Novel fiber-optical accelerometer for monitoring tasks of power generators

Transcription

1 B4.2 Novl fibr-optical acclromtr for monitoring tasks of powr gnrators Michal Villnow, imns AG, Corporat Tchnology, Erlangn, Grmany Dr. Thomas Bosslmann, imns AG, Corporat Tchnology, Erlangn, Grmany Dr. Michal Willsch, imns AG, Corporat Tchnology, Erlangn, Grmany Prof. Brnhard chmauss, Lhrstuhl für Hochfrqunztchnik, Univrsity Erlangn-Nurmbrg, Erlangn, Grmany 1. Introduction Du to th incras in nrgy dmand and th concpt of sustainabl nginring, modrnizations of lctric powr plant installations bcom mor and mor important for two rasons: first to improv th nrgy fficincy and scond to sav costs and matrials. Howvr, th oprating point of lctric machins lik powr gnrators gts closr to its physical limit. To prvnt damags and to vrify optimization masurs, snsors and monitoring systms ar particularly ssntial. Thrfor a novl fibr-optical acclration snsor has bn dvlopd. In this papr th snsor s principl of opration is xplaind, laboratory tsts ar shown, th snsor bhavior is charactrizd and ral lif masurmnts ar prsntd. 2. Function Principl Th imns Fibr Optical Acclromtr (FOA) is basd on th intnsity modulation of light. Th nd surfac of a bar multimod fibr is placd towards a tiltd mirror. Thrby th fibr is mountd into a fibr frrul and fixd by a solid adhsiv bond. A dfind lngth l of th fibr sticks out at th nd of th frrul lik a cantilvr bar, as dpictd in Figur 1a). Fig. 1: working principl of th imns Fibr Optical Acclromtr (FOA) Whn light is launchd into th fibr th light lavs th wavguid at its nd surfac, is rflctd by th mirror and partially coupld back into th fibr. Du to th aprtur of th fibr, th light bam is radiatd conically divrgnt, thus only a small prcntag of th rflctd light is coupld back into th fibr. Whn th snsor had is xposd to an acclration a, as dpictd in figur 1b), th protruding fibr nd carris out a rlativ motion with rspct to th mirror. In consqunc of th gomtric configuration of th tiltd mirror, th distanc btwn fibr nd surfac and mirror changs du to th motion. Basd on ths distanc changs th optical path of th conical light bam gts shortr and th rflctd light bam is lss widnd. Hnc mor light is coupld back into th fibr. Whn th fibr is dflctd in th opposit dirction, th distanc btwn fibr nd surfac and mirror incrass, whrby lss light is coupld back E N O R + T E T C o n f r n c s E N O R P r o c d i n g s 2 7 7

2 into th cor of th fibr. As a rsult th intnsity I of th rflctd and coupld light is modulatd according to th occurring acclration. To charactriz th prformanc and snsitivity of th snsor, th modulation ratio M r (hncforth just calld modulation) has to b calculatd, taking into account th background intnsity I av and th masurd acclration a, as shown in quation 1.1. Rsonanc Frquncy Du to th protruding lngth of th fibr l, its diamtr d f, th dnsity and th spcific E-modulus E, th rsonanc frquncy of th snsor had is st to a spcific frquncy. By stting up th balanc of forcs and momnts of a fibr surfac lmnt, th rsonanc frquncis of th protruding fibr can b calculatd. Equation 1.2 shows th corrlation btwn th rsonanc frquncy f rs of th protruding fibr and its physical paramtrs in good approximation. With rgard to using th FOA to masur vibrations on th nd windings of a powr gnrator, th rsonanc frquncy of th snsor should b st to a valu, which is far nough away from th masurmnt frquncy. Normally a gnrator runs with a nominal frquncy of 50 Hz, in som parts of th world with a nominal frquncy of 60 Hz.g. in th U... Du to th altrnating magntic fild th inducd vibrations, mainly causd by magntic forcs, show a frquncy twic th nominal frquncy, thus up to 120 Hz. To calculat a modal analysis of a vibration mod, th snsor should b abl to masur at last th scond harmonic. In consqunc th rsonanc frquncy of th FOA has to b highr than 240 Hz, or vn mor than 300 Hz to nsur an adquat margin to sparat vibration frquncis from th snsor s rsonanc frquncy. nsitivity Normally th vibrations occurring in an lctric ngin should not xcd th acclration limit of 10 m/s², corrsponding to a displacmnt of 50 micromtrs pak-to-pak at 100 Hz. In som cass th acclrations at som locations can b much highr. Hnc th objctiv targt is to raliz a snsor that can masur acclrations ovr a wid rang of about 100 m/s² having a rsolution of 1 m /s² or 0.1g. Th snsitivity of th FOA is dfind by th movmnt of th fibr nd placd towards th mirror. Th mor th fibr nd is displacd th mor th light intnsity is modulatd du to distanc changs btwn th fibr nd surfac and th mirror. Equation 1.3 shows th dpndncy of th fibr dflction x at its nd. Hr q is th lin load causd by th fibr s mass and I is th ara momnt of inrtia. Th ara momnt of inrtia is givn by th quation 1.4 according to a symmtric circular surfac. It can b rcognizd that th dflction dpnds on som matrial constants and mainly on th protruding fibr lngth l. To achiv a highr dflction and thrfor a highr modulation of light intnsity, th protruding fibr lngth has to b longatd. Howvr this rquirmnt contradicts to th bhavior of th snsor s rsonanc frquncy, which limits th lngth of th fibr. Finally a trad-off btwn th snsitivity of th snsor had and th rsonanc frquncy has to b mad. As a rsult th rsonanc frquncy is st to 350 Hz, which shows a sufficint distanc to th scond harmonic of most lctric machins. Th latral dflction at th fibr nd is lss than 1 micron pr m/s², but th rsulting modulation is still about s²/m. Dirctional snsitivity Information about th dirction of occurring forcs is vry important for th dsign of lctric ngins and th dimnsioning of support lmnts. To masur th dirction of.g. nd winding vibrations, th acclration snsor has to b dirctional snsitiv. This is achivd by th gomtric arrangmnt of th mirror. Figur 2 shows a simplifid drawing of th snsor had with two diffrnt viws. Figur 2a) shows th sid viw, also shown bfor in figur 1. Figur 2b) givs a top viw on th gomtric configuration of th snsor had. (1.1) (1.2) (1.3) (1.4) E N O R + T E T C o n f r n c s E N O R P r o c d i n g s 2 7 8

3 Fig. 2: simplifid gomtric snsor had configuration from two diffrnt viws By dflcting th fibr in th x-z-plan th distanc btwn fibr nd surfac and mirror changs (s 1 > s) lading to a light modulation. Whn th fibr prforms a movmnt in th y-z-plan, no significant distanc changs btwn fibr nd surfac and mirror occur (s 1 = s). Th rason for this is vry simpl, basd on th lmntary trigonomtric functions. Th distanc chang z = s 1 -s can b calculatd by th tangnt function, with th tiltd angl of th mirror and th dflction x, shown in quation 1.5. inc th mirror is not tiltd in th y-z-plan, th angl of th mirror is zro ( = 0 ). Consquntly th distanc chang also rturns zro ( z = 0) by quation 1.5. This gomtric corrlation is illustratd in figur 2b). Bsid this distanc chanc th fibr gts bnt du to acclration forcs and th fibr s mass, so th fibr nd surfac tilts during th dflction. Th bnding angl at th nd of th protruding fibr can b calculatd by quation 1.6. With a typical acclration of a = 10 m/s² th bnding angl of th fibr nd surfac rsults to (l) , which is lss than on angular scond. As a consqunc th influnc of th bam distortion causd by fibr bnding can asily b ignord for acclrations of up to 100 m/s². Considring all gomtric corrlations and simplifications th snsor had is in vry good approximation (rror lss than 2%) only snsitiv for dflctions in th x-dirction. 3. nsor dsign and matrials Th snsor had consists of non-mtallic matrials lik GRP (glass fibr rinforcd plastic), Tflon, cramic and poxy rsin. Thus this typ of snsor can b usd for masurmnts undr harsh conditions,.g. high tmpraturs of up to 150 C and strong magntic filds that prvail in powr gnrators. Figur 3 shows a pictur of th snsor had and svral componnts of th innr body. (1.5) (1.6) Fig. 3: nsor housing and snsor had lmnts Th housing of th snsor is mad of laminatd fabric, so it is wll insulatd and applicabl vn for high voltag tasks. Th housing has a quadratic cross sctional ara, which dfins th snsitiv and nonsnsitiv axs of th snsor had. Th innr snsor body is built from cramic tubs with diffrnt diamtrs. All tubs fit prfctly togthr with littl tolrancs, so th snsor can b built by modular lmnts. Th mirror is mountd on th nd surfac of th outr cramic tub. To bond all parts of th snsor had, a hat curing poxy rsin glu is usd. Th cabl lngth of th snsor can b customizd to suit spcific nds. Th fibr is protctd by a dimnsionally stabl Tflon tub. o th snsor can asily b usd vn undr rough conditions.g. in E N O R + T E T C o n f r n c s E N O R P r o c d i n g s 2 7 9

4 powr plant installations. Anothr advantag is th thrmal rsistanc of Tflon of up to 220 C, so th cabl can b placd vn along machin hot spots. An E-2000 angld fibr connctor with a dust covr is usd to protct th connctor s fibr surfac. 4. Intrrogation unit and signal procssing A modular intrrogation unit was dvlopd to rad out th fibr optic acclromtr. Evry singl masuring channl is built on its own circuit board, ralizd as a slid-in card for a 19 rack unit. All in all 16 channls can b run by on singl intrrogation unit. Th latst vrsion is shown in figur 4a). Fig. 4: a) Intrrogation unit with 16 slid-in snsor cards b) Block diagram of snsor signal light path To illuminat th snsor had, a LED with a cntr wavlngth of 820nm and a spctral width of 50nm is usd. Th LED light is coupld into a multimod glass fibr and guidd by a 3dB fibr couplr to a connctor at th front plat, whr th snsor can b pluggd in. Th modulatd light, coming back from th snsor had, is guidd to a photodiod by passing th 3dB fibr couplr, as shown in figur 4b). Th photo currnt is convrtd to a voltag signal by a transimpdanc amplifir. To guarant an idal snsor oprating point, th light intnsity is rgulatd by a controllr which attuns th LED currnt to all attnuation influncs along th light path. Hnc th snsor can still b usd vn if its cabl is dformd or bnt around dgs with small curv radii. Th voltag signal of th snsor can now b procssd by diffrnt filtr and normalization options. To supprss th rsonanc frquncy of th snsor a low-pass filtr can b activatd. To liminat th fluctuations in snsitivity of ach snsor had causd by production tolrancs, a lvl adaption is ralizd so vry snsor shows th sam bhavior and signal output. In addition a normalization is implmntd to calibrat th snsor signal to a constant factor of 10 mv/(m/s²). Th ditd signal can now b rcordd by an xtrnal transint rcordr or an oscilloscop. Howvr, this fibr optic acclration snsor shows a frquncy-dpndnt bhavior, so it is ncssary to carry out a mathmatical post-procssing whn th xact vibration amplitud of signals with suprposd vibrations at diffrnt frquncis is ndd. Th frquncy rspons of a rprsntativ snsor, calibratd to 10mV/(m/s²) for 100Hz, is shown in figur 5. snsitivity [mv/(m/s²)] frquncy [Hz] Fig. 5: Frquncy rspons of th fibr optic acclromtr A rcordd transint signal can b sparatd into its frquncy componnts by a Fourir transform. In th frquncy domain an amplitud adaption can b carrid out corrsponding to th invrs frquncy rsponc of th snsor. Finally th signal can b transformd back to th tim domain which rsults in a corrctd vibration amplitud. As an xampl a typical vibration signal is shown in figur 6a), its corrsponding Fourir transformd signal is shown in 6b) and th corrctd tim domain signal in 6c). E N O R + T E T C o n f r n c s E N O R P r o c d i n g s 2 8 0

5 Fig. 6: a) Typical transint signal; b) Fourir transform signal; c) Corrctd tim domain signal 5. Fild masurmnt rsults inc 2009 svral fild tsts hav bn prformd to dmonstrat th snsor functionality undr ral lif conditions, to prov th thrmal robustnss and magntic ruggdnss of th snsor and finally to opn up nw filds of application. In 2009 th first imns Fibr Optical Acclromtrs hav bn placd at th nd winding connctors of a 300 MVA hydro-powrd motor-gnrator of a watr pump storag powr plant. Fourtn snsors hav bn mountd on svn diffrnt masuring points, masuring th tangntial and radial vibration componnts. Th masurmnts hav bn prformd at diffrnt gnrator machin conditions, lik short circuit, opn circuit, bas load and ovr xcitation. During all masurmnts no snsor faild or causd any typ of problms lik partial discharg. Fig 7: FOAs mountd on th nd winding connctor of a 300 MVA gnrator Whil rcording all snsor signals during a machin run-up, th frquncy and th amplitud of th machin s rsonanc paks can b calculatd. Th according masurmnt is shown in figur 8a). As a consqunc th influnc of nhancd rsonanc vibrations on th durability of th machin can b stimatd. Th oprator of a powr plant is now abl to avoid oprating points with incrasd vibration amplituds to improv th liftim of th machin. Fig 8: a) Machin run-up, opn circuit b) End winding vibrations, bas load/ovr xcitation By using fibr optic vibration snsors dirctly placd on highly strssd machin parts lik stator bars or supporting lmnts, small damags can b dtctd arly so that machin failur or a complt dstruction can b prvntd. E N O R + T E T C o n f r n c s E N O R P r o c d i n g s 2 8 1

6 In figur 8b) th vibration amplitud of svn snsors is shown, whil th gnrator is running in bas load with an ovr xcitd rotor. At this oprating point th highst acclration amplituds hav bn masurd, rising up to 25 m/s² and mor. Looking at th shap of th masurd vibration curv shown in figur 8b), it can b ralizd that th machin dos not only oscillat at its fundamntal mod but at additional highr mods. Fig. 9: End winding vibrations during machin run-up of a 40 MVA comprssor motor of a gas transfr and distribution station A furthr fild tst was mad during th run-up of a compltly rnwd 40 MVA comprssor motor. Thr th snsors had to work in an xplosion protctd ara. Thrfor a spcial fdthrough was constructd to guarant th gas-proofd saling. Th rcordd vibration amplituds masurd by fiftn FOAs ar shown in figur 9. It has bn rcognizd that this machin has a lot of incrasing rsonancs at diffrnt constllations of rotating frquncy and comprssion output powr. Howvr, th maximum vibration amplitud nvr xcdd a valu of mor than 20 m/s². 6. Conclusion A novl fibr optic acclromtr calld FOA has bn dvlopd and invstigatd. Th working principl is basd on th modulation of light using a simpl configuration of only on fibr and on mirror. No moving parts ar ncssary which lads to a vry tough but also small snsor housing. Du to th choic of spcial non-mtallic matrials th snsor can b usd undr harsh conditions lik magntic filds, high voltag or tmpraturs of up to 150 C. Bcaus of a passiv oprating snsor concpt using a low powr LED, th acclromtr can vn b usd in an xplosion protctd ara without furthr prcautions. Furthrmor, th snsor can b usd for monitoring tasks of high acclrations.g. suddn short circuit vibrations of up to 700 m/s² by changing th snsor dimnsioning. By calculating th mchanical bhavior of th snsor had, an optimizd configuration was found to kp th rsonanc frquncy and th snsitivity high. vral prototyps hav bn built to liminat production rrors and to charactriz rsidual production tolrancs. To th prsnt day svral fild masurmnts hav bn mad to prov th durability and to tst th snsor bhavior undr ral lif conditions. As a rsult of ths succssful masurmnt campaigns, th complt snsor systm turnd out to b a usful nlargmnt of xisting monitoring systms to prvnt damags and machin failurs. [1] R. O. Cook and C. W. Hamm, Fibr optic lvr displacmnt transducr, Applid Optics,Vol. 18, Issu 19, pp , 1979 [2] J. Kalnik, R. Pajak, A cantilvr optical-fibr acclromtr, nsors and Actuators A: Physical, Volum 68, Issus 1-3, Pags , EurosnsorsXI, 1998 [3] K.-H. Grot and J. Fldhusn, Dubbl Taschnbuch für dn Maschinnbau, 22. Aufl. pringr Brlin Hidlbrg Nw York, 2007 [4] G. Conforti, M. Brnci, A. Mncaglia, and A. G. Mignani, Fibr optic vibration snsor for rmot monitoring in high powr lctric machins, Appl. Opt., vol. 28, no. 23, pp , Dc. 1, [5] J.M. Lópz-Higura t al., impl low frquncy optical fibr acclromtr, Journal of Lightwav Tchnology, VOL. 15, NO. 7, July 1997 E N O R + T E T C o n f r n c s E N O R P r o c d i n g s 2 8 2