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The Story Of Electrical Energy, Pt.3 Alternators can be regarded as the central machines in power stations. They convert the mechanical energy of the turbines into electrical energy. To do so, alternators require very heavy excitation currents and advanced methods of cooling to keep the internal heat generated under control. By BRYAN MAHER As we noted in the first episode of this series, an alternator has two sets of windings: the stator or stationary windings in which the huge voltages and currents are generated, and the rotor which provides the rotating magnetic field. It is the interaction of this field with the stator windings that produces the 30 SILICON CHIP electrical output. The magnetic flux density is very strong, typically 2 to 2.5 Teslas. This is more than twice the flux density present in the voice coil gap of modern loudspeakers which have very large permanent magnets. In big alternators, the magnetic field is produced by very large DC currents fed to heavy coils of silverbearing hard-drawn copper wound on the rotor. These "excitation" currents are usually fed to the coils of the spinning rotor via carbon brushes which run on two large sliprings mounted on (but insulated from) the main shaft. Exciting currents In earlier systems, excitation currents were usually provided by DC · generators driven by the main turbine shaft. Alternatively, in some installations the exciter was driven by a separate small steam turbine. The field coils of the exciting generator were usually supplied by a smaller DC generator called the pilot exciter, as shown in Fig.1. This practice gave good control of the excitation voltage but DC LEFT: THE 500 MEGAWATT turboalternators at Wallerawang power station, NSW are hydrogen and water cooled. In the background is the steam turbine of one unit and in front of it, the main alternator. The large rectangular housing in front of the main alternator is the main exciter. In the foreground are the pilot and starting . exciters. PILOT EXCITER OC GENERA TOR MAIN EXCITER DC GENERATOR COMMUTATOR ~ ARMATURE COMMUTATOR SOFT CARBON __, BRUSHES VOLTAGE REGULATOR generators with commutators require considerable maintenance. 3</> HIGH VOLTAGE 50Hz OUTPUT Modern exciters To eliminate the need for any commutators, modern systems have small alternators to generate an auxiliary AC supply. This is fed to silicon rectifiers to provide the DC excitation for the rotor of the main alternator. Fig.2 is a block diagram of the excitation system controlling one of the 500MW alternators at Wallerawang power station. Fig.2 shows a steam turbine (A) driving the main 500MW alternator (B). This has its exciting currents provided by the main exciter (E). This is a 2.6 megawatt 3-phase alternator which feeds a bridge rectifier (D) consisting of multiple silicon diodes. The bridge rectifier's DC output is fed to the rotor of the 500MW alternator by the sliprings (C). Since the main exciter is also an alternator, it must have its rotor supplied by DC currents. These are supplied by the 90kW pilot exciter (H). It too. is an alternator but it is EXCITATION STARTER GENERATOR (K) MAIN AL TERNA TOR FIG.1: THIS IS THE CLASSIC METHOD of excitation as used in olde1· alternators. A DC generator is used to generate the current for the rotor field of the main alternator. This exciter is then controlled by the pilot exciter which is another DC generator. This has the advantage of being easy to control but DC generators require lots of maintenance to their brushes commutators and sliprings. rather unusual. As shown in Fig.2, its DC field coils are mounted on the stator while the AC output is taken from the rotor. This may seem a little weird but electrically, it is immaterial whether the DC field or the AC output winding rotates. So the pilot exciter is an "inverted" alternator. The pilot exciter's 3-phase output is taken from the triple sliprings (J) and then goes to two rectifiers (G & M). M is a small silicon diode bridge supplying the stationary DC fields of the pilot exciter (H). G is a much larger thyristor bridge which supplies a variable DC voltage (up to 74V and 160 amps) to the rotor field coils of the main exciter E, via sliprings (F). Voltage regulation Just as the alternator in a car needs to have a voltage regulator, so that the car's electrics and battery will not be damaged, then so too the very large alternators in our power stations. If they didn't have voltage regulation, the mains voltage would vary enormously according to the load. In Fig.2, voltage regulation is provided by the thyristor bridge rectifier G. It controls the variable output of the pilot exciter and therefore controls the output of the main exciter alternator. And since the main exciter directly controls the field windings of the main alternator, the thyristor bridge G 90kW 3<b PILOT EXCITER (H) (LI 2.6MW MAIN EXCITER (E) AC 500MW MAIN AL TERNA TOR (B) MAIN EXCITER SLIP-RINGS (F) 3000RPM STEAM TURBINE (A) SPEED GOVERNOR 235VAC 220A 100Hz 3.,; ~"7""-j--1 74V 160A 420V 3850A 100Hz 3o , AC (G) VOLTAGE REGULATOR (0) FULL-WAVE RECTIFIER OUTPUT 500MW 22kV l15437A 3</> 50Hz FIG.2: THIS IS THE METHOD OF FIELD excitation used more commonly today. In fact, this is the block diagram for the 500MW alternators at Wallerawang power station, NSW. The main exciter is an alternator and it is controlled by a pilot exciter which is an "inverted" alternator - its output is taken from the rotor rather than the stator. SEPTEMBER1990 31 MAIN EXCITER (E) PILOT EXCITER (H) MAIN ALTERNATOR (B) MAIN SHAFT '-c:-=_=_ :::::1--l AC CABLES INSIDE HOLLOW MAIN SHAFT FIELD COILS (G) oc 3Q 400Hz (Al MAIN SHAFT 1 ..,_ 1 ~:i.~.1.~J WINrii~lll VO LTAGE REGU LATOR 400Hz AC currents generated in its stator are rectified and controlled by a 6-tpyristor full wave bridge (G). (In a 3-phase bridge rectifier, s ix diodes or thyristors are required). The direct currents so derived energise the stator field coils of the main exciter (E). This is an inverted alternator wherein AC currents of a few thousand amps are generated in its rotor coils. Heavy cables, running inside the hollow shaft, carry these large alternating currents to a a silicon diode bridge located within a wheel (X) mounted on the shaft. The resulting rectified DC flows via copper conductors through the shaft to the rotor windings of the 300MW alternator (B). So the rotor field windings are excited without any brushes or sliprings being used. Having no brushes anywhere considerably reduces maintenance and makes for more compact, lighter machines. Each turbo-alternator weighs only 700 tonnes. The 300 megawatt output of each machine is conducted by 500mm diameter hollow aluminium busbars to the alternator transformers outside the building in the station switchyard. Here the voltage is stepped up to 275kV for transmission to Brisbane, the Gold Coast, Kareeya (near Cairns in the deep north) and all points on Queensland's east coast. STEAM TU RBINE DIODE WHEEL (X) 16.2kV 12.6kA 285/JOOMW FIG. 3: MORE MODERN SYSTEMS, such as the Gladstone power station in Queensland, use an excitation system which is completely free of brushes, commutators or sliprings. Note the use of the diode wheel to rectify the main exciter's output before it is fed to the rotor of the main alternator. therefore controls the output of the whole system. G is called the Automatic Voltage Regulator (or A VR) of the complete turboalternator. citer via rectifier fL). As the voltage generated by the pilot exciter builds up, it begins to supply its own fields through rectifier M. Starting excitation Brushless excitation We have seen how the main alternator has its field coils driven by the main exciter which in turn is controlled by the pilot exciter. Well, the pilot exciter is an alternator too and you guessed it, it has it own exciter, called the excitation starter . This is a small 12-pole 300Hz permanent magnet generator. When the turbines are being run up to speed, it supplies the auxiliary DC field coils on the pilot ex- A particularly interesting excitation method is used at Gladstone power station in Queensland. Here each of the six 300MW turboalternators is excited by a completely brushless system. No sliprings, brushes or commutators are used. Fig.3 shows this clever scheme. The pilot exciter (H) in this case is a 3-phase 16-pole alternator with a permanent magnet rotor. The Synchronising alternators VIEW INSIDE THE TURBINE HALL of Gladstone power station, in the far north of Queensland. The 285MW alternators are all hydrogen cooled. 32 SILICON C I-IIP Most power stations have a number of alternators running and sharing the load. Also the electricity grid system links all power stations in the state together. This means that all the alternators in the state's power grid are effectively connected in parallel. As you might imagine, connecting and running dozens of alternators together in a system grid is not simple task. lil order to connect all the alternators in parallel, five requirements must be met. All machines must: (1). Generate the same voltage; (2). Run at exactly the same speed; (3). Generate the same frequency; (4). Produce a pure sinewave voltage waveform; and BIG ALTERNATORS REQUIRE big on/off switches. This set of 3-phase circuit breakers can break a current of 250,000 amps. Each breaker measures approximately 4 metres long and 3 metres high. (5). Must be in phase with each other (ie, the generated sinewave must rise and fall with exactly the same timing in all machines). This last requirement is called synchronisation. Starting an alternator When an additional alternator is to be connected into the system, the normal procedure for starting is as follows: (1). Steam is applied to the turbine to bring it very gradually up to temperature and full speed. This takes many hours because of the large thermal mass of the machine and the very high operating temperature. While the machine is coming up to operating temperature, it is not electrically connected to the system (ie, the alternator circuit breaker is not yet closed). l2). A synchroscope is used to measure the phase difference between the voltage generated by the incoming machine and that of the state grid system. (3). If the measurements reveal any difference in phase timing, the in- coming machine must be speeded up or slowed (more or less steam applied). Usually only slight changes are needed but the phase correspondence with the other system alternators must be exact. (4). When synchronism is achieved and held for a stabilising period, then and only then is the alternator circuit breaker closed. The new machine is now connected in parallel with the system but as yet is carrying no load. (5). After perhaps an hour or more of unloaded running for temperature equilibrium, the incoming alternator is made to supply some share of the system electrical load. The total time for this procedure for a large alternator of, say, 500 megawatts, is around 8 hours. For this reason, alternators are usually kept running all the time, whether or not they are supplying power to the system. In this way, they make up the "spinning reserve" of the system. Load sharing To make a synchronised alter- nator supply a greater share of the system load, more steam is admitted to the turbine. It is not done by increasing the alternator's output voltage, as you might expect. No, increasing the excitation of one machine running in parallel with many others would not raise its voltage nor increase its power load share. More excitation would only cause that alternator to supply a larger portion of the out-of-phase component of the state load. Feeding more steam to the turbine makes it push harder against the mechanical braking effect of the electrical power load on the alternator. That alternator then takes a greater share of the system power load. Excitation must then be increased to compensate for the demagnetising effect that the stator reaction has on the rotor field. We can explain this process of load sharing by using an analogy. Consider 15 people pushing a car up a steep hill. The pushers correspond to all the parallel alternators in the power system. The effort needed to propel the car may be likened to the state's electrical power load. All these volunteers grab the car and move with it. They are all 'in sync'. If you make a move to help, you may walk with the car in perfect synchronism, yet do no work. To take a greater share of the load, what must you do? You walk at the same speed but just push harder! You have turned on more steam! If for any reason one alternator in a system should lose steam supply and attempt to slow down, that machine no longer carries any load. Rather, the rest of the system alter~ators will force it to stay in sync; ie, at full speed. The errant alternator is said to be "motoring" on the system. Obviously this is not the way a steam power station normally runs as it constitutes an extra load on the rest of the alternators. It would normally never happen. Faulty synchronisation Should an alternator be running too slow when the circuit breaker is closed to join it to the state electricity grid, all hell would break loose! Very large currents would inSEPTEMBER 1990 33 turbo-alternators of ever greater capacity are being installed worldwide. But they can't just get bigger and bigger. Severe constraints exist in the size and weight of alternators that can be transported from the manufacturers' plants to power station sites. For this reason, ongoing development must produce alternators of higher power rating without much increase in mass and dimensions. This demands greater current density in windings and stronger magnetic fields to generate more volts per turn. But more amps per square cm of conductor means that more heat is generated and therefore more effective cooling methods must be devised. Means of cooling THIS PHOTO, TAKEN DURING the construction of a 500 megawatt alternator, shows the teflon pipes which carry the de-ionised water to cool the stator windings. Teflon plumbing and de-ionised water must be used because the stator winding operates at thousands of volts above the machine's frame. Note the heavy bracing which secures the stator windings. stantly flow. Enormous power would surge into the slower machine from the others in an effort to pull it into synchronism. With a hundred or so tonnes rotating at 3000rpm in each alternator, even a small speed alteration translates into enormous momentum and kinetic energy changes. Huge mechanical shock waves would reverberate within each unit from its stator core to the foundations. For these reasons, great care must be taken in ensuring that each alternator is exactly synchronised before it is connected to the system. Circuit breakers From time to time, faults and short circuits do occur in the statewide electricity grid systems. When this happens, enormous fault currents can flow during the time 34 SILICON CHIP taken for the circuit breakers to open. For this reason, faster, stronger and bigger switches are constantly being developed, especially for alternator circuit breaker service. One of the photos included in this article shows the alternator circuit breaker units installed at Meppen power station, Germany. Each these circuit breakers, made by Brown Boveri et Cie, measures around 4 metres long and 3 metres high and is capable of interrupting fault currents of up to 250,000 amps in 50 milliseconds. That's some switch! Power density The demand for electrical energy increases exponentially with the years - around 5 % per annum in industrialised countries. Therefore, With alternators rated up to 60 megawatts or so, air cooling is used. In the basement below the machine are mounted large motordriven centrifugal fans which blow filtered fresh air at high speed through the alternator, to cool the stator and rotor windings. But for bigger alternators, air cooling just can't do the job. For these machines, hydrogen cooling is used. Pure hydrogen gas at around eight times atmospheric pressure is circulated through passageways in the stator and rotor cores. Thus, the windings are indirectly cooled. But hydrogen is highly inflammable and can form dangerously explosive mixtures with air, so what about the safety aspect? Because the hydrogen in the alternator is at a pressure well above the outside air, any leakage will be of hydrogen leaking out, not air leaking in. So the possibility of an explosive mixture is very low. And, of course, the outer casing of the alternators is fitted with hydrogen detectors to warn of any potential hazards. But why use a dangerous gas like hydrogen anyhow? Hydrogen gas is used as a coolant because it is much lighter than air and because it has much greater specific heat. Because hydrogen is so much lighter than air, even when at eight times atmospheric pressure, alternators spinning in a hydrogen at- THIS IS A SECTION cut from an alternator's stator coil. It is wound using multiple flat copper tubes through which water is circulated for cooling. Each flat tube is about 9 x 4mm. The outside insulation is 8mm thick and consists of a mica/glass/epoxy resin material. mosphere have greatly reduced windage losses. And because hydrogen has 14 times greater specific heat and eight times better thermal conductivity than air, it can collect and carry the heat away from alternator windings very efficiently. because each winding is running at tens of thousands of volts above ground! Now you can see why deionised or distilled water is used if ordinary water were used, it would be a short circuit at these high voltages. Water cooled stators Under normal load, the stator current applies a mechanical braking torque against the rotation of the rotor. The steam turbine does work by pushing against this force. But every action implies an equal and opposite reaction from the stationary component; ie, the stator coil itself. Therefore, the winding continuously experiences a sideways For alternators of 350 megawatt to 1.5 gigawatt capacity, even more cooling is required. So in addition to the hydrogen cooling just described, the stator windings are water cooled. Instead of using solid copper conductors, the stator windings are wound from flat copper tubes. Each turn of the winding terminates in a manifold and deionised water is pumped from grounded pipes through teflon hoses to the manifolds. The teflon hoses are absolutely necessary Winding forces force of up to 12 tonnes, and this squeezes the stator copper bars against their insulation and the core slot walls. The insulating materials used must be mechanically strong enough at running temperature to withstand these forces. However, much more violent forces are possible within an alternator, when faults occur. As noted above, short circuits can occur in the state grid system due to lightning strikes or accidents. The excessive currents then flowing in the alternator stator conductors can produce enormous destructive forces on the windings. To prevent the coils from being torn apart, large steel clamps are provided. These are bolted solidly to the frame as illustrated in one of the photos accompanying this article. Next month, we'll have a look at the high voltage power lines used transmit the power to the end users. Acknowledgements Special thanks for photos and data to the Electricity Commissions of NSW and Queensland, the management and staff of Wallerawang Power Station, ABB and ASEA Reviews, Electronics and Power, and C. A. Parsons Ltd. ~ THIS PHOTO OF A LARGE AL TERNATOR rotor shows the fans which help circulate the cooling hydrogen. The hydrogen circulates through passages in both the rotor and stator to cool the windings indirectly. SEPTEMBER 1990 35