Six-phase induction machine operating as a standalone self-excited induction generator

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1 Six-phase induction achine operating as a standalone self-excited induction generator Kael Nounou, Khoudir Marouani, Mohaed Benbouzid, Bekheira Tabbache To cite this version: Kael Nounou, Khoudir Marouani, Mohaed Benbouzid, Bekheira Tabbache. Six-phase induction achine operating as a standalone self-excited induction generator. IEEE ICGE 4, Mar 4, Sfax, Tunisia. pp.58-63, 4. <hal-358> HA Id: hal Subitted on 3 Jul 4 HA is a ulti-disciplinary open access archive for the deposit and disseination of scientific research docuents, whether they are published or not. The docuents ay coe fro teaching and research institutions in France or abroad, or fro public or private research centers. archive ouverte pluridisciplinaire HA, est destinée au dépôt et à la diffusion de docuents scientifiques de niveau recherche, publiés ou non, éanant des établisseents d enseigneent et de recherche français ou étrangers, des laboratoires publics ou privés.

2 Six-phase induction achine operating as a standalone self-excited induction generator Kael NOUNOU Ecole Militaire Polytechnique CM-UER-ET Boj El-Bahri-646, Algiers, Algeria Khoudir MAROUANI Ecole Militaire Polytechnique CM-UER-ET Boj El-Bahri-646, Algiers, Algeria arouani_khoudir@yahoo.fr Mohaed BENBOUZID University of Brest, EA 435 BMS, Rue de Kergoat, CS 93837, 938 Brest Cedex 3, France Mohaed.Benbouzid@univ-brest.fr Bekheïra TABBACHE Ecole Militaire Polytechnique CM-UER-ET Boj El-Bahri-646, Algiers, Algeria laid_tabache@yahoo.co Abstract This paper deals with the use of ultiphase induction achines in renewable energy applications such as wind and hydropower. Thus, soe preliinary test results carried out on a six-phase induction achine operating as a stand-alone self-excited induction generator and supplying various loads under different conditions are presented. Firstly, the dynaic odel of the power generation syste is developed considering the agnetizing inductance saturation and excitation capacitors sizing to ensure the excitation task. Then, siulation and experiental results carried out on a 5.5 kw six-phase squirrel-cage induction generator are presented and discussed. Keywos Six-phase induction achine, self-excited induction generators, agnetic saturation, renewable energy. I. INTRODUCTION Induction achines are widely used in various areas and vast range of energy conversion applications ranging fro low power devices as doestic appliances to large industrial drives. Consequently, since the benefits of induction achines are well known and aply deonstrated, currently, they are faced to utations towas new applications as renewable energy generation. These trends are otivated by different advantages that ade these achines as potential candidates to eet applications presenting severe exploitation conditions as in wind and hydro energy conversion. The ain criteria are siple and rigid structure allowing a great robustness with low investent and aintenance costs. Nevertheless, the control principle is difficult and coplicated because the induction generator can only be fed through the stator side which constitutes the excitation and generation part at the sae tie. Thus, at present induction generators are particularly used in sall and isolated power plants based on wind turbine or hydroelectric generators [-3]. However, the power electronics developents in conjunction with control theories have encouraged researchers to conduct substantial works and to bring attention to the opportunity of eergent applications of induction generators. Aong the new challenges, ultiphase induction achines are considered as proising solution for renewable energy applications [4]. Accoingly, apart the fact that high phase oer drives possess soe advantageous features as copared to conventional three-phase drives, such as reducing the aplitude and increasing the frequency of torque pulsation, reducing the rotor haronic currents, power segentation, high reliability and increased power [5], the nuber of phases can be used as an additional degree of freedo to ake this kind of achines suitable for renewable energy applications. The ain objective of this paper is to present soe preliinary test results carried out on a six-phase induction achine operating as a stand-alone self-excited induction generator and supplying various loads under different conditions. Hence, this paper consists of three parts: the first one concerns the odeling of the energy generation syste taking into account the effect of the agnetic circuit saturation on the achine paraeters, and the second part presents the siulation results of the obtained odel and finally the thi part is dedicated to the presentation of soe experiental results conducted on a 5.5kW six-phase squirrel cage induction achine supplying different electrical loads. II. POWER GENERATION SYSTEM DESCRIPTION The energy conversion syste consists of a six-phase induction generator (SPIG) with stator windings separated into two identical three-phase winding sets, and delta-connected excitation capacitor banks linked to each set. The generator is driven by a prie over and is feeding static loads as shown in Fig.. This syste presentation is adopted in oer to get a general representation odel which can be later used to develop control strategies siilar to that of doubly-fed induction generator. So, one three-phase winding set can be used for excitation purpose as control winding, while the second one can be used as power generation winding. Prie over SPIG Excitation capacitor bank Fig. : Self-excited six-phase induction generator power generation syste. C C o a d o a d

3 III. MODEING OF THE SIX-PHASE INDUCTION GENERATOR The real odel of the induction generator is exactly siilar to that of an induction otor and the only difference lies in placing a inus sign before the current phase sybolizing the generator ode instead of a otor one [6]. Also, the odel of the six-phase generator (SPIG) can be easily pointed-out based on that of the six-phase otor, oreover, as the stator winding is separated into two identical three-phase winding sets, the usual Park transfor can be applied to each three-phase set separately, adopting the usual siplification assuptions [7-9]. Thus, the SPIG odel expressed in the synchronous reference frae is decoposed into two ain sub-odels for the stator side and one sub-odel for the rotor side as in a following. A. Stator voltage equations The stator voltage sub-odels are noted (sd -sq ) for the first stator winding set and (sd -sq ) for the second stator winding set, as follows: sd sd= RsIsd+ ωsφ sq sq sq= RsIsq+ +ωsφ sd sd sd= RsIsd+ ωsφ sq sq sq= RsIsq+ +ωsφ sd B. Rotor voltage equations The rotor sub-odel is noted (-rq) and written as follows: = = R I + ωφ = = R I rq + +ωφ r r rq rq r rq r C. Stator and rotor flux equations Also the transfored stator and rotor flux are presented accoing to the three sub-odels as follows: () () (3) In oer to siplify the flux equations, the rotor flux Φ r and current I variables oved to stator side are introduced instead of Φ r and I r variables. The paraeter is the agnetizing inductance which expresses the relation between the total agnetizing flux φ and current I, as follows: φ = (7) I The agnetizing current is given by: ( ) ( ) sd sd sq sq rq I = I I + I + I I + I (8) The agnetizing inductance is a non-linear function of the agnetizing current I and depends on the saturation level. The agnetizing inductance constitutes an iportant paraeter for the generator odel and it can be deterined using the achine real agnetizing curve. So, its analytical expression can be written as follows [6], []. c I + c c 6 = (9) c4 c3i + c5 c,c,c,c,c,and 6 Where: given in Appendix. D. Modeling of excitation capacitors c are constant pareeters The excitation capacitors terinal voltage equations can be also represented in the Park reference frae as follows []: dsq = Icq ωssq C d sd = Icd+ωssd C d sq = Icq ωssq C d sd = Icd +ωssd C r () Φ Φ Φ Φ Φ Φ sd= ls sd+ sd sd + sq= ls sq+ sq sq + rq sd = ls sd + sd sd + sq = ls sq + sq sq + rq = lr + sd sd + rq = lr + sq sq + rq (4) (5) (6) I,I,I,and I cq are the current coponents Where: cd cq cd flowing through the excitation capacitor C and C connected at the terinals of stator winding sets and, respectively. The coplete odel of the SPIG is established accoing to a particular schee considering the stator winding as two three-phase sets sharing the sae agnetic circuit. As entioned above, this syste presentation is adopted in oer to get a general representation odel which can be later used to develop control strategies siilar to that of a doubly-fed induction generator. So, one three-phase winding set can be used for excitation purpose as control winding, while the second one can be used as power generation winding. Given that this paper is only liited to the presentation of soe

4 preliinary test results carried out on a six-phase induction achine operating as a stand-alone self-excited induction generator and supplying various loads under different conditions, the SPIG odel is also tested under other control techniques and the results will be reported in a subsequent paper. So, the next sections expose the siulation results of the self-excited SPIG followed by the developed odel experiental validation. oltages () as,bs,cs I. SIMUATION RESUTS The siulation of the whole odel of the SPIG (Fig.) is perfored for different operating cases under various conditions. Hence, the SPIG voltage and current wave fors at the terinal of the stator windings are firstly presented at no-load and then for a resistive (R) load as well as for a resistive-inductive (R-) one. Also, the influence of echanical speed and the excitation capacitor variations on the voltage generation are shown. A. No-load siulation results The siulation results of Fig. 3 at no-load case show the initial state of the self-excitation startup of the SPIG realized with the generator driven at a constant echanical speed of rp and capacitors values of C = C = 4μF. It is observed fro Fig. 3-a that the terinal voltage has an exponential shape which starts fro an initial zero value until a steady-state having a axiu value of. The voltage exponential evolution is related to the agnetizing state of the generator which starts fro an initial value, corresponding to the residual rotor flux or to shunt excitation capacitance initial charge, until an equilibriu point where the generated and the capacitor terinal voltages becoe equal. Generally, this intersection point corresponds to the state of agnetic circuit saturation. Also, the Fig. 3-b curve shows that the current circulating between the capacitor and the stator windings presents the sae shape as for the stator voltage, but it is in advance of a phase angle of 9 equivalent to a purely capacitive load, as shown in Fig. 3-c) The agnetizing inductance described by (9) with its curve presented in Fig. 3-d leads to a non-linear relation function of the agnetizing current and depends on the generator circuit agnetic state. B. Siulation results with resistive load The siulation results shown in Fig. 4 displays the terinal voltage (Fig. 4-a) and the load current (Fig. 4-b) after a startup operation of the SPIG, followed by a connection to a purely resistive load of Ω at instant t = sec. So, the current steady-state is established instantaneously, whereas the axiu value the terinal voltage is decreased fro to 86. This is a noral operating case in absence of a voltage regulation loop. Thus as the load current is increased the voltage drop in the stator winding resistance increases and consequently the capacitor voltage decreases which eans that the excitation voltage decreases and causes terinal voltage decreasing as consequence Currents (A) (a) Stator terinal voltages. Ias,Ibs,Ics (b) Current circulating between capacitors and stator windings. (H) oltage (), Current(A) as/4 & Ias (c) Superposition of stator current and terinal voltage =f(i) I (A) (d) Magnetizing inductance curve. Fig. 3: Siulation results of the self-excitation startup of the SPIG at no-load with the generator driven at rp and C = C = 4μF.

5 5 as 5 as 5 5 oltage () 5-5 voltage () (a) Effect of a resistive load on the stator terinal voltages. Current (A) idl (b) Resistive load current. Fig. 4: Siulation results of the self-excited SPIG with a resistive load Ω applied at t = sec. Current (A) (a) Effect of a R- load on the stator terinal voltages idl (b) Resistive-inductive load current. Fig. 5 : Siulation results of the self-excited SPIG with a R- load ( Ω, H) applied at t = sec. C. Siulation results with a resistive-inductive (R-) load The siulation results with a R- load coposed of a resistance of Ω in series with inductance of H connected at t = sec, are presented in Fig. 5. It can be observed that the terinal voltage (Fig. 5-a) displays severe attenuation fro to 65, which affects the load current (Fig. 5-b). This can be interpreted by the fact that the inductive load consues high reactive power and the generator ay also collapse if the available reactive power is not sufficient. oltages () rad/s rad/s 3rad/s D. Influence of echanical speed variations In oer to show the influence of the echanical speed variations on stator terinal voltages of the sel-excited SPIG, tests are realized for three values of speeds, 46 and 4rp at no-load, as shown in Fig. 6. The excitation capacitors are fixed at C = C = 4μF. It is evident that the no-load terinal voltage increases with the echanical speed, but agnetic saturation will ipose an upper liit of the voltage at which the achine can operate. It is observed that the voltage frequency increases with speed, whereas the tie of self-excitation decreases. E. Influence of excitation capacitor variations Also the influence of excitation capacitors on generated voltage is also studied and tests are realized for different values of capacitors C and C of 4, 45 and 5μF. The drive speed is fixed at rp. Figure 7 shows that the terinal voltage increases slightly with the excitation capacitor, and the tie of self-excitation decreases Fig. 6: Influence of echanical speed variations on generated voltage. oltages () µF 45µF 5µF Fig. 7: Influence of excitation capacitor variations on generated voltage.

6 . EXPERIMENTA RESUTS Experients are carried-out on an experiental test bench to check the siulation results. The laboratory test bench consists of a dual star induction achine (DSIM: 5.5kW, 6 poles) configurable as syetrical or asyetrical six-phase induction achine operating as generator and coupled to a DC otor used as prie over as shown in Fig. 8 []. A deltaconnected capacitor bank of 45μF is linked across the terinals of the induction achine in each three-phase sets feeding electrical loads. Furtherore, the experiental tests on the SPIG are done for the sae operating cases and conditions as well as the siulation tests presented above. (b) Zoo on the stator terinal voltages for one stator winding set (/div). Fig. 8: Photography of the experiental test bench. A. No-load experiental results The experiental results depicted by Fig. 9 at no-load case show the initial state of the self-excitation startup of the SPIG realized with the generator driven at a constant echanical speed of rp and capacitors values of C = C = 45μF. It is observed fro Fig. 9-a that the terinal voltage exhibits exactly the sae wave fors as for the siulation case with an exponential shape which starts fro an initial zero value until a steady-state having a axiu value of. Also, the stator terinal voltages are well balanced and regular, as shown in Fig. 9-b for one stator winding set. It is observed that the generated voltage frequency depends on the echanical speed. As well, the Fig. 9-c curve shows that the current circulating between the capacitor and the stator windings presents the sae shape as for the stator voltage, but it is in advance of a phase angle of 9 equivalent to a purely capacitive load. (c) Superposition of stator current (blue curve: 3A/div) and terinal voltage (orange curve: /div). Fig. 9: Experiental results of the self-excited SPIG at no-load with the generator driven at rp and C = C = 45μF. (a) Effect of a resistive load on the stator terinal voltages (/div). (b) Superposition of the resistive load current (blue curve: A/div) and terinal voltage (orange curve: /div). (a) Stator terinal voltages (/div). Fig. : Experiental results of the self-excited SPIG with a resistive load applied at t = sec.

7 B. Experiental results with resistive load The experiental results shown in Fig. displays the terinal voltage (Fig. -a) and confir the siulation results for that case and shows that the terinal voltage decreases to approxiately 9 after a connection of a resistive load, and causes the decrease of the excitation capacitor current (Fig. -b). Also, the resistive load current superposed with the terinal voltage are exactly in phase. C. Siulation results with a resistive-inductive (R-) load The experiental results, with a R- load connected at t = sec are presented in Fig.. They also confir the siulation results. It can be observed that the terinal voltage (Fig. -a) displays also severe attenuation fro to 7, and affects the load current (Fig. -b) which is in lagging phase equivalent to the R- load. I. CONCUSION In this paper a series of siulation and experiental tests were carried-out on a six-phase induction achine operating as a stand-alone self-excited induction generator and supplying various loads under different conditions. So, the non-linear atheatical odel of the generator has been presented considering the agnetizing inductance saturation. Then, excitation capacitors were sized to ensure the excitation task by supplying the required reactive power. The perforances of the generator were strongly influenced by the excitation capacitors, drive echanical speed and the type of load connected to its terinals. As the self-excited induction achine is an open-loop operating ode and the generated voltage aplitude and frequency depend on the rotating speed and the size of the excitation capacitors, therefore this operating ode is suitable for loads which are not sensible to voltage and frequency variations. Otherwise an adaptation stage is necessary between the generator output and the load in oer to adapt the aplitude and frequency. Since, the SPIG has high nuber of phases, control strategies siilar to that of doubly-fed induction generator can be developed and applied in oer to adjust the generator output voltage and frequency. So, one stator winding set can be used for excitation purpose as control winding, while the second one can be used as power generation winding and this ake this kind of achines suitable for renewable energy applications. REFERENCES (a) Effect of a R- load on the stator terinal voltages (/div). (b) Superposition of the R- load current (blue curve: A/div) and terinal voltage (orange curve: /div). Fig. : Experiental results of the self-excited SPIG with a R- load applied at t = sec. APPENDIX The constant coefficients of the agnetizing characteristics of SEIG are as follows: c =.53, c =.98, c 3 =.68, c =.48, c 5 = 8.67, c 6 = [] D. B. Watson, J. Alaga, and T. Dense, Controllable d.c. power supply fro wind-driven self-excited induction achines, Proc. IEE, vol. 6, no., pp , 979. [] J. B. Patton and D. Curtice, Analysis of utility protection probles associated with sall wind turbine interconnections, IEEE Trans. Power Apparatus and Systes, vol., no., pp ,98. [3] G. Raini and O.P. Malik, Wind energy conversion using a self-excited induction generator, IEEE Trans. Power Apparatus and Systes, vol., no., pp , 983. [4] F. Bu, Y. Hu, W. Huang, S. Zhuang and K. Shi, Control Strategy and Dynaic Perforance of Dual Stator-Winding Induction Generator ariable Frequency AC Generating Syste With Inductive and Capacitive oads, IEEE Trans. Power Electron., vol. 9, no. 4, pp , Apr. 4. [5] E.A. Klingshirn, High phase oer induction otors-part I: Description and Theoretical consideration, IEEE Transactions on power Apparatus and Systes, ol., pp.47-53, 983. [6] Singh GK, Yadav KB and Saini R.P. Analysis of a saturated ultiphase (six-phase) Self-excited induction generator, International Journal of Eerging Electric Power Systes, vol 7, no 4, 6. [7] T. A. ipo, A dq odel for six phase induction achines Proc. ICEM 8, pp , Sep.98. [8] P. as, Sensorless vector and direct torque control, Ed. Oxfo University Press, 998. [9] K. Marouani, Contribution à la coande d un entraîneent électrique à base de oteur asynchrone double étoile, Phd. thesis (in french), Ecole Militaire Polytechnique, Algiers, Algeria, Jun.. [] A. Nesba, Caractérisation du phénoène de la saturation agnétique de la achine asynchrone, Phd. thesis (in french), Ecole Nationale Polytechnique, Algiers, Algeria, Jan. 7. [] K. Natarajan,, A. M. Sharaf,, S. Sivakuar and S. Naganathan, Modeling and control design for wind energy power conversion schee using self-excited induction generator, IEEE Trans. Energ. Conv.,, 987,, vol. EC-, no. 3, pp. 56-5, Sap. 98. [] K. Marouani, H. Guendouz, B. Tabbache, F. Khoucha and A. Kheloui, Experiental investigation of an eulator "Haware In the oop" for electric naval propulsion syste, st IEEE-Mediterranean Conference on Control and Autoation (MED), Chania, Greece, 3.