Design and Analysis of Brushless Self-Excited Three-Phase Synchronous Generator

Transcription

1 European Association for the Development of Renewale Energies, Environment an Power Quality (EA4EPQ) International Conference on Renewale Energies an Power Quality (ICREPQ 12) Santiago e Compostela (Spain), 28th to 30th March, 2012 Design an Analysis of Brushless Self-Excite Three-Phase Synchronous Generator M. O. Oliveira 1-2, A. S. Bretas 1, F. H. García 1, L. A. Walantus 2, H. E. Muñoz 2, O. E. Perrone 2 an J. H. Reversat 2 1 Electrical Engineering Department UFRGS, Feeral University of Rio Grane o Sul 103 Osvalo Aranha Avenue, Porto Alegre-RS (Brazil) Phone/Fax numer: , s: moliveira@ece.ufrgs.r, aretas@ece.ufrgs.r, fhez@ece.ufrgs.r 2 Energy Stuy Center to Development CEED UNaM, National University of Misiones 327 Juan Manuel e Rosas Street, Oerá-Mnes. (Argentina) Phone/Fax numer: , s: walantus@fio.unam.eu.ar, hmunioz@fio.unam.eu.ar, perrone@fio.unam.eu.ar, hreversat@fio.unam.eu.ar Astract. This paper presents the main features of esign, construction, performance analyses an experimental stuies of a rushless self-excite three-phase synchronous generator. The asic construction, principle of operation, an exciting characteristics are escrie. In propose machine the rotor wining is shorte through ioes an the self-excitation is reache y a slip etween rotor an stator wining uilt with ifferent pole numers. Computer simulations performe with MATLAB software were use to verify the mathematical moel of the rushless self-excite synchronous generator an the achieve results showe similar ehavior with experimental values. Key wors Brushless generator, synchronous machine, self-excite system, half-wave rectifie, excite capacitor. 1. Introuction Synchronous generators an the associate control system constitute one the most important an complex projects presente in the electric power system. These machines generate electricity which is then transporte to consumption center through transmission lines. This generators type is typically projecte with the slip rings whose terminals can e use with a steay source power for excitation. The caron rushes are the most critical parts of the power generator ecause they eman an important maintenance level, an therefore are preferre than the rushless generators to supply energy to critical loas. The rushless generators usually consist of a permanent magnet to eliminate rotating excitation wining, caron rushes an slip rings [1]. On the other han, the small capacity generator works in severe operation conitions, therefore, it is essential that its construction is simpler an roust. Also, self-excitation an self-regulation of voltage in a synchronous generator is important to reuce the maintenance level an the control system complexity in case of large loa variations. In this sense, many project an stuies of synchronous generators have een propose in the last years. Nonaka an Kesamaru [2] escrie a new rushless generator without exciter rotating where the stator has two inepenent winings with ifferent pole numers. The first is the single-phase loa wining an the secon is the exciter wining in irect current (DC). This generator has the avantage of presenting an almost constant output voltage for large loa variations an ifferent spees. On the same theme, Inoue et al. [3] proposes a novel rushless generator with three-phase armature winings on the stator, one fiel wining an one exciting wining with five time as many poles of the armature wining on the rotor, an a three-phase reactor connecte to the terminal of the armature wining. The isavantage is the nee of an external attery since the capacitors system is not fully efficient. More recently, Chan an Lai analyze the steay state ehavior of a single-phase self-excite generator where the operation equations are erive using the symmetrical component metho [4]. Hooke an Jeeves metho is use to etermine the machine variales. A more etaile iscussion of some excitation control systems for rushless synchronous generators is presente in [5]-[7], an ifferent methos of voltage regulation in generators are propose in [8]-[10]. In this context, the ojective of this paper is to present an iscuss the results otaine from the moeling, construction an testing of a synchronous generator with asynchronous excitation (SGEA) an with an uncontrolle rectifie rige. On the last, the terminal voltage of the machine is regulate through the fiel current. The test results otaine with the prototype test are acceptale an encouraging RE&PQJ, Vol.1, No.10, April 2012

2 2. Moelling of the Brushless Self-Excite Synchronous Generator Although the fiel wining of a traitional synchronous generator is in the rotor an the fiel wining of a homopolar generator is locate in the stator, oth machines share the same terminal characteristic an can e escrie with the same parameters groupe [1]. The classic moel of separately excite synchronous machines can e represente in rotational coorinate -q [10]. However, etaile moeling of synchronous generator for transient stuies shoul inclue a variale phase moel, orthogonal -q axis moel an finite element analysis [9], [2]. Fig. 1 shows the asic circuit of the rushless self-excite synchronous generator esigne in this work. The stator of this machine has two winings, the loa wining Wa an the auxiliary self-excitation wining Wc, connecte to a variale capacitor C. The rotor has only one fiel wining W f, shorte with ioes D. For a given output frequency f, a voltage is inuce on the W f y the reverse fiel ue to the armature reaction. This voltage is then rectifie in halfwave to otain the excitation voltage of the synchronous generator. Fig. 1. Electric circuit for rushless self-exciting three-phase synchronous generator. The asic steay-state equations for moeling the rushless self-excite synchronous generator were otaine from [11]. The following simplifying assumptions were consiere: Negligile wining mmf (magnetomotive force) an saliency harmonics. Rotor cage equivalent to sinusoially istriute wining on each axis. Constant rotor spee. Negligile rotor fiel wining resistance so that fiel flux is consiere constant. Other important aspects for moeling the rushless self-excite synchronous generator are presente in [12]- [15]. A. D-q Moel for Single Phase Synchronous Machine Steay state armature current is given y the following equation [11]: I = I cos( nω t + Φ ) (1) a an r an n= 1 an the exciter wining current has the form: I = I cos( nω t + Φ ) (2) n r n n= 1 where I an, Φ an, I an, an Φ n are unknow. The temporal an spatial variation of flux linkage etween the exciter wining an armature is efine y the following expression: ψ a cos( ωrt + δ ) sen( ωrt + δ ) = ψ sen( ωrt + δ ) cos( ωrt + δ ) where the M matrix is efine as: [ M ] r Z q mq I 0 0 t r xq 0 I ω q r r I 0 x + I Z m x m t 0 ω x ψ fr fr where the parameters x q, x, Z mq, Z m, x m y x fr were calculate as etaile in [11]. The armature wining flux linkage, Ψ a, from (3) is sustitute into the armature voltage expression: (3) (4) ψ a R Ia 0 t ω + = (5) where R is the loa resistance. The exciter wining flux linkage, Ψ, is sustitute into the exciter voltage expression: ψ + xls I + r I + ω X c I t = 0 t ω t (6) where X c is the capacitive reactance referre to the armature wining. Accoring to [11], the resulting equations consist of equations of funamental frequency an an infinite series of harmonics voltages. The equation of excitation current an armature are given y the following expressions: I e = K Ψ e δ γ (7) j Φ 1 j( + 1) 1 1 fr n j( nδ + φk ) jφn n k= 1 n π k = 3 n 1 I e = K I e (8) I e K I e = (9) jφa1 j( Φ 1+α1 ) a1 a RE&PQJ, Vol.1, No.10, April 2012

3 I e K π K I e = (10) jφan n j( Φ n + αn ) an an k = 3 k 1 where K n, γ n, K an an α n are functions of the machine parameters. The loa angle, δ, an fiel flux, Ψ fr, are unknowns. Thus, if the fiel wining resistance is neglecte then an assuming the fiel ioe commutates when time t=0, it is possile to otain: r Ψ fr xm r Z m I I fr = I 0 t= 0 + = x x + x x t fr m lfr m r 2 r m m fr t = ω t ω t (11) x I Z I e = = 0 (12) e x I Z I = 0 (13) t t t 2 r 3 r fr m m ω t 0 ω = where I fr is the fiel current an e fr is the fiel voltage applie across the fiel ioe. 3. Electric Circuit an Mechanical Structure of the Synchronous Generator Propose The propose SGEA was uilt using the mechanical structure of an asynchronous motor of 5HP, 1000 rpm. This generator is asynchronously self-excite, ie, the rectifie alternating voltage use in the generator excitation is otaine through a slip etween the rotor an stator winings. The stator consists of an auxiliary fiel wining in parallel with a capacitor ank an a stator wining loa. The voltage inuce in the rotor fiel wining is one-phase an half-wave rectifie. A. Operating Principle Fig. 2 shows the configuration an arrangement of the rotor an stator winings of the propose synchronous machine. When the generator starts to turn, it is inuce on the single-phase wining an electro-motive force (fem) ue to the resiual magnetism of the rotor. Thus, a current I C will start flowing in the circuit forme y the parallel etween single-phase wining an the capacitor C connecte to it. The I C current, known as magnetizing current of selfexcitation, is reinforce y the capacitor current once the generator reaches steay state. It is important to recall that a ifference exists etween the variales relate to magnetic fiels ue to the main fiel spee (4 poles) is 1500 rpm an auxiliary fiel spee (6 poles) is 1000 rpm. The generator rotation spee is kept stale at 1500 rpm so there is a ifference in favor of the excitation current I C. The single-phase wining of six poles takes the capacitor the reactive power for excitation an the capacitor size is etermine in this esign. The fiel excitation current of the stator (I C ) prouces a magnetic fiel B through the air gap an inuces on the rotor wining a new fem responsile for creating the main excitation fiel. B. Stator Circuit Characteristics The stator consists of the auxiliary an loa wining. Both winings have a sinusoial istriution of magnetic flux in the air gap prouce y the stator current of the machine. In this circuit, it is very important the orientation of the magnetic axis etween the two winings for the machine work correctly. In this sense, the angle etween the magnetic axis of the loa wining (power coil) an auxiliary wining (connecte to the capacitor ank) shoul e 90 electrical egrees. C. Rotor Circuit Characteristics The most appropriate is to make the rotor magnetic flux a uniform istriution along the air gap an a magnetic ensity B with constant geometric istriution. Thus, as the rotor rotates at a constant spee, the magnetic flux in the generator varies in pulses form with a frequency twice the stator frequency. This variation causes that the turns see (or e cut) y a relative flux that varies sinusoially in space an time. In practice, the rotor current is not exactly pulsing an sinusoial. This prolem must e compensate y the rotor magnetic circuit shape, for example, changing the polar heas or ecreasing the air gap. This situation occurs with the eformation of the excitation circuit current ue to the presence of harmonic components. The generator oes not work if there is not a certain resiual magnetism in the core. If there is no resiual magnetism, the excitation process is not initiate an therefore in this project the resiual magnetism is generate y placing small permanent magnets in the polar heas. D. Brushless Excitation System Fig. 2. Stator an rotor wining single-phase configuration. Excitation systems with DC or AC exciters an rectifiers must e transferre from these facilities to the fiel wining of the generator, emaning slips rings an rushes. This makes oth the esign of these evices as their maintenance more ifficult, increasing the egree of complexity with the increase of the nominal power of generators. The main prolems are relate to cooling of the collector rings an the useful life of the rushes. To solve this prolem successfully, an excitation system without slip rings was projecte RE&PQJ, Vol.1, No.10, April 2012

4 E. Rotor Wining Connection Fig. 3 shows the connection iagram of the rotor wining. Here, each coil consists of a half-wave rectification circuit, connecte (shorte) to a ioe rectifier to form alternating poles. These connections are mae from the exterior of the rotor wining set, allowing quick an easy implementation to carrying out the corresponing measurements of voltage (V) an current (A). This configuration gives the excitation current which is the main generator fiel. The asynchronous auxiliary wining is connecte in series to form 6 poles in single phase configuration. This wining has a resistance of 33.9 Ω. The polar heas of the inuctor wining on the rotor are place alternately an separate y 10 slots which house the partial sie coil, where each coil consists of 60 conuctors. On the other han, the raial length of the air gap etween rotor an stator is 2.5mm an the necessary inuction on the rotor machine is 1.15 Tesla. The amperes-turns for meter (A-turns/m) necessary to etermine the magneto-motive force (mmf) in each circuit of the generator were otaine through the magnetization curve (B-H) of the machine stuy, otaine through of the laoratory test an shows in the Fig. 5. Fig. 3. Connection iagram of the rotor wining. F. Prototype Construction Fig. 4 shows part of the construction process of the prototype rushless self-excite generator. Part (a) presents the aaption process of the slots on the ase machine which later enale the construction of the stator wining (). It can e oserve the mechanic asic esign of the rotary rectifier ioes in (c) an its mounting on the rotor in (). Part (e) shows the stator an rotor wining efore final assemly an part (f) presents the connection of rushless self-excitation synchronous generator on the tale test. Fig. 5. Magnetization curve (B-H) for generator stuy. 4. Test Results an Discussion The mathematical moel use in computer simulation was implemente in MatLa environment [16]. Fig. 6 illustrates the rotor wining current ehavior (excitation current of machine) efore passing through the half-wave rectifier circuit. The capacitor current I C variation, flowing through the auxiliary stator wining it is also shown in this figure. Fig. 6. Temporal variation of the rotor an capacitor currents without loa connecte on generator. Fig. 4. Prototype construction process. Fig. 7 shows the temporal variation of the magnetizing inuctance of the generator uring self-excitation. It is note that after 0.2 secon of simulation, this inuctance aruptly ecreases in relationship to its initial values 1662 RE&PQJ, Vol.1, No.10, April 2012

5 generating transient eformation on voltage wave of the generator as shown in Fig. 8. It is note that the voltage wave istortion occurs perioically each 1/2 cycle. Fig. 7. Temporal variation of magnetizing inuctance uring selfexcitation of the SGEA. Fig. 9. Armature voltage vs excite voltage for propose machine with excitation capacitance of 100µF an without loa. Along the self-excitation process, the terminal voltage of the generator changes from transient to steay state reaching a peak value of 320 Volts, as shown in Fig.10. In this figure, it can e oserve the eformation cause y the magnetizing inuctance variation. Fig. 8. Temporal variation of single-phase voltage of the generator uring self-excitation process with loa of 100Ω. A. Prototype Valiation: 4 Pole Machine with Varying of Excitation Voltage Tale I lists the numerical values for armature voltage as well as voltage an current exciter otaine through tests in laoratory. These values were otaine for 4 pole configuration with 1500 rpm. Tale I. Measurement armature volts for ifferent excite current. (4 poles, 1500rpm) Excitation Parameters R-phase S-phase T-phase Vexc [V] (I exc ) [A] [V] [V] [V] Fig. 10. Temporal variation of the terminal voltage generator. Fig. 11 shows the temporal variation of the loa current of the SGEA to loa of 250W.The waveform of this current is similar to the waveform terminal voltage generator, ie, has the same perioic istortions. Fig. 11. Temporal variation of the loa current to loa of 250W. Fig. 9 shows a plot of the output armature voltage of the machine vs excitation voltage having an excitation capacitance constant of 100µF RE&PQJ, Vol.1, No.10, April 2012

6 5. Conclusions This paper presents the esign, construction an asic analysis of a synchronous generator with rushless asynchronous excitation. The results presente are part of a research project evelopment in the Engineering Faculty of the National University of Misiones, Argentina. The rushless self-excite three-phase synchronous generator is of simple construction, showing roust an has a low maintenance level mainly ecause not having slip rushes in the excite system. The mathematical moel use in this stuy was aequate, since the simulation results were consistent with the experimental results, representing consistently the expecte ehavior of the generator. In this sense, it was verifie that the generator output voltage remaine constant for alance electrical loa variation. Due to the variation of magnetizing inuctance uring the self-excitation of the generator, funamental components an other harmonics are generate in the magnetic flux an the rotating fiel. This results in istorte voltages an currents in the terminal outputs of the generator. In relation to the mechanical construction, the results relate with torque an spee generator were not presente in this paper, however in test evelopment a significant viration was oserve in the structure uner loa variation conition. On the other han, steay overheating occurre in the pole pieces leaing to a consequent reuction in the life of the machine. The great ifficulty which ha to e overcome for the evelopment of the excitation system without rushes was the intensity of centrifugal efforts which rectifier an protection evices woul e suject. The use of higher frequencies for AC exciter increase the level of the excitation voltage ramatically improving the efficiency of the system. Generator, IEEE Transaction on Energy Conversion, vol. 11, no. 3, Septemer [6] R. Erceg, G. Erceg, S. Tesnjak, Digital excitation system for a small rushlees synchronous generator, Proceeing of the IECON, vol. 1, pp , [7] R. Erceg, G. Erceg, T. Izotic, Using igital signal processor for excitation system of rushless synchronous generator, Proceeing of the IECON, vol. 3, pp , [8] B. Raelo an W. Hofmann, Optimal Reactive Power Splitting with Douly Fe Inuction Generator for Win- Turine, Proceeing of DEWEK 2002, CD. Wilhelmshaven, Germany, Octoer, [9] Ion Bolea, The Electric Generators Hanook: Synchronous Generator, CRC Press, vol. 1, Boca Raton FL, [10] B. Raelo, W. Hofmann, M. Tilscher, A. Basteck, Voltage Regulator for Reactive Power Control on Synchronous Generators in Win Energy Power Plants, Proceeing of NORPIE, Tronheim, Norway, [11] R. M. Cuzner, S. C. Rao, Valiation Stuy of Mathematical Moel for Brushless, Capacitor-Excite Single-Phase Synchronous Generator, Proceeing of Inustry Application Conference IAS, vol. 2, pp , [12] T. Fukami, T. Kono, T. Miyamoto, Performance Analysis of a Self-Regulate, Self-Excite, Brushless Three-Phase Synchronous Generator, In Proceeing of IEMD, pp , [13] S. Nonaka, T. Kawaguchi, A New Variale-Spee ac Generator System Using a Brushless Self-Excite-Type Synchronous Machine, IEEE Transaction on Inustry Application, vol. 28, no. 2, pp , March/April [14] F. Shiata, T. Kohrin, A Brushless, Self-Excite Polyphase Synchronous Generator, IEEE Transaction on Power Apparatus an Systems, vol. PAS-02, no. 8, pp , August [15] F. Shiata, N. Naoe, Characteristics of Brushless an Exciterless, Self-Excite Synchronous Generator, IEEE Inustry Application Society Annual Meeting, vol. 1, pp , [16] The Mathworks Inc. Mathworks matla. [On line]. Availale in: Acknowlegment The authors thank Engineering Faculty of National University of Misiones for their cooperation in constructing an test of stuy machine. References [1] E. Akpinar, D. Buakçi, A moel of homopolar sunchronous generator feeing 3-phase rige rectifier, Department of Electrical an Electronics Engineering Turkey. Availale in: [2] S. Nonaka an K. Kesamaru, "Analysis of New Brushless Self-Excite Single-Phase Synchronous Generator y Finite Element Metho", IEEE Transaction on Inustry Application, vol, 30, no. 3, pp , May/June [3] K. Inoue, H. Yamashita, E. Nakamae, T. Fujikawa, "A Brushless Self-Exciting Three-Phase Synchronous Generator Utilizing the 5 th Space Harmonic Component of Magneto Motive Force through Armature Currents", IEEE Transactions on Energy Conversions, vol. 7, no. 3, pp , Septemer [4] T. F. Chan, L. L. Lai, A Novel Single-Phase Self- Regulate Self-Excite Inuction Generator Using a Three- Phase Machine, IEEE Transaction on Energy Conversions, vol. 16, no. 2, June [5] A. Gohwani, M. J. Basler, A Digital Excitation Control System For Use on Brushless Excite Synchronous 1664 RE&PQJ, Vol.1, No.10, April 2012