Preventing lightning damage in bearings by using mechanical preloading

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ElectricPowerSystemsResearch153(2017)152–158

ContentslistsavailableatScienceDirect

Electric

Power

Systems

Research

jou rn a l h om ep a g e :w w w . e l s e v i e r . c o m / l oc a t e / e p s r

Preventing

lightning

damage

in

bearings

by

using

mechanical

preloading

Frank

Leferink

a,b,∗

a_University_of_Twente,_Enschede,_The_Netherlands b_THALES,_Hengelo,_The_Netherlands

a

r

t

i

c

l

e

i

n

f

o

Articlehistory: Received9May2016

Receivedinrevisedform30July2016 Accepted3October2016

Availableonline13October2016 Keywords: Lightning Arcing Bearing Boundaryﬁlmlubrication

a

b

s

t

r

a

c

t

Highpositionedsystemssuchaswindturbinesorradarsystemonboardashipcanbeeasilystruckby

lightning.Thelightningcurrenthastobeconductedviadeﬁnedpathstopreventdamage.Akeyelement

toprotectisthebearingsystembetweentherotatingandstationarypart.Providingabypasscurrent

pathviaadditionalmeasuressuchasaslipringistheconventionalwayofprotectingbearing.Thearcing

duetohighvoltagedifferencebetweenrollersandracewayishoweverthemaincauseofdamagetothe

bearingsystem.Itiscommonlyassumedthat,ifthewindturbineblades,ortheradar,isrotating,the

lubricationbetweentherollingelementsandtheracewayisanon-electricalconductinghydrodynamic

lubricationlayer,andthushighvoltagescanbedeveloped.Butifthebearingissufﬁcientlypreloaded

itisstillprovidingaconductivepathviaboundarylubrication.Noarcingoccurs,andnodamage.The

conceptofpre-loadingthebearingsystemhasbeenevaluatedusingmanyexperimentsonstationary

androtatingbearings,andafterperformingendurancetesting.

(http://creativecommons.org/licenses/by/4.0/).

1. Introduction

Windturbinescanbeeasilystruckbylightning[1–7].Themost commondamageistheelectroniccontrolsystem,whilethemost visibledamageisthelightningdamagetothecompositeblades. Thebearingsystemisalsoavulnerablepart[8–11],althoughless investigated.Theriskofradar equipmentbeingstruckby light-ningismuchhigherinthelittoral,i.e.watersnearthecoast,than inthebluewaterareas,farawayfromthecoast[12].Naval ves-selswereoperatinginthebluewaterbutthisischangingrapidly tothelittoral.Radarsystemsareoftenlocatedatahighposition andarethereforealsovulnerabletobestruckbylightning.Alsofor rotatingradarsystemsthebearingsystemisthekeycomponent betweentherotatingandstationarypartsofaradar system.No publicdatawasfoundonlightningdamagetoradarbearings,but theproblemcanbecomparedwiththatofbearingsonwind tur-bines[12].Themain-shaftbearingisoneofthemostinvolvedparts [8],andlightningdamagestothesebearingscanresultinhighcosts ofmaintenance.InthemainstandardIEC61400-24[1]onlyvery genericinformationcanbefound,suchasthatthebearingshould beprotected.InRef.[13]lightningtestsonanelectricvehicleare

∗ Correspondenceto:UniversityofTwente,EWIFaculty,P.O.Box217,7500AE Enschede,TheNetherlands.

E-mailaddress:frank.leferink@utwente.nl

describedshowingthatthelightningcurrentpathflowsbetween themetalalloyofthewheelandground.Nofurtherinvestigation onthebearingisreported.InRefs.[2–4,8]itissuggestedthatthe damageofthemain-shaftbearingiscausedbyflashoverswithin thebearinglubricant,orarcing,resultinginpitting.Thepossibly following highcurrent afterthearc couldresultin overheating andwelding,butthislatterphenomenonisnotdescribedin lit-erature.Ina conventional system,slip ringsareconductingthe lightningaroundthebearing,in ordertopreventarcing.Thisis alsotheconventionalapproachprotectingarcinginradarsystem bearings.Theslipringconsistsofaseriesofcarbonbrushelements dividedoverthecircumferenceoftheradarantennadrivecreating anelectrical conductivepathfor thelightningcurrent.The dis-advantageofthesebrushelementsisthewear,resultinginvery regularmaintenance,forinstanceonceperyear.Furthermorethe weariscausingdust,thuspollution.Aslipringaroundthemain shaftisalsothegenericmeanstoprotectthebearingofwind tur-binesagainstlubricantflashovers,sincethebearingisaverycritical componenttoexchange.Becauseofthisprotection,practical expe-riencewithlightningdamagetowindturbinebearingsisscarce, windturbine bearingsarenot normallycheckedafterlightning strikes[2,3].Fewinvestigationsofthedamagingeffectoflightning currentonbearingshavebeencarriedout.InRef.[11],thetests wereexaggeratedbyreducingthenumberofrollingelements,to forcethecurrentthroughonly1or2suchelements.Butit was

http://dx.doi.org/10.1016/j.epsr.2016.10.004

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Fig.1. Electricalimpedancesofthereducedscalebearingmodel,from[10].

alsoconcludedthatthedamagesproducedbyanarccanbebigger thanthecurrentconductiondamage.Inthefollowingsectionsthe stationarybearing,boundarylubricationandrotatingbearing,and lightningexperimentsonarotatingpreloadedbearingsolutions arediscussed,showingtheadvantagesofmechanicalpreloadingof bearingtopreventdamageduetolightningcurrents.

2. Stationarybearing

Anewtrendinarchitectureduringthelastfewyearsistheuse ofretractableroofsinstadiums.Theseroofstructureshavenofixed connectionstoground;insteadtheyhavethousandsofmoving con-tactpoints.Traditionally, forabuildingorafixed-roof stadium, lightningrodsaregroundedtotheroofsteel,theroofsteel con-nectedtocolumns,andthecolumnsconnectedtoagroundgrid. Butforaretractableroofitislikelythatthecurrentwillfindits waytogroundviabearings.ThisproblemisdescribedinRef.[14]. Lightningcurrenttestshavebeenperformedonagreasedbearingin stationarycondition,anditwasfoundthatnodamageoccurredto rollingelementsandraceways.Theretractableroofhasbeenused inmanyareaswherelightninghashitthestructurewithout damag-ingthestructure.Thissituationiscomparabletoastationaryradar, i.e.aradarwhichisnormallyrotatingbutisinanon-activemode. Theweightoftheradarinaparkedpositionispressingthroughthe lubricantanda directcontactbetweenrollingelementsandthe racewayismade.Thennoarcingispossible,andnopittingresults. ComparableconclusionshavebeendrawninRef.[11]where light-ningcurrentswereconductedthroughbearings.Theidenticaltests toarotatingandastationarybearingonlyresultedindamagetothe rotatingbearing.Itwasconcludedthatthecauseofdamageisarcing betweenrollingelementsandracewaysatthebreakdownpoints throughtheinsulatinghydrodynamiclubricationlayerpresentin therotatingbearing.ThiswasalsoconcludedinRef.[15],and con-firmstheassumptionthatthearcingresultinginpittingisthecause ofdamage,andnotthecurrentwhichmightcauselocallyheating. Ofcourse,incasenearlyallrollingelementsareremoved,suchasin Ref.[11],thenonlyonerollerhastoconductthecurrentresulting inaveryhighlocalcurrentdensity,andinevitablydamage.

In Refs.[9,10] modelsfor thebearing impedance havebeen developed,basedonextensivemeasurementsofa1:20reduced scalemodeloftypicalmainshaftwindturbinebearings.To repli-catethemechanicalloadconditions,alsoscaledmechanicalforces, tilt-momentandrotationspeedthatappearsinrealscalemainshaft bearingswereused.Fig.1showsthemeasuredimpedancesofthe

reducedscalebearingmodel,forparked,idling,partialloadandfull (wind)load.Intheparked-standingmode,thebearingimpedance showsalowimpedance(ohmic)behaviorthatcanbejustified con-sideringthatthebearingelementsareinmetalliccontactdueto thenullrotationspeedthatdoesnotallowfortheformationofthe lubricantfilm.Butintheidling,partialandfullwindload condi-tions,thebearingcanbemodelledbyacapacitancesuggestingthat thereisfilmlubricationbetweentherollerandtheraceway.This isinlinewiththeassumptioninRef.[5],whichassumesafilmof 50␮mthicknessattheoperationalspeedofawindturbine(20–30 RPM).

3. Boundarylubrication

Ifaradarantennaisrotatingahydrodynamiclayeroflubricant isdevelopedinthebearingwhichwillresultinaninsulatinglayer. Ifalightningstrikestheantenna,ahighvoltageisdeveloped,and flashoverwilloccur,causingaspark,whichwillultimatelyresult inpittingontheraceandrollersevenatrelativelylowcurrent lev-els.Topreventdamage,carbonbrushesinaslipringareoftenused toprovidealowimpedancecurrentpath.Preloadedbearingshave beenusedmanytimestoreducethevariationsinmovements,to attaintherequiredbuilt-instiffnessandrunningaccuracy. Experi-mentswithanelectrostaticdischargegunonbearingsshowedthat stationarybearingswereconducting(asexpected),rotating bear-ingswerenotconductingduetotheoilfilm(asexpected),butthat rotatingpreloadedbearingswerealsoconducting.Thislasteffect isagainsttheconventionalknowledgeandunexpected.However, theactualfrictionregimesforslidinglubricatedsurfaceshavebeen broadlycategorizedintosolid/boundary,mixed,orfluidfriction, onthebasis oftheStribeckcurve[16–18].In general, unbreak-able lubricating films are required to preventintimate contact betweenmatingsurfaces,whichareproducedbyfluidfilm lubrica-tion.Inpractice,however,thetransitionfromfluidfilmlubrication toboundarylubricationoccurswithincreasingloadordecreasing relativevelocity,whichleadstoanincreaseinthecoefficientof fric-tion.Stribeck,andothers,studiedthevariationoffrictionbetween twoliquidlubricatedsurfacesasafunctionofadimensionless lubri-cationparameterN/P,whereisthedynamicviscosity(Ns/m2_),

Ntheslidingspeed(m/s)andPtheloadprojectedontothe geo-metricalsurfacewhichisusuallyloadperunitlengthofbearingin N/m.AnexampleisshowninFig.2.Intheﬁlmlubricationregime therollingelementsandracewayarecompletelyisolatedviaathin layeroflubricationoil.Intheboundarylubricationregimetheﬁlm

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154 F.Leferink/ElectricPowerSystemsResearch153(2017)152–158

Fig.2.Stribeckcurve.

thicknessformedissigniﬁcantlysmallerandtheloadwillbecarried bytheasperities,ratherthanbythelubricant.

TheStribeckcurve providesa qualitative explanationof the transitionbutitsquantitativeevaluationmethodshavenotbeen establishedyet.InRefs.[19,20]thecompleximpedanceofa bear-ingsystemwasmeasured,withtheobjectivetocalculatethefilm thicknessandbreakdownratio.Itwasconcludedthatinthefull fluidfilmlubricationregime,thefilmseparatesthetwosurfaces, andtheimpedanceisveryhighandcapacitive,likeasshowninRefs. [9,10].Whentheoilfilmstartstobreakdown,i.e.isapproaching boundarylubricationtheimpedanceshowsalowvalueintheorder oflessthan1.Thisresultconfirmsourexperimentsperformed onpreloadedbearing,whereitwasimpossibletocreateasparkgap becausethepreloadingpreventedfullfluidfilmlubrication. 4. Resistanceandlightningtestsonbearing

Theanalysis and experiments described before suggest that preloadedbearingscanconductlightningcurrentswithout dam-age,andcarbonbrushelementsarethusnotneeded.Resistance measurementsandlightningcurrentexperimentshavebeen car-riedoutonnormalandpreloadedbearingstodeterminetheeffect ofpreloading.

Theresistancebetween thetwo bearing raceways, withthe rollingelementsinbetween,wasmeasuredonanormalbearing usingamilli-ohmmeter.Theresistancewaslessthan1mforthe staticbearing,i.e.notrotating.Thislowimpedanceisachievedasa resultofthedirectcontactbetweenrollingelementsandthe

race-Table1

Lightningpulseparameters.

Pulsename Peakcurrent Risetime Timetohalfvalue Duration

A 100kA 9␮s 240␮s 700␮s

B 2kA 200␮s 500␮s 3ms

C 300A – 10ms 200ms

way.Whenthebearingwasrotatingtheresistanceincreasedto morethan1000duetotheﬁlmlubrication.Thesametestwas performedonamechanicalpreloadedbearing,asusedforaradar system.Radarsystems areusingpreloadedbearings toprevent wigglingandassureaccuratetrackingofobjects.Thispreloaded bearingshowedaresistanceoflessthan1m,forstaticand rotat-ingconditions.

Lightningcurrenttests,includingthefastApulse,theBpulse, andthehigh-energyCpulse,havebeenperformedonnormalroller bearingsandpreloadedrollerbearings[21].Thelightningcurrent generatorisdescribedinRef.[22]andconsistsofaseriesofthree pulsesasdescribedinRefs.[23,24]:

• A-pulsetestsetupforhighcurrent,consistingofacascade capac-itorbank,forshortstrokeelectricalcurrentsof100kA,

• B-pulseartiﬁciallineintermediatecurrenttestsetup, capacitor-coilcircuit,forcurrentsof2kA,

• C-pulsecontinuingcurrenttestsetup,3-phasediodebridge cir-cuitforDCcurrentsof,inthistest,300A.

Theparameters forthetestsare listedinTable1.Theseare measuredvaluesforthetestsonthebearingandthedifferences betweenthevariousexperimentswaswithin±5%.

Toperformacompletelightningtest,thetestsetupis subse-quentlyconnectedtoeachofthetestsetupsandloadedwiththe maximumoutput.

4.1. Conventionalbearing

Asinglelightningcurrenttestwasperformedontheﬁrst, nor-mal,bearing.OnlytheA-pulsewasusedforthestaticbearing.No changesinrotationandresistancewasobserved.Thenthenormal bearingwasrotatingatnormalspeed,inﬁlmlubricationcondition, andthelightningA-pulsetestwasrepeated.Severedamagetothe bearingwasobserved.Mechanicalvibrationincreasedandacoustic

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Fig.4.Pittingintheinnerandouterracewayduetothelightningcurrent.

Fig.5.Testsetupfortestingarotatingpreloadedbearing.

noiseincreased.Afterdismantlingthenormalbearing,thedamage couldbeobservedontherollingelement,asshowninFig.3.The damagedinnerandouterracewayisshowninFig.4.

4.2. Preloadedbearing

Highvoltagetestshavebeenperformedtotestthe effective-nessofsparkgapsmountedexternaltoaradarsystem.Using a mechanicalpreloadedbearingit wasnotpossibletogeneratea spark,becausethebearingwascreatingacontinuousconductive path.Althoughtheconceptofboundarylubricationisknown,this resultwasnotexpectedbecausethecommonassumptionwasthat evenatlow speedthere isfilmlubrication,creatinganisolated layer.Theconceptofusing asufficientlypreloadedbearing has beenfiledinapatent[25].Aspecialtestsetupwasbuilttotest atypicalpreloadedbearingconfigurationundernormaloperating conditions,rotatingatapproximately25RPM,asshowninFig.5. Tosimulatethecurrentstypicalforalightningstroke,thesetup wassubjectedtoanA,BandClightningpulseasmentionedin Table1.Theaxialpreloadonthebearing-systemwasthesameas thepreloadingwiththeradarsysteminstalled.Measurementsof thebearingresistancebeforeandaftereachlightningcurrenttest showedvaluesbelow1m,typicalbetween300and700␮.

Tomonitortheconditionofthebearingsystem,axial,radialand tangentialaccelerationshavebeenmeasuredbeforeandaftereach lightningpulsetest.Thisisaverywellknowntechniqueto

esti-mateifthereissomedeviation,e.g.pitting,intherollingelement and/orraceway.Thevibrationshavebeenmeasuredwithasample frequencyof2kHzandarecordlengthof10s.Fromthisrawdata andFFTspectrum,theRMSvalueofthevibration(acceleration)was calculated,asshowninFig.6.

Thedifferencesbetweenthesubsequentmeasurementsareof thesameorderofmagnitudeasthestandarddeviationoftheset ofmeasurements,sonochangeinvibrationbeforeandafterthe lightningtestwasobserved.

Totesttheremainingoperationallifeofthebearingsystem,an endurancetestwasalsoperformed.Assumingthatwearis equiv-alenttothetotalnumberofrevolutions,thebearingsystemwas settorotateatamuchhigherspeedthantheoperationalspeedin ordertocompressthetotaltesttime.Thebearingtemperaturewas monitoredtoassurenormaloperatingconditions.Theendurance testwasstoppedafter4weeksathighrotationalspeed, represent-ingmanyyearsofnormaloperation.Therootmeansquare(RMS) valuesofthemeasuredvibrationsduringtheendurancetestare showninFig.7andTable2.

Thedifferencesbetweenthesubsequentmeasurementsareof thesameorderofmagnitudeasthestandarddeviationofthesetof measurementresults.Therearenodifferencesintheshapeofthe spectrathatindicateanydeteriorationofthebearingsystem.

Since theresultsof thevibration measurementsindicate no deviationsandthatbearingsystemisnotdamaged,onlytheupper bearingofthetestsetupwasdisassembledfollowingtheendurance

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156 F.Leferink/ElectricPowerSystemsResearch153(2017)152–158

Fig.6.Resultsofvibrationmeasurementsduringthelightningtests.

Table2

RMSvaluesofmeasuredvibrationsduringtheendurancetest.

Duration[h] 0:31 0:58 2:04 3:37 5:52 29:52 29:52 53:52 53:52 170:58

Axial 0.79 0.78 0.77 0.81 0.80 0.78 0.83 0.79 0.74 0.71

Radial 0.40 0.41 0.42 0.64 0.61 0.63 0.51 0.50 0.46 0.43

Tangential 0.62 0.63 0.66 0.68 0.69 0.73 0.91 0.76 0.70 0.67

test.TheresultsareshowninFig.8.Itisclearthatthetestedbearing isfreeofpittingorweldingspots

5. Conclusion

Therollingelementandracewayofa bearingsystemcanbe damagedbylightning.Theprimarycauseispittingcausedbythe arcingbetweentherollingelementsandracewayatthebreakdown pointsthroughtheinsulatinglubricationlayerpresentinarotating

bearing.Ifabearingisstationaryandloadedwithsomemass,then thisweightissufﬁcienttopressthroughthelubricantsuchthata directcontactbetweenrollingelementsandtheracewayismade. Nodamagehasbeenobservedbymanyresearchers,andthe pre-sumedweldingisnotfoundtobeadominantfactor,ifmanyrolling elementsare,inparallel,incontactwiththeraceway.

Experimentswithanelectrostatic dischargegunonbearings showedthat stationary bearingswere conducting,and rotating unloadedbearings were not conducting due tothe oilﬁlm, as

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Fig.7.Resultsofvibrationmeasurementsduringtheendurancetestwithincreasedrotationalspeed.Note:thegraphshavebeenshiftedslightlytogivea3Dimpression. Thescalesshouldbereadasifeverylinestartsonthebottom-leftcorner.

expected.However,rotatingpreloadedbearingswerealso conduct-ing,whichisagainsttheconventionalideasthatthereisalwaysa lubricatingfilmpresent.TheStribeckcurveshowsthatthe tran-sition from solid/boundary lubrication to fluid film lubrication occurs,dependingofthedynamicviscosityandslidingspeedand inversetotheload.Radarsystemsareusingpreloadedbearings topreventwigglingandassureaccuratetrackingofobjects. Var-ious resistancemeasurements and lightning current tests have beencarriedout todeterminetheeffectof preloadedbearings. Theboundarylubricationispresentifthebearingissufficiently preloaded,whichassuresacontinuouselectricalcontactbetween rollingelementsandracewayandresistancemeasurementsshow

valueslessthan1m.Thelightningexperimentsperformedshow noincreaseofvibration,whichis ameasureofpossiblepitting. Endurancetestshavebeenperformedtoensurethatevensmall pittingwasnotpresent.Aftertheendurancetestthebearingwas dismantledandinspectedvisually.Nodamagewasfound,while acomparablenormalbearing,withoutpreloading,show consider-abledamage.Thelackofdamageforthepreloadedbearingproves thevalidityofthepreloadingconcept,toprotectabearingsystem againsttheriskoflightningdamage.

Furtherresearchcouldbeperformedontheinﬂuenceof light-ningcurrent amplitudes, includingthe extremecurrents found insomeregionsintheworld,onpossibledamage.Alsothe

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rela-158 F.Leferink/ElectricPowerSystemsResearch153(2017)152–158

Fig.8.Ballbearingsandbearingofthedrive,usingthenormalpreloadedsituation:nodamage.

tionbetweenthevariousparametersintheStribeckcurveandthe resultingresistanceisaninterestingtopicforfurtherresearch. References

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