FPS Wind Turbine Rotor Balance Article PublishC

8/8/2019 FPS Wind Turbine Rotor Balance Article PublishC

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WIND TURBINE ROTOR BALANCE

Everyone is familiar with the problems associated with poorly balanced rotating equipment.When buying new tires for a car, you would not think of having them installed without firsthaving them balanced. An unbalanced tire creates vibration forces that can be feltthroughout the entire vehicle. A balanced tire is created when a small weight is placedopposite the heavy spot on the tire. This small weight is the difference between a bouncy,dangerous ride and a smooth one. The same can be said about a rotor on a wind turbine. Awind turbine rotor which is not balanced creates vibration forces that affect the mechanicallife and operating performance of a turbine. Therefore, an acceptable level of wind turbinerotor balance should be established as a part of any in-situ blade repair, wind turbine

commissioning, end-of-warranty inspection, or Reliability-Centered Maintenance program.

Problems Caused by Unbalanced Turbine Rotor

An unbalanced wind turbine rotor results from a mismatched weight distribution of theindividual blades which make up the rotor. There may be several consequences of anunbalanced rotor on a wind turbine.

First, in light winds, a turbine may have trouble coming on line or may be operating as amotor rather than a generator, both leading to a loss of revenue potential for the wind farmoperator. Both situations are often misdiagnosed as having a defective controller.Increased expenses are incurred by deploying unnecessary maintenance resources tocorrect the defective control. In the case where the turbine is coming on line, anunbalanced rotor accelerates and decelerates during each revolution. When the rotor isclose to synchronous speed, the heavy blade may speed the rotor up before the turbinecomes on line. The controller sees the rotor moving too fast to safely connect and shutsdown instead. In the case where the unbalanced turbine is motoring, the generatorexperiences loss of power during half of the rotor rotation and gain of power during thesecond half of rotation. The controller sees this as a satisfactory condition to stayconnected. Both issues are a problem if the controller is unable to compensate for thefluctuating speed. The solution is to properly balance the rotor. However, achieving aprecision balance on a wind turbine rotor is easier said than done and falls outside of theskill set of most wind farm maintenance personnel as well as those who are specificallyemployed by general industry to balance rotating equipment.

Second, a wind turbine with an unbalanced rotor will lose some of its low wind productioncapability. For any given level of unbalance, it requires power to rotate it at a given shaft

speed. This power requirement robs the rotor of speed for a given wind velocity and delaysits synchronization with the utility. Consequently, it will require more wind velocity toachieve synchronization as compared with a balanced rotor. Again, an unbalanced rotorresults in a loss of revenue potential for a wind farm operator which could have beenrealized otherwise by an acceptably balanced wind turbine rotor.

Third, the out of balance rotor continuously applies fluctuating loads to all of the mechanicalcomponents of a wind turbine e.g. main shaft, main bearings, gear box, drive train supportstructure, yaw system, tower, and bolts and even the foundation. In order to visualize the

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effect of the unbalanced force on a rotor, remember the unbalanced ceiling fan thatoperates on high speed and wobbles uncontrollably. If there wasnt a flexible mounting tothe ceiling, these forces would eventually cause the ceiling fan to fall as was the case whenthe flexible mount was not an installation requirement. The rotor weight of a ceiling fan is

on the order of tens of pounds. The rotor weight of a wind turbine can be greater than72,000 lbs. However, there is no flexible mounting for the wind turbine rotor. Themechanical structures which support the rotor are designed to keep the rotor in a fixedlocation. These constantly fluctuating loads add to the fatigue loading and shorten themechanical life of these systems. Remember, fatigue life is finite. When you use it upthrough these wildly fluctuating loads, its gone. You unknowingly spend fatigue life on anunbalanced rotor which could have otherwise been spent on producing revenue. Prematureexpenses and loss of production will be incurred by the wind farm operator through thereplacement of a major component or entire system as a result of this fatigue failure.

Rotor unbalance is a leading contributor to the need for frequent and costly maintenanceaction on yaw systems and fastening hardware. The unbalanced force on the rotor causes areaction on the yaw system twice per revolution, accelerating the wear on the yaw gear

teeth through impact loading and adding to the fatigue loading of the tower shell andmounting bolts. Yaw brakes are used to limit this impacting on the gears. However, thebrakes do nothing to limit the loads transferred to the tower. For perspective, a typicalturbine will have over 100 million of these two per revolution loading events in its life.

Thats a lot of pounding and stressing on your machine components.

Causes of Wind Turbine Rotor Unbalance

Rotor unbalance results from an unmatched weight distribution between each of the bladeson a rotor. Each blade on a hub produces a moment or a torque on the main shaft. Themoment or torque can be represented as a weight mounted on a weightless arm at somedistance from the centerline of rotation (main shaft). Ideally, the moments of each blade onthe rotor are equal around the center of rotation. When these moments are identical, thereis no net unbalance force on the rotor. The moment of a blade is the product of the distanceof its center of gravity to the center of rotation and its total blade weight (see Fig. 1).

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Fig. 1

Ma = Tw X DcgMa = Moment of blade A

Tw = Total Blade Weight of ADcg = Distance of Blade As center of gravity from centerline of rotation perpendicular to theforce of gravity

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Cg = Center of Gravity - The point on the blade where, kinematically, the physical structurecan be substituted with its total weight acting on a theoretically infinitely rigid arm ofno mass.

It is for this reason that efforts are attempted to match the blades as closely as possibleduring blade manufacture.

Any changes which affect the weight distribution of the blade will affect the moment of ablade and, consequently, the balance quality of the rotor. Significant changes in blademoment can result from everything from manufacturing to blade repair. Examples of issuesleading to blade repair are: structural cracking, gel coat cracking, lightning damage, gunshot damage, shipping damage, storage damage, ultraviolet environmental degradation,etc. Changes in blade moment may also happen during normal operation. Examples thatproduce changes in weight distribution and total blade weight are: oil and grease from pitchsystems, wind erosion, water absorption, internal debris accumulation from poorconstruction, animals/nests from unprotected ground storage. Examples which produce

unequal blade moments at time of manufacture are: poor production techniques, weakconstruction process controls, inconsistent manufacturing tools, etc. Over the twenty yeardesign life of a wind turbine, there is a high probability that a wind turbine rotor will beoperating with a higher degree of unbalance than desiredcaused by any combination ofthese factors. Perhaps most concerning is that without good analysis, you, as an operator,may never know the extent of your balance problems.

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Balance Quality

The international ISO 1940-1 standard is a widely accepted standard for selecting rigid rotorbalance quality. It effectively grades and describes acceptable limits of rotor unbalance as a

function of its rotating mass and rotational speed.

The lower the grade number, the better the rotor is balanced. For example, an ISO balancequality G0.4 would apply to a spindle of a precision grinder while an ISO G1600 would applyto the crankshaft of a large two-cycle, rigidly mounted engine. While a rotor balance qualitystandard has not been adopted by the wind turbine industry, an ISO G16 or better mayprove to be a practical and acceptable target.

The chart above is an adaptation of the ISO 1940-1 standard for the area of wind turbinerotor operation. To use the chart above, you would select the desired balance quality andthe maximum operating speed of the wind turbine rotor. At the point where the operatingspeed intersects the desired balance quality line, find the corresponding value of e on they- axis. Multiplying this value of e by the total weight of the rotor will give you themaximum amount of residual rotor unbalance allowed for the desired ISO balance quality.For example, if a rotor weighed 5,000 lbs and was operating at 60 rpm, for a G16 balancequality, it could have no more than approximately 500 in-lbs of residual unbalance. This is

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equivalent to one blade having an extra weight of 2 lbs located 250 inches away fromcenter of the hub.

Rotor Balance - Static Blade Balancing

All three blades must create the same moment around the center of rotation when mountedon a wind turbine hub. This is accomplished by weighing the blades and then addingcalculated weights such that each blade exhibits the same moment around the center ofrotation. The inherent assumption is that if the blades are balanced, the assembled windturbine rotor will be balanced. There is a phrase for those that ASSUME.

A generally accepted balancing practice for a blade repair shop or for field determination ofblade balance requires weighing the tip and root ends of each blade through the use offlexible straps supporting the blade at prescribed distances and a crane weighing scale.Root and tip weights are then matched to the heaviest root and tip measurements by addingthe appropriate amount of weight. This process makes the total blade weights the same andthe moments of the roots and tips the same around the center line of rotation. Staticallybalancing the blades in this fashion will produce equal blade moments. However, it does sowith more weight than is necessary. It will not only effectively increase the mass of therotor more than required but will place additional bending fatigue on the blade as thiscorrective weight at the tip has to be supported against gravity every half revolution of therotor. This extra weight is operationally counter productive.

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Static Blade Balancing Errors

The assumption that a precision rotor balance will be achieved through static bladebalancing must be challenged in the balancing process itself. The level of precision

attainable by the static balancing of the blades will be limited by the accuracy of themeasurements as well as the application of weights.

Weight Measurement Error:

A crane scale is generally accurate to within 0.5% of full scale. If the full scale is 10,000 lbs,the reading is only accurate to +/- 50 lbs. Accuracy refers to how closely the measuredvalue of a quantity corresponds to its absolute or true value. Precision expresses the degreeof reproducibility between repeated measurements. The accuracy and precision of thereading are both important. If the precision of a reading (repeatability) is within 10% of thedeviation that you intend to measure, you can achieve good static balance as long as theblades are being measured with the same scale, and the blades are installed on the samerotor. Care must be taken to ensure that a blade is not matched to another blade that was

weighed by a different scale, or it may potentially introduce twice the 0.5% difference intothe calculations. The question which must be answered is whether or not the rotor will havean acceptable balance quality resulting from the inherent measurement errors of theprocess and the selected crane scale. Using the previous example, lets look at respectiveweight errors from the tip measurement with just one blade as it relates to rotor balancequality on an assembled rotor:

Distance Measurement:

A large blade may require a 4 flexible lifting strap. It might be difficult to accuratelymeasure the distance from the root to the center of a large strap, especially on long bladeswhere measurement device error will be introduced. If there is any appreciable amount of

wind, it may not be possible to pick up the blade at all because of the instability as well asconcerns for safety. Additionally, any efforts to hold it steady will certainly introduce weightmeasurement errors.

Frontier Pro Services can use a patent-pending method to statically balance blades that aredetached from the rotor which produces a minimum corrective weight solution. A correctiveweight reduction of greater than 5:1 over the root and tip method can be demonstrated.Of course, having a balanced rotor with the minimum amount of weight contributes to wind

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turbine life. Additionally, the process reduces the potential errors which can be introducedby as much as 50%.

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Observations of Wind Turbine Rotor Unbalance

Visual

A rule of thumb method for the identification of turbines which are candidates forbalancing involves the observation of a group of identical turbines during wind start up.

Those that have a higher degree of unbalance will always be the last to start spinning andcontribute to production. This assumes that each of these turbines is experiencing similarwind speeds. In absence of a more precise and objective assessment, this method willidentify the worst of the worst. However, if all of the turbines are similarly and grosslyunbalanced, there will be no differentiation as this is a relative determination.

Auditory

Forces related to rotor unbalance will be audible as a modulation of the audible gear meshfrequencies in the gearbox can oftentimes heard at the base of the tower. During eachrevolution of the rotor during generation or motoring, the rotor unbalance will make the highfrequency sound of the gearbox get loud and soft during each revolution of the rotor as theheavy spot of the rotor moves against and with gravity.

Static Rotor Balancing

This method of balancing a wind turbine rotor starts by bumping or motoring the turbinerotor on a calm day such that it can rotate through a complete revolution and coast. As anunbalanced rotor coasts to a stop, the rotor will repeatedly tend to come to rest and oscillatearound the section of the rotor which is heavy. The heavy spot can be a single blade or itcan lie somewhere between two blades. The results of this procedure may be somewhatelusive because any wind will either tend to keep the rotor spinning or affect therepeatability of the process.

Determining the corrective solution then becomes a trial and error process, adding trial

weights to the light section of the rotor to offset the heavy section and repeating the coastdown step again to determine the effect. Friction of the drive system works against theachievement of good rotor balance using this method as it negatively affects repeatabilityand sensitivity of the results. The efforts involved with this method versus the resultsachieved generally render it an ineffective method for solving a balance problem on windturbine rotors.

Dynamic Rotor Balancing

For general industry, rotating equipment (fans, pumps, gears, motors, etc.) can be balancedusing a transducer (displacement, velocity, acceleration) and a shaft reference to measurethe response of the shaft supporting structure or the shaft itself to the unbalance forces inrelationship to a physical location on the shaft. The rotational speed of general industryequipment is typically above 600 rpm. The process involves recording the amplituderesponse, typically with an accelerometer, at the turning speed (1X rpm) of the rotor and itsrelationship to a physical shaft reference. A trial weight is then added to the rotating objectand the new 1X rpm amplitude response and shaft reference relationship is measured. Byknowing the response to the trial weight, a corrective weight solution can then be calculatedto nullify the unbalance.

This same principle can apply to wind turbine rotors and several companies offer correctivebalancing of wind turbine rotors using this method. However, employing this method forwind turbine rotors is, at best, risky. The typical speed of a wind turbine rotor is below 60

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rpm or 1 Hz and falls into a range of limited usable sensitivity for most standard transducersand associated data acquisition equipment. The reduction in sensitivity will necessarily limitthe level of balance quality which can be achieved. The accessibility of the main shaft,where the 1X rpm amplitude response is measured, presents an issue as it may be elevated

off of the ground by distances as great as 300 feet. Safety protocol may prevent personnelfrom being in this area during operation. Data acquisition is further complicated by thepresence of non-steady wind and spurious structural responses which may limit datacollection to periods of low wind. While several companies may balance a turbine rotor withthis method, a simple assessment or validation of rotor balance using these methods maybe cost prohibitive. As compared with static blade and static rotor balancing, a better rotorbalance quality can be achieved dynamically within the limits of the traditional balancinginstrumentation employed, technician skill level, and other factors.

Frontier Pro Services utilizes Dynamic PowerPro Balance, a patent-pending method andinstrumentation to dynamically assess the degree of wind turbine rotor unbalance andprovide for a corrective solution. This proprietary method does not employ the traditionalsensors used in balancing and is not subject to the inherent sensitivity limitations. The

technology performs throughout the motoring and generation range of the turbine.Furthermore, this technology allows both cost effective assessment and correctionunattainable by traditional methods. As such, balance quality can be assessed and realizedas a part of any in-situ blade repair, wind turbine commissioning, end-of-warrantyinspection, or Reliability-Centered Maintenance (RCM) program. The balance assessmentcan be made without any installation of trial weights and does not require turbine downtime. While Frontier Pro Services targets to achieve a balance quality of G16, the utilizationof this technology, as well as the process of installing the corrective weights internally to theblade, has enabled final balance qualities of better than G5.

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Installation of Corrective Blade Weights

Correcting a blades weight requires that a precise amount of weight e.g. metal bars, leadshot, or lead sheet, be bonded internally to the blade at a prescribed distance from the hub

to avoid any disturbances to the aerodynamics of the rotor. The corrective weight could beinstalled near the hub. However, the amount of corrective weight required for a givensolution decreases linearly in proportion to the distance from the hub. Therefore, ifcorrective weight is installed internally to the blade, the installation as close as practical tothe blade tip reduces the complexity of bonding and blade repair because the size of theweight used for correction is minimized.

One method of installation is to place a slurry of resin and lead shot in a container andinternally bond it to the blade. An alternate method is to pour the slurry through a smallhole into the blade cavity. However, simply pouring the slurry of resin and lead shot into alarge void does not guarantee that the weight will end up where you want it or that it willstay in place. This is the birthplace of so-called Blade Rats, a condition where the looselead shot rattles up and down the internal length of the blade. When corrective weight is

added, it should be kept as close as possible to the center line of the blade to minimizetwisting effects.

In all cases of corrective weight installation, it is important to guarantee that the weightcannot break loose inside the blade, that it is precisely weighed, and that its center ofgravity is precisely located. A mass not precisely weighed or located will not achieve thebalance quality prescribed. An improperly bonded weight could be disastrous as it canhammer back and forth with every revolution, not only causing internal damage to the bladebut causing an unbalanced rotor as well. It is always important that the results of thecorrective weight installation are confirmed through the dynamic assessment of rotorbalance.

Knowledge of blade construction and a comprehensive composite repair skill set are bothrequired for implementation of a satisfactory corrective weight solution. These skills and

techniques are generally beyond the capability of typical maintenance crews. On turbineswhere blade access cannot be realized with a man basket, rope access skills are anotherrequirement if the corrective weight solution is to be done in-situ. The composition of theblade should be known and proper fiberglass/composite repair techniques are necessary topatch any holes put in the blade during the installation of the corrective weight. Todaysmodern machine blades are complicated composite structures. When a blade set can costmore than $300k for a matched set, these procedures and repairs are best left to theexperts.

The value of being able to install corrective weights on an unbalanced rotor in-situ cannot beoverstated. For a large turbine, rotor removal and installation could be higher than $210Kconsidering crane mobilization, crane rental, and labor. This conservative estimate assumesperfect weather conditions and local crane procurement. It does not consider the costs of

implementing the corrective solution nor the one week outage time. Furthermore, by usingtraditional methods of balancing and correction, there is no assurance of a desired rotorbalance quality. Of course, we could always ASSUME.

With the proprietary Dynamic PowerPro Balance, Frontier Pro Services can not onlyperform a cost effective balance assessment but can provide a cost effective, in-situcorrective solution and assure that the solution is acceptable. Cost-conscious wind farmoperators can now elevate their turbines to new levels of performance and reliability bymitigating a condition pervasive to this industry.

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Conclusion

Wind turbines are highly engineered mechanical devices designed to have balanced rotors.Unacceptable levels of rotor unbalance accelerate mechanical fatigue on everything

connected to itbearings, shafts, pitch systems, gearboxes, generators, yaw drives, towers,and even foundations. This excessive fatigue loading will cause performance issues on startup and shut down. Unbalanced rotors result from manufacturing, blade repairs, or bladealterations as well as events which can occur during operation. There is no debate; the costof accelerated mechanical wear, delayed startup, and wind turbine motoring are real. Like asilent cash register, your costs accumulate on every sub-optimal turbine, every day.Precision rotor balancing can drastically reduce the continuous profit drain on youroperation.

While static blade balancing is designed to achieve a balanced rotor, it will not achieve theobjective due to measurement and process errors. Additionally, static blade balancingrequires that the blades be removed from the rotor, which is expensive, time consuming,and risky. Traditional dynamic balance assessment and correction doesnt provide the

precision necessary to get optimal results. Thus, the best way to assure peak performanceis to utilize Frontier Pro Services Dynamic PowerPro Balance system. As a part of overallreliability and performance management of a wind farm, establishing acceptability criteriafor rotor balance is essential for blade repair, wind turbine commissioning, end-of-warrantyinspection, or reliability-centered maintenance program. Management of rotor balancequality can now be cost effectively realized through the services and technology of FrontierPro Services.

For information on how Frontier Pro Services can help you with wind turbine rotor balanceassessment and correction, or wind turbine asset management, please contact:

Authors:

Mark Dawson, P.E.

Senior [email protected]

Michael LenzProcess & Asset ManagerAsset Maintenance & Condition [email protected]

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mailto:[email protected]:[email protected]:[email protected]:[email protected]

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