RELIABILITY AND CONDITION MONITORING OF A WIND TURBINE

M. Mohsin Khan, M. Tariq Iqbal, Faisal KhanFaculty of Engineering and Applied Sciences, MUN, St.John’s. NL. A1B 3X5

AbstractWind is one of the cheapest and cleanest sources of

energy. However, large and frequent fluctuations in windintensity and directions can cause serious problems inharvesting this energy. Wind turbines are subjected tomany unexpected environmental loads, which can becatastrophic in nature for the wind turbine system. Like anyother industrial equipment wind turbines also require sometype of monitoring system which is able to predict the upcoming faults of the most sensitive components of the systemto save it from a major disaster. This paper highlights theongoing research on reliability analysis and conditionmonitoring system required for a small-scale wind turbinesystem AOC15/50, which is widely used in Atlantic Canadaand USA. The paper describes the importance of safetysystem and lay ground work for sensor specification, sensormounting and configuration requirements for magnetic tipbrake and yaw bearing which were proved to be the leastreliable components in an extensive reliability analysis. Thepaper describes condition monitoring instrumentation, dataacquisition system and data analysis methodology.

Wind turbines, Renewable Energy, ConditionMonitoring, Fault prediction, Instrumentation and Measurement.

1. Introduction

The need of an independent condition monitoringsystem has proven to be necessary for manyelectromechanical systems. Wind turbines installed inremote location need constant monitoring of differentvariables. Some variables like wind speed and direction,which are primarily important for basic operation are alwaystaken into account, however due to changes in environmentother parameters, which under normal situation may notpose any harm, may require attention. Condition monitoringsystem for wind turbines has been proposed by a numberauthors [5] and [6]. Most of the large commercial windturbines have some sort of condition monitoring system butsmall wind turbines lack this feature due to additional costinvolved. With reference to the work [1], an extensive anddetailed reliability analysis was carried out for a 50kWsmall wind turbine AOC15/50. That shed some light on thereliability and sensitivity of some of the AOC15/50components while operating under normal conditions. Theresult in Table-1 describes the reliability value of windturbine component discussed in [1]. Tip brake and yawbearing have been identified as the most venerable

components in the analysis and are consistent with theproblems faced by other systems in operation.

Table-1: Component Reliability and Failure rate/hr.

Component Reliability Failure rates

Tip break R = 0.5334 1.00 x 10-4

Yaw bearing R = 0.9013 0.115 x 10–4

Generator R = 0.99305 0.769 x 10-6

Gearbox R = 0.9944 0.63 x 10-6

Parking Brakes R = 0.999 2.16 x 10-6

Blades R = 0.9068 1.116 x 10-5

Blots R = 0.9068 1.116 x 10-5

Hub R = 0.9068 1.116 x 10-5

Tower and anchorBolts

R = 0.9997 1.000 x 10-7

With advancement in the filed of renewable energy it isdeemed necessary to make system more reliable thusachieving higher availability. There is a need for a conditionmonitoring system to predict the up coming faults and toshutdown the system if necessary. In the following proposedcondition monitoring system with all its components isdiscussed.

2. Condition Monitoring Parameters

The next important task after identifying thesensitive components is to specify suitable parameters formonitoring of these components. These parameters will beused to select sensors for collection of data using a dataacquisition system and will be used for monitoring. Eachcomponent exhibit a unique attribute related to it, which isaffected during its use. That attribute can be translated into ameasurable parameter like voltage or current, which mayvary depending upon different operating conditions. Thischange will provide information about the operationconditions of different parts of the system. As explainedearlier the components proved unreliable need a monitoringsystem for them. Three parameters selected for thesecomponents are brake current, strain and vibration.

2.1 Current MeasurementThe spoilers or the tip brake are mounted on the tip

of each blade that provides aerodynamic drag whilestopping. They are a part of regular stopping mechanismand have to be deployed when stop cycle commences.

0-7803-8886-0/05/$20.00 ©2005 IEEECCECE/CCGEI, Saskatoon, May 2005

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However it has been discovered that tip brake beingmechanical in nature, have some problems. They are proneto failure during regular operation especially when they areclosed. The centrifugal forces acting on them can also leadto a failure. Even in case of magnets activated for keepingthem close, brakes seem to open partially or deploy withoutan instruction from the systems PLC. This type of situationcan cause problem towards a smooth operation and energyproduction. On closed observation it was found that before acomplete failure the spoiler would experience mechanicalplay. This play will introduce some change in the supplycurrent. The change in current is more significant as thebreak (which essentially is a metal plate connected to springmechanism and magnet) will vibrate while rotating. Due tothe centrifugal force, plates may disconnect from themagnets sending a current surge in the supply as shown inFig-1. These variations in current can be recorded to keeptrack of the health of every tip brake. Any sudden variationsor a pattern observed by the safety system can be recordedand appropriate action i.e. shutdown the wind turbine maybe taken if more then two brakes are vibrating. Vibrationinduced however current variations, while the system inoperation with brakes not deployed will appear in the mainpower supply to the brakes. The main supply to the brakes israted at 120V/5A. Therefore a current variation of 0-5Ampis expected. In [2] it is shown the brakes work on a constantAC current value, which implies that any sudden changes incurrent can be detected.

Fig-1: Expected variation in the brake current.

2.2 Strain MeasurementStrain measurement is mostly meant for structural

loading however it will be serving a slightly differentpurpose. From analysis in [1] the second most sensitive partof AOC 15/50 appears to be the yaw bearing. It has beenobserved in similar and other system of larger capacity, thatyaw bearing is a prime source to service disruption. Thathappens in case of bigger yaw bearings, but AOC 15/50 hasa yaw bearing not exceeding a diameter of 2 feet. Thiscustom designed bearing under normal operating conditionsexperiences some unusual moments on it self. Similarbearings have been a cause of failure in many documentedcases in other system. The reliability data also suggest the

same finding. Being the passive nature of yaw mechanismin AOC15/50, the operating conditions of yaw becomesvenerable, as there is nothing to reduce the increased forcesin sever weather and wind conditions. Such circumstancesmay cause some irreversible damage to the bearing’s shape,and later on can result in a major system failure. However ifsuch a change is detected before hand, a lot of potential losscan be saved. Before proceeding toward instrumentation thefollowing analysis is deemed necessary. With reference tostructural details discussed in [1] and [3], the following willbe helpful in designing and calibrating any appropriatesensing arrangement. From Fig 2, which is the reducedscaled drawing provided by the manufacturers anddimensions available in [3] we will proceed as follows;

Fig-2 Drive Train and Yaw support with Tower top

m1 = Mass of Generator ≅≅≅≅ 120 Kgm2 = Combined mass of Three blades at hub ≅≅≅≅ 450Kg

The downward force exerted by both masses as ignoringthe mass of gearbox is given by;

F1 = 1.176K N and F2 = 4.410K NWith moment of force indicated in Fig-2 as L1 and L2 forboth forces respectively the moment of force on the rotatingring of bearing would be;

M1 = 514.73 Nm and M2 = 4890.48 NmThus creating an inherent imbalance of moment of about4375.752 N.m. It is also important to note from Fig-2 thatthe center of gravity is also slightly towards the Hub ratherthen being on the center of rotation. This also adds to theimbalance effect on the bearing. Rotor disk thrustphenomenon is discussed in detail in [1]. This thrust canalso affect the imbalance conditions. Assuming that whilerotating, the blade experience the maximum thrust whenthey are in a vertically upward position. At the same timethe other two blades are at an angle of 120’ from it and in aposition where they might not be receiving an equal thrust,thus creating a net downward moment on the hub end of theassembly. To keep the analysis conservative a worst-case

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scenario of downward moment due to the difference ofthrust to be the total thrust is considered. It means that noforce is acting on other blades. From [1] we have the thruston rotor disk is given as;

∆T = T = 49.394k.NMT = 54.775 x 103 Nm

Hence the total bending moment on bearing comes out to be;

Mββββ = 59.1513 K NmWhich translates into stress θθθθ of 0.18640 Mpa. UsingHook’s law to observe change in length per unit lengthwhen the above-mentioned stress is applied;

E =εϕ

(i)

Where εεεε strain and E as young modulus of the material(steel 316), we have a strain value;

After studying the dynamics of bearing which is a 8-pointslip bearing designed for heavier loads, the mechanics oftransfer of force to the outer bearing ring (mounted totower) may not be linear, however it is assumed that strainwill be transmitted to the outer ring (on which themeasuring instrumentation will be done) with samemagnitude. For sensitivity and calibration purposes and byconvention this value is converted into in/in units andcomes out to be;

εεεε = 0.00003799 inch/inFor an available strain gauge with a gauge factor of 2.0 andR = 120Ω the relation is given in [4] as;

∆∆∆∆R = εεεε. (GF) .R (ii)∆∆∆∆R = 0.009118 Ω∆V ∝ ∆R

From ohm’s law, which implies that there will be aproportional change in the value of voltage with changes inresistance. This provides a basis for a changing parameterfor detection.

2.3 Vibration MeasurementVibration measurement has now become a norm in

modern condition monitoring systems. The use of such asystem is not directed by any analysis most of the time,however to determine the health of rotating machinery a realtime vibration measurement technique is required. There aremany existing monitoring systems available in markets bydifferent vendors. These systems use Fast FourierTransform (FFT) routines to compute the FFT and displaythe magnitude spectrum on the monitor or system display.Most of these systems are custom made and have to becalibrated, moreover they are installed as a softwareprogram/application on a Windows/Dos based PC alongwith a separate data acquisition card that will take the real

time data samples and compute the FFT spectrum. Thesetypes of systems are suitable for larger systems that have astaggering installation and repair costs. However for smallersystems like AOC15/50 the addition of a separate dataacquisition and monitoring system will come out as a hugeprice issue if one system is to be provided with each windturbine unit. Keeping in view the financial restrictions thereis a need to develop a cost effective and equally efficientsolution. An effort is being made during this research todevelop such a system that is able to keep track of vibrationactivity for the whole unit and thus be able to predict suddenchanges in frequency spectrum. These changes will beindicative of any up coming faults with in the machine aswell as of imbalance with in the frame of turbine that can bea result of yaw movements. FFT is an improved way ofcomputing discrete time Fourier transform (DTFT) which isthe crux of the solution. DTFT can be computed on anyband limited time varying signal. From its name it is clearthat discrete time sample values are required. Mathematicalrepresentations of Fourier transform of a continuous timeand discrete time signal are given by following equations;

A continuous time signal xc (t) can be represented in adiscrete time representation as follows;

cx (t) = kx (n∆Ts) (iii)

NX (ω) =−

=

1

0

N

kkx ϖjke− (iv)

Where Ts be the sampling time and Xn is the DFT of thediscrete signal xk.

Now ω= nωs; ωs = 2N

π(v)

Which gives us the final relation as under;

NX (nω) =−

=

1

0

N

kkx sjkne ϖ− (vi)

This theoretical interpretation of DTFT can be implementedin many ways in code. Fourier transform represented inabove equations are known as exponential Fouriertransforms, the trigonometric series can also be used tocompute the same spectrum. In this particular caseCompact Trigonometric Fourier is used in code to computethe Fourier transform the relations used are given as under;

an cos nωst + bn sin nωst = Cn cos ( nωst +θn ) (vii)

The magnitude and Phase spectrum for the aboverepresentation given below is used in the code;

Cn = 22nn ba + (viii)

θθθθ = tan –1 (n

n

a

b−) (ix)

εεεε = 9.65 x 10-7 m/m

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Phase spectrum is always given a secondary importance asmagnitude spectrum provide enough information in term ofharmonics and amplitude of each harmonic, which changeswith machine health and upcoming faults.

3. Instrumentation and Data Acquisition

The instrumentation required from abovediscussion is obvious. Instrumentation involves sensors forcurrent, strain and vibration. Typically vibration is measuredon rotating machinery by accelerometer, which can bemounted anywhere on the drive train. All three sensors havetheir own working specifications, so the signals have to beconditioned before the data is recorded and they have to bemade compatible with the single board computer. Threesignals were conditioned using three different analogcircuits for suitable data acquisition. The block diagram forthree circuits is as under;

Fig-3 Instrumentation Board Layout

Each signal conditioning circuit is selected after consideringthe input range of the single board computer (SBC), whichis 0-5volts Dc. The SBC is selected keeping in considerationthe speed and computation capability required for FFTroutines and sampling rates. The processor operates at40MHz clock frequency with and ADC at 66kHz, which ismore then what is required. The sampling will be done atslower rate to accommodate the SBC memory issue, as it isonly 256K. Selected SBC (Pico Flash system) comes with abuilt in DOS utility which makes it easy to operate and user-friendly than other conventional mirco-contorllers. C++ isused to make .exe file for application. This file is loadedinto memory and a batch file is programmed in DOS, whichinclude the path of .exe file, which runs at startup of SBC.This unorthodox methodology of using SBC has proven tobe less problematic and more flexible then conventionalmethods of data acquisition by micro controllers. A pictureof the proposed measurement setup along with theexperimental inputs is given in Fig 4.

4. Conclusions

This paper describes the ongoing efforts on reliabilityanalysis and development of a condition monitoring systemrequired for a small-scale wind turbine system AOC15/50.

The paper describes the importance of safety system andlays groundwork for sensor specification, sensor mountingand configuration requirements the least reliablecomponents of AOC15/50. The paper describes proposedcondition monitoring instrumentation, data acquisitionsystem and data analysis methodology.

Fig-4 Instrumentation Test Bench

Since the research is still in progress the results of theanalysis and performance of the proposed are not included.

Acknowledgements

The authors would like to thank the NaturalSciences and Engineering Research Council (NSERC),Canada for the financial support towards this research.

References

[1] M. Mohsin Khan, Tariq Iqbal, Faisal Khan “Reliabilityanalysis of a HAWT” NECEC Proceedings Oct-2004.

[2] Atlantic Orient Corporation Limited “AOC 15/50Operating and instruction Manual.” Revised Versin5.0.

[3] Byers and Snyder’s “Engineering Mech. OfDeformable bodies”

[4] B.P.Lathi “Modern and Analog Dig. Communication”3rd Edition.

[5] P. Caselitz, J.Giebhardt “ Fault Prediction Techniquesfor offshore Wind farms maintenance & repairstrategies” ISET, Div of Energy Conversion andControl Engineering.© ISET 2003

[6] P. Caselitz, J.Giebhardt “Advanced conditionmonitoring system for Wind Energy converters” ISET.Proceedings of EWEC’99, Nice, France.

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