Fatigue damage of steam turbine shaft at asynchronous connections of turbine generator to electrical network

Transcription

1 Journl of Physics: Conference Series PAPER OPEN ACCESS Ftigue dmge of stem turine shft t synchronous connections of turine genertor to electricl network To cite this rticle: A P Bovsunovsky 5 J. Phys.: Conf. Ser. 6 Relted content - Turulence Impct on Wind Turines: Experimentl Investigtions on Wind Turine Model A Al-Adi, Y J Kim, Ö Ertunç et l. - Wind turine wke trcking nd its correltions with wind turine monitoring sensors. Preliminry results S Aurun, E. Torres Grci, M Boquet et l. - Mgnus wind turines s n lterntive to the lde ones N M Bychkov, A V Dovgl nd V V Kozlov View the rticle online for updtes nd enhncements. This content ws downloded from IP ddress on 5//9 t 4:5

2 Ftigue dmge of stem turine shft t synchronous connections of turine genertor to electricl network A P Bovsunovsky Ntionl University of Food Technologies, Deprtment of Mechnicl Engineering, Stndrdiztion nd Certifiction of Equipment, 6 Volodymyrsk Str.,, Kyiv, Ukrine E-mil: povsunovsky@gmil.com Astrct. The investigtions of crcks growth in the frctured turine rotors point out t theirs ftigue nture. The min reson of turine shfts ftigue dmge is theirs periodicl strtups which re typicl for stem turines. Ech strtup of turine is ccompnied y the connection of turine genertor to electricl network. During the connection ecuse of the phse shift etween the vector of electromotive force of turine genertor nd the vector of supply-line voltge the short-term ut powerful rective shft torque rises. This torque cuses torsionl virtions nd ftigue dmge of turine shfts of different intensity. Bsed on the D finite element model of turine shft of the stem turine K-- nd the mechnicl properties of rotor steel there ws estimted the ftigue dmge of the shft t its torsionl virtions rising s result of connection of turine genertor to electric network.. Introduction. The nlysis of virtion of stem turine shft usully focuses upon their trnsverse virtions, the intensity of which is especilly significnt when pssing through the criticl speeds of shft rottion. At the sme time, in certin cses nmely torsionl virtions of turine shfts re much more dngerous. The evidences of the ltter re the ctstrophic filure of the rotor of medium pressure stem turine t the power plnt in Glltin (USA) in 974, cused y the growth of crck for severl yers under the cyclic torsion [], s well s the destruction of the stem turine in Kshirski therml sttion (Russin Federtion) in []. The min resons of the turine shft torsionl virtions re the short circuit of turine genertor, the connection of turine genertor to electric network, nd the dynmic instility of the system turine genertor electric network [-4]. Short circuits of turine genertors hppen severl times during the opertion time. Under certin conditions short circuit cn cuse the instnt destruction of turine [], ut the ccumultion of ftigue dmge of turine shft s result of short circuit is unlikely. At the sme time, the nlysis of crck propgtion in the destroyed or decommissioned turine shfts mkes it possile to conclude tht crcks developed over long period of time [, 5]. The resons of ccumultion of ftigue dmge of turine shfts should e sought in theirs multiple strtups, which re typicl for stem turine units operting in flexile mnner. The predetermined numer of such strtups during the opertion time is, ut in some cses it cn e prolonged to 5 nd more. At ech strtup t lest one connection of turine genertor to electric network occurs. At this ecuse of the difference of the phse shift ngle etween the vector of electromotive force of turine genertor nd the vector of electric network voltge the short-term rective electromgnetic shft torque rises, resulting in the Content from this work my e used under the terms of the Cretive Commons Attriution. licence. Any further distriution of this work must mintin ttriution to the uthor(s) nd the title of the work, journl cittion nd DOI. Pulished under licence y Ltd

3 turine shft torsionl virtions of vrious intensity. The intensity of torsionl virtions my e so high tht they cuse the ftigue dmge of the shft. According to the estlished prctice, the evlution of residul life of stem turines is minly sed on the nlysis of therml stress stte of rotors nd on the study of degrdtion of mechnicl properties of mterils of turine structurl elements. Thus the ftigue dmge of turine shft due to the torsionl virtions is not tken into ccount, which my e the reson of significnt errors of estimtion of ctul dmge of turines structurl elements. The importnce of the prolem of torsionl virtions of turine shfts is emphsized in ref. [6]. The im of the present reserch ws to investigte the fctors which ffect the ftigue dmge of turine shft, s well s to ssess the level of this dmge cused y torsionl virtions rising t the connections of turine genertor to electricl network.. Model of stem turine shft. The stem turine K-- is complex mechnicl system consisting of high pressure rotor (HPR), middle pressure rotor (MPR), low pressure rotor (LPR) nd turine genertor (figure ; the genertor is not shown in this figure). The numers, nd in figure point out the most stressed res of turine shft the ftigue dmge of which hs een the suject of reserch. HPR MPR Figure. Stem turine K--. LPR The study on forced virtions of such mechnicl system ws performed numericlly with -D finite element model (figure ). As n oject for the reserch ws selected the shft of turine K- - ( MW). The model ws mesh of 5 eight-node elements. The detiled description of the model nd the numericl methods used for the solution of the prolem cn e found in ref. [7]. The scheme of the shft loding is presented in figure, nmely the torques rising in HPR (M hp), MPR (M mp), LPR (M lp) nd the rective torque (M r) which rises t connection of turine genertor (TG) to electricl network. Totl operting shft torque is determined s follows Moper M hp M mp Mlp. () Clcultions of the electromgnetic torque t the connection of the turine genertor TGV- to the electric network of voltge KV showed tht the rective torque strongly depends on the phse shift ngle (the so clled synchronizing ngle) etween the vector of electromotive force of the turine genertor nd the vector of electricl network voltge []. The results of clcultion of time dependence of the reltive vlue of rective shft torque t different vlues of synchronizing ngle re shown in figure. As cn e seen, the rective shft torque increses in direct proportion to the vlue of the synchronizing ngle nd my exceed up to 6 times the operting shft torque. Since these functions re dmped, the lmost complete ttenution of the electromgnetic processes in turine genertor occurs in out 4 seconds.

4 M r /M oper HPR Мhp MPR Мmp Мlp 5 5 o 6 o LPR Мr -5-9 o = o TG t, sec Figure. -D finite element model of the turine K-- shft. Figure. Time dependences of the rective shft torque t different vlues of ngle.. Torsionl virtions of turine shft t synchronous connection of turine genertor to electricl network. Clcultions of forced torsionl virtions of turine shft were performed for the synchronizing ngles =, 6, 9 nd. The level of dmping of the mechnicl system ws estimted y the logrithmic decrement of virtion. As fr s its ctul vlue is unknown it ws ccepted in clcultions tht =.. Such high level of dmping ws justified y the presence of t lest four mechnisms of energy dissiption, nmely the energy dissiption in the mteril, structurl energy dissiption, the resistnce of the ir-stem medium nd the resistnce of turo genertor itself. The connection of turine genertor to electricl network cn e successful nd unsuccessful. In the cse of successful connection the turine genertor is not disconnected from the electricl network nd the rective shft torque grdully vnishes. But if in the process of the connection the intense virtions of turine rise, the turine genertor is disconnected from electricl network, the result of which is the lmost immedite disppernce of the rective torque nd of the virtion of the turine s well. The ltter is the cse of the so-clled unsuccessful connection. The ttempt of connection is repeted for s long s it would e successful one. Torsionl virtions of the turine shft t successful nd unsuccessful connection of turine genertor to electricl network hve some specific fetures. Thus t successful connection of turine genertor torsionl virtions of the shft re mintined t reltively high level for lmost the entire durtion of the rective torque ction (figure 4). The most intense virtions re excited in the cse of the ngle =, s fr s the rective torque reches mximum vlue. With the decrese of the synchronizing ngle from to the mplitude of torsionl virtions of the shft drops in hlf. The most intense stresses re oserved in res nd. The ftigue limit of RMA rotor steel t norml temperture is = MP (it is shown in figure 4, c nd d y horizontl line). As cn e seen, t certin conditions the stress level my exceed the ftigue limit of rotor steel, tht is the reson of ftigue dmge of the shft. It is evident tht the higher is the synchronizing ngle the more intense is the ftigue dmge of the shft. As the rective electromgnetic torque ttenutes the torsionl virtions of the shft grdully reduce. At high level of dmping in the mechnicl system ftigue dmge occurs only t first few cycles of virtion s fr s the intensity of stress quickly drops to the sfe level. In the cse of unsuccessful connection of turine genertor to electricl network the intensity of torsionl virtions of the shft depends not only on the synchronizing ngle, ut lso on the durtion of the rective shft torque t r.

5 М r /М oper 4-4 = o М r /М oper 6 - = o mx, MP c = o t, sec mx, MP Figure 4. Time dependences of the rective torsionl moment (а, ) nd sher stresses (c, d) in res, nd of the shft t successful connection of turine genertor. d = o t, sec The development of torsionl virtions of the shft when trying to connect the turine genertor to electricl network nd susequent its emergency disconnection t different durtion of the rective shft torque is demonstrted in figure 5. As cn e seen, the intensity of forced virtions in oth cses (t r=.7 nd.9 sec) is lmost the sme. However, the difference etween the intensity of free virtions reches severl times. So the durtion of the rective torque strongly ffects free virtions of the shft. 4. Clcultions of ftigue dmge of turine shft. The ftigue dmge of turine shft s result of torsionl virtions ws evluted sed on the hypothesis of Plmgren-Miner [9] n D, () N i id where n is the numer of dmging cycles of virtion, in which sher stresses exceed the ftigue limit of rotor steel ; N id is the numer of cycles till the frcture under the stress imx. In connections of turine genertor the totl ftigue dmge ws determined s D totl=d. The limiting stte of the rotor steel is ttined in condition D totl=. Clcultions of cyclic dmge of the shft were performed with the ftigue curve for the rotor steel RMA, which ws otined t symmetric cycle of loding nd for norml temperture []. The influence of scle effect, temperture nd symmetry of cyclic deformtion on the ftigue properties of the steel ws tken into ccount y mens of correction fctors N c N mx КsKKt v u, () where =,; v=76,; u=,6 5 ; c=,4; =,4; N is the numer of loding cycles; 4

6 M r /M oper 6 - t r =.7 sec M r /M oper 6 - t r =.9 sec mx, MP mx, MP t, sec c d Figure 5. Time dependences of the rective torque (а, ) nd sher stresses (c, d) in res, nd of the shft t unsuccessful connection of turine genertor (=). K s is the scle fctor; K is the symmetry fctor; K t is the temperture fctor. Bsed on the experimentl reserch [] the vlues of scle nd temperture fctors were excepted to e К s=.5 nd К t=.7. The symmetry fctor ws determined y the formul m K, (4) where = MP; =.9; =.55 []; m is the men stress. The results of clcultion of the shft s ftigue dmge s result of torsionl virtions for successful connections of turine genertor to electricl network re shown in the tle. As cn e seen, in the cse of synchronizing ngle = the totl ftigue dmge of the shft does not exceed 6%, so there is significnt reserve of residul durility. The increse of synchronizing ngle cuses the growth of dmging cycles nd, consequently, the spred of ftigue dmge. Most dngerous sitution is oserved in the cse = when even high level of dmping is not le to prevent the rotor steel to ttin the limiting stte. In ll cses eing considered the re of the shft prcticlly is not sujected to the ftigue dmge. At unsuccessful connection of turine genertor to electricl network it is necessry to tke into ccount the durtion of the rective shft torque. In this cse the virtions consist of forced virtion under the ction of rective torque nd of free virtion rising fter the disconnection of turine genertor from electricl network. The dependencies of totl dmge, s well s dmge due to forced nd free virtions of the shft on the durtion of the rective torque re shown in figure 6. As cn e seen, the curves re qusiperiodic with the trend of grdul reduction of the mximl dmge. At smll dmping (=.) the dmge of re is higher thn of re, ut t high dmping (=.) the dmge of re exceeds tht one of re. The dt on dmge of re is not shown ecuse it does not exceed 5%. It is lso evident from figure 6 tht if the level of dmping would e low, the dmge ccumulted minly t free virtions (figure 6,, ). High dmping rpidly reduces theirs intensity to 4 t, sec 5

7 the sfe level nd dmge ccumultes minly t forced virtions (figure 6, c, d). It is necessry to note tht ech point of the curves in figure 6 ws otined under the ssumption tht the durtion of rective torque t every of connection ws the sme. Oviously, in prctice this vlue is different t ech connection. Tle. Ftigue dmge of turine shft t successful connection of turine genertor to electricl network,. Ares of the shft n D totl Therefore, more relistic ssessment of dmge should e performed sed on the ssumption tht the durtion t r is distriuted equiproly in certin rnge t r=t r,k t r, (t r, nd t r,k re the initil nd end vlue of the rnge, respectively). The initil vlue of the rnge ws tken to e t r,=.5 sec. The step of chnge of durtion t r ws. sec. The verged over the rnge dmge ws determined y the formul K D totl, i ( tr, i ) D i v, (5) tr where D totl,i is the totl dmge t the durtion of the rective torque t r,i. D totl D totl =. = =.. = t r, sec t r, sec c d Figure 6. Dependences of totl ftigue dmge of turine shft in re (а, c) nd (, d) on the durtion of the rective torque t unsuccessful connection of turine genertor (=): free virtion (dsh dot line); forced virtion (dot line); sum (solid line). 6

8 As cn e seen from figure 7, the verged dmge over the lifetime of the turine is quite smooth chrcteristic nd depends wekly on the verging rnge. This chrcteristic cn e used to ssess the ftigue dmge of the shft s result of torsionl virtions through the turine lifetime. D v D v = = Figure 7. Dependences of verged ftigue dmge of turine shft in re on the rnge of durtion of the rective torque t unsuccessful connection of turine genertor: free virtion (dsh dot line); forced virtion (dot line); sum (solid line). It is prcticlly unlikely tht ll connections of turine genertor to electricl network would e successful. The more relistic scenrio should count for possiility of certin numer of unsuccessful connections which is different for vrious stem turines. Figure presents the results of verged dmge ssessment for two hypotheticl cses: first cse considers successful connections nd second one unsuccessful connections. It should e noted tht the ttempts to connect the turine genertor to electricl network continues until it will e successful. Therefore, in the clcultions there ws tken into ccount tht unsuccessful connections were ccompnied y the sme mount of successful ones. In ll cses presented in figure the verged dmge is strongly dependent on the synchronizing ngle nd on the numer of unsuccessful connections. The scenrio of dmge ccumultion presented in figure y solid line is optimistic in term of the numer of unsuccessful connections. The ctul numer could e much higher thn..5. t r, sec t r, sec D v D v 4 6, o 4 6 Figure. Dependences of verged ftigue dmge of turine shft in re () nd () on the ngle : successful connections (dsh line); unsuccessful nd successful connections (solid line). Comprison of figure, nd demonstrtes tht the ftigue dmge of re is more intense thn of re of the shft. Therefore, the conclusion out the possiility of sfe opertion of the turine should e done sed on the nlysis of ftigue dmge ccumulted in re. In such wy, o 7

9 the limiting stte of the rotor steel cn e chieved in condition 75 if ll connections would e successful nd in condition 5 if the numer of unsuccessful connections would e (verticl dsh lines in figure, ). It is evident tht the higher is the numer of unsuccessful connections the lower is the synchronizing ngle. Consequently the sfety of stem turines in opertion to gret extent is ffected y the numer of unsuccessful connections nd y the synchronizing ngle. The vlue of synchronizing ngle should e controlled t every connection of the turine genertor to electricl network. 5. Conclusions. The results of reserch hve shown tht the evlution of ftigue dmge of turine shft should e sed on the ccount not only of the trnsverse ut lso of the torsionl virtions occurring t the connection of turine genertor to electricl network. The intensity of the ltter depends essentilly on the synchronizing ngle, durtion of the rective shft torque nd dmping level. The torsionl virtions of turine shft re the reson of turine shft ftigue dmge. In generl cse the ftigue dmge is function of three prmeters: the synchronizing ngle, the numer of unsuccessful connections nd the durtion of the rective shft torque t unsuccessful connections. The ssessment of ftigue dmge of turine shft t torsionl virtions for successful connections of turine genertor to power network showed tht t high level of dmping in the mechnicl system the limiting stte of the rotor steel cn e chieved in condition 75. Ech unsuccessful connection decreses this vlue y out.. The efficient wy to control the extent of ftigue dmge of turine shft is the on-line mesurement of the shft virtions nd the use this informtion to ssess the dmge sed on the model proposed in the present work. References [] Krmer L D nd Rndolph D D 976 Anlysis of the Tennessee vlley uthority, Glltin unit N turine rotor urst ASME-MPC Symp. on Creep-Ftigue Interction [] Zgretdinov I Sh, Kostyuk A G, Trukhnii A D nd Dolzhnskii P R 4 Filure of the MW turine unit of the stte district power sttion t Kshir: cuses, consequences nd conclusions Teploénergetik [] Dorfmn L S nd Truelj M 999 Torsionl monitoring of turine-genertors for incipient filure detection Proc. of the sixth EPRI stem turine/genertor workshop, St. Luis, Missouri (August 7 ) 6 [4] Stein J nd Fick H 9 The torsionl stress nlyzer for continuously monitoring turine genertors IEEE Trns. PAS-99() Mr.-Apr. 7 7 [5] Brell S, Bellogini M, Bonirdi M nd Cincer S Filure nlysis of stem turine rotor Eng. Fil. Anl. 5 9 [6] Tetsuro Kkinoki, Ryuichi Yokoym, Goro Fujit, Koru Koyngc, Toshihis Funshi nd Kwng Y Lee 4 Shft torque oserver nd excittion control for turine genertor torsionl oscilltion Elect. Pow. Syst. Res [7] Bovsunovskii A P, Chernousenko O Yu, Shtefn E V nd Bsht D A Ftigue dmge nd filure of stem turine rotors y torsionl virtions Strength Mter. 4 [] Bovsunovsky A P, Kuevd V P, Kuevd Yu V nd Shtefn E V Ftigue dmge of stem turine shft t synchronous switching on of turo genertor to power network Vir. in Eng. Technol [9] Plmgren A 94 Life of ll erings Zeitschrift des Vereines Deutscher Ingen [] Bovsunovskii A P Assessment of ftigue dmge in stem turine shfting due to torsionl virtions Strength Mter [] Kudryvtsev I V nd Numchenkov N E 97 Ftigue strength chrcteristics of steel 5KhNZMFA in reltion to solute dimensions nd stress concentrtions Strength Mter. 6 9