Vibration Control Of: WIND TURBINES

APPLICATIONSAP-Power-Wind Turbines-13aVibration Control of:Wind TurbinesWind power is popular. The market for wind turbines is expanding rapidly and with it is an increasing demand forturbines to be installed in urban areas. This can be problematic due to turbulence and the lack of consistent wind andmeans that they need to be located above the city on the roofs of buildings or other structures.Placing wind turbines on roofs has a number of challenges such as; Access Capacity to support the applied forces Anticipated requirement for specialised support structures for stability Proximity to high spec building use such as penthouses Risk of resonance in the building structure Vibration isolation performance is very dependent on the turbine type,the building design, and building useEach turbine design has its own operating characteristics and moreimportantly each roof structure and potential mounting arrangementwill vary meaning that a bespoke system needs to be designed based anumber of factors and must include; Structural Integrity vs VibrationControl vs Operating Performance vs Practicalities.POWER GENERATION - WIND TURBINESWINDTURBINES

POWER GENERATION - PUMPS & COMPRESSORS01Design Consideration 01Shock and Vibration affecting operatingperformanceRoof structures tend to be light in weight and strengthand therefore are good transmitters of vibration. Usuallyspecialist wind turbine support frames need to be designedto provide sufficient strength by connecting the frame to themain columns from the building which in turn creates a directtransmission path from turbine to the main building structure.Vibration from wind turbines can either generate vibration or,worse, resonate parts of the structure causing vibration andreradiated noise disturbance within the building.The vibration of the turbines can come from two sources: Unpredictable mechanical friction, unbalancing of therotor, wind gust related. Depending on the maximumrotor speed, these vibrations will tend to be below 10Hz. The second type of predictable vibration is linked to thegenerator operation, and micro pulses due tothe electronics of the turbine. These vibrationswill range from 100Hz to 400Hz (varyingwith the rotor speed). Depending on the stiffness of themounting system, 2nd and 3rd harmonics canappear on the vibration spectrum. Therefore, we need toconsider the complete range of vibration from 100Hz to 1.2kHz (400Hz*3rd harmonic). However, the energyfrom the 2nd and 3rd harmonics are a lot lower than thefirst one.Most human ears can hear sounds of frequencies rangingbetween 20Hz and 20kHz. However, the frequency responseof the human ear is such that low frequency sounds appearless loud than high frequencies. This is commonly accountedfor by weighting the sound pressure level. A-weighting,for example, provides a relatively accurate measurementof perceived loudness. Vibration transmission from windturbines into a building is undesirable because it can be heard.Farrat has a significant amount of experience in thesuccessful design of vibration control systems for roofmounted wind turbines. The solutions in each case weredriven by the constraints but generally resulted in a boltedthrough or full decoupled push-pull connection using highdamping elastomeric materials.Factors to consider in obtaining a solution are;1. Positioning- Since structural integrity is critical and suchsupport frames cannot be avoided then vibration controlneeds to be incorporated somewhere between the turbinemast and the building structure. From a structural pointof view they need to be located to ensure that the structureretains sufficient stiffness. The forces from the windturbine can lead to torsion and shear loadings. From anacoustic point of view they should be above theroof which will be the most efficient transmitter of vibrationenergy as noise into the building and from a practical pointof view they should not be affected by, or affect theinsulation and waterproofing systems on the roof.

023. Modelling - The entire system needs to be consideredas a 3d model rather than a simple mass on springsisolation model. Rocking and modal behaviour of theturbine structure have to be considered. For any roofmounted wind turbine application it is very importantto establish the operating frequencies of the turbine aswell as the natural frequency of the mounting, roof andbuilding structure to see what the combined effectsmight be. This usually needs to be done with modelling.Dynamic modelling of the entire system is also importantwhen you consider that once resilient materials have beenincorporated into a connection the natural frequency ofthe frame will alter. The natural frequency of beams andmasts will decrease once they are resiliently connected asopposed to if they were rigidly connected.Head Thrust652. Allowable movement - A primary constraint invibration control of wind turbines is allowable movement.A small movement at the base of a turbine will beamplified at the top of the mast so a first question toconsider is how much movement can be permitted atthe top of the mast and then what is the max allowablemovement at the point where the vibration controlsolution is to be applied. Often this value will be very lowand therefore means that, high resilience, low frequencysolutions such as springs or pads cannot be used. Addingmass by way of an inertial block is a very effective way ofreducing vibration disturbance (2x mass 0.5 vibration),increasing stability by lowering the centre of gravity andallowing the use of lower frequency isolators but as weknow roof structures are lightweight and usually cannotpermit significant addition of mass.provide suitable resilience at normal wind speeds whilststill being able to accept peak wind loadings. The isolationconnection design should be pre-compressed to ensurethat as the connection deflects or up-lifts it does notloose tension in the connection. Pads must stay incontact with frame even under peak load (6-7x normalload). This puts an extra emphasis on the anti-vibrationmaterial to be able to withstand very high loads whilststill offering resilience at low loads.WeightTotal moment4. Natural frequency - is derived from mass of systemrather than applied wind loads (wind load should not beconsidered as a mass) so the dynamic spring constantof the bolted through (push pull) connection is critical toreal performance. Dynamic natural frequency does notnecessarily equate to the calculated (mass spring) naturalfrequency.5. Damping - Damping plays a crucial role in wind turbineisolation as it dissipates energy in a vibrating systemand converts that energy to heat. It is essential tocontrol the potential high levels of transient vibrationand shock, particularly if the system is excited at, ornear, to its resonant frequency. Damping is requiredwhere movement of the supported equipment must beminimised especially at resonance and when shock is tobe absorbed.6. Light weight but high forces - Roof mounted windturbines tend to be very light whilst peak wind speedsare significantly higher (6-7x) than typical wind speedsso it is difficult to design a mounting system that canPOWER GENERATION - PUMPS & COMPRESSORSShock and Vibration affecting operatingperformance (Cont)

POWER GENERATION - PUMPS & COMPRESSORS03Shock and Vibration affecting operatingperformance (Cont)7. Bespoke vs off the shelf - Many off the shelf windturbine isolation systems are designed for installation onthe ground or concrete and brick structures (which arevery good to damp vibration) which means they may notoffer adequate vibration control performance for steelstructures.Example of Farrat’s success and experience in designing highperformance vibration control solutions for roof mountedwind turbines we were asked to provide an alternative designto an existing application where a competitors solution wasinstalled but not providing adequate performance.The following is an exert from the conclusions of thevibration consultant’s findings;“The 8 hour average results demonstrate that thetypical background vibration levels fall well belowthe ASHRAE guideline levels for Residential (goodenvironmental standards) of 2.032e-4m/s (8000µIn/s).In addition, over the key frequency band of 10-15Hz thatwas seen previously to be responsible for vibration issues,the modified system design coupled with Farrat Isolationresulted in a 53.8% and 72.5% reduction over the February2011 and September 2011 surveys respectively.”In another example the findings were as follows;“Levels of vibration in the flat immediately below theturbine was well below that known to cause nuisance ordiscomfort. The turbine’s control panel dominates thevibration measured in the flat in the X, Y and Z directions.Furthermore they did not show any correlation to windspeed.No measurable impact on noise levels both externally andwithin the flat due to operation of the turbine Vibrationlevels within the flat are well below the comfort criteria for24hr working.”SolutionsFarrat Damping Pads (NBR, Squaregrip, IMBR)

APPLICATIONSAP-Power-Wind Turbines-13aVibration Control of:Wind TurbinesWind power is popular. The market for wind turbines is expanding rapidly and with it is an increasing demand forturbines to be installed in urban areas. This can be problematic due to turbulence and the lack of consistent wind andmeans that they need to be located above the city on the roofs of buildings or other structures.Placing wind turbines on roofs has a number of challenges such as; Access Capacity to support the applied forces Anticipated requirement for specialised support structures for stability Proximity to high spec building use such as penthouses Risk of resonance in the building structure Vibration isolation performance is very dependent on the turbine type,the building design, and building useEach turbine design has its own operating characteristics and moreimportantly each roof structure and potential mounting arrangementwill vary meaning that a bespoke system needs to be designed based anumber of factors and must include; Structural Integrity vs VibrationControl vs Operating Performance vs Practicalities.POWER GENERATION - WIND TURBINESWINDTURBINES

WIND TURBINES Wind Turbines AP-Power-Wind Turbines-13a Wind power is popular. The market for wind turbines is expanding rapidly and with it is an increasing demand for turbines to be i