Method of Shorted Turn Monitoring in the Turbogenerator Rotor Winding

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1 Pro. of the 5th WSEAS/IASME Int. Conf. on Eletri Power Systems, High Voltges, Eletri Mhines, Tenerife, Spin, Deemer 16-18, 25 (pp ) Method of Shorted Turn Monitoring in the Turogenertor Rotor Winding M. ROYTGARTS Turogenertor Deprtment OJSC Power Mhines Brnh Eletrosil 139, Moskovsky pr., 19615, St-Petersurg RUSSIA Astrt: - The represented method of monitoring the shorted turn in the winding of the turogenertor rotor is sed on the nlysis of spetrl struture of eletromotive fore indued y mgneti field of the rotor in the indution sensor. Results of nlytil nd numeril lultions of mgnetomotive fore nd mgneti field of the rotor with shorted turns in the winding re given. The dmged oil is determined y omprison of lultions nd experimentl dt. Reserh results of the turogenertor t the power sttion re shown. It is shown lso tht shorted turns nd ir gp eentriity influene similrly on the mgneti field nd mgneti fores. Key-Words: - Turogenertor, rotor winding, shorted turn, mgneti field, spetrl nlysis. 1 Introdution Aording to the inspetion exeuted y the ompny Genertorteh, In. [1], more thn 5 % of turogenertors t power sttions re operted with shorted turns in the rotor winding nd hve no prolems with it. However genertors with more thn 5 % shorted turns show inresed virtions, restrition of pity, sudden swithing-off, dmges of retining rings. There is tendeny of nnul inrese of the numer of shorted turns in the investigted mhines. Thus, it is neessry to detet duly the shorted turns in the rotor winding nd perform ontinuous monitoring of the proess development t opertion of the turogenertors. Now there re two si methods for the ontinuous monitoring of short iruits of rotor windings t power sttions: the monitoring of the form of the eletromotive fore (EMF), indued in the indution sensor, pled in ir-gp [2,3,4], or the nlysis of urrent struture in sttor winding used s mesuring winding [3,5]. The monitoring with the help of indution sensors, offered y Alright D.R. [1,2], hs reeived wide reognition. Indution sensors for mesurement of rdil nd tngentil field omponents re pled t distne of 4%-6 % of ir-gp height from the surfe of the rotor. The form of output voltge or eletromotive fore, indued y the rotting mgneti field in the sensor, repets tooth kink of the rotor. In se of shorted turns in the rotor winding the height of orresponding peks nd dips vries, the form of indued EMF is not so symmetri reltive to zero. Using these hnges one n determine oils with shorted turns. The speil time-dely lines llow to ompre wve forms of two poles t n osillogrph. Advntge of the method is reltive simpliity nd lrity of the monitoring, ut the method is effetive for turogenertors with rther smll numer of turns in the rotor oils. 2 The Spetrum Monitiring With inrese in the numer of turns in rotor oils, the shorted turn of oil eomes less notiele in EMF urve of the sensor. For exmple, if the rotor oil hs 2 turns, then t short iruit of one turn the hnge of the urve of the EMF sensor s ompred with the defetive oil will mke up no more thn 5% nd n e ttriuted to the inury of mesurements. Even if t present there re no prolems with opertion of the turogenertor, short iruit not reveled duly n e the eginning of the filure proess. Sujetivity of the qulittive onlusion ording to rther smll hnge of the EMF urve form omplites the nlysis; there my e the miguous onlusions out presene of shorted turns. In this onnetion we hve offered dditionl fetures of the shorted turns monitoring in the rotor winding t opertion of the turogenertor, sed on the spetrl nlysis of EMF indued in the sensor

2 Pro. of the 5th WSEAS/IASME Int. Conf. on Eletri Power Systems, High Voltges, Eletri Mhines, Tenerife, Spin, Deemer 16-18, 25 (pp ) [6,7]. Bsed on the frequeny struture nd reltive size of seprte spetrum omponents in frequeny re we hve rrived t onlusion of presene of shorted turns. Criterion of servieility of the rotor is prtil sene of even hrmoni omponents in the EMF spetrum of the sensor. At presene of even hrmoni omponents, the oil with shorted turns is determined y their omprison with spetrum struture of EMF of fultless turogenertor nd with results of numeril modeling of possile short iruits. The offered method does not require dditionl equipment for the turogenertor, it is onvenient for lgorithmiztion nd is development of the trditionl indution monitoring. The mesuring devie is the nrrow-nd spetrum nlyzer or eletroni digitl osillogrph with omputer. Also two opposite onneted indution sensors with the identil frequeny hrteristis n e used, instlled t identil distne from the surfe of the rotor nd shifted for pole division over the irle of the sttor. As the spetrl nlysis in frequeny re supplements the nlysis of the mesured signl form of the sensor in time, the ury of the monitoring inreses. By wy of explntion we shll onsider the si eletromgneti rtios. 3 Mgnetomotive Fore In no-lod ondition the turogenertor mgneti field is reted y urrent in the rotor winding representing onentri oils, pled in rdil slots of the rotor. Mgnetomotive fore (MMF) of the rotor winding is distriuted symmetrilly mong poles, it hs two xes of symmetry for the period of expnsion nd onsequently n e sumitted s Fourier series ontining odd hrmoni omponents, multiple to the numer of pole pirs 4Fq f ( ϕ ) = 1 k k sin ( np ϕ ) (1) π n wn n 1,3,5,... where the distriution ftor for oils with identil numer of turns nd identil slots pith k wn 2 p z π n 2 πnp = sin γ sin (2) 2 z ' 2 Ftor of dislosing the slot πn πn k n = sin. (3) 2τ 2τ Here F = IW /( 22 ) is the MMF, neessry for trnsferring of mgneti flux through one ir-gp, I - rotor urrent, W - the numer of turns in the oil, 2 - the numer of prllel rnhes of the rotor winding, = z / 4 p - the numer of oils per pole, q 2 τ- polr division of the rotor, z 2 - the numer of slots with winding, z 2 - the numer of slot points of the rotor, - width of the groove of the rotor, p - the numer of pole pirs, γ = z 2 / z 2 - shre of wound prt of the rotor. The first oil hs the gretest pith, lst q - ordingly the lest pith. The shorted turn ompnying prtil or full swithing-off of the turn n e presented y superposition of symmetri distriuted MMF of the winding nd opposite MMF of the shorted turn. Thus, spetrum MMF will ontin generlly odd nd even hrmonis, oth the greter nd the smller order (for multipolr mhines) onerning the numer of pole pirs. Ftors of expnsion will depend on the rrngement of the oil on pole nd on the rrngement of the pole itself. Let, for exmple, short iruit hs tken ple in ny k oil of l-st pole. The totl mgneti flux in twodimensionl mgneti field t the period of expnsion is equl to zero. Tht is there is no onstnt omponent of the field. Then MMF expnsion of the shorted turns of this oil in hrmoni series will e f kl 1 ( ϕ ) = [ nkl os( nϕ ) + nkl sin( nϕ )] (4) π n where the ftors of expnsion re [ τ (2k 1)t ] πn(2l 1) πn nkl = os sin, n = 2p 2pτ (5) πn(2l 1) πn[ τ ( 2k 1)t ] nkl = sin sin, n =. 2p 2pτ Here F kl is MMF, orresponding to the turn-toturn short iruit, t - tooth division of the rotor. For the ipolr mhine the expression of the expnsion ftors eomes simpler, even nd odd hrmonis re seprted. Fig 1, 2 shows the spetr of even hrmonis of MMF used y short iruit in 1-st nd 9-th oils of the ipolr mhine. Ourrene of the even hrmonis of MMF, whih hs not een onneted to the numer of the mhine poles pirs, testifies the infringement of eletri symmetry in the rotor nd is n ttriute of shorted turns in the rotor.

3 Pro. of the 5th WSEAS/IASME Int. Conf. on Eletri Power Systems, High Voltges, Eletri Mhines, Tenerife, Spin, Deemer 16-18, 25 (pp ) φ Ηφ φ Figure 1. Even hrmonis of MMF(), Hϕ(), EMF() in the sensor t fult in the first oil Figure 2. Even hrmonis of MMF(), Hϕ(), EMF() in the sensor t fult in the ninth oil. More sophistited nlysis of eletri symmetry of the rotor n e mde ording to liner urrent lod, tht is ording to derivtion of hnging MMF long irle of the rotor. Liner urrent lod of the rotor MMF is reeived y differentiting (1) t ngulr oordinte 4Fq J( ϕ ) = kwnkn os( npϕ ). (6) τ 1,3,5,7,... Aordingly from expression (4) the liner urrent lod of the shorted turn of k oil nd l pole is the following. J kl ( ϕ ) = nkl sin + pτ ( nϕ ) os( nϕ nkl ) (7) Formuls (6), (7) re reeived for the sttionry rotor. At rottion of the rotor, MMF nd liner urrent lod wves of the rotor winding rotte together with the rotor t the sme ngulr speed ω = ω / p, where ω - irulr frequeny. For liner urrent lod of symmetri winding it is reeived: 4Fq J( ϕ,t ) = τ k wn 1,3,5,7,... k n os n ( ω t pϕ ) (8) Corresponding expression for rotting wves of liner urrent lod of the shorted turn is: J kl( ϕ,t ) = nkl os[ n( ω t ϕ ) ψ nkl ] (9) pτ 1,2,3,4,... where с nkl, y nkl re determined y ftors nkt, nkt. 4 Mgneti Field nd EMF in the Sensor Liner urrent lod of the rotor s oundry ondition unequivolly determines mgneti field t no lod ondition of the turogenertor. At indefinitely high mgneti permeility of steel the tngentil omponent of mgneti field density is equl to urrent lod on the rotor surfe nd spires to zero on the surfe of the sttor. These oundry onditions orrespond to the solution of Lple's eqution for tngentil omponent of mgneti field intensity in ir-gp. In view of possile shorted turns in the rotor winding the solution looks like: H ϕ ( r, ϕ, t) nkl pτ = r n r n 1 r ( ) ( ) ( ) r1 r r1 r 2n 1 ( ) r 1 os n [ n( ω t ϕ) ψ ], (1) where r, r1, r - respetively, ore rdii of the rotor, sttor nd rdil oordinte of the sensor instlltion. Expression (1) is pproximte nd n e used for the qulittive nlysis of the mgneti field. From the omprison (1) nd (9), ording to properties of potentil funtions, it is seen, tht t no lod ondition the tngentil onstituent of the mgneti field density ner to the rotor surfe is onvenient to nlyze the distriution of liner urrent lod (fig.1, 2) with the purpose of nkl

4 Pro. of the 5th WSEAS/IASME Int. Conf. on Eletri Power Systems, High Voltges, Eletri Mhines, Tenerife, Spin, Deemer 16-18, 25 (pp ) determintion of shorted turns. Expression (1) n e speified y the serrtion of ores nd sturtion, however for reeption of quntittive results it is expedient to use numeril methods. It is neessry to note lso, tht t sturtion for determintion of the field it is neessry to set resulting liner urrent lod. Anlysis of MMF y using rdil omponent of the mgneti field intensity in the stedy-stte short iruit ondition is possile lso. The rdil omponent of the field is wekened t height of the gp muh less, thn tngentil one, therefore the sensor foused on rdil omponent of the field, n e pled t the greter distne from the rotor surfe. In the lod onditions the nlysis eomes omplited, s dditionl fields of the high urrent hrmonis, for exmple, from nonliner retifier lod of the genertor re imposed on fields of eletri nd mgneti symmetry of the rotor. Complex mplitude of EMF E, indued in the sttionry sensor y vrile mgneti field, is determined from (1) ording to Mxwell lw E = w dφ dt = jw ω µ S nh (11) p p p n where w p nd S p re the numer of turns nd setion of the mesuring sensor, H n nd Ф re hrmonis of the field intensity nd the mgneti flux whih is tking ple through the sensor. From (11) it is seen, tht the mplitude of hrmoni EMF in the sensor is proportionl to the time order of the field hrmonis, oiniding with sptil order of MMF nd the rotor urrent lod hrmonis. In this onnetion the wekened high hrmonis mplified in EMF of the sensor in omprison with hrmonis of low orders nd they re expedient for using in the dignosti purposes. It is shown on fig. 1, 2. Prtilly offered monitoring t power sttions mens periodi determintion of the form nd spetrum of the sensor signl nd omprison with the referene dt reeived t enh tests of the given mhine. At ourrene of even hrmonis for definition of the oil with shorted turns numeril lultion should e mde in view of rel distriution of urrents in turns of oils, s well s tking into ount the properties of onstrutive mterils, serrtion of ores, geometril sizes of turogenertors with the postproessor hrmoni nlysis of lultion results of the field in the lotion of the sensor. Fig. 3,, represents distriution of the mgneti flux density long the setion of the rotor t no lod ondition t nominl exittion oth t sene of short iruits, nd t shorted turns in vrious oils. Fig. 4, shows omprison of results of numeril modeling of tngentil omponent of the mgneti field in the ir-gp t sene of short iruits nd t short iruit of one of 18 turns in the oil. It is prtilly impossile to determine shorted turn ording to the form of urves in this se. At the sme time t frequeny spetr of EMF, mde t Fig. 4,d for the sme ses the ourrene of even hrmonis is visile t shorted turn. The spetrum of even hrmonis hs hnges depending on oil, whih hs shorted turn. But ording to the results of numeril experiments, position of the shorted turn t height of the oil hnges spetrl struture of the field nd EMF in the sensor little. At use of the spetrl monitoring of shorted turns with the help of the indution sensor it is neessry to exlude the inuries onneted to influene of possile mgneti symmetry of the rotor, the inury of mesuring devies nd numeril modeling, espeilly t the nlysis of the high hrmoni of the spetrum. At shorted turns in slot prt of oils there is n dditionl xil omponent of the field tht demnds numeril lultion of the threedimensionl mgneti field. The tsk eomes omplited lso t simultneous shorted turns in different oils. 5 Influene of Air Gp Eentriity As n exmple of ourring in prtie devitions from idelized design module let us exmine the eentri rotor position reltive to the sttor ore (stti eentriity) nd reltive to xis of rottion (dynmi eentriity). The vlue of the eentri ir gp etween the rotor nd the sttor eing restrited with the first term of smll prmeter expnsion my e presented y the formul: [ 1 ε os( ω ϕ )] δ = δ ε t, (12) where: δ uniform ir gp, ε - reltive eentriity, ω ε - ngulr speed equl to zero t stti eentriity nd ngulr speed of the rotor rottion ω' t dynmi eentriity. The seond term of formul (12) defines dditionl rotting hrmoni omponents of the symmetri rotor winding mgneti field H ε tht sptil orders differ from MMF hrmonis orders y 1 nd frequenies oinide with MMF frequeny y ω' t dynmi eentriity. [( nω ± ω ) t ( np ± ) ϕ] Hε = (13) Hεmn os ε 1 1,3,5,...

,, Pro. of the 5th WSEAS/IASME Int. Conf. on Eletri Power Systems, High Voltges, Eletri Mhines, Tenerife, Spin, Deemer 16-18, 25 (pp468-473).15.1.5 Bt, T.5.1.15 3 6 9 12 15 18 21 24 27 3 33 36.

15 2 25 3 35 4 45 5 d Hrmoni numer - even hrmonis - odd hrmonis Figure 3. Mgneti flux density long the setion of the rotor Figure 4.

5 ,, Pro. of the 5th WSEAS/IASME Int. Conf. on Eletri Power Systems, High Voltges, Eletri Mhines, Tenerife, Spin, Deemer 16-18, 25 (pp ) Bt, T Degree.1.5 Bt, T Degree 6 Hrmoni mplitude, db Hrmoni Numer - even hrmonis - odd hrmonis 6 Hrmoni mplitude, db d Hrmoni numer - even hrmonis - odd hrmonis Figure 3. Mgneti flux density long the setion of the rotor Figure 4. Mgneti field nd EMF spetr t sene nd t one of 18 shorted turn In ordne with the formuls (12), (13) it follows, tht t dynmi eentriity the EMF even hrmonis similr to EMF hrmonis from turn-toturn short iruits in the rotor winding re indued in the mesuring sensor. Thus, the tion of the rotor ody dynmi mgneti eentriity on the sensor is similr to the tion of the rotor winding eletril symmetry. As result of similrity, the mgneti fores t turn-toturn short iruits in the rotor winding re ppered.

6 Pro. of the 5th WSEAS/IASME Int. Conf. on Eletri Power Systems, High Voltges, Eletri Mhines, Tenerife, Spin, Deemer 16-18, 25 (pp ) In unsturted mhines the mplitudes of dditionl field hrmonis from the ir gp dynmi eentriity re proportionl to the hlf of the eentriity reltive vlue [8]. Due to high ury of the high-speed turogenertor rotors lning, the error dded y the ir gp eentriity to the shorted turn dignostis is insignifint. 6 Results of Tests With the help of the given method in onditions of the power sttion the monitoring of winding short iruits in the rotor of the turogenertor with hydrogen ooling on whih there ws n ssumption of presene of shorted turns is rried out. The turogenertor is mintined more thn 4 yers without ny prolems onneted with shorted turns. Eh oil of multiple-turn rotor winding hs 18 turns. It is diffiult to drw onlusion on presene of shorted turns in the rotor winding using the form of EMF urve in the indution sensor of the given turogenertor (fig. 5). At the sme time the nlysis of spetrl struture of EMF in frequeny re speifies unequivolly the presene of shorted turns (Fig. 5). Now the further monitoring of the given turogenertor is mde to tre the evolution of proess nd to exlude serious filure. 7 Conlusion It is shown, tht in turogenertors with multipleturns oils of the rotor winding the trditionl EMF, V Hrmoni mplitude, db time, µ - even hrmonis - odd hrmonis Figure 5. Form of EMF urve () nd spetrl struture of EMF () in the indution sensor. monitoring of shorted turns ording to the shpe of EMF urves in the indution sensor is omplited. The offered dditionl monitoring inludes the spetrl nlysis of struture of indued EMF in the sensor nd omprison of spetr nd urves of EMF, tken t the ftory-mnufturer, with the results, reeived during opertion. Aording to the presene of even hrmonis in spetrum it is onluded out shorted turns in the winding of the rotor. For determintion of the oil with short iruit the test dt re ompred with results of numeril modeling. It is shown tht rotor winding shorted turns like s ir gp eentriity exite unidiretionl mgneti fores. At sene of therml imlne, ppreile inrese of urrent, losses nd heting in the field winding, the opertion of turogenertors with the shorted turn n e llowed t the ontinuous monitoring of the winding ondition. Referenes [1] Alright J.D., Alright D.R. Genertor Field Winding Shorted Turns: Oserved Conditions nd Cuses. EPRI Interntionl Conferene on Eletri Genertor Preditive Mintenne nd Refurishment, Jnury 2-21, 23 / Orlndo, Florid, USA, p.1. [2] Alright D.R. Interturn short-iruit detetion for turogenertor rotor windings, IEEE Trns., PAS-9, 1971, pp [3] Wood J.W. nd Hindmrh R.T. Rotor winding short detetion. IEE Proeedings, Vol. 133, Pt.B, No.3, My 1986, pp [4] Woshngg E. Turogenertor Field Winding Shorted Turn Detetion y AC Flux Mesurement. IEEE Trnstions on Energy Convertion, Vol.9, No.2, June 1994, pp [5] Yonggng L., Zho H., Heming L. The New Method on Potor Winding Interturn Short-iruit Fult Mesure of Turine Genertor. Proeedings of the IEMDC, June 1-4, 23/ Mdison, Wisonsin, USA, pp [6] Roytgrts M. Method of dignostis nd ontrol of turn-to-turn fults in the rotor of synhronous mhine. Russin Ptent, No with priority dted [7] Roytgrts M. Detetion of turn-to-turn fults in the rotor winding of running turogenertor on the sis of the rotor mgneti field spetrum nlysis. Eletrosil, No.43, 24, pp (In Russin). [8] Heller B., Hmt V. Hrmoni field effets in indution mhines, Prgue, 1977, p.351.

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