New Control Strategy of Three-Phase Five-Level NPC Rectifier - Inverter System for Induction Machine Drive

Transcription

1 Availale online at Energy Procedia 18 (2012 ) New Control Strategy of Three-Phase Five-Level NPC Rectifier - Inverter System for Induction Machine Drive Raia GUEDOUANI a,*,bachir FIALA a,e.m BERKOUK,M.S. BOUCHERIT a Laoratoire des Systèmes Electriques et Industriel, University of Sciences an Technology Houari Boumediene B.O. Box 32 El Alia Ba-Ezzouar, Algiers, Algeria. Laoratoire de commande des Processus Ecole Nationale Polytechnique BP182, 10 avenue Hassen adi. El harrach, Algiers, Algeria. Astract This paper proposes a control strategy of a three phase five-level doule converter for induction motor drives. The converter consists of the five-level NPC rectifier, DC link, and the five-level NPC inverter. In this control strategy, the DC link voltages are controlled y using a closed loop with an optimized stailization system called clamping ridge. It provides a fast and flexile control of the converter capacitor voltage. This method will redress the imalance of DC link voltage. This control strategy is completely independent from the load control, leading to a simpler implementation. The three-phase five-level NPC rectifier-inverter system is an ideal interface etween a utility and renewale energy sources such as photovoltaic or wind generator. Keywords: Five-level NPC converter, PWM strategy, DC link voltage, regulation voltage, unity power factor, stailization system, induction machine, renewale energy. 1. Introduction In recent years, multilevel power converters for high power applications have een actively investigated [1-3]. In particular, three-level drive systems have een put to practical uses [1-2-3]. * Corresponding author. Tel.: address: guedouaniraea@yahoo.fr (R.GUEDOUANI) Pulished y Elsevier Ltd. Selection and/or peer review under responsiility of The TerraGreen Society. Open access under CC BY-NC-ND license. doi: /j.egypro

2 Raia Guedouani et al. / Energy Procedia 18 ( 2012 ) The general function of the multilevel inverter is to synthesize a desired AC voltage from several levels of DC voltages. For this reason, multilevel inverters are ideal for connecting either in series or in parallel an AC grid with renewale energy sources such as photovoltaic or fuel cells or wind generator. Additional applications of multilevel converters include such uses as medium voltage adjustale speed motor drives, static var compensation, dynamic voltage restoration [3].. Several topologies under serious consideration from industry are developed [3], however the Neutral Point Clamped (NPC) structure, proposed y Naea and al [2], remains the popular circuit due to its similarity to a conventional two-level voltage source inverter, and its ease of control [1-2-3]. The five-level inverter appears to offer interesting options for higher power drives without need for simultaneous device switching [2-3-4]. The output voltage waveform of the five-level NPC inverter is composed of intermediary voltage levels, which are typically otained from capacitor voltage sources. However, the major prolems with this configuration are in achieving four alanced voltage within the DC voltage source. Different solutions have een proposed to overcome this prolem [ ]. A first approach [3] is to provide a single DC link voltage (from a three phase rectifier), and sudivide it into four equal voltage levels y using a split ank of capacitors [7]. The prolem with the split capacitor arrangement is that during normal operation a net mean current is drawn from nodes 2 and 4 of the DC input voltage source y the load, and this will cause the link capacitors to charge or discharge, causing an imalance in the DC input voltage source levels. During transient state, or if the PWM scheme and device switching characteristics are slightly unalanced etween the output phases, a net mean current may e drawn from the neutral point, node 3, again leading to a capacitor voltage imalance[3-5-6]. The easiest approach is to simply supply the DC link voltage with three-phase five-level NPC rectifier. We have proposed in [8] three-phase PWM five-level NPC rectifier as input stage of the three-phase fivelevel NPC voltage source inverter. We have shown in [8] that this first solution is not enough to maintain the equal voltage division mainly when a large load torque is applied. In this paper, we propose a feed-ack control, in dq rotating frame, of DC link voltage for five-level NPC rectifier-inverter system. The proposed control strategy uses closed loop with an optimized stailization system called clamping ridge. It permits to have the capacitor voltage alancing with network unity power factor. In the first section, we elaorate the knowledge and control models of threephase five-level NPC rectifier-inverter system. The second section presents the triangular-sinusoidal control strategy using four carriers. In the third section, we propose a control strategy of a three phase five-level doule converter for induction motor drives y using a closed loop with an optimized stailization system called clamping ridge. The last section shows some important results of the proposed control strategy. As an application, we study the speed performances drive of the three-phase high power induction machine fed y this system. 2. Analysis of Five-level NPC Rectifier/Inverter System 2.1. Topology Fig.1 shows the main circuit configuration of the three-phase five-level doule converter. It consists of the five-level rectifier, the DC link, and the five-level inverter feeding the induction motor. The rectifier and inverter employ the neutral point clamping (NPC) structure. Each converter composed of three arms [6]. Each leg of this structure has six pairs of switching s is devices in series and two in parallel. The diodes let to have zero voltage for V im (V im is the voltage of the phase relatively to the middle point M).

3 1384 Raia Guedouani et al. / Energy Procedia 18 ( 2012 ) The DC link consists of four capacitors in series. For a DC link voltage of Vdc, if the voltage across each capacitor is Vdc/4, the voltage stress across each switching device will e limited to Vdc/4. Thus this topology would e suited to high voltage and high power applications. i d2 p i d2 s a3 s a7 Uc1 1 V S1 ia s a2 Uc2 i d1 2 i d1 n V S2 V S3 i s a1 a c M i d0 i d0 A B C ic s a4 s a5 s a8 Uc3 i d3 i d3 s a6 Uc4 3 Phase a Phase Phase c 4 Phase A Phase B Phase C Rectifier i d4 N i d4 Inverter DC Link ia IM Fig 1. Schematic of a three-phase five-level rectifier/inverter system The topology analysis of such structure shows that seven different configurations E n (n is the numer of the configuration) possile [6] and are illustrated in Tale 1. Tale.1 Different configurations of Five-Level NPC converter Configurations Electrical quantity E 0 = {ø} I i = 0 A E 1 ={s i1 s i2 s i3 } V im = U C1 +U C2 E 2 ={s i1 s i2 s i7 } V im = U C1 E 3 ={s i1 D i1 } V im = 0 E 4 ={s i4 s i5 s i8 } V im = -U C3 E 5 ={s i4 s i5 s i6 } V im = -U C3 - U C4 E 6 ={s i4 D i0 } V im = 0 Five complementary laws are possile for five-level NPC converter. The optimal one is [6-7-8]:

4 Raia Guedouani et al. / Energy Procedia 18 ( 2012 ) S i 4 Si2 S (1) i5 Si1 S i6 Si3 S is : control signal of the semi-conductor s is ; i = (a,,c) for the rectifier and =( A,B & C) numer the phase of the inverter; s: numer of switching. U ck : capacitor voltage of the DC link (k=1, 2, 3 & 4) Knowledge model In controllale mode, we define for each semi-conductor s is of the converter the connection function F is as follows [6-8]: 1 if sis is turned on F i S 0 if sis is turned off By using the proposed complementary law, the output voltage of the rectifier/inverter V km relatively to the middle point M, is given y the following system [8-11]: V F F F U F F F U U F F F U F F F U U (3) im i1 i2 i3 C1 i1 i2 i3 C1 C 2 i4 i5 i6 C 3 i4 i5 i6 C 3 C 4 The system (3) shows that the five-level NPC converter is equivalent to four two-level or two threelevel NPC converter in series. The DC input currents (i d1, i d2, i d3, i d4, i d0 ) of the three-phase five-level NPC inverter expressions, using the load currents (i A, i B, i C ), are given as: id1 F17 ia F27 F37 id 2 F11 ia F21 F31 id F18 ia F28 F38 id 4 F10 ia F20 F30 i d 0 ia id1 id 2 id3 id 4 3 (4) The rectified currents (i d1, i d2, i d3, i d4, i d0) of the three-phase five-level NPC rectifier expressions, using the input network currents (i a, i, i c ), are given as follow: i d1 F 17 ia F 27 i F 37 ic i d 2 F11 ia F21 i F31 ic i d3 F 18 ia F 28 i F 38 ic i d 4 F10 ia F20 i F30 ic i d 0 ia i ic i d1 i d 2 i d3 i d 4 Where F is is switching control of the switches rectifier. (2) (5)

5 1386 Raia Guedouani et al. / Energy Procedia 18 ( 2012 ) The systems (3), (4) and (5) are used to estalish the model of the five-level NCP rectifier/inverter system in matla-simulink environment. 3. Multilevel Control Strategy Different triangulo-sinusoidal strategies are developed for multilevel converter [1-7-8]. These strategies are extended from two-level carrier-ased PWM techniques to multilevel inverters y making the use of several triangular carriers and one reference signal y phase. For N-level inverter, (N-1) carriers with the same frequency f c and same peak to peak amplitude A c are shifted y Tc N 1 with T c is the period of the carrier. The reference wave form has peak to peak amplitude A m and frequency f m, it is centred in the middle of the carriers signal. This reference is continuously compared with each carrier. In this paper, we develop triangulo-sinusoidal strategy using four carriers. This strategy uses the property that the three-phase five-level NPC rectifier/inverter is equivalent to four two level converters in series[1-7-8]. This strategy is characterised y the amplitude modulation index m a and the frequency ratio m f, which are defined as: Am f m a, m c A f (6) c fm The algorithm of this strategy can e summarized as follow: Step 1: Compute of the intermediate voltages U p1 then ViM1 if V U then V U C refi p1 im1 0 U p2 then ViM 2 2 U C U p2 then ViM 2 U C (7) if V U then V refi p3 im 3 U p3 then ViM 3 0 U C U p4 then ViM 4 U C U p4 then ViM 4 2U C Step 2: Compute of the out put voltage V refi V 4. Control Process V 4.1. DC voltage control V V im1 im 2 im 3 im 4 The control flow of five-level rectifier/inverter system is as follows. Fig. 2 shows the control circuit structure of DC voltage of five-level converter. First, the DC link voltage V dc, which is the potential difference etween the P and N level, is detected. The error etween V dc and its command value V * dc passes the PI controller, after which this value is multiplied to the gain K (K=V dc /V ds ). This later is determined y using the instantaneous power conservation principle, with neglecting the rectifier losses, in dq frame. This value then ecome the direct input line current command I * d.

6 Raia Guedouani et al. / Energy Procedia 18 ( 2012 ) Input current control The errors etween the line current references (i * d, i * q), in rotating dq frame, and detected line currents (i d, i q ) are inputted into PI controllers. Then, we otain the commands (V * a, V *, V * c) which are used as references signals of PWM control strategy. Thus, The PWM control strategy determines switching levels to control switches in the main circuit of the five-level rectifier. In this paper, we use the triangulosinusoidal strategies with four ipolar carriers. The network power factor is controlled y imposing the quadrature line current i * q=0a. n V s1 V s2 V s3 R L R L R L i sa i s i sc Three phase five level NPC rectifier i d i c N i d V dc Three phase five level NPC inverter IM i sa i s i sc P V dc V a V V c V * dc i * q=0 V * q V * d i d i * c i q i d i * d K i * d Fig. 2. Control circuit of five-level NPC rectifier 4.3 DC link voltage stailisation In order to remedy the prolem of fluctuation due to the inverter input DC voltages drift, an optimized DC link voltage stailization system, shown in Fig. 3, is proposed. We suggest a solution which consists in estalish a alancing ridge etween the rectifier and the intermediate filter. Capacitor voltage equalization control should e implemented to restrict the charge-discharge current to the allowale cell limitations in the capacitor string. Fig. 3 shows another variant of the alancing circuits called mixed optimized clamping ridge. This circuit is capale of transferring the electric load from the capacitor which has high voltage towards neighour capacitor that has low voltage. This circuit is placed in parallel with every condenser of the DC link. It consists of a idirectional semiconductor T k and an inductance L ck with a resistor in series R ck, which serves to stailize the DC voltages. This circuit was suggested in a concern reducing the losses y effect Joule even to cancel them. If the voltage U ck gets higher then an imposed reference U * ck, the switch T k is opened to slow down the charging of capacitor C k. The model of the intermediate filter with the stailization ridge is defined as follow: C1 Uc 1 ic1dt ( i d2 id2 il1s1 ) dt C 2 Uc 2 ic 2 dt ( id 1 i d2 id2 id1 il2s2) dt (8) C3 Uc 3 ic3dt ( id 4 i d4 id4 id3 il3s3) dt C4 Uc 4 ic4dt ( i d4 id4 il4s4) dt

7 1388 Raia Guedouani et al. / Energy Procedia 18 ( 2012 ) i d2 U c1 i d1 The rectifier side U c2 U c3 i d0 i d3 The inverter side U c4 i d4 Fig. 3. Optimized DC link voltage stailization system With: ilk 1 Lk. ( SkUck RckiLk ) dt (9) The switches are controlled as follow: * Uck U c xk with k=1, 2, 3 & 4 (10) if x 0 then S 1 i 0 else S 0 i 0 (11) 5. Simulation Results k x k L k x k In the following simulations, the machine works without load, then at the moment t =17s a load torque, equal to 80 % of its nominal torque ( n =130 N.m), is applied. For these essays, we use the triangulosinusoidal with four carriers as command strategy for oth multi level converters. We show perfectly the instaility of the DC input voltages (Fig.4.a) without using voltage stailization system. This instaility increases when a load torque of the induction machine is applied. However, these voltages are practically equal y pair (Uc1 = Uc4, Uc2 = Uc3)(Fig.4.a), then their differences are not very large (Fig.4.a). This property permits to eliminate the homopolar component of the output voltage of the five-level NPC VSI and let it symmetrical (Fig. 4.). The cascade two three-level PWM five-level NPC rectifier/inverter loads an induction motor. Using vector control (with flux constant), the speed control performances are shown in Fig. 5. The speed and the electromagnetic torque follow their references. However the electromagnetic torque has very important undulations (Fig.5.). The results show the importance of the staility of DC input voltages of the rectifier-inverter system to have good performances for the induction machine speed control. L k DC Voltage Ucx(V) Output voltage of five-level VA(V) Fig. 4. a) DC Link voltage without stailization system, ) Output voltage of five-level VSI without use voltage stailization system

8 Raia Guedouani et al. / Energy Procedia 18 ( 2012 ) (rp/mn) * (rp/mn) Fig. 5. a) Representation of the induction machine speed, ) Representation of the induction machine Torque (N.m) * (N.m) In order to demonstrate the feasiility of the proposed control method, the closed loop with the voltage stailization system has een tested y simulations. The Fig. 6, 7, and 8 show the performances of the feedack control of the three-phase five-level NPC rectifier/inverter with voltage stailization system. In the following simulations, the machine works without load, then at the moment t =15s a load torque, equal to 80 % of its nominal torque ( n =130 N.m), is applied. We note that, the controlled voltage V dc (Fig. 6.a.) follow perfectly its reference (V * dc=20000 V). Therefore, the different input DC voltages of the five-level NPC inverter ecome constant as it is shown in Fig.6., and practically equal. Their differences are very small (Fig.7). In consequence, the output voltage of the five-level NPC VSI is symmetrical and stale as depicted in Fig. 8. The Fig. 9 show direct i d and quadrature i q line input current and their references (i * d, i * q ). We note that the direct current id follow its reference around 2000 A (Fig 9.a). Although, the quadrature line current iq oscillates around zero (Fig 9.). Thus, the phase voltage and its corresponding line network current are shown in Fig.10.a. The two waves are practically in phase. Finally, the speed control of the induction machine, fed y this system, is represented on the Fig.10-. We remark that the undulations on the performances (torque, currents i d & i q ) of the machine disappear and those performances are improved y using the mixed optimized clamping ridge. DC Voltage Vdc& V * dc(v) DC Voltage UcK(V) Induction Machine Speed (rp/mn) Induction Machine Torque (N.m) Fig. 6. a) Controlled DC voltage with closed loop, ) DC Link voltages with the stailization ridge 6. Conclusion In this paper, a control theory for DC link voltage alancing of three-phase five-level rectifier-inverter system has een presented. Simulations results were shown to verify the analysis and to demonstrate the following advantages of the proposed control:

9 1390 Raia Guedouani et al. / Energy Procedia 18 ( 2012 ) While it can generate a five level staircase waveform without the use of transformers, the five-level converter generates almost sinusoidal voltage and current waveforms even at fundamental switching frequency. - The voltages on the DC link capacitor are well alanced with very small ripple. - The system has low harmonic in the line network current and also this current is in phase with the corresponding phase voltage. - In this system, the switching stress is low. - The use of the voltage stailization system permits a economic and simple electronic implementation, whereas in the space vector modulation control the computational urden, the complexity of the algorithms and the numer of instructions are drastic. - They are an ideal interface etween a utility and renewale energy sources such as photovoltaic or wind energies. The DC voltage error Fig. 7. a) Error U c1 -U c2 & U c3 -U c4, ) Error U c1 -U c4 & U c2 -U c3 The DC voltage error (V) Output voltage of five-level NPC Line current of the induction machine Fig. 8. a) Output voltage of five-level VSI voltage stailization system, ) Line current of the induction machine The direct input line id & i*d(a) The quadrature input line iq& i*q(a) Fig. 9. a) Direct line input current i d and its reference i * d, ) Quadrature line input current i q and its reference i * q

10 , Input line current(a) & the phase voltage (V) Raia Guedouani et al. / Energy Procedia 18 ( 2012 ) V s1 (V) i sa (A) Torque of the induction machine gy ( ) Fig. 10. a) Line input current i sa and the corresponding phase voltage V sa, ) Representation of the induction machine torque (for r=100kn.m at 15s) (N.m) * (N.m) References [1] N.Kimura, T. Morizane, K.Taniguchi, T.Oono, Multi-modulation signal PWM control for multi-level converter, The 11 th International Power Electronics and Motion control conference, EPE-PEMC 2004, Riga, Latvia, 2-4 Septemer [2] Z. Pan, F. Zheng Peng and all, Voltage Balancing Control of Diode-Clamped Multilevel Rectifier/Inverter Systems, IEEE transactions On Industry Applications, Vol -41. N 6, Novemer/Decemer 2005, pp [3]L. M Tolert, F.. Zheng Peng, Multilevel converters as utility Interface for Renewale Energy Systems, Power Engineering Society Summer Meeting, augaust IEEE, Vol. 2, pp [4] T. Ishida, K. Matususe, Fundamental Characteristic of Five-Level Doule Converters With Adjustale DC voltages for Induction Motor Drives, IEEE Transactions On Industrial Electronics, Vol.49, N 4, August 2002, pp: [5] O. Bouhali, B. Francois, E.M. Berkouk, C. Saudemont, DC Link Capacitor Voltage Balancing in a Three-Phase Diode Clamped Inverter Controlled y a Direct Space Vector of Line-to-Line Voltages, IEEE Transactions on Power Electronics, Vol. 22, Issue 5, Sept [6] R.Guedouani, B. Fiala, E. Berkouk, M. Boucherit, A New Control Algorithm for Four Three-Phase AC/DC PWM Voltage Source Rectifiers-Five-Level Neutral PoinT Clamped Inverter System., Journal the Mediterranean Journal of Measurement and Control, Vol.4, N 4,ISSN ,Octore 2008, pp [7] R.Guedouani, B.Fiala, E.M. Berkouk, M.S. Boucherit Modeling and Control of MultileveL Three - Phase Current Source nverter International Aegean Conference on Electrical Machines and Power Electronics, ACEMP 11 & Electromotion 11 joint conference, Istanul Turkey, Septemer 2011, pp [8]R.Guedouani, B.Fiala, E.M. Berkouk, M.S. Boucherit, " Feedack Control of Two PWM VS Rectifiers Five-level NPC Voltage Source inverter. Application of Induction Machine drive." special issue N 2 Novemer 2010, Journal of Electrical Systems ISSN , pp Appendix A. The input and output filters parameters are given in the following tale. Input filters parameters Output filters parameters Clamping ridge parameters L = 1 mh L c =80 mh C 1 = C 2 = C 3 = C 4 =80 mf R = 0.25 R c =1