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The Story Of Electrical Energy, Pt.5 Until now, this series has dealt with very large coal burning power stations but there are many regions which are far from natural energy sources. A prime example is Darwin which is now using a highly efficient gas/steam combined cycle system. By BRYAN MAHER Though Darwin is the smallest of our capital cities, it shares with its harbour the longest and most romantic history. The magnificent inlet we know as Port Darwin was discovered in 1644 by Captain Abel Janszoon Tasman of Lutjegast, Holland, who was exploring our northern waters. 98 SILICON CHIP Sailing in the Zeemeeun, he mapped our coastline from Cape York to North West Cape with incredible accuracy. He named Van Diemen Gulf and visited areas around present day Darwin. The first Englishman to enter what is now Darwin's harbour was Captain Phillip Parker King, gamely sailing in 1819 with his tiny 85-tonne ship , HMS Mermaid. Electrical beginnings For many years , the batteries of the overland telegraph line were the only sources of electrical energy in the Northern Territory. Then in 1914, the first engine-driven generator in the Territory was installed to service Darwin government offices and drive a freezer. By 1920, the Town Hall and Victoria Hotel boasted electric lights and a few town streets were lit. Then in 1922/23, Felix Holmes built an oil engine driven power station to supply energy to all Darwin. When he died , his station was taken Left: this is the Channel Island Power Station in Darwin. Fuelled by natural gas, it has five gas turbines. The five stacks for the gas turbines can be seen next to the main turbine hall but two of these stacks are not normally used. Instead, the exhaust gases for the two associated turbines are fed into heat exchangers to power two steam turbines. The exhaust gases are then vented from the two stacks between the buildings. over by the Town Council in 1930 and seven years later by the NT Administration. In 1939 this, the only generating plant in Darwin, had a capacity of 160kW. By mid-1940, a second station equipped with twin English Electric alternators lifted Darwin's total capacity to 660kW. These were driven by 6-cylinder, liquid cooled, vertical stroke diesel engines. Within two years, additional machines were installed. A lMW Fullager and two smaller Crossey diesel-alternators raised the station's capability to 2.5MW as the city grew during the war years. To supply military needs, one of the units was moved to Alice Springs and a smaller machine to Katherine. Additional electricity supplies for armed forces establishments were provided by a multiplicity of smaller generating plants, some semi-portable. Ex-US Army diesel generating sets were given to the NT Administration at the end of World War 2 but they proved troublesome due to poor wartime maintenance. As late as 1960, the Administration operated power stations at Darwin, Katherine and Alice Springs. Peko mines supplied their own needs plus the town of Tennant Creek. Around 1961, Darwin's electricity situation became critical and a new power station at Stokes Hill was planned. This was completed in 1962 and supplied the city's needs until 1987. All machines up to 1987 were diesel powered but the need to freight in expensive diesel fuel kept the price of generated electricity a lot higher than in other states. Another factor on the minds of the administration was Darwin's history of destructive hurricanes sweeping in from the Timar Sea. The new Channel Island power station uses gas turbine driven alternators, fuelled by natural gas from the Amadeus Basin, 1600km to the south. This view of the station shows the large exhaust stacks for three of the turbines. In the foreground is the metering equipment for the natural gas. In 1974, Cyclone Tracy destroyed most of the overhead electricity distribution network, and damaged substations and some of the generators in the power station. However, most of the high voltage lines and all underground lines remained intact. After the devastation of Tracy, electricity authorities from all over Australia and the defence forces sent approximately 500 electricians and linesmen to assist the Northern Terri- tory Administration staff in restoring power. Cyclone proof underground power transmission was installed in the suburbs of Tiwi, Anula, Wulagi and Wanguri, as well as the city central business district. Berrimah power station Following the development of gas turbines of megawatt capability in Europe and the USA, the Northern NOVEMBER 1990 99 Not all natural gas fuelled generating sets in the Northern Territory are gas turbines. This new installation at Tennant Creek uses a 340kW spark ignition prime mover made by Waukesha. Territory Electricity Commission, formed in July 1978, began construction of the Berrimah power station. The installation was to use two gasturbine driven alternators, a departure from the prevailing system of diesel driven units. Gas turbines will run on just about any fuel. However, that statement should be modified by saying that the efficiency of a gas turbine is a function of the type of hydrocarbon on which it is fed and is very dependent on the ambient temperature. This factor is important in the tropical climate of Darwin. However, the efficiency of an engine is not the only factor in the price of generated electricity. Just as important is the fuel cost per kilogram, delivered to the site. Therefore, the existence of natural gas below Central Australia is today a vital factor in the Northern Territory electric energy story. Natural gas from deep wells in the Amadeus Basin in Central Australia, 300km south-west of Alice Springs, is now reticulated by a 2000km underground pipe system. The Darwin power stations receive their fuel from this source and gas is also laid on to smaller stations at Tennant Creek, Katherine and the privately owned 100 SILICON CHIP Cosmo Howeley power station. Liquid natural gas (LNG) is supplied to the power station at Pine Creek. However, most installations also retain some distillate fired machines. Channel Island Because of the city's continual growth, a new and larger coal-fired power station was planned in 1981 for Channel Island. Originally used as a quarantine station and leprosar- . ium, this small land mass, roughly 1.7km long and 900m wide, is situated within Port Darwin harbour, 13km south of the city. Naturally, a coal-fired power station in a state without coal deposits would pose an economy problem. Approximately 500,000 tonnes of coal would have been needed annually. Then, in 1984, very large gas turbines became available and so site preparation was stopped and the power station re-designed to use this new technology. Gas supply is by reticulation from the Amadeus Basin wells. Physics of gas turbines Basically, a gas turbine consists of an air compressor stage and a power turbine section on the one shaft which drives the external load. Air is drawn in, compressed, then mixed with combustible fuel gas. To start the engine with the shaft turning, an electric spark ignites the air-fuel mixture, creating a flame which reaches very high temperatures - 800-1400°C. The resultant sudden expansion produces high pressures within the combustion chamber. The hot gases escape by flowing through the turbine blade wheels, driving them around to produce mechanical power. The turbine shaft must drive the compressor stages and the rotary mechanical load; ie, the alternator. The power turbine may consist of four or five blade-wheels with fixed blade assemblies between them. The air compressor section {;an have as many as 9-12 blade-wheels and complementary fixed blade sets. The fixed blades direct the flow of gases onto the rotary blades. The largest blades are found in the air inlet and exhaust gas outlet areas where pressures are lowest. Conversely, small blade wheels are used near the centre of the machine where pressures are highest. High temperature materials Gas turbines were known in principle but not used for decades until machine parts capable of withstanding the high temperatures could be manufactured. The flame in the combustion chamber is far too hot for many metals. The resulting corrosion, added to the abrasive action of extremely fine dust entering with the air supply, can play havoc with blades and bearings. The metals now employed include alloys of titanium (a metal won by beach sand mining), and also chromium and vanadium. Research in the past three years has produced turbine blades and bearings made from ceramics such as silicon nitride and boron carbide. The efficiency of any steam or gas engine is maximised when the temperature of the gas entering the power turbine blades (or cylinder, as the case may be) is highest and that of the exhaust gas is lowest. This is expressed by the equation shown in Table 1. One big disadvantage of gas turbines is that the heat content in the exhaust gases is very high. Exhaust temperatures may be as much as 4006000C. However, this heat does not have to be wasted. It can be used to turn water into steam to drive another turbine and alternator. This is the 13.6MW ASEA gas turbine installation at the Ron Goodwin Power Station at Alice Springs. The Northern Territory is gradually changing to natural gas for electric power generation, using the rich gas fields of the Amadeus Basin in Central Australia. Combined cycle systems Because the gas turbine's waste heat indirectly drives a steam plant, the total power output is higher for the same fuel input. Thus, the combined cycle system (gas turbine + associated steam turbine) has increased efficiency. The first use of this system in Australia was in the Channel Island power station, Darwin. Here the exhaust heat · from two 32MW gas turbo-alternator sets is used to boil water in two water-tube heat exchanger boilers. The steam produced drives a 34MW steam turbo-alternator. Thus, the combined cycle system generates a total of lO0MW of electrical power using only the fuel supplied to the gas turbines. Gas turbine section The combined cycle system at Channel Island uses two of the five John Brown (UK) Ltd gas turbines. The 550°C hot exhaust gases from each flow via horizontal 5-metre square ducting to a heat exchanger. In this large compartment, 15-20 metres high and 6 metres wide, filled with three sets of water pipes, the hot exhaust gases boil water to steam. The steam pipes then re-enter the heat exchanger at the point where the flue gasses are hottest, so the steam is Looking not much more imposing than a deluxe caravan, this is actually the ASEA FT35C 13.6 megawatt gas turbine generating set which was installed at the Ron Goodwin Power Station at Alice Springs at the end of 1987. It supplies base load power to the township. NOVEMBER 1990 101 Table 1: Efficiency of Rankine Cycle Turbine Efficiency = Work Output = Heat Input - Heat Output Heat Input Heat Input Note 1: this simplified equation does not take into account superheating and reheating in the case of steam systems. Note 2: all temperatures are in the absolute scale; viz: zero degrees absolute= -273.16°C. As absolute zero cannot be reached, engine efficiency is always less than 1 (ie, <100%). Note 3: being a Rankine cycle (rather than a theoretical Carnot), the above equation cannot reduce to (T(in) - T(out))/T(in). superheated to 485°C at a pressure 4.2MPa. After passing through the heat exchanger, over superheater and HP tubes, economiser, LP and feedwater tubes, the flue gases have given up most of their heat to the steam system. The exit flue gases, now down to 145°C, pass up a large vertical vent to the atmosphere. The steam derived from the two heat exchangers drives a 3000rpm Mitsubishi steam turbine, directcoupled to a Brush 2-pole 34MW alternator. rel cage pony motor supplied from the station auxiliary AC busbars. This motor drives the gas turbine shaft via an hydraulic torque converter coupling with variable slip. As the speed builds up , the fuel gas is ignited, increasing the speed to the full 5100rpm, whereupon the starter motor is shut down. Should some electrical calamity such as a severe lightning strike or a major breakdown stop the whole power station, a "black start" is necessary. Under this very rare condition, the station auxiliary bus would also be dead and the only lighting would be from batteries. To overcome this problem, black starts at Channel Island use a Norwegian Kongsberg gas turbine. This machine has no electric pony motor but is started by compressed air. Once up and running, this emergency generator supplies the station lights and auxiliary AC busbars, enabling the other machines to be electrically started in the normal way. The compressed air reservoir is always kept up to pressure by an electrically driven compressor, though in an emergency a separate diesel driven compressor provides the air supply. Should all this fail, electrical supply is available for a black start from Berrimah power station, 40km away. Here, two Stal Laval gas turbines can supply Darwin via the 132kV powerline system. Hudson Creek substation As well as providing steam from their exhaust heat, each gas turbine, running at 5100rpm, also drives a 3000rpm 32MW alternator via a helical gear box of 51:30 ratio. These 11.5kV 50Hz alternators, made by the Brush Company of England, use the brushless excitation method, outlined in chapter 3 of this series (September, 1990). An inverted exciting-alternator on the same shaft has its AC output rectified by a diode wheel to provide the low voltage heavy DC current needed to excite the main alternator rotor. As the excitation currents run via cables inside the hollow shaft, no sliprings or brushes are needed. The 220MW output of Channel Island power station runs via three 132kV lines to Hudson Creek substation. Each line is capable of carrying the entire 1300 amps load in an emergency. Two of these lines use the bundle-of-two conductor arrangement. The construction of this substation was the major part of the $7 million works implemented during 1989. At Hudson Creek, a 125MVA autotransformer provides a 66kV supply for Pine Creek and other mining areas. At Alice Springs, new ABB Stahl Laval GT35 gas turbines and generators run on a mix of 97% natural gas and 3% distillate. A total generating capacity of 53MW is installed and the gas is supplied by a separate pipeline from Palm Valley wells. Control system High voltage lines The machines at Channel Island power station are each controlled by a General Electric Speedtronic electronic speed, power and voltage regulator. A distributed computer bus system oversees excitation, and controls load and reactive current sharing between generators . The usual starting procedure for each gas turbine is via a 3-phase squir- The new 132kV transmission line, completed and put into service on 12th November, 1989, now supplies power from Channel Island to Katherine. As well as augmenting the Katherine power station, this power line also supplies electricity to consumers along the 350km route . In normal operation, Channel Island is used as a base load station Gear driven alternators 102 SILICON CHIP This photo shows a 30MW gas turbine during assembly at the ABB works in Switzerland. The blades, shaft and bearings of gas turbines are subjected to extreme temperatures from the burning gases. To combat the corrosion and abrasion from the high temperature gases, the blades are usually made from titanium alloys. have complex redundancy built in because there are many alternative lines feeding each area. But in the vastness of the Northern Territory, this would be totally uneconomical. So, in common with many other electric supply authorities worldwide, live line maintenance is undertaken as a routine operation in the NT. To perform live line work, linesmen must be specially trained. Long manipulation rods, jib poles and lifting tackles made of non-hygroscopic plasticised non-conducting materials are used. One of the photos in this article shows a crew on a transmission tower changing a faulty strain insulator set on a 66kV line. The power is on and full load current is flowing through the cables. The red insulated manipulation rods shown in the photograph can be fitted with special tool ends such as spanners, nut drivers, hooks and cable clamps. Working from a safe distance, the linesman, holding one end of the long rod, hooks the other end into the live bolt, nut or cable clamp. A twisting motion can then unscrew or tighten, insert or remove the part as required. Changing insulators Live line maintenance at 66kV! And you thought that 240VAC was hazardous. Specially made insulated manipulating poles and highly trained staff means that live line maintenance is a routine procedure, not only in the Northern Territory but all around Australia. because of the much higher efficiency of its lOOMW combined-cycle output. Katherine's generators assist in times of peak load or if a breakdown occurs at Channel Island. Among the new power lines are an 80km lightweight 66kV line to Pine Creek, plus others to Jabiru town, the Ranger uranium mine and the Alligator River region. A 22kV line feeds the Jindalee Over-the-Horizon Radar Station north of Alice Springs. In total, the Northern Territory has built 2290km of overhead and 400km of underground high voltage lines, plus 2200 substations. These service 43,000 consumers who consume 900GWh annually. Live line maintenance On any electrical system, power interruptions are a cause of annoyance to householders and lost production in industry. Some systems Tension insulator strings are under tonnes of strain due to the weight and tightness of the span conductor, plus wind forces. To change such an insulator set, as is being done in the photo, first the mechanical strain must be taken up by a temporary jury rig. Then, working carefully, the linesmen uncouple and remove the damaged insulator string. A new insulator set is then bolted into place and the jury rig removed. All this is achieved with the line fully alive but the workmen safely at ground potential. Other live line work which can be carried out includes attaching or removing tee junctions, using liveline cable clamps. Also, new power lines can be erected above existing live high voltage lines. Acknowledgements Special thanks to the Northern Territory Power and Water Authority, to Public Relations manager Michael Nott, and to Max Wagland, Ian Campbell, ABB and ASEA Journals, Action and Review and Electricity Trust of SA. ~ NOVEMBER 1990 103