Notes Topic 10

Notes Topic 10 - 1 2 3 4 5 6 7 8 9 INTRODUCTION WIND...

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Unformatted text preview: 1 2 3 4 5 6 7 8 9 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 OPERATION CONTROL OF WIND TURBINES 10 WIND TURBINE SAFETY 10.1 Wind Energy Safety Record The wind turbine industry has a proven safety record but there are occasional cases of turbine component failure e.g. the Growian machine in 1982 (see Topic 5) and it is vital that a wind turbine is not only designed with safety in mind but employs a safety control system to sense emergency conditions and protect the turbine. Note that the overall risk to public safety from an operating wind turbine is extremely low; if standing within 200m of a wind turbine, the probability of being struck by a fragment of that turbine is comparable to the probability of being struck by lightning. 10.2 Safety Control Systems 1 Introduction.................................................................................................................................................. 1 2 Wind Resources ........................................................................................................................................... 1 3 Wind Turbine Components and Concepts ................................................................................................... 1 4 Wind Turbine Aerodynamics....................................................................................................................... 1 5 Wind Turbine Blade Design and Blade Manufacture .................................................................................. 1 6 Wind Turbine Mechanical Design ............................................................................................................... 1 7 Generators .................................................................................................................................................... 1 8 Grid Connection and Power Conditioning................................................................................................... 1 9 Operation Control of Wind Turbines ........................................................................................................... 1 10 WIND TURBINE SAFETY ..................................................................................................................... 2 10.1 Wind Energy Safety Record.................................................................................................................. 2 10.2 Safety Control Systems ......................................................................................................................... 2 10.3 Designing for Safety.............................................................................................................................. 2 10.4 Emergency Condition Sensing .............................................................................................................. 4 10.4.1 Vibration ......................................................................................................................................... 4 10.4.2 Temperature.................................................................................................................................... 5 10.4.3 Rotor Over-speed............................................................................................................................ 5 10.5 Rotor Speed Control.............................................................................................................................. 6 10.5.1 Aerodynamic Methods of Over-speed Control............................................................................... 7 10.5.2 Mechanical Braking........................................................................................................................ 9 10.5.3 Dynamic Braking.......................................................................................................................... 10 10.6 Lightning ............................................................................................................................................. 10 10.7 Operation and Maintenance Safety ..................................................................................................... 10 Figure 10-1 A service person wearing a harness connected to a fall-arresting device....................................... 3 Figure 10-2 A ball and cup style mechanical vibration sensor........................................................................... 4 Figure 10-3 Illustration of a cooling system for a car ........................................................................................ 5 Figure 10-4 An optical-type speed measurement sensor mounted on the low-speed shaft of a wind turbine ... 6 Figure 10-5 Illustration of ailerons on an aircraft............................................................................................... 7 Figure 10-6 Hydraulic disc brake used on the high-speed shaft......................................................................... 7 Figure 10-7 Blade tip in braked position ............................................................................................................ 8 Figure 10-8 Two-bladed wind turbine deploying tip brakes .............................................................................. 8 Figure 10-9 Pitch control system using rotating masses as used by Westwind Turbines .................................. 9 Figure 10-10 A disc brake installed on the gearbox of a Tacke turbine (note the teeth on the outside diameter of the brake disk used to measure rotational speed) .................................................................................. 10 10-1 A wind turbine control system not only manages the normal functioning of the machine but it must also monitor the operation of individual components and initiate any necessary control strategies to protect the integrity of the turbine if dangerous conditions arise. Since the power in the wind blowing across the turbine rotor’s swept area follows a cubic law, any small increase in wind speed above rated wind speed has the potential to overload components. Therefore, high wind speeds obviously pose the most significant risk to the reliable operation of a wind turbine. If any serious irregularities are sensed, the control system will signal an emergency shutdown, braking the rotor and signalling the fault to the turbine manager via the SCADA system. The safety controller is often designed to act as an independent system from the supervisory control system. This approach builds a degree of redundancy into the system that greatly reduces the probability of an emergency situation going uncontrolled due to control failure. Actuators that are critical to the safe operation of the turbine are often designed to be inherently fail-safe such as brake callipers and pitch actuators. This design philosophy is taken to minimise risk of equipment failure if there is a control system malfunction. For this reason logic controllers or microprocessors are seldom used, instead a hard-wired fail-safe circuit comprising a series loop of normally closed relays that are electrically (or hydraulically) held open when the turbine is operating normally is preferred. For example in most turbines, energy (electricity or hydraulic pressure) is required to disengage the mechanical brake acting on the rotor shaft, if the power supply is lost the brake is automatically applied. This fail-safe approach is often taken on the design of any subsystem that controls the mechanical power of the rotor. Critically important sensors are often duplicated and comparisons are made between the output measurements of like devices. Much time and effort is taken in the design stage to minimise unnecessary equipment in an effort to keep the number of individual components to a minimum, consequently reducing the potential for errors and breakdowns. 10.3 Designing for Safety An essential element when designing individual components as well as the complete wind turbine system is ease and safety of maintenance. Numerous standards must be adhered to such as those pertaining to the design of machines (safety guards etc.) and specific standards focused on particular aspects of wind turbine service safety, such as IEC 61400-1. Particular areas that are addressed are: • Ground to Nacelle Access Many early wind turbine designs used lattice towers and access to the nacelle was via a ladder mounted on the side or within the tower structure. Therefore any service person attempting to climb to the nacelle was subjected to the weather for the entire climb. This obviously has serious safety implications particularly if attempting a climb in windy or wet conditions. This is one of the reasons why nearly all wind turbines which need nacelle access now use tubular towers with the access ladder within the tower. Safety regulations 10-2 stipulate that a safety harness must be worn when using a ladder over a specific height. Intermediate platforms or landings may also be used to break up the overall ladder length and keep it within regulations. Climbing to the nacelle of most wind turbines requires the use of some form of fall-arresting device. This device comprises a body harness and a lanyard that is fixed to the ladder. Some designs use a shock absorbing lanyard and a track that the lanyard is attached to so that it may be drawn along as the person climbs (see Figure 10-1) below). • Electrical safety These requirements are covered by relevant standards for industrial electrical practise and nothing pertains solely to wind turbines. Figure 10-1 A service person wearing a harness connected to a fall-arresting device 10.4.1 Vibration Vibration poses one of the most significant risks to the premature failure of a turbine and it is the actual means by which most catastrophic wind turbine failures occur. Since a wind turbine is predominantly a rotating machine, an unbalance in any rotating system has the potential to cause significant vibrations. If the frequency of the vibration coincides with the natural frequency of part of the turbine’s structure, then there is the potential to excite quite large amplitude oscillations. Wind turbine design engineers investigate and model the interaction of these systems to prevent this from occurring. Significant changes, however, may occur over the life of the turbine that can alter the dynamics - changes to the masses of the blades, for instance. The imbalance may stem either from aerodynamical, mechanical or electrical means. For example, if the mass of a blade is altered, possibly by lightning damage or blade icing, the imbalanced rotation may cause excessive vibrations, forcing high amplitude oscillations (particularly in components with similar natural frequencies) and potentially causing structural failure. Aerodynamic imbalances may stem from yaw misalignment (the difference between the rotor orientation and the mean wind direction) or pitch variation between blades. (Source: Miller safety products, see e.g. http://www.bacou-dalloz.com/ ) • Rotor locking device The rotor must be stopped and locked before service work is carried out on the rotor or a component of the power train. The standard mechanical brake is used to stop the turbine before work is carried out and prevent the rotor from moving, but a separate device is required if maintenance is to be carried out on the brake itself. A service-locking device is normally provided to mechanically lock the rotor after the brake has stopped the turbine. This device should be incorporated into the low speed shaft and normally takes the form of a steel pin that slides into a hole on the blade hub and is rigidly fastened to the machinery bed of the nacelle. The rotor locking device operates on the low speed shaft so that work may be carried out on the “ downstream” side of the power train, such as on the gearbox couplings, brake etc. This device also prevents any accidents, such as the turbine being put back on line while being serviced. Some turbines also have a similar device or an additional service brake override on the yaw system to prevent the turbine from yawing when servicing is underway. 10.4 Emergency Condition Sensing Sustained vibrations are of concern to the operating life of the turbine as cyclic forces may induce fatigue type failures. Vibration measuring equipment such as accelerometers are often installed within the nacelle. If certain limits are exceeded an alarm is raised or the turbine is shut down until service personnel can assess the situation. Many turbines have a simple mechanical vibration sensor mounted within the nacelle. The sensor uses a small ball that rests in a ring. A wire or chain attaches the ball to the arm of a lever switch. The ring and ball is designed in such a way that if there is any significant vibration, the ball will roll out of the ring and the weight of the ball will activate the lever switch shutting the turbine down. This simple safety device was first installed in the Gedser machine, invented by Johannes Juul in 1957 (see Topic 1). Figure 10-2 A ball and cup style mechanical vibration sensor • Fire extinguishing Fire poses minimal risk in wind turbines, but nonetheless many large wind turbines have automatic fire extinguishing systems. • Emergency ejection systems Many large wind turbines have an emergency escape system that can be used if service personnel must evacuate the nacelle, such as in the event of a fire. A type of abseiling system is often used so that the service personnel may eject from the nacelle. (Source: http://www.spintelligentlabs.com/tech_pap/vibsw.pdf) • Machinery guards As with all machinery, especially rotating machines, guards are used to protect personnel from the dangers of rotating shafts, gears, motors etc. 10-3 10-4 10.4.2 Temperature Large wind turbines can have a numerous temperature sensors located within the mechanical and electrical components such as the generator, gearbox, main shaft bearings and yaw and pitch motors to name just a few. The temperature of many components may be measured to give warning in advance if a fault occurs, which can then be rectified before serious damage may result. Often ambient temperature and pressure are also recorded. Many large-scale wind turbines have an external cooling circuit, incorporating a fluid pump and a heat exchanger to limit the temperature of the gearbox and generator. Some manufactures use water-cooled generators, where water is pumped through a jacket within the generator to an air/water heat exchanger in a closed loop fashion (similar to the cooling system of an automobile, see Figure 10-3). Likewise gear oil temperature is often kept within limits by pumping the hot oil through an air or water-cooled heat exchanger. Temperature sensors mounted on key components of the cooling system allow remote assessment of the performance of the cooling system. Figure 10-3 Illustration of a cooling system for a car corrective measures were applied. For a given torque, power is directly proportional to rotor speed, thus the faster the rotor turns the higher the power produced – also causing run-away. Manufacturers of rotor blades specify a maximum speed of rotation for which the blade may operate. Operation above this limit may compromise the operation life of the blade and lead to structural failure. The centrifugal force causing an axial tensile load in the blade root is proportional to the square of the rotor speed. Therefore any overshoots in rotor speed may have significant consequences to the structure integrity of the blade root. If a blade failure occurs and a part or full blade is shed, the vibration caused by the extreme rotor imbalance will almost certainly cause a catastrophic failure. If the electrical load of a turbine, operating near its rated level, is lost, it is feasible that the rotor speed may double in as little as three or four seconds. The power in the rotor will also have doubled to a level that may be above the restraining -capacity of the mechanical braking system. For this reason it is essential that the rotor speed sensing equipment and control system have sufficiently quick response to prevent these situations from arising. Rotor speed is normally measured on the low speed shaft (for turbines using a gearbox) or electronically within the generator of a synchronous machine. Commonly optical type or magnetic hall-effect sensors are used (see Figure 10-4). If the rotor speed or acceleration reaches a pre-determined limit the controller will signal a counter action, such as pitching the blades or applying the mechanical brake. Figure 10-4 An optical-type speed measurement sensor mounted on the low-speed shaft of a wind turbine (Source: www.enginewity.com/ questions.htm) Sustained overload of the turbine or operation in high ambient temperatures may result in excessive gearbox and or generator temperatures. Extreme gearbox temperatures may lead to premature bearing failure, accelerated gear wear and a reduction in the service life of the oil. Generator overheating may also cause premature bearing failure, irreversible demagnetising of permanent magnets or a breakdown in winding insulation that may lead to internal short-circuiting and a burn-out. The information from the thermal sensors is often available as real time information (or recorded as historical data) and is accessible via the SCADA system. This information is often used as a very rough indication of the electro-mechanical condition of the turbine. 10.4.3 Rotor Overspeed Since the power in the wind follows a cubic law, during high winds a turbine may produce significantly more power than its rated level. Topic 9 described various methods that may be used to limit the power generated by the rotor to prevent overloading the power train. Stall regulation, for example, relies on torque being applied to the rotor, by the generator, to limit its rotational speed. If the rotor accelerates beyond this margin, the effect of stall may be delayed and the power generated by the rotor will rise dramatically, exacerbating the situation and resulting in what is termed run-away. Similarly, if the electrical load (grid) were disconnected while the turbine was in operation, the rotor would immediately start to accelerate unless 10-5 (Source: http://www.oemdc.com/english/services/index.htm) 10.5 Rotor Speed Control For legal/insurance reasons, wind farm planners must use turbines that have been certified and have been designed to comply with the relevant international standards. The international standard relevant to safety and control of turbines with greater than 40m2 rotor area is IEC 61400-1. This standard dictates that all wind turbines must have at least two independent methods of rotor speed control: • Aerodynamic The power in the rotor may be limited or reduced, by reducing the amount of lift developed by the aerofoil. This may be done by varying the blade pitch angle or by the use of air brakes or ailerons (see Figure 10-5) on the blades to increase drag. Reducing the blade efficiency or output is the ultimate method of reducing rotor power production and any other method is only a means of dissipating the energy that is generated. 10-6 Figure 10-5 Illustration of ailerons on an aircraft (Source: http://nasaexplores.com/lessons/02-019/5-8_1.html) have used numerous methods of actuation over the years but the principal of operation has remained similar. Because of the fail-safe design requirement, power is normally fed to the latching mechanism to hold the brake in the normal run position. If the controller senses an over-speed, the latch system is released and the tip brake rotates into the braked position usually under spring tension. Some turbines use hydraulic pressure to latch the blade tips in the run position, when deployment is needed the hydraulic pressure is vented. After the tip brakes have been deployed and the rotor has slowed, the mechanical brake usually is then applied to bring the rotor to standstill. Applying the mechanical brake after the tip brakes have slowed the rotor greatly reduces drive train loads and the demand on the brake. Figure 10-7 Blade tip in braked position • Mechanical Mechanical brakes are often used on one of the power transmission shafts within the nacelle. Commonly disk brakes are used that are electrically or hydraulically held off (see Figure 10-6), in a fail safe fashion, and may be positioned on either the main shaft or the high speed shaft, if the turbine has a gearbox. As mentioned above, all large wind turbines must have a service braking system whereby the rotor can be locked in a fixed position after the mechanical brake has halted the rotor. This rotor-locking device may be used during servicing when personnel may need to perform maintenance on the drive train, rotor or blades. (Source: http://www.olsenwings.dk/geninf.htm) Figure 10-6 Hydraulic disc brake used on the high-speed shaft Figure 10-8 Two-bladed wind turbine deploying tip brakes (Source: http://www.indutrans.dk/uk/wind.htm) 10.5.1 Aerodynamic Methods of Overspeed Control As previously mentioned, limiting the aerodynamic performance of the rotor is the ultimate method of rotor power and speed control and thus nearly all modern, large-scale, turbines have some method of aerodynamic power control. The industry uses numerous methods of aerodynamic control from blade pitch variation to aerodynamic brakes. These methods of control depend on the type of turbine (i.e. synchronous/asynchronous generator, fixed/variable speed etc.) and may either be passively or actively controlled. Modern pitchcontrolled turbines use aerodynamic control (pitching blades to reduce lift) during their normal operating regimes, whereas stall-regulated turbines often use a method of aerodynamic control as an emergency measure in the event of over-speed when the generator cannot provide adequate torque to hold the rotor in stall. (Source: http://www.galeforce.nireland.co.uk/Boreas.htm) 10.5.1.1 Pitching blade tips Some designs allow the blade tip to be rotated through 90 degrees to the braking position to provide sufficient drag to slow the rotor in emergency conditions. See Figure 10-7 and Figure 10-8. Manufacturers 10-7 On pitch-controlled turbines, such as the Enercon range of turbines from Germany, the entire blade can be rotated into a feathered or stalled position. This design uses three mutually independent pitch-control motors (one for each blade) to vary the pitch angle of the blades. In this way a degree of redundancy is created thus eliminating the need for an additional safety system. If the turbine must be stopped the blades are fully 10-8 feathered to slow the rotor before a mechanical brake is applied. By feathering the blades, the thrust acting on the turbine is also minimised. Since the amount of thrust on the turbine may be controlled, large cost savings can be achieved by using a turbine and tower that are lightweight compared to the very conservative and robust designs needed for stall-controlled designs. Some small turbines use a passively controlled pitch mechanism to prevent rotor over-speed. Commonly, centrifugally actuated ‘bob weights’ provide the energy to operate the pitch mechanism once a certain rotational speed has been reached. Depending on the design the blades may either be pitched into a feathered or stall position. The French manufacturer Vergnet and the U.S. Jacobs turbine use this method for the control of their range of turbines. Westwind Turbines also use this method of over-speed control on their 20kW wind turbine, see Figure 10-9. brake pads onto the disk halting the rotor. Because of the immense power that must be dissipated as a rotor is stopped, the pads are usually made from special metal alloys designed to resist brake fade at high temperatures. The braking torque places heavy loads onto the transmission system, hence it is only used for emergencies or when servicing the turbine. Figure 10-10 A disc brake installed on the gearbox of a Tacke turbine (note the teeth on the outside diameter of the brake disk used to measure rotational speed) Figure 10-9 Pitch control system using rotating masses as used by Westwind Turbines (Source: http://www.svendborg-brakes.dk/) (Source: Westwind Turbines) The aerodynamic braking system cannot completely stop blade rotation for all conditions. It therefore must be used in conjunction with another form of braking if a complete securing is needed. The aerodynamic brake, however, is normally designed so that the resulting free-running rotational speed is lower than the normal operation speed even if the mechanical brake should fail. 10.5.2 Mechanical Braking Most turbine designs have a mechanical brake that may be used to stop or hold the rotor for shutdown or for safety reasons during servicing. In general the brake should be positioned on the rotor main shaft, where direct control of the rotor may be achieved even if the gearbox or a coupling fails. In practice, however the brake is commonly installed on the high-speed shaft of the gearbox. In this location the braking torque is reduced due to the higher shaft speed, hence the size of the brake may be reduced. The most common type of brake used is a disk brake, usually manufactured from steel. As previously mentioned, the brakes are of the fail-safe design, meaning that energy is needed to disengage the brake. Energy is usually hydraulic pressure or electricity. If the energy supply fails, powerful springs clamp the 10-9 10.5.3 Dynamic Braking Some turbine designs employ electro-dynamic braking whereby a low impedance load is switched over the output of the generator. In doing this, a large restraining torque is produced within the generator that stalls the blades and in turn brakes the rotor. The SouthWest Windpower, AIR 403 is an example of a small wind turbine (400W) that employs dynamic braking. The generator loads the rotor by shorting the stator windings progressively. A dynamic braking system has the potential to generate large amounts of heat within the generator as the windings dissipate the energy, hence this type of braking is normally used in conjunction with another form of power control such as pitching the blades. To effectively provide dynamic braking an oversized generator needs to be provided. 10.6 Lightning As wind turbines are progressively becoming larger the risk of lightning damage has also become an issue and statistics show that lightning damage is a major contributor to downtime in the operation of wind farms. The blades in particular, are of high risk and a strike will usually result in catastrophic damage unless necessary precautions have been taken. For more information on blade lightning damage, see Topic 5. A significant other area of damage resulting from a direct or nearby lightning strike is to the turbines electrical systems and control circuitry. The presence of lightning may induce high current and voltage spikes into electrical cabling causing severe damage to nearby or interconnected equipment unless properly protected. 10.7 Operation and Maintenance Safety The wind industry has a relatively clean record with respect to worker casualties. No member of the public has ever been injured or killed by a wind turbine [Ref:1]. Possibly the greatest potential for injury occurs 10-10 during the construction phase as the turbines are being installed. This is due to the increased number of personnel on site and frequent lifting of equipment. Strict Occupational Health &Safety measures should be adhered to during all phases of the installation and operation of the wind turbines. After the turbines have been installed, service personnel should be suitably trained before attempting any maintenance tasks. The main risks associated with working on wind turbines and associated equipment are: • • • • • Working at elevated heights High voltages Rotating machinery Exposure to extreme or unpredictable weather Lifting equipment with cranes It is the responsibility of the company who is involved with the wind turbine project to compile a Risk Management Plan. This plan lists and assesses the potential risks associated with each stage of the project and advises what actions should be taken to avoid or minimise these risks. The Risk Management Plan will rank the consequence of the potential risk and the probability of the incident occurring. The safety officers who author this plan, will decide the plan of action depending on these two factors. For example, some hazards are extremely unlikely but the consequence may be dire thus it may be decided that an alternative route be followed to avoid the risk. It is impossible to completely eradicate the potential for all accidents, but by adhering to a well-compiled safety plan the chances of accidents will be dramatically reduced. McIntyre & Ingram [Ref: 2] review the industry’s risk management performance and investigate some of the risks facing a developer during the construction phase of a wind turbine development. The authors describe an alternative of a self-erecting wind turbine that would dispense with the potential safety hazards associated with large cranes. Because of the potential dangers to human life, suitably trained personnel should only work on wind turbines. This is especially true when raising or lowering equipment or when working in an elevated position. Often a short meeting is held between workers and a safety officer, prior to site work starting, to address the potential risk that personnel may be exposed to that particular day and instruct all workers of the methods to minimise these risks. During this meeting the floor may be opened to the site workers who then can discuss any incidents that may have occurred or changes to the Risk Management Plan that may be needed due to alterations in the work plan. References: [1] Irish Energy Centre, Renewable Energy Information Office. Fact Sheet – Environmental (and Other) Impacts of Wind Turbines. [2] McIntyre, S. & Ingram, J. Wind turbine construction using a jacking crane, Proceedings from Wind Power Europe Conference, Madrid, December 2002. Useful websites: http://www.lightningsafety.com/nlsi_lhm/wind1.html 10-11 ...
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