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1 the service magazine of the PRÜFTECHNIK Group In this issue: CMS look closely at wind turbinesField balancing of rotor bladesCMS as a service tool for windturbinesReducing vibration with alignmenttargetsExperiences made by a MonitoringCenterMonitoring low-speed drive traincomponentsNews Wind turbines are subject to vibration –some more than others. Insiders knowwhich types of systems run stably,which were only built in small numbersbecause of vibration issues, and which –such as variable speed systems – are atrisk of vibration when they rotate closeto the tower’s natural frequency. Oftenthese vibrations can be minimized bybalancing the system. Field balancing of rotors, machinesand fans is common practice in machineand system engineering. Years ago,PRÜFTECHNIK developed suitable mea-suring equipment and can offer one- andtwo-plane field balancing as a service.Therefore, it is worth considering hav-ing the rotor blades of a wind turbinebalanced, especially since there are wind turbines with relatively strong, lowfrequency vibrations and because rotorblades can be balanced using one-planebalancing. Preliminary trials haveshown that PRÜFTECHNIK measuringequipment is sufficiently sensitive toperform linear measurements and bal-ancing at 0.1Hz and higher.To offer an example: An operator re-ported frequent shutdowns triggered by a vibration monitor integrated in thenacelle. The initial response would be toassume that the vibration monitor wasnot adjusted properly and to attempt tocorrectly adjust the device. The nextstep might be to search for a new, less vibration prone location in the nacellefor mounting the device. But not in thisparticular case. Here, PRÜFTECHNIK was immediately contracted to measurethe vibrations and to reduce their mag-nitude by means of field balancing. Fig-ures 1–3 show various stages of this work – from attaching the balancing beltto measuring the imbalance and fixingthe additional masses.One-plane field balancing was perfor-med in four steps: 1. Diagnosis measurements The first step was to measure andevaluate the low frequency amplitudespectra of the vibration velocity. Theresults showed that there was a rota-tional speed range within the rated op-erating range of the turbine in which thetower’s natural frequency underwentparticularly strong excitation. This madeit clear that even small imbalance massesor other disturbances would cause therotor/nacelle/tower system to vibrate. > Fig.1: Mounting the balancing belt on a rotor blade Condition Monitoring Service Field balancing of rotor blades Dr. Edwin BeckerWind power plants are more and moreseen as an alternative to global warm-ing. Alternative drive concepts are cur-rently discussed to make life expectancy of such systems even longer. Fully ratedconverters, synchronous generators,permanent generators, and gearboxes with more than two planetary stages or with a hydraulic preliminary stage areonly a few catch phrase words in thisdiscussion. At the end of the day realapplication will show if these concepts will succeed. But one thing is clearalready – same as with currently usedconcepts – Condition Monitoring willalso play an important role. We already had a previous issue of Telediagnosis focused on the wind power industry, butdo it now again to show you whatchanges in the monitoring concepts were made in this very dynamic market.PRÜFTECHNIK News CMS look closely at wind turbines No. 12 – Focus: Wind turbines 2 the service magazine of the PRÜFTECHNIK Group 2. Imbalance run In the imbalance run, a foreign mass isapplied to the system in order to “get toknow the system”. Additional massesare mounted on a rotor blade and mea-surements are made of the rotational vibrations by means of accelerometers. Also, rotational speed and phase aredetermined using optical sensors. Be-cause the system described here exhibit-ed resonance excitations, only small im-balance masses could be attached. Toreduce aerodynamic imbalance influ-ences, the wind turbine had to be run ina non-critical state and without pitch-ing. 3. Balancing runs The balancing runs themselves wereperformed using the balancing programin the VIBXPERT ® measuring device.The measuring program was configuredso that only three fixed positions existedfor applying the additional masses –namely, the three rotor blades. The sug-gested balancing masses were mountedon the premounted balancing belt andthe balancing run was repeated. 4. Repeatdiagnosis measurements Following the balancing runs, the di-agnosis measurement is repeated andcompared to the initial condition. In thisparticular case, a reduction in the vibra-tion load of about 40% could beachieved in the resonating state. Thediagnosis measurements also revealedthat the natural resonance of the tower was excited by aerodynamic imbalances.The operator was advised to electrically block the critical speed ranges to pre- vent the aerodynamic imbalances fromcausing resonance.Other causes for very low frequency imbalances and disturbance excitationsin wind turbines are shown in Figure 5. Fig.5: Imbalance in the wind turbine can have many causes and results in premature wear. Rotor with rotor bladesMass imbalances Uneven rotor blade massesUneven mass distribution in the rotor bladeFlange and pitch errors in the hubHub imbalancesEccentricities of the entire rotorBent shaftWater penetration/icing Drive train Mass and moment imbalance in the generator, coupling or brake disk Aerodynamic imbalances Blade angle errorsUneven rotor blade profile formsRotor blade damage and effects ofrepairs on rotor bladePitch/cone errorIndirect incident flowExternal location-related excitations(gusts, lee turbulence from obstructions)Fig.2: Measuring the imbalanceFig.4: The work – and the technician – are finished!Fig.1: Mounting the balancing beltFig.3: Attaching a determined balance mass 3 the service magazine of the PRÜFTECHNIK Group Condition Monitoring Application CMS as a service tool for wind turbines Johann Loesl, Managing Director of PRÜFTECHNIK Condition Monitoring An increasing number of wind turbinesare being equipped with ConditionMonitoring Systems to provide the op-erator with data on the condition ofthe wind turbine on a running basis.Especially in the case of offshore sys-tems but also in online systems, earlyinformation on changes in the operat-ing conditions can lower the risk ofproduction standstills. Condition Moni-toring Systems (CMS) handle all rele-vant operating parameters and signifi-cantly contribute to making the opera-tion of wind turbines profitable In its latest guidelines, the Germa-nische Lloyd (GL) not only more closely defined the requirements of a CMS buteven prescribed the use of Online Condi-tion Monitoring in certified offshore wind turbines.In addition to these certified systems,there are service tools and monitoringapproaches in use internationally thathave been proven to be highly valuable,and which we will now describe. Mobile Condition Monitoring Mobile measurements are used whenan online CMS has not been installed.This type of measurement is useful when conspicuous behavior has beenidentified and special measurements areneeded to determine the specific cause.These methods can be classic structure-borne sound measurements, but alsonoise and displacement analyses. In any case, it is essential that repeat measure-ments be made in the same speed andload range. Most wind turbine control-lers offer a special mode that runs at aconstant load beginning at a certain wind speed and deactivates automaticcontrol – at least in part. With hand-heldmeasuring equipment such as VIB- XPERT ® , which is used for measure-ments in many wind turbines, it is evenpossible to record data at variablespeeds. In this case it is important thatthe measurement time is sufficiently long to ensure that the damage eventsare fully recorded. When this results inthe identification of vibration exciters, itis often useful to determine the actualdegree of damage. Video-endoscopic ex-aminations are an excellent tool in thehands of a qualified service technician.Small wind turbine manufacturersand experts use mobile measuringequipment such as VIBXPERT ® to mea-sure the running and vibration behaviorduring repeat test runs and to identify deviations.When mobile measurements are per-formed by PRÜFTECHNIK ConditionMonitoring, special care is taken to en-sure that a load of at least 20% ispresent and that the measurements areof sufficient duration. Figure 2 showsthe system hierarchy of a typical windturbine drive train. Characteristic mea-surement locations were defined on thegenerator, the gearbox and the mainbearing and combined with measure-ment and analysis functions. To keep themeasurements repeatable without re-quiring special knowledge of the ma-chinery, the parameters are saved as atemplate that can be loaded into thedevice when needed.Many operators and manufacturers of wind turbines are faced with the ques-tion of whether to train inhouse special-ists in the field of machine diagnosis orto contract external companies.PRÜFTECHNIK has addressed this con-cern by developing the Condition Moni-toring Partner Concept (CPK). The sys-tem operator collects the measurementdata according to certain specificationsand transmits them for diagnosis to thePRÜFTECHNIK Monitoring Center, which has been GL-certified since early 2007. All preparations, such as the se-lection of measurement locations on the wind turbine and the creation of tem-plates for the measurement equipmentare performed by PRÜFTECHNIK. Using VIBXPERT ® , the data are collected by service technicians of the operator ormanufacturer – people who usually need to be on-site anyway. This proce-dure saves time and money, for exampleby lowering travel costs for the diagno-sis specialists. If necessary, however, theclient employees can be trained to per-form the diagnoses themselves. Whencomplicated problems arise, they canfall back on PRÜFTECHNIK specialists atany time. RoutineCondition Monitoring CPK using a hand-held device has the Fig.1: Noise analysis on the output bearingFig.2: Directory tree for mobile measurementsFig.3: EMC-based installation of a VIBRO-WEB ® XP under a 2500 kW generator 4 the service magazine of the PRÜFTECHNIK Group disadvantage that the machine condi-tion can only be determined at a particu-lar time and under particular condi-tions. Faults that only occur under mar-ginal conditions are not always detect-ed. This can be remedied by temporarily installing an online CMS. A ‘portable version’ of the VIBROWEB ® XP collectsmeasurement results under operatingconditions over a certain time periodand delivers the data via the network tothe PRÜFTECHNIK Monitoring Center. Alternatively, data can also be collectedon a USB stick, which is later read out inthe Monitoring Center. This procedure isa cost-effective alternative to the perma-nent installation of an online CMS and issuitable for wind farms with small windturbines.Temporary Condition Monitoring isalso used to track the behavior of ma-chines under critical conditions or, forexample, in the case of gearboxes withprior damage. Additionally, the systemcan be used to determine alignmenttargets (see p. 5 ff.). TTS = TemporaryTelediagnose Service The procedure described above is re-ferred to as TTS, or Temporary Telediag-nosis Service. If a system has a problemor if a prototype is being tested, forexample, PRÜFTECHNIK can devise asuitable monitoring concept that com-bines vibration measurements with therecordings of displacement, torques,temperatures and other parameters anddisplay them clearly in an evaluationsoftware. Standard online monitoring The best solution for monitoring a wind turbine is a permanently installedonline CMS. The basic features of thesystems and monitoring procedure areessentially laid out in the directives of the GL. The following PRÜFTECHNIK online CMS were certified by GL in thisregard:–VIBROWEB ® XP–VIBGUARD ® (distributed by µ-Sen)–WinTControl ® (Flender/Winergy).These systems were developed prima-rily according to criteria such as longmeasurement times, data acquisition indifferent power ranges and continuousimprovement of the evaluation algo-rithms. Data management In addition to adapting the onlineCMS to the given requirements, a fur-ther major challenge is presented by data management. Figure 4 shows howPRÜFTECHNIK fundamentally imple-ments the task in all online CMS:The data from the wind turbines aretransmitted within the wind farm viaEthernet and sent to the PRÜFTECHNIK Monitoring Center as a file attachmentin an eMail. On a secure Internet con- Abb.4: Data management concept with remote service Preview Our next issue will focus on turbomachines –Temporary telediagnosis service onturbo drives–Vibration measurements on turbo-gears–Run-up and coast-down analyses–Evaluation of orbits and displace-ment analyses Fig.5: Small and compact – VIBNODE ® CMS nection, the operator and MonitoringCenter have access to the databasesstored on a central server. Cost-effective alternative In instances in which a certified sys-tem is not necessary or is not economi-cally justifiable, the more cost-effective VIBNODE ® CMS is an alternative. Utilizing synergies The many years of experience thatPRÜFTECHNIK has in the ConditionMonitoring and Service fields can beideally combined in partnerships. Nor-dex AG and PRÜFTECHNIK ConditionMonitoring are going down this roadtogether. EthernetLANInternetDSL Firewall SMTP servereMaileMail OperatorMonitoring Center 5 the service magazine of the PRÜFTECHNIK Group Alignment Application Reducing vibration with alignment targets Dr. Edwin Becker, Ole Holstein Alignment errors in wind turbinedrive trains are among the main causesof vibration. They are easy to identify by evaluating the vibration velocity spec-tra: If the amplitudes at the single and/or double frequency of the generatorshaft are too high, the most recent align-ment reports should be checked or theshaft alignment should be measured us-ing laser-optical alignment equipment(see Figure 1). Alignment to zero? Drive trains in a wind turbine exhibithighly variable alignment between theflexibly mounted generators and gear-boxes. The generator shaft shifts towardthe gear output shaft depending on loadand speed. To ensure that power can betransmitted during all alignment condi-tions that arise during operation, systemmanufacturers usually use couplings with a very large working range. Here itis important that alignment conditionsduring operation actually remain withinthe working range of the coupling inuse. This can be accomplished usingcouplings as shown in Figure 2 that arecapable of a high degree of displace-ment. However, even this type of cou-pling has limitations when it comes toparallel offset, which manifests itself ingreater restoring forces, stronger vibra-tions and premature wear.Using modern laser measurementmethods, a near perfect shaft alignmentcan be achieved between the gearboxand the generator with the system at astandstill. In reality, however, it is notdesirable to align flexibly mounted drivetrains to “zero”. Rather, both compo-nents must be misaligned while at astandstill so that the ideal shaft align-ment is obtained duringoperation. To achievethis, the direction of thedisplacements that oc-cur during operationmust be known. Thesedata can be entered asso-called target values inall PRÜFTECHNIK align-ment systems. Alignment targetvalues – takenfrom where? The alignment target values can eitherbe taken from the operating manual orobtained from the system manufacturer.However, the wide variety of drive traincomponents and basic frame typesmakes it difficult to provide generalquantitative figures.The actual alignment target valuescan be determined using the PERMA-LIGN ® alignment monitoring system:For this purpose, highly sensitive lasersensors are mounted on the generatorand gearbox to measure the displace-ments of the machines in intervals of one second.The measurements can be recordedand further evaluated with any onlineCMS from PRÜFTECHNIK. Thus, cus-tomers who already have a WinTCon-trol ® , VIBROWEB ® XP or VIBGUARD ® online system can simply connect the Fig.2: The shaft of a wind turbine drive train is aligned using a laser systemFig.3: The four degrees of freedom in alignmentFig.1: Vibration velocity spectrum withincreased amplitude at twice the rotationalFig.4: Target values can be entered into thealignment system – with the correct signparallel offset,horizontalparallel offset,verticalangular offset,verticalangular offset,horizontal
