Notes Topic 7 - 1 2 3 4 5 6 1 2 3 4 5 6 7 INTRODUCTION TO...

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Unformatted text preview: 1 2 3 4 5 6 1 2 3 4 5 6 7 INTRODUCTION TO WIND ENERGY WIND RESOURCES WIND TURBINE COMPONENTS AND CONCEPTS WIND TURBINE AERODYNAMICS WIND TURBINE BLADE DESIGN AND BLADE MANUFACTURE WIND TURBINE MECHANICAL DESIGN 7 GENERATORS The electric generator within a wind turbine converts the rotor torque (mechanical energy) into electrical power. The generators used in all modern wind turbines, irrespective of capacity, are either asynchronous (induction) or synchronous generators. Both types of generators exhibit different inherent characteristics and consequently different control strategies are used. Therefore the wind project planner must assess the different electrical characteristics of the turbines offered to best suit the utility’s needs. Introduction to wind energy......................................................................................................................... 1 Wind Resources ........................................................................................................................................... 1 Wind Turbine Components and Concepts ................................................................................................... 1 Wind Turbine Aerodynamics....................................................................................................................... 1 Wind Turbine Blade Design and Blade Manufacture .................................................................................. 1 Wind Turbine Mechanical Design ............................................................................................................... 1 GENERATORS ........................................................................................................................................... 2 7.1 Asynchronous or induction machines ................................................................................................... 2 7.1.1 Synchronous Speed......................................................................................................................... 6 7.1.2 Slip.................................................................................................................................................. 6 7.1.3 Reactive Power and the Power Factor ............................................................................................ 7 7.1.4 The asynchronous motor as a wind turbine generator .................................................................... 8 7.1.4.1 Twin Speed Machines ................................................................................................................. 9 7.1.5 Speed variation with asynchronous generators............................................................................. 10 7.1.5.1 The principal of speed variation using rotor energy.................................................................. 10 7.1.5.2 Dynamic Slip Control................................................................................................................ 12 7.2 Synchronous Machines........................................................................................................................ 12 7.2.1 Synchronous Generators in Wind Turbines.................................................................................. 13 7.2.1.1 Synchronous generators in small scale turbines........................................................................ 14 7.2.1.2 Synchronous generators in large scale turbines ........................................................................ 14 Figure 7-1 The two main components of an induction motor – rotor and stator................................................ 2 Figure 7-2 The stator windings of a Bonus 2MW machine................................................................................ 3 Figure 7-3 Illustration of a wound rotor for an induction motor ........................................................................ 4 Figure 7-4 Illustration of a squirrel-cage rotor for an induction motor .............................................................. 4 Figure 7-5 Cut away view of an asynchronous (induction) motor. .................................................................... 5 Figure 7-6 The relationship between rotor speed and current flow for a grid-connected induction machine.... 6 Figure 7-7 The power draw can be resolved into two components – real power and reactive power ............... 7 Figure 7-8 Generators being assembled at an Enercon factory .......................................................................... 8 Figure 7-9 Torque speed curve for an asynchronous machine ........................................................................... 8 Figure 7-10 Asynchronous machine with a wound rotor and slip rings.......................................................... 10 Figure 7-11 Schematic of Dynamic Slip Control ............................................................................................. 12 Figure 7-12 Illustration of a synchronous machine .......................................................................................... 13 Figure 7-13 Schematic diagram of the cross-section of an Enercon E-66 nacelle ........................................... 15 Both types of electric machines have a non-rotating assembly called the stator, which is normally fastened within the body or frame of the generator. In a conventional electric machine the rotating assembly called the rotor turns within the stator and, in the case of a wind turbine, is driven by the rotor blades either directly or via a gearbox. All utility style turbines ‘export’ power in three-phase AC, (50 or 60Hz) usually at 690V. Step up transformers normally located at the base of the turbine tower, may then be used to increase the generated voltage to that of the high voltage transmission lines of the utility’s distribution network. Small battery charging turbines commonly use direct driven (no gearbox) permanent magnet, synchronous generators because of their high capacity to weight ratio. The three-phase AC output is then rectified before battery connection. Within each of the two main classes of generator, asynchronous and synchronous, lie many design variations, created to produce different operational characteristics. This has resulted in the use of a myriad of different generator types and configurations, all striving for the lowest life cycle cost of electricity production. It is the job of the wind turbine installer/planner to weigh-up the pros and cons of each system offered to arrive at the optimum solution for the particular installation. 7.1 Asynchronous or induction machines Asynchronous motors, often referred to as induction motors, are the most commonly used electric motor in industrial applications today. This is because of their relatively simple rugged construction, availability and low cost. The asynchronous motor may be used as both a motor, if fed with an AC supply, or a generator if supplied with mechanical energy as torque. The motor consists of two main parts, the stator and the rotor (see Figure 7-1). Figure 7-1 The two main components of an induction motor – rotor and stator Table 7-1 Synchronous speeds (in rpm) for common induction motors. ........................................................... 9 (Source: http://www.tpub.com/neets/book5/18c.htm) 7-1 7-2 The stator is made up of a stack of round pre-punched laminations (thin insulated iron sheets) which when assembled resembles an open ended cylinder. The lamination stack is then pressed into the motor body which may be made of aluminium or cast iron. The inner surface of the stator has a number of deep radial slots or grooves that run axially around the internal diameter of the laminations. The stator also consists of a series of coils of copper wire, called windings (see Figure 7-2), the number of which must be a multiple of three. The windings are positioned into these pre-mentioned lamination slots. The arrangement of the windings or coils within the stator determines the number of poles that the motor has. The windings provide a path for the A.C. current to flow which in turn produces the rotary magnetic field that will cause the rotor to turn. The winding configuration, slot shape, and lamination material all have an effect on the performance of the motor. The voltage rating of the motor is determined by the number of winding turns on the stator and the power rating of the motor is determined by the losses which comprise copper loss and iron loss, and the ability of the motor to dissipate the heat generated by these losses. The stator design determines the rated speed of the motor and most of the full load, full speed characteristics. Figure 7-3 Illustration of a wound rotor for an induction motor (Source: http://www.tpub.com/neets/book5/18c.htm) Figure 7-2 The stator windings of a Bonus 2MW machine If the rotor winding consists of solid bars that are joined either end by a shorting ring, it is known as a "squirrel cage rotor" motor (see Figure 7-4). Named because the cage of the rotor resembles the cage that squirrels or mice use to play with when in captivity, the squirrel cage rotor motor is the most common type in use today. Figure 7-4 Illustration of a squirrel-cage rotor for an induction motor (Source: Renewable Energy World July-August 02, p70) The rotor consists of laminations and a "winding" that depends on the specific type of motor. The rotor is supported on a steel shaft running in bearings mounted in the body of the motor. If the rotor has a winding similar to that of the stator (copper coils) it is known as a "wound rotor motor" (see Figure 7-3). The wound rotor motor has a set of slip rings that are used to transfer the current from the rotating rotor to an external circuit for control purposes. 7-3 (Source: http://www.tpub.com/neets/book5/18c.htm) The squirrel cage windings are made up of rotor bars passed through the rotor, from one end to the other, around the circumference of the rotor. The bars protrude beyond the ends of the rotor and are connected together by a shorting ring at either end. The bars are usually made of aluminium or copper. The bars position relative to the surface of the rotor, their shape, cross sectional area and material determine the rotor’s characteristics. Essentially, the rotor windings exhibit inductance and resistance. A bar with a large cross sectional area will exhibit a low resistance, conversely a bar of a small cross sectional area will exhibit a relatively high resistance. Likewise the resistance will also depend on conductor material. Positioning the bar deeper into the rotor increases the amount of iron around the bar and consequently increases the inductance exhibited by the rotor. The impedance of the bar is dependant on both resistance and inductance, thus two 7-4 bars of equal dimensions will exhibit different impedance depending on their position relative to the surface of the rotor. Figure 7-6 The relationship between rotor speed and current flow for a grid-connected induction machine The name induction motor stems from the motors method of operation, whereby the stator windings induce a current flow in the rotor windings due to the stator’s magnetic field moving relative to the rotor. The induced current in the bars of the rotor’s cage produces an electromagnetic field that is then pulled along by the rotating magnetic field of the stator. Figure 7-5 Cut away view of an asynchronous (induction) motor. 7.1.1 Synchronous Speed The motors synchronous speed is the motor’s theoretical maximum speed that would result in no torque output as the rotor would be stationary relative to the stators magnetic field and in theory no current would flow. In actual operation, rotor speed always lags the magnetic field’s speed, allowing the rotor bars to cut magnetic lines of force and produce useful torque. Even in free run conditions the actual rotational speed of the motor is slightly lower than the synchronous speed. This is due to the small amount of torque that is needed to overcome bearing friction and other mechanical losses such as windage caused by air resistance of the rotor and cooling fan.. The motor’s synchronous speed in revolutions per minute is determined by the excitation frequency f in Hertz and the number of stator poles p, shown in the equation below. Equation 7-1 ns = 120 f p 7.1.2 Slip Since the induction motor produces no useful torque when operated at its synchronous speed, it is limited to operate at speeds slightly below its synchronous speed when used as a motor. This difference between the motor’s operation speed n and the synchronous speed ns is known as slip s and is usually defined as a percentage of the synchronous speed. The slip is dependant on the load applied and the type of motor construction but is typically limited to about 2% and a maximum of 5%. (Source: http://www.controleng.com/archives/1999/ctl1201.99/9912bb.htm) Equation 7-2 The diagram below shows the effect of different rotor speeds on a grid connected asynchronous motor. If the motor is connected to an AC supply (the grid) it will rotate at its synchronous speed ns, which is related to the frequency of the supply and the number of stator poles in the motor. If the motor is ‘loaded’ by applying a force opposing the direction of rotation, the current drawn from the supply grid will increase to provide additional torque to drive the mechanical load. Now, if the load is reduced and a torque is gradually applied in the direction of rotation, the current being drawn by the motor will reduce. When the motor is being rotated at exactly it’s synchronous speed it will be in equilibrium and no current will flow. If the torques is increased further until the motor is rotated faster than its synchronous speed, the motor will then start to act as a generator and current will flow into the grid. 7-5 s= ns − n × 100 (%) ns Typically an asynchronous machine with a given rated power, the nominal slip will increase as the pole count increases. A motor with a relatively large slip value is termed soft while a motor with minimal slip is termed stiff. A relatively soft generator is advantageous in wind turbine as the slip accommodates small speed variations caused by torque fluctuations in gusty conditions. Machines that have a low rated slip, result in a high geartrain mechanical loads due to the almost rigid grid coupling and as a result these designs need to be stronger (usually meaning heavier) to overcome the higher loadings, than those turbines equipped with higher slip generators. Since turbine rotor torque is proportional to electrical power (P = T.ω), torque fluctuations may be reduced, by allowing the rotor to slip. The fluctuations of power are then stored as kinetic energy in the rotor. This has two main advantages: 7-6 1) 2) drive train torque fluctuations may be reduced, which then allows for lighter, less massive designs. turbines with higher slip will result in lower power fluctuations, which is preferred by utilities operating weak grids. 7.1.3 Reactive Power and the Power Factor One disadvantage of using an asynchronous generator is that reactive power is consumed from the connected grid to magnetise the stator. Utilities penalise users of reactive power, hence it is advantageous to reduce the amount drawn to maximise generation profits. The amount of reactive power (kVAR) drawn is dependant on the real power output of the machine and often results low power factors. Power factor is the ratio of real power (kW) to apparent power (kVA). 7.1.4 The asynchronous motor as a wind turbine generator Most early wind turbine manufacturers and many still today use induction motors as generators (see Figure 7-8), capitalising on the motor’s reliability and the advantage of being readily available off-the-shelf in many different capacities. Another major advantage of asynchronous generators is that they produce utility compatible power, directly, without the need for expensive power electronics for grid interfacing which have only become commercially available in recent times. Figure 7-8 Generators being assembled at an Enercon factory Equation 7-3 PF = Re alPower = cos Θ ApparentPower Figure 7-7 The power draw can be resolved into two components – real power and reactive power (Source: To be confirmed) The behaviour of an asynchronous generator may be described by a torque-speed curve. Figure 7-9 shows the torque-speed curve for a typical asynchronous machine. The characteristics of the asynchronous machine is largely dictated by the electromechanical design Figure 7-9 Torque speed curve for an asynchronous machine The vector diagram above illustrates the components of a power draw. The vector addition of reactive power and real power is apparent power. The angle between the apparent power vector and the horizontal axis (real power) is given by theta. The cosine of the angle theta is equal to the power factor. Capacitor banks are often used with asynchronous machines to negate the effect of phase shift that occurs as magnetising current is drawn to provide the reactive power for the generator. A control system is used to switch banks of capacitors in and out as needed to keep the power factor as close to unity as possible (or within the utilities requirements). It is important however that the amount of capacitance never be enough to self -excite the turbine if the grid is down. 7-7 7-8 Synchronous speed: n1 Operating as a motor: MAM start up torque; MSM pull-up torque; MKM pull-out torque; MNM rated torque; nKM speed of rotation at pull-out; nNM rated speed of rotation. Operating as a generator: MNG rated torque; MKG pull-out torque; MSG pull-up torque; nNG rated speed of rotation; nKG speed of rotation at pull-out. (Source: Heier, S., Grid Connection of Wind Energy Conversion Systems, p121) Of particular interest to wind turbine generators is the pullout torque MKG, also referred to as the breakdown torque, which occurs when the rotor inductive reactance is equal to the rotor resistance. This characteristic of the asynchronous generator allows one of the simplest methods of controlling the aerodynamic forces on the turbines rotor and hence the ability to limit the peak power of the turbine. This method of control is called passive stall regulation or stall control and is covered in more depth in Topic 9. Typically the pull-out torque of an induction generator is about 2.5 times the rated torque MNG. If this value is exceeded in operation, the speed control effect of the grid will be lost, and could result in dangerous “run-away’ conditions. The most common asynchronous generators used in wind turbines are either four or six pole induction motors. In order to produce electrical power the generator must be driven above its synchronous speed. The synchronous speeds in RPM for common induction motors are shown below. Table 7-1 Synchronous speeds (in rpm) for common induction motors. 4 pole 6 pole 8 pole 50Hz 1500 1000 750 60Hz 1800 1200 900 effect may also be accomplished with a single generator with dual stator windings. In this design, a smaller capacity six pole winding may be selected in low to moderate winds and then the second, high capacity four pole, winding selected in higher wind strengths. This design results in a higher efficiency, primarily at lower than rated wind speed, that generally have a higher occurrence. Turbines incorporating this design will have a higher overall efficiency for two reasons: 1. the rotor may then operate at one of two speeds allowing the rotor to operate closer to the optimum tip speed ratio for a given wind speed, producing higher aerodynamic efficiency. 2. the selected generator operates closer to its rated power level, hence with higher electrical efficiency. Since a dual-speed turbine’s rotor will rotate slower at low wind speeds (when the smaller generator is engaged), aerodynamic noise from the rotor tends to be significantly lower than for a single speed turbine operating at its single high speed. Incorporating twin generators significantly increases the lifetime power production of a wind turbine, but fundamentally the turbine is still not operating ideally for all wind speeds. Various designs are used to increase the variable speed characteristics of asynchronous generators in order to reduce mechanical loads, power fluctuations and increase energy capture. As previously discussed, increased generator slip allows the rotor torque transients to be absorbed and creates a so-called ‘softer’ interface with the grid. In some designs, high slip allows a limited variable speed operation which means the rotor operates closer to its optimal tip speed ratio. Furthermore output power fluctuations may be minimised during gusty conditions. Various methods such as dynamic slip control and double fed asynchronous generators are used to achieve increased speed variation of asynchronous generators. 7.1.5 Speed variation with asynchronous generators Various methods of increasing generator slip have been used by wind turbine manufactures to reduce drive train torque transients and grid power fluctuations (flicker) associated with standard short-circuited rotor asynchronous generators. In order to drive the generator at these relatively high speeds, turbines using asynchronous generators must use gearboxes to step-up the rotational speed of the rotor. Since an asynchronous generator’s rotational speed (and consequently the turbine’s rotor) is fundamentally locked relative to the grid frequency, a relatively constant speed operation will result. Hence turbines employing asynchronous generators, which are directly connected to the grid, fall into the class of fixed speed turbines and this characteristic has become a feature of the traditional ‘Danish’ design. One of the major disadvantages of fixed speed turbines is that their efficiency is a maximum near the rated power of the generator and is significantly lower at partial loads. Furthermore, the maximum power coefficient of the turbine’s rotor occurs at a single tip speed ratio. Hence the aerodynamic efficiency of the rotor is lower for a fixed speed turbine than for a variable speed turbine because the rotor will only be operating at it’s optimum point at one particular wind speed. 7.1.5.1 The principal of speed variation using rotor energy Asynchronous generators with increased slip control usually have wound rotors as opposed to the common short-circuited rotor of the squirrel cage design. Current is commonly transferred to the rotor windings via brushes and slip rings on the rotor shaft as shown in Figure 7-10. Figure 7-10 Asynchronous machine with a wound rotor and slip rings. 7.1.4.1 Twin Speed Machines To overcome the obvious inefficiencies of operating the turbine at one fixed rotor speed, it is common for fixed speed turbines to be fitted with two generators. One small generator for low speed, light wind operation, and one large capacity with a higher pole count designed for rated power generation. The same 7-9 7-10 When the stator of the asynchronous machine is supplied with an AC voltage a rotating magnetic flux is generated. The magnetic flux generated is dependant on the magnetic circuit reluctance, the number of turns in the stator windings Ns and the stator current Is. This rotating magnetic flux creates an electromotive force or emf in the rotor windings. The ratio of rotor emf Er and stator emf Es is given by Equation 7-4 where ωs is the stator electrical frequency in rad/s (synchronous speed) and ωm is the rotor speed of rotation in rad/s. The number of rotor and stator turns is denoted by Nr and Ns respectively. Equation 7-4 Er Nr (ωs − ωm) = Es N s ωs 7.1.5.2 Dynamic Slip Control A method of dissipating power in the rotor is by increasing its resistance. By increasing the resistance of the windings, the amount of slip is increased since more power will be dissipated. In practise, a wound rotor is used with the windings connected externally via slip rings on the rotor shaft. A series of external resistors may be switched into the circuit to vary the resistance of the rotor. An electronic controller is often used that allows a variable amount of power to be diverted to a resistor in order to control the amount of slip. Since power is dissipated in the resistor system, efficiency decreases as the amount of slip increases and higher amounts of slip require larger capacity and hence more costly power electronics. Therefore this configuration only lends itself to relatively low slip values of approximately 10%. This method of slip control is normally only used transiently; hence losses and cooling requirements are relatively small. Since no frequency converters are used, dynamic slip control is a cost effective method of producing marginal speed variation only, which is used for absorbing torque transients in gusty conditions and not for variable speed operation. Since slip s is defined as: Equation 7-5 s= The most common methods of dissipating rotor power, thus varying the operational characteristics of the slip-ring rotor asynchronous machines, are described below. More examples, specifically those controlled by power electronics are given in Topic 8. (ωs − ωm ) ωs Figure 7-11 Schematic of Dynamic Slip Control Equation 7-4 may be re-written as: Equation 7-6 Er Nr = .s Es Ns The rotor and stator rms current behave like a transformers primary and secondary windings: Equation 7-7 Ir Ns = Is Nr Thus the ratio of rotor power and stator power may be defined as: (Source: Heier, S., Grid Connection of Wind Energy Conversion Systems, p110) Equation 7-8 Using brushes, particularly on the high-speed motor shaft, creates added complexity and an added maintenance concern. This has led one turbine manufacturer to a novel solution. IrEr =s IsEs Equation 7-8 shows by dissipating more power in the rotor windings a higher slip is achieved. Thus to vary the rotor speed it becomes simply a matter of changing the power dissipated in the rotor. In simple terms, if the resistance of the rotor increases it becomes more difficult for the rotor current to flow. Thus the rotor speed has to increase to increase the current flow for a given torque. It should also be noted that as the rotor resistance increases and hence slip increases, the efficiency of the generator decreases. 7-11 Vestas Optislip Vestas, a leading Danish manufacturer has developed an innovative variable slip generator marketed as Optislip. This design allows up to 10% generator slip by varying the rotor resistance as outlined above. The maintenance issues of high-speed brushes and slip rings have been overcome by mounting the resistors and the switching system on the rotor itself. Control signals are fed to the rotor via fibre-optic components, rather than conventional slip rings, hence reducing the maintenance concerns associated with brush wear. 7.2 Synchronous Machines Synchronous generators are characterised by the rotor having a magnetic field consisting of a number of poles. The magnetic field may be produced by permanent magnets or by DC current fed to the rotor windings via slip rings and brushes on the rotor’s shaft (see Figure 7-12). Most large machines (>30kW) have wound 7-12 field rotors that enable the strength of the magnetic field to be altered in order to alter reactive power. The stator usually consists of a lamination stack holding a set of three phase windings similar to that of the asynchronous machine. Figure 7-12 Illustration of a synchronous machine turbines where the electrical connection to the grid is de-coupled by the use of an intermediate circuit. This allows the synchronous generator to operate independently from the effects of the grid. 7.2.1.1 Synchronous generators in small scale turbines Today nearly all battery charging (low capacity) wind turbines use direct driven, permanent magnet, synchronous generators. The advantages of this type of generator are: • High power to mass ratio • Possible to manufacture high torque/low speed generators for direct drive applications. • Relatively simple battery charge control system. • True variable speed operation. (Source: http://tpub.com/neets/book5/18b.htm) Synchronous generators tend to be more costly than a comparable capacity asynchronous generator. This reflects their differing methods of construction and their complexity. The fundamental characteristic of the synchronous motor is that their rotor speed is always rigidly locked-to and proportional to the frequency of the connected grid. Therefore if a synchronous machine is directly grid connected it will become a constant speed machine and turn at exactly the synchronous speed -hence its name. By the same token, if a synchronous machine is being driven open circuited, the output frequency and voltage is proportional to the rotors speed of rotation. The trend to use synchronous generators in large utility scale wind turbines is progressively increasing. This is due to advancements in power electronics permitting cost effective use of large capacity power conditioning and control systems. Suitably controlled synchronous generators also have the ability to allow full variable speed operation and reactive power control, which is a significant advantage in large capacity wind turbine installations. 7.2.1 Synchronous Generators in Wind Turbines Synchronous generators are commonly used in small battery charging wind turbines and more recently are being used in large-scale grid connected turbines. The theory of operation of the generators in these two classes of turbines is basically the same and the difference lies in the power electronics used for either battery charging or grid connection. Some early turbine designs used synchronous generators that were directly grid connected. This resulted in definite fixed speed operation because of the rigid electrical characteristics. Wind turbine designers soon discovered that this configuration produced high levels of output power fluctuations, caused by wind gusts, due to the inherent very stiff characteristics (due to the absence of slip) . Mechanical wear and tear on these machines was also excessive due to the drive train torque fluctuations. Therefore directly grid connected synchronous generators are now not used. Instead synchronous generators are used in some variable speed 7-13 The generators of such turbines usually employ a rotating magnetic field within (or around) the stator windings. This layout eliminates the need for brushes and slip rings within the generator since the electric current is generated within the stationary windings. The stator is normally wound for three-phase AC operation, which permits relatively efficient generation and transmission of the generated power over long distances to the rectifier/controller and the storage batteries. The generator output is variable voltage and variable frequency, both of which are proportional to the speed of the rotor. For battery charging applications the AC output of the generator is rectified and the resulting DC is fed to the batteries. Since there is no external AC to influence the operation of the generator, variable speed operation is permitted. This allows the generator (and turbine rotor) to operate at variable speeds permitting efficient operation at the optimum blade tip speed ratio. This also reduces torque fluctuations of the rotor in gusty conditions (lowering mechanical stresses), which permits a lighter, more cost effective turbine design. The recent advent of commercially available high strength, rare earth, permanent magnets (neo-dymium iron boron) has allowed for even greater generator power densities over the traditional designs using ferrite magnets. The two main generator design topologies are shown below. Both of these designs fall under the class of radial flux machines. 7.2.1.2 Synchronous generators in large scale turbines Direct Drive Some manufacturers of large-scale grid connected turbines (notably Enercon and Lagerwey), manufacture purpose built, direct-drive, synchronous generators. (The advantages associated with the omission of the gearbox have been discussed in Topic 6.) Although the advantages of the direct drive concept seem obvious, the realisation of this design has been hampered by the high cost of custom building the generator and expense of the power electronics necessary for variable speed operation and power conditioning before grid connection. Currently all commercially available, large-scale, turbines use the conventional layout of synchronous generators, where the magnetic field is provided by electromagnets on the rotor. The magnetic field is produced by DC power fed to the rotor via slip rings and brushes. Using electromagnets produces a significant advantage in that the amount of reactive power supplied by the machine may be controlled by the magnitude of this field current. As this field current increases, the generator passes from consuming to producing reactive power. One of the sobering factors of the direct drive topology, being used in large-scale turbines, is the very high number of poles required. For example a MW sized machine operating at 20RPM would need 300 poles to achieve a generator frequency of 50Hz, subsequently requiring a very large diameter generator. Because 7-14 these machines are indirectly coupled to the grid (with the aid of frequency converters) it is not essential that the output frequency be the same as the grid, thus the number of poles may be reduced. However, as the frequency is reduced the cost of the converter generally increases. As a result, a high number of poles are often used, producing generators with a similar appearance i.e. large diameter, narrow, ring-shaped generators (see Figure 7-13). Figure 7-13 Schematic diagram of the cross-section of an Enercon E-66 nacelle (Source: http://www.afm.dtu.dk/wind/smep/enercon.html) References: [1] Heier, S., Grid Integration of Wind Energy Conversion Systems, John Wiley and Sons Ltd, UK, 1998. [2] Renewable Energy World Magazine July-August 02, p68-70 [3] Rizzoni, Giorgio. Principles and Applications of Electrical Engineering. 3rd ed. pp864-881 Useful websites: www.vestas.com.dk 7-15 ...
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This note was uploaded on 06/09/2011 for the course PV 5053 taught by Professor Aasd during the Three '11 term at University of New South Wales.

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