Fullscreen HTML5Med Res (??MB)HTML4

The Story of Electrical Energy, Pt.17 When people think of Brazil they tend to think of huge rivers and rainforests but this country is also the greatest industrial nation and exporter of manufactured goods in the southern hemisphere. In recent years, Brazil has built a hydroelectric power station and high voltage DC link which is the largest in the world. By BRYAN MAHER Brazil, the largest country in South America, is exceeded in size only by the USSR, China, Canada and the USA. With over 130 million people, Brazil has half the population of the Latin continent. By any standards, it is a major industrial nation. Brazil has huge mineral resources, including iron ore reserves of 60 billion tonnes. Naturally, it has an extensive railway system, with 33,000 kilometres of track, of which 1400 kilometres is electrified. In 1945, Brazil's total electricity consumption was less than 1.6 gigawatts. Today, industrial growth has pushed consumption above 43GW with no sign of slackening. Of this, hydroelectric plants supply 40GW (93%). Projections forecast a 160GW demand by the year 2010 and meeting this demand could cost about $100 Itaipu's alternators are among the biggest in the world, as this photo of a 2060tonne rotor shows. It has 66 poles and rotates at more than 90 RPM to generate 715 megawatts. 8 SILICON CHIP billion over the next 20 years. The megalopolis of Brazil, indeed of all South America, is the Greater City of Sao Paulo. This massive urban area sprawls about 80 kilometres wide and is home to 11 million people. Situated on the Tropic of Capricorn, up on the southern escarpment 770 metres above sea level, it is 53 kilometres west of its port Santos and 320 kilometres southwest of Rio de Janeiro. The Amazon & hydro power The Amazon, the world's greatest river by far, is navigable for the full 3 700km east-west width ofBrazil. This enormous watercourse, more than 100 metres deep and 80 kilometres wide in parts, discharges water into the sea at the phenomenal rate of 160,000 cubic metres per second. The Amazon region has 47% of all drainage basin area in Brazil. But because of the low gradient of the river (10mm per kilometre or 0.001 %) and its slow speed (3-8km/h), the main Amazon stream has only 7% of Brazil's hydroelectric potential. Table 1 shows the total calculated hydroelectric potential of Brazil to be an enormous 210 gigawatts (equivalent to about 200 times that of all of Tasmania's hydro stations). But of the total, more electricity could be generated by the Southern Parana system than by the sum of all others including the Amazon. Fed by voluminous tributaries, the Parana forms the second largest water drainage in the South American continent. Flowing southward in the hinterland, the Parana near the town of Guaira is 4.2km wide. Beyond this point, at Salta del Guaira, the water drops over the escarpment in the mighty Guaira falls, one of the most breathtaking sights on the continent. ltaipu power station The obvious energy potential of this This artist's impression shows the great sweep of the Itaipu dam project. At left are the spillways which must handle almost the full flow of the river in flood times. To the right of the curved section is the power station which has 18 turboalternators giving a total output of 12.6 Gigawatts, more than the total grid capacity of New South Wales! mighty river system prompted an investigation of the hydroelectric potential in 1966 at an isolated spot on the Parana known as ltaipu. Plans called for the building of a colossal dam and power station. With a generation capacity of over 14GVA (12.6GW), this plant alone is larger than the sum of all power stations in New South Wales! However, it is 800km from the industrial suburbs of Sao Paulo, so long power lines were inevitable. To complicate matters further, the Parana River runs through other countries and forms the border with Paraguay. Cooperation between Brazil and Paraguay resulted in the ltaipu Treaty, signed on April 26th, 1973. This described the project to be built and acknowledged the joint ownership by both countries. Within 12 months, topography studies were complete, together with predictions of the power level and time schedule. The bi-national construction authority, now called Itaipu Binacional, contracted for the purchase and installation of machinery, materials and services. Preference was given to tenders from Brazil and Paraguay. So vast were the civil engineering works, that thirteen companies from both countries cooperated in their execution. The manufacture and installation of the giant turbines and generators was carried out by a consortium of six Brazilian, one Paraguayan and six European firms, including Brown Boveri/ ASEA (later ABB), Alsthom, Bordella, Voith and Siemens. The sequence of construction Consider the size of this massive undertaking. At the location of the proposed dam, the Parana was (and still is) a massive swift flowing river, more than half a kilometre wide, 30 to 90 metres deep, with the water racing past at two million tonnes per minute (33,000 tonnes per second). How on earth would you start building a dam to block it? There is no way a coffer dam could hold back that torrent even if you could get in there to build one! That was the first problem the builders faced. Their method solved the problems in an ingenious sequence of construction covering 13 years: (1) Between 1975 and 1978, they dug a 2km-long diversion channel, 150 metres wide and 90 metres deep. It was blocked upstream and downstream by temporary concrete arch dams and two rock plugs. In just under two years they dug out 2,600,000 cubic metres of sand and gravel, plus 30 million tonnes of hard rock. Simultaneously, outside the rock plugs, underwater excavation of 1.5 million tonnes of rock was performed using special equipment. In the dry space between these temporary coffer dams, the permanent concrete diversion structure was then built, incorporating 12 sluice openings. jA NUAR Y 1992 9 (4) When all walls were built up to full height, the spillway gates were closed. So great is the water flow in the Parana River that the reservoir filled to capacity in 12 days! Installation of the power house machinery then proceeded. The first turbogenerator became operational in 1983 and the 18th unit by 1988, provision being left for two future machines. Size comparison Drainage Basin Fraction of Brazil's Area Hydroelectric Potential Amazon 47% 15GW Sao Francisco 7% 38GW The entire dam wall measures 8km long, with the highest section being 196 metres above the riverbed at its centre. To gain a realistic appreciation of the height and length of this giant structure, let us compare it with something we all know well. If the Itaipu Dam were placed across Sydney Harbour, it would be a solid wall one and a half times the height of the Harbour Bridge and over 250 metres thick at its base. This immense barrier would extend all the way from Redfern on the south side to St Leonards on the north. Itaipu Dam raises the river 120 metres above it1? natural level, forming a lake 170km long on the upstream side and up to 16km in width. Its maximum surface area is more than 1400 square kilometres. Of this, 57% is in Brazilian territory and the rest is in Paraguay. From the total catchment area of a little less than a million square kilometres, the Parana River inflow to the dam varies with the seasons from about 33,000 tonnes per second to 72,000 tonnes per second at peak flood times. Of this, 14,000 tonnes per second flows through the 18 715-megawatt Francis water turbines in the power station. The spillway, rated at a maximum flow of 62,200 tonnes per second, must be capable of passing all excess water in flood times. Such is the bulk of water flowing down the Parana river that the power plant is essentially a run-of-the-river operation, with the reservoir water level r€Jmaining approximately constant. East Coast 7% 21GW 50Hz & 60Hz generation Parana 11% 121GW All Others 28% 15GW Total 100% 210GW This photo shows a 2000-tonne alternator stator being lowered into place during construction of the Itaipu project. Note its size in relation to the men below. (2) With the rock plugs removed, the concrete coffer dams at both ends of the diversion channel were simultaneously l>laslecl oul, allowing water to enter the diversion channel. For the next four years, the river flowed via the diversion channel and through the diversion structure sluices. The main river channel was then blocked upstream and downstre,a m by coffer dams. Construction of the main dam, power station and spillway then proceeded in the dry river bed. At the same time, the left and right wing dams were completed. (3) Filling of the reservoir occurred in 1982. To effect this, the steel sluice gates of the diversion structure were closed and plugged with concrete. The diversion structure was then enlarged until it became part of the main dam and power house. TABLE 1: HYDROELECTRIC POTENTIAL IN BRAZIL 10 SILICO N CHIP Because the Paraguayan electricity system works on 50Hz and the Brazilian on 60Hz, Itaipu generates power at both frequencies. Nine alternators with 66 poles and running at 90 RPM (actually 90.9091 RPM) produce the conductors are concentrically enclosed in (but insulated. from) continuous grounded pressure-tight metal outer pipes. The whole assembly of large diameter piping is filled with sulphur hexafluoride gas at high pressure. This SF 6 gas is extremely inert chemically, non-toxic, non-flammable and has very high dielectric strength. Each of these properties is essential for switchgear applications. So even at the extremely high voltages of 525kV at Itaipu, gas insulated busbars and switchgear result in a compact indoor installation instead of the huge outdoor switchyard which would otherwise be needed. As the circuit breaker contacts are completely encased in grounded metal, RFI generated during switch openings is suppressed. Therefore, microelectronics may be used for control circuits and can be mounted close to the high voltage equipment. This is just not possible with conventional high voltage switchgear. And being always immersed within dry unreactive gas , the conductors, insulators and contacts will not be corroded by the warm humid atmosphere expected around a hydro power station. 50Hz to HVDC conversion The size of the ltaipu project is so large that it is difficult to comprehend. Here we see a 525kV gas insulated conductor (looks just like a big pipe) and behind that is a 525kV lightning arrestor. In the background are some of the penstocks that feed the turbines. Each penstock has an inside diameter of 10.5 metres (more than 34 feet) 50Hz supply. The other nine alternators have 78 poles each and run at 92RPM (actually 92.30769 RPM) to produce the 60Hz supply. So no fancy frequency changing is required to produce both power frequencies. But the story becomes much more complicated with regard to distribution, as we shall see. Each machine develops its output at 18kV but the currents are enormous - more than 26,000 amps per phase. Enclosed hollow conductors carry the huge currents from each alternator to 18kV /525kV step-up transformers. These very heavy 18kV conductors are hollow for two reasons. Firstly, the currents are so enormous that skin effect is appreciable and thus solid conductors would give no benefit. Secondly, it allows the circulation of cooling fluids. The 60Hz units supply the Brazilian national system by transformation from 525kV to 765kV. Most of this AC power is transmitted by 3phase lines to Sao Paulo. The 50Hz machines send power to the Paraguayan state grid at 525kV and, by further transformation, at 220kV. Gas insulated switchgear The 525kV AC outputs from each transformer are carried by gas insulated busbars and switchgear. The live While the generating capacity at Itaipu is evenly split between 50Hz and 60Hz, not all the 50Hz power is required by Paraguay. The excess 50Hz power is sold to Brazil to supplement the supply to Sao Paulo. But, as we have seen, the Brazilian system runs at 60Hz. Therefore , the 50Hz power is converted to high voltage DC at Itaipu, transmitted over the 800km to Sao Paulo, then converted from DC to 60Hz 3-phase AC supply. To achieve this, they built the world's largest high voltage DC transmission system, between 1984 and 1987. Two DC power lines, each rated at 3.15 gigawatts , run in parallel over the route. Each line consists of two multi-bundled conductors rated at 2650A; one conductor at +600kV (with respect to earth) and the other at -600kV. This centre-earthed arrangement effectively gives a 1.2 million volt link while limiting the potential stresses at insulators and equipment to half this voltage. To convert the AC to DC, multiple JA N UARY 1992 11 light-triggered water-cooled silicon thyristors are used. These third generation GTO units are a far cry from earlier gear. Thyristor ratings This photo of the ltaipu project shows the turbine hall section of the dam. Notice the huge penstocks feeding each turbine. The turbine hall is about 1km long. The capability of a thyristor to handle large currents is governed mainly by its cathode area. Today, silicon wafers 100mm in diameter are commercially available. This makes possible the manufacture of thyristors with a cathode area about 13 times larger than the active area of the first HVDC thyristors used in the Gotland DC link in 1970, as described in the March 1991 article of this series. The thyristors used each carry about 3300 amps under normal conditions but under fault conditions they must safely carry up to 30,000A during the four or five cycles that elapse before the overcurrent circuit breakers can open. The very high fault current capability of these modern GTO thyristors allows optimisation of the accompanying transformer design. No longer need transformers possess high reactance in order to limit short circuit currents. A transformer with lower output impedance means less copper loss in the windings and a big saving in transformer cooling equipment. Third generation thyristors also have much improved voltage capability. Off-state voltage ratings are now as high as 7kV per unit. This is two or three times the value possible in earlier designs. ASEA now make thyristors which are capable of carrying the full current of all present and projected HVDC systems. Thus, no paralleling of thyristors is required. Light-triggered thyristors Looking more like a chemical plant than a power station, these are some of the gas insulated (sulphur hexafluoride) bus bars and switchgear inside the ltaipu project. Because it uses gas insulated switchgear throughout, there is no large high voltage switchyard; just miles and miles of big pipes! 12 SILICON CHIP However, even with voltage ratings of 5kV or 6kV per thyristor, hundreds of such units must be connected in series to withstand the full 600kV of the Itaipu line. They are clamped into water cooled heatsinks, with teflon hoses connecting them to an external source of distilled water. Naturally, all thyristors in the series stack must be turned on and off simultaneously, even though some thyristors will be elevated at plus or minus 600kV above earth. Therefore, all the thyristors are triggered optoelectronically with laser light pulses via optical fibre glass cables. This photo shows the Parana River in full flood, with the spillways handling more than 62,000 tonnes of water per second. At the same time, the total flow through the 18 turbines is 14,000 tonnes per second! In the centre of the silicon wafer is a second auxiliary thyristor. The infrared light signal first switches on the auxiliary thyristor which then electrically triggers the main power thyristor. For the entire ltaipu project, ASEA used 20,000 high power thyristors rated at 7kV off-state voltage and 4kA on-state current. So ltaipu stands as the world 's biggest power station, running the world's biggest and heaviest alternators and feeding the world's biggest high voltage DC link over the greatest distance. Truly, ltaipu is a mind boggling engineering project on a vast scale. sc Acknowledgements A view inside the Valve Hall at ltaipu, showing two of the 16 valve assemblies. Each valve assembly contains 384 thyristors in a full-wave bridge configuration; as needed for a 600kV DC line. ltaipu is the starting point for the biggest and longest high voltage DC link in the world (800km to Sao Paulo). Special thanks to Ms Maria Nicholl, the Embassy of Brazil, for photos, diagrams and data. Acknowledgements also to IEEE Spectrum, ABB/ASEA Journal and Action, Phillip Vaughn-Williams, Goverflo do Estado Sao Paulo, Dr Geoff Cochran, Dr Mike Gore, ltaipu Binacional. JANUARY 1992 13