Notes Topic 9 - 1 2 3 4 5 6 7 8 INTRODUCTION WIND RESOURCES...

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Unformatted text preview: 1 2 3 4 5 6 7 8 INTRODUCTION WIND RESOURCES WIND TURBINE COMPONENTS AND CONCEPTS WIND TURBINE AERODYNAMICS WIND TURBINE BLADE DESIGN AND BLADE MANUFACTURE WIND TURBINE MECHANICAL DESIGN GENERATORS GRID CONNECTION AND POWER CONDITIONING 1 2 3 4 5 6 7 8 9 9 WIND TURBINE CONTROL 9.1 Introduction Control of wind turbines is important in order to optimise output, limit high mechanical loads, and prevent over-power output. If a turbine were uncontrolled it would operate inefficiently, resulting in far from optimal energy production and would certainly fail in high wind conditions. Introduction.................................................................................................................................................. 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 .................................................................................................................................................... 1 Grid Connection & Power Conditioning ..................................................................................................... 1 WIND TURBINE CONTROL..................................................................................................................... 2 9.1 Introduction ........................................................................................................................................... 2 9.2 The Fixed Speed Concept...................................................................................................................... 5 9.2.1 Fixed Speed, Passive Stall Control................................................................................................. 5 9.2.2 Fixed Speed, Active Stall Control .................................................................................................. 6 9.3 The Variable Speed Concept................................................................................................................. 8 9.3.1 Variable Speed, Stall Control ......................................................................................................... 8 9.3.2 Variable Speed, Active Pitch Control............................................................................................. 8 9.3.3 Partial Variable Speed Control ..................................................................................................... 10 9.4 The Role of Wind Turbine Sensors in Control Systems ..................................................................... 10 9.4.1 Wind Speed Measurement............................................................................................................ 10 9.4.2 Wind Direction Measurement - Yaw Control .............................................................................. 11 9.4.3 Active Yaw ................................................................................................................................... 11 9.4.4 Passive Yaw (Furling) .................................................................................................................. 13 9.5 SCADA systems.................................................................................................................................. 13 Figure 9-1 Control schematic for a pitch-regulated wind turbine ...................................................................... 2 Figure 9-2 Wind farm central supervisory control system used by Turbowinds, in Belgium............................ 4 Figure 9-3 The Nordex N50/800kW – a machine that use passive stall regulation ........................................... 6 Figure 9-4 Comparison of power curves for a generalised passive and active stall regulated turbines ............. 7 Figure 9-5 Schematic of BONUS Energy A/S 1MW nacelle showing pitch bearing and pitch gearbox .......... 9 Figure 9-6 A WAS425 ultrasonic wind sensor from Vaisala. The wind sensor works on the principle that wind speed affects the transit time of ultrasound from one transducer to another. ................................... 10 Figure 9-7 An anemometer and wind direction vane mounted on the rear of a turbine’s nacelle.................... 11 Figure 9-8 Electrically operated yaw control system showing ring gear ......................................................... 12 Figure 9-9 Schematic of a passive yaw system ................................................................................................ 13 Figure 9-10 An example of the display of a SCADA system........................................................................... 14 9-1 The control of a wind turbine may be separated into two important and closely associated systems –the supervisory control system and the safety/emergency system. The level of complexity of the total control system is related to the rated capacity and to the particular design of the turbine. Since the main input into the system, wind, is by nature random, it is the task of the supervisory controller to sample averaged parameters such as wind speed and wind direction and determine the necessary control measures, which are then sent as inputs to high-speed dynamic controllers. While operating in wind speeds up to cutout level, the supervisory controller may signal such procedures as start-up, grid synchronisation, blade pitch (e.g. see Figure 9-1) and generator speed variation, to name a few. If the wind speed exceeds cutout level or an operational fault is sensed (such as rotor over-speed, excessive vibration etc.) the supervisory control system may signal a safety sequence to limit power production or initiate a shut-down to minimise dangerous loads and protect the turbine and or components from failure. If the supervisory control system fails, the safety controller will override the operation of the turbine and shut the turbine down. Wind turbine safety control systems are discussed in more detail in Topic 10. Figure 9-1 Control schematic for a pitch-regulated wind turbine (Source: To be confirmed) When a group of turbines forms a wind farm, another control system commonly referred to as a SCADA (Supervisory Control and Data Acquisition) system can coordinate the operation of the wind farm. SCADA systems may be remotely accessed for data retrieval and for displaying information on each wind turbine in the network (see 9-2 Figure 9-2). SCADA systems enable individual machines to be started and stopped, control sequences altered and the operation history to be downloaded. Wind farm managers rely heavily on this information for the efficient operation and maintenance of the turbines. Figure 9-2 Wind farm central supervisory control system used by Turbowinds, in Belgium The control methods and systems used in a modern, utility-scale wind turbine are often very complex and in most cases are managed by a number of microprocessor based control systems. Signals are processed from a multitude of different sensors, measuring variables such as wind speed, blade pitch angle, rotor speed, generated voltage, power, frequency, temperature of electronic and mechanical components etc. The supervisory controller determines whether any operational adjustments need to be made, either to improve performance (aerodynamic or electro-mechanical) or to protect the turbine from excessive loads if dangerous fault conditions arise. Conversely, the control systems used on most small-scale turbines are comparatively less complex. Small turbines often use passive methods for control, reflecting their simplicity and cost. In most cases wind turbines operate over long periods without human supervision, hence the control system must be capable of reliably supervising the day-to-day operation of the turbine and be able to perform a high level of fault reporting and self-diagnosis. Many manufacturers market their turbines on their high availability; therefore the operational reliability of the complete system is paramount as is accurate fault diagnosis. Since the power of the wind varies with the cube of the wind speed, the mechanical power developed by the rotor in high wind strengths may easily exceed the rated capacity of the power train and generator. For this reason one important role of the control system is to limit the power produced by the rotor to safe levels. There are a number of different control regimes used by manufacturers of wind turbines. The control strategies differ largely due to the design of the turbine (i.e. variable speed, fixed speed, variable pitch etc.). Given that the designs of the turbines may differ, the purpose of the control system remains largely similar. In general its objective is to: • • • tune the operating parameters to extract the optimal energy from the rotor for the given conditions from start-up to shut-down limit the maximum power produced to prevent mechanical or electrical overload, once rated power has been reached Initiate safety control measures to protect the turbine from damage or failure if normal operational control is lost or if a fault condition is sensed. (Source: http://www.turbowinds.com/ control.html) The general control strategy of a wind turbine depends under which broad class the turbine falls -fixed speed or variable speed. Fixed speed turbines, as the name implies, operate at one (or two) relatively constant rotor speeds. Conversely, the rotor speed of variable speed turbines may be adjusted to manage power production. As wind turbine technology has developed over the years, many different control strategies have been developed and tested, all with a common goal - to reduce the life cycle cost of power generated. Many designs have been tried that have realised gains in efficiency but have resulted in expensive, impractical or unreliable products. It should be stressed that although turbine conversion efficiency is important, it is the life cycle cost of the electricity generated that is paramount. The heart of the controller is usually a microprocessor that operates in a closed loop fashion where the various signals from the sensors are processed and control signals are sent to the actuators to tune the 9-3 9-4 operation of the turbine. The amount of sensors and actuators used is dependant on the general control strategy of the turbine. In order to extract the maximum energy from the wind, the rotor must be operated at optimum conditions (i.e. tip speed ratio & angle of attack). Operating the rotor outside these conditions will result in less than optimal power production. Much research has been done to quantify performance levels of variable speed operation versus fixed speed operation. It generally believed that full variable speed operation increases energy capture by up to 10% over the fixed speed operation. Research has shown that this figure is largely dependent on the local wind regime [Ref:1]. 9.2 The Fixed Speed Concept Currently, the majority of grid-connected turbines installed in the world fall into the fixed speed, stall regulated class. However, modern variable speed turbines are increasing in popularity as the technology matures and the cost of the necessary power electronics decreases. The fixed speed concept relies on the phenomenon of aerodynamic stall regulation to limit power production in high wind speeds. The onset of stall may be either passive (fixed pitch) or actively (limited variable pitch) controlled. The obvious advantage of using stall control is the simplicity of the control system, relating to a low manufactured cost and increased reliability over a pitch control system. Although this design is inherently simple, it is also far from optimal in terms of energy conversion for a number of reasons. 1) The rotor operates at one fixed speed, thus its aerodynamic efficiency is reduced because the rotor operates at non-optimal tip speed ratios. 2) The aerodynamic behaviour of a blade in deep stall can be unpredictable and can lead to transient loads (dynamic stall – see Topic 5). 3) A generator maximum efficiency occurs near its rated power, however wind turbines frequently operate at 15-20% rated power. Hence the electrical efficiency is compromised by fixed speed operation. 4) Mechanical losses within the gearbox decrease overall efficiency. One way of increasing the aerodynamic efficiency over operating at a single fixed speed is to introduce a second rotor speed that may be selected in low wind speeds. To achieve this a second smaller capacity generator is installed that operates at a lower synchronous speed or, alternatively, a pole-changing generator may be installed (see Topic 7 for more details). 9.2.1 Fixed Speed Passive Stall Control Wind turbines, which operate at one or two fixed rotational speeds, and are without blade pitch control mechanisms, use passive aerodynamic stall to limit rotor power production. The fixed speed stall regulated concept forms the basis of a simple design philosophy often referred to as the ‘Danish Concept’ (see Topic 1). Other general features of this design are a 3-bladed, upwind rotor, fixed (or rigid) rotor hub, gearbox and an asynchronous generator. length from the root to the tip. Predicting the onset of stall on a wind turbine blade is a complex area of aerodynamics and is difficult to predict particularly in unsteady or turbulent conditions. Topic 4 contains more details on aerodynamic stall. For reliable stall control operation, sufficient grid load must be guaranteed to provide adequate torque on the generator to maintain the drive train at its design speed and to operate the rotor in stall mode. If the consumer load is insufficient or the grid is disconnected, run-away conditions may occur. In order to have ample pullout torque (braking torque to prevent rotor acceleration in gusts), turbines using stall control usually require asynchronous generators with a higher rated output than that of the turbine capacity. Similarly the gearbox needs to be oversized to manage the torque transients caused by these power fluctuations. Passive stall control presents the simplest method of wind turbine control, reflecting the reduced complexity and cost of the control/actuator system required. Essentially the operational control system of a passive stallregulated turbine, in its most simple form, includes a speed sensor on the generator shaft and a contactor (or soft start thyristors) for grid connection. As the wind increases from standstill, the rotor begins to turn and the torque of the rotor is used to accelerate rotating masses of the drive train and overcome friction in the bearings and gearbox. The generators rotational speed is measured by a speed sensor mounted on the highspeed shaft (output shaft of the gearbox). When grid synchronous speed is reached, the contactor (or soft start) is energised and the turbine starts exporting power. If a pole-changing generator is used, the controller first engages the low-speed winding and monitors the power produced. If the average power produced over a given time exceeds the rated power of the low-speed winding the controller then engages the high-speed winding. To prevent undue gearbox wear caused by frequent speed changing, the sampled time interval in generally in the order of five to ten minutes. Nordex is one leading manufacturer of passive stall regulated turbines (see Figure 9-3) and NEG Micon currently manufacturers a 1.5MW turbine with this conventional technology. As turbines have become progressively larger in capacity, however, passive stall control is becoming less favoured; with more manufacturers preferring sophisticated active aerodynamic control strategies, which offer precise power control. Figure 9-3 The Nordex N50/800kW – a machine that use passive stall regulation (Source: www.nordex.dk) Stall control relies on sufficient torque being developed by the generator to hold the rotor at a constant rotational speed governed by the synchronous speed and the slip of the generator. Thus there is compatibility between stall control and a grid ‘tied’ asynchronous generator. As the wind speed increases the angle of attack of the wind will increase since the blade speed remains relatively constant. At a certain point, the laminar flow over the blade becomes turbulent, due to the excessively high angle of attack. As this turbulence increases, lift is lost and consequently rotor power is reduced. This phenomenon is termed stall. The aerodynamic profile of the blade is carefully designed so that stall occurs progressively along the blade 9.2.2 Fixed Speed Active Stall Control In practise the onset of stall is difficult to predict because of the random nature of wind (turbulence, air density etc.). Therefore to ensure run-away does not occur, the generator and gearbox must be adequately oversized to allow for over-production if the effect of stall is delayed. By mechanically altering the blade pitch, stall may be more accurately controlled, allowing closer control over rotor torque. Since the degree of uncertainty is significantly reduced, conservative factors of safety used in the drive train design may 9-5 9-6 consequently be reduced. Thus the components used in the power train of an actively stall-regulated turbine may be of reduced capacity than a comparable capacity passive stall-regulated turbine. In practise, once rated power is reached, stall is induced by pitching the blades towards stall i.e. in the opposite direction of an active pitch-controlled turbine. In order to effect stall, the pitch angle need only be changed a few degrees, this enables a less complex blade-pitching mechanism than in an actively pitchcontrolled turbine which may require blade pitching of up to ninety degrees. Thus active stall regulation produces smoother and more controlled power production at rated power level than a passively stallregulated turbine but without the cost and complexity of full active variable pitch. Figure 9-4 Comparison of power curves for a generalised passive and active stall regulated turbines POWER kW Active stall control RATED POWER start-up and increased light wind performance. Since the advantage of passive stall controlled concept is the simplicity of the pitch control system, machines of this design rarely have fine control over their pitch angle throughout the range. Generally they have a defined start-up pitch, normal operation pitch and then limited pitch control (over a few degrees) to control stall at rated power. NEG-Micon and Bonus are two manufacturers who use this technology. As with passive stall regulated machines, a two-speed generator is often used to increase energy capture at low wind speeds. 9.3 The Variable Speed Concept 9.3.1 Variable Speed Stall Control This configuration is seldom commercially used. One advantage of variable speed turbines is that the power fluctuations, from wind gusts, may be absorbed as rotor speed fluctuations. When this occurs the excess energy is ‘stored’ as kinetic energy in the rotating mass of the rotor. But in order to realise aerodynamic stall the generator must be able to provide sufficient torque to prevent over-speed and a dangerous run-away condition. To achieve this the generator and converter system must then be sufficiently oversized, creating an un-necessarily costly system. 9.3.2 Variable Speed Active Pitch Control Active variable pitch control, or pitch regulation, is increasingly becoming more common on many largescale turbines, especially those machine employing variable speed generators. As described in Topic 4, Section 4.3.2, variable pitch enables precise power control by altering the aerodynamic performance of the rotor. Passive stall control Pitch variation for early start up WIND SPEED m/s Figure 9-4 highlights the effect of active stall control, compared to passive control. It should be noted that the power production of a passive stall regulated turbine decreases as the wind speed increases due the rotor going into deeper stall as the angle of attack of the incident air stream increases over the rotor. Conversely the power curve of the active stall controlled turbine is flatter, reflecting the more accurate control of stall enabling control of rotor power at rated level up to cut out wind speed. Since this method of control is primarily used to limit output power, and not to increase rotor efficiency as in pitch regulated turbines, some turbine designs employ pitch variation on only an outer portion of the blade, leaving the root section of the blade stationary which may reduce the cost of the mechanism. Stall reduces the lift on the blade, hence limiting the power generated, but does not significantly reduce the thrust acting on the blade since the blades are kept relatively flat onto the wind and their projected area in the direction of the wind is relatively constant. Thus both active and passive stall regulated turbines tend to be structurally heavier than active pitch controlled turbines, where the blades can be moved into the feather position to decrease the thrust load on the rotor. Some designs allow a higher angle of attack to be selected at low wind speeds, similar to that of an active pitch controlled turbine but without the infinite control. This increases the torque of the rotor enabling earlier 9-7 The advantages of using active pitch versus stall control are; a) A high level of control over the aerodynamic performance and power generated by the rotor. b) Lower start-up wind speed achievable by coarsely pitching the blades for increased torque. c) The loads acting on the rotor of a pitch-controlled turbine are substantially lower to that of a fixed pitch turbine particularly in high wind speeds. This reduction in load allows for a corresponding reduction in strength, hence material used in the structure of the turbine, consequently leading to a lower manufactured cost. d) Active pitch control technology is generally used in conjunction with variable speed generators. This combination allows for the use of slower response blade pitch actuators (compared to the active stall regulation/ fixed speed concept) since overshoots in power due to wind gusts are stored as kinetic energy in the rotor, while the power converter keeps the exported power constant. e) The blades may be pitched to aerodynamically brake the rotor or set so that no torque is produced (this is also possible with active stall control). Designing for independent control per blade (for redundancy) allows for the use of a smaller, hence lower cost, emergency brake to prevent runaway if the electrical load is lost. These advantages do come at a cost however, most significantly added control system complexity leading to increased cost of these components. Increased complexity may also introduce reliability issues and added maintenance expenses. Most large turbines employing active pitch control use full span (complete blade length) pitch variation. With this type of system, the blade root is mounted on an axial bearing that allows the blade to rotate around its longitudinal axis on the rotor hub. The pitching system may be manipulated by hydraulic actuators mounted on the rotor hub or electrically by individual motors that rotate the blades via a ring gear attached to the blade pitch bearing. The control system is able to accurately determine the actual blade pitch angle, via the actuator and to alter this by a fraction of a degree, to alter the aerodynamic performance. 9-8 Figure 9-5 Schematic of BONUS Energy A/S 1MW nacelle showing pitch bearing and pitch gearbox 9.3.3 Partial Variable Speed Control A compromise between variable speed and fixed speed may be found with the limited variable speed design. This concept uses an asynchronous generator with some form of slip control to provide a narrow band of speed variation. Variable slip generators are generally used in stall-regulated designs and result in smoother power production at and above rated wind speeds than a similar design with a standard asynchronous generator. See section Topic 7, Section 7.2.5.1 and Section 7.2.5.2 for further information on variable slip designs. Another advantage in using variable slip technology with active stall control lies in the fact that the pitch changing mechanism may have a slower response time than need when used with a conventional generator. This is because power fluctuations may be absorbed as rotor speed variations above synchronous speed without the need to critically alter blade pitch to tune power output. This means smaller, less costly actuators may be used and response is not as critical, wear is reduced. Manufacturers using this technology are Vestas and Nordic (specifically the Nordic 1000). 9.4 The Role of Wind Turbine Sensors in Control Systems Nacelle Arrangement 1. Spinner 2. Rotor hub 3. Blade 4. Pitch bearing 5. Pitch gearbox 6. Main bearing 7. Main shaft 8. Top controller 9. Gearbox 10. Brake disc 11. Brake caliper 12. Coupling 13. Generator 14. Meteorological sensors 15. Yaw ring 16. Yaw bearing 17. Yaw gearbox 18. Nacelle bedplate 19. Canopy 20. Generator fan (Source: http://www.bonus.dk/uk/produkter/tekbe_1mv_design.html) Active pitch control enables the optimum blade pitch angles to be adjusted for the full operation range of the turbine. This results in an increased rotor coefficient of performance in normal conditions leading to greater power production, when compared to a fixed pitch turbine. As rated power is reached the blades may be pitched to the feather position to reduce efficiency, hence limiting rotor power (and speed) in strong winds. The blades are normally positively pitched (leading edge into the wind) to decrease the angle of attack above rated power. The advantages of doing this are: a) The blades lose lift due to an inefficient angle of attack; therefore the power of the rotor may be limited. b) The mechanical loads on the blades are reduced due to the excess power being shed and by minimising projected area of the blade ‘seen’ by the wind. This reduces the loads transferred to the turbine structure and importantly the drive train. c) As the blades become more positively pitched, the bending strength of the blades increases in the fore-aft direction (the wind direction) hence protecting the blades from excessive bending stresses in extreme winds. The pitch controller design is mainly centred on proportional-integral-derivative (PID) controllers. This type of controller regulates the error, or the difference between the measured input and the desired input. The variation in the value of this error with respect to time (along with its derivative and integral) provides a signal to the actuator that affects the control process in a closed loop fashion. The bandwidth or sensitivity of the control function is of considerable importance to the designers of the control system. If the bandwidth is too narrow, the blade pitch servomechanism will be subjected to excessive operations leading to premature wear and possible high torsional blade loads. 9-9 9.4.1 Wind Speed Measurement On most utility scale wind turbines, anemometers are mounted on or near the nacelle to measure the wind speed. Conventional rotating cup-type anemometers are predominantly used, but ultrasonic units, with no moving components, are becoming popular because of their inherent high reliability (see Figure 9-6). Figure 9-6 A WAS425 ultrasonic wind sensor from Vaisala. The wind sensor works on the principle that wind speed affects the transit time of ultrasound from one transducer to another. (Source: http://www.vaisala.com/) Most modern wind turbines have two anemometers. One is used to measure the wind speed and the other is used to cross check. If the error between the two anemometers is outside a given value an alarm is raised. The wind speed data may be used in a number of ways depending on the design of the turbine. It may initiate a start-up at a predetermined mean wind speed, or shut down procedure at cut-out wind speed. On variable pitch and variable speed turbines the wind speed data may be used to determine the rotor operating parameters (blade pitch angle and rotor speed). 9-10 Peak and mean wind speeds are recorded and the data used for validating performance predictions or for tuning the operation of the turbines for optimal output for the particular site. Figure 9-8 Electrically operated yaw control system showing ring gear Figure 9-7 An anemometer and wind direction vane mounted on the rear of a turbine’s nacelle (Source: To be confirmed) 9.4.2 Wind Direction Measurement Yaw Control On medium and large utility-scale turbines, a wind vane is normally mounted near the anemometer on the turbine’s nacelle to measure wind direction (see Figure 9-7). The controller samples the indicated wind direction over a period of time and determines the average wind direction. If the rotor is not aligned with the measured wind direction, the controller signals the yaw motor to rotate the nacelle to face into the wind. This feature is termed active yaw. 9.4.3 Active Yaw The nacelle is connected to the tower top via a slewing bearing that allows the nacelle to rotate on the tower. One or more electric or hydraulic motors are usually mounted on the nacelle’s machine platform. These yaw motor(s) act on a ring gear which is bolted to the tower side of the slewing gear (See Figure 9-8) allowing a controlled rotation of the nacelle in either direction. Since all gears have a certain amount of backlash (freeplay), most turbines have a brake that ‘locks’ the nacelle yaw position once the yaw motor has finished aligning the nacelle with the wind. This brake reduces wear on the yawing gears, potentially caused by nacelle oscillations taking up backlash in the gears. The yawing must be done slowly to minimise gyroscopic loads, on the blade roots and rotor shaft, due to changing the axis of the spinning rotor. (Source: Renewable Energy World Magazine Vol. 5 No. 4, p72) The error measured between the true wind direction and the blade rotational axis is termed yaw misalignment and, if significant, may be damaging both structurally and performance-wise. Large yaw errors not only reduce the output of the turbine because of reduced rotor projected area but also induce additional cyclic stresses into the blade/hub structure. These stresses are created because the rotating blade will experience a different relative velocity, hence differing aerodynamic load, if it is rotating into the wind or away from it due to yaw error. If the yaw error is significant, this may create considerable additional cyclic loads that may promote fatigue damage. If the wind direction changes through 360 degrees, which is a common daily occurrence in many areas, the main electric cable running down the tower from the generator will twist. Depending on the length and size of the cable a certain amount of cable twisting is permissible, but repeatedly over twisting may fatigue the conductors within the cable. Small wind turbines commonly have slip rings that allow the nacelle to repeatedly rotate without twisting the main power cable. It is impractical, however, to manufacture slip rings to transmit the high current generated by large wind turbines. Therefore to prevent the power cables from twisting, a counter records the number of turns that the nacelle makes, when a limit is reached the turbine is shut down and the yaw motor then revolves the nacelle to un-twist the cables. 9-11 9-12 Most small turbines use passive yaw to align the rotor with the wind. Commonly a tail fin is connected to the nacelle that aligns the rotor with the prevailing wind. This simple method is not used on large turbines because turbulent winds often cause sudden wind shifts, which would cause the nacelle to yaw rapidly. The rotor’s moment of inertia is relatively high for a large turbine and any sudden movements may create large gyroscopic loads that would compromise the structural longevity of the structure - hence actively controlled yaw systems are always used on large capacity turbines. turbines and alter the operational set points to alter performance. The historical data may be used to provide information on process trends for fault diagnosis and to display availability reports of each particular wind turbine. Critical alarm messages may also be set via the SCADA system to mobile phones as SMS text messages, detailing the alarm. Further information on a commercially available SCADA package may be accessed at www.garradhassan.com/scada. Figure 9-10 An example of the display of a SCADA system 9.4.4 Passive Yaw Many small wind turbines (up to about 30kW in capacity) use passive yaw to orient the turbine into the wind. Wind turbines of this design use a freely rotating nacelle and a tail fin to keep the rotor facing the wind. By allowing some control of the angle between the rotor axis and the tail fin axis (which are normally parallel) the projected area of the rotor may be varied. By yawing out of the wind the projected area of the rotor varies with the cosine of the yaw angle. Hence the rotor power may be theoretically reduced to zero at a yaw error angle of 90 degrees. This enables yawing of the rotor to limit power production. This method is commonly called furling. Furling, or passively yawing the rotor out of the wind at high wind strengths, was first used in a widespread fashion in multi-bladed, water-pumping windmills. In order to create a moment to turn the rotor, the axis of rotation of the blades is offset to the nacelle yaw axis (tower axis). Since the centre of pressure of the rotor is then a distance from the yaw axis, a moment is created which will tend to turn the rotor as seen in Figure 9-9. This type of control is often slow and unpredictable, particularly in gusty or turbulent conditions; therefore a relatively large safety factor is used when designing the rotor of turbines employing this system. (Source http://www.pcorp.com.au/html/projects/scada.pdf) Figure 9-9 Schematic of a passive yaw system References: [1] Hoffmann R. A comparison of control concepts for wind turbines in terms of energy capture. Darmstadt University. Feb 2001. Yaw axis (tower) WIND [2] Hand M.M. Variable speed wind turbine controller systematic design methodology – A comparison of non-linear and linear model based designs. NREL July 1999. (www.nrel.gov/wind/25540.pdf) Furl offset Useful websites: http://www.pcorp.com.au http://www.garradhassan.com http://www.neg-micon.com/downloads/nm72C_1500_gb.pdf NORMAL OPERATION FURLING http://www.bonus.dk/images/PDF/CombiStall%20UK.pdf http://www.nwp.se/english.htm http://www.vestas.dk 9.5 SCADA systems As the level of penetration of wind power into the grid increases, utilities require greater control over the operation of the turbines to comply with power quality standards and prevent instances such as power surges due to wind gusts etc. SCADA systems allow remote access to the supervisory controllers of most modern turbines allowing access to both live and historical data, as well as the ability to start and stop particular 9-13 http://www.garradhassan.com/scada 9-14 ...
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