length. As lines heat up either from resistive power loss or hot weather (or both), they expand in length and therefore hang lower. The reverse occurs at night when the loading is least and the weather is coldest. Construction crews must allow for these effects when erecting lines. Tensioning must be governed by the ambient temperature at that moment and the expected hottest and coldest temperatures. Aerial earth High above all conductors, suspended from the highest points on towers, runs the aerial earth conductor. This lighter cable is there to protect the main conductors from lightning strikes. Acknowledgemeuts Grateful thanks to the public relations managers and staffs of the Electricity Commissions of NSW and Queensland, to ABB and to ASEA for data, photos and permission to publish. ~ On the alert, accurate and safe Fluke 80 Series multimeters come w ith a large number of "alert" functions designed to ensure safe, easy use . The "Input Alert" prevents accidental damage through wrong lead connection . An automatic power cut-off (after 30 minutes idle) extends battery life. Thi s may be overridden in 36 hour record mode. And the readouts couldn't be easier. Fluke's patented "Touch Hold "™ feature, w hen activated, ca ptures, locks and displays each measurement, yet leaves you both hands fre e to position t he probes . In t he relative mode, measurements can be made relative to your own reference point . And there's a maximum/minimu m alert, plus automati c storag e of max/min reading s. And there's much more. Why not be alert .to all the special feature s of Fluke 80 Series multimeters . . . call your local Fluke distributor today. FLUKE AN D PHILIPS - THE T & M ALLI ANCE [e PHILIPS OCT0BER1990 101