IMPACT OF SYNCHRONIZATION METHODS ON THE PERFORMANCE OF SELF-EXCITED INDUCTION GENERATORS

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1 IMPACT OF SYNCHRONIZATION METHODS ON THE PERFORMANCE OF SELF-EXCITED INDUCTION GENERATORS C. Rech, R. F. de Camargo, M. de Campos, F. Saladori, G. V. Leandro, J. C. O. Bolacell Uniersidade Regional do Noroeste do Estado do Rio Grande do Sul - UNIJUÍ CEP: Ijuí, RS Brazil cassiano@ieee.org, robinson.camargo@unijui.edu.br Abstract This paper analyzes digital sensorless control systems used to regulate the output oltages o a sel-excited squirrel-cage induction generator. Dierent mechanical sensorless synchronization methods are ealuated and their perormances are compared under seeral load conditions, including unbalanced and nonlinear loads. Simulation and experimental results are included to demonstrate the impact o distinct digital control systems on the output oltages distortion and the unbalance actor. Keywords Induction generator, synchronization method, oltage control. I. INTRODUCTION It is well-known that an induction machine can generate power when the rotor speed is greater than the speed o the stator rotating magnetic ield, and when it is connected to an external reactie power source. In stand-alone systems, the reactie power to create the stator magnetic ield can be supplied by a capacitor bank []. A squirrel-cage induction machine is attractie or small and medium power generation systems due to its low cost, robustness, sel-protection capacity and high power density (W/kg). On the other hand, the amplitude and the requency o the output oltages o a sel-excited induction generator (SEIG) depend on the load. Seeral solutions hae been presented to regulate the output oltages o induction generators in stand-alone systems. Among these alternaties, it is usual to employ a pulsewidth modulated oltage-source inerter (PWM-VSI) connected in parallel with the induction generator and the ac loads []-[7]. The PWM-VSI operates as a static reactie power compensator, which absorbs or injects reactie power to the system according to the load to regulate the ac bus oltages. Distinct control techniques hae been deeloped to control the shunt connected PWM-VSI and, consequently, to regulate the output oltages under dierent load conditions [3]-[7]. Digital control systems without any mechanical sensor are particularly interesting, because the cost o a mechanical sensor is signiicant when compared to the cost o a squirrel-cage induction machine or small and medium power generation systems [7]. In addition, control systems are usually implemented in dq reerence rame or threease electrical machines applications, since the tracking problem is changed to a regulation problem, making possible to use well-known proportional-integral () controllers to regulate the control ariables. To conert the ariables rom abc stationary reerence rame to dq synchronous reerence rame, it is necessary to use a synchronization method to compute the synchronization signals employed in the transormation. Neertheless, the impact o distinct synchronization methods on the oltages generated by a stand-alone generation system based on a sel-excited induction generator has not yet been presented. To ill this gap, this paper analyzes digital sensorless control systems used to regulate the output oltages o a selexcited squirrel-cage induction generator. Distinct open-loop synchronization methods are ealuated and their perormances are compared under seeral load conditions, including unbalanced and nonlinear loads. Simulation and experimental results are included to demonstrate the impact o distinct synchronization methods on the unbalance and distortion o the output oltages. II. DESCRIPTION OF THE GENERATION SYSTEM A simpliied diagram o the system under study is shown in Fig.. It is composed o a squirrel-cage induction generator excited by a three-ase capacitor bank and a pulsewidth modulated oltage-source inerter. The dc bus o the inerter is composed o electrolytic capacitors as the dc oltage source. The induction generator is connected to the PWM inerter ac side through ilter inductances, which composes a secondorder ilter with the capacitor bank to minimize high-order harmonic requencies generated by the VSI. The PWM-VSI should inject or absorb a reactie power leel to regulate the induction generator output oltages, independent on the ac load power. The PWM-VSI acts as a capacitor and it injects reactie power in the system when the induction generator output oltages are smaller than the reerence oltage. On the other hand, the PWM-VSI absorbs reactie power rom the generation system when the ac bus oltages are higher than the desired alue. The sensorless digital control system employed in this paper uses the dq synchronous reerence rame, and, thereore, the eedback ariables in abc axis must be transormed to the dq axis, using the ollowing transormation: cos( θ) cos( θ) sin ( θ) cos( θ) sin ( θ) Tdq = sin ( θ) sin ( θ) cos( θ) sin ( θ) cos( θ) () According to (), a synchronization method should be utilized to obtain the synchronization angle θ or the sine and cosine signals used in abc to dq transormation. Fig. presents a simpliied block diagram o the PWM- VSI control system. The error between the dc bus oltage and its reerence alue is the input o a controller, which generates the d-axis reerence current (i d ). The d-axis inerter current (i d ) controls the actie power low through 307

2 Prime Moer Induction Generation a L a i a Three-ase PWM Conerter PM IG c b ab bc L b L c i b i c - C dc C C AC Loads C Synchronization methods sin(θ) cos(θ) abc dq PWM Generator dc sin(θ) cos(θ) abc dq i d i q ac and dc oltage controllers DSP TMS30F8 Fig.. Simpliied diagram o the generation system. d dc dc d d id i d i q i q the PWM inerter and, thereore, this control loop directly aects the dc bus oltage leel. Similarly, the error between the induction generator output oltage in the d-axis and its reerence alue is the input o another controller, generating the q-axis reerence current (i q ). The q-axis inerter current (i q ) controls the reactie power low through the PWM-VSI, aecting the amplitude o the ac oltages generated by the system. The errors between the reerence currents obtained rom the outer loop oltage controllers and the measured currents are the inputs o current controllers, which generate the control actions in the dq axis. This paper is ocused on the analysis o synchronization methods used in the abc to dq transormation and ice-ersa. Thereore, as the same controllers are employed or all synchronization methods presented hereinater, the analysis and design o the controllers in not presented in this paper. III. SYNCHRONIZATION METHODS Synchronization techniques can be classiied as closedloop [8]-[] or open-loop []-[5] methods. In the closedloop methods the synchronization angle is obtained through a closed-loop structure, to lock the estimated alue o the ase angle to its actual alue. On the other hand, open-loop synchronization methods are simple, since they do not use mechanical sensors or position/speed estimation methods. The synchronization angle or the normalized synchronization u d u q sin(θ) cos(θ) dq abc u a u b u c PWM generator Fig.. Block diagram o the ac and dc oltage controllers. ector is obtained directly rom the ac oltages [], [3] or estimated ac oltages [4]. This paper analyzes our openloop synchronization methods that employ only two line oltages at the induction generator terminals. A. Method I For three-ase three-wire systems, a synchronization ector can be obtained rom the measurement o only two ac line oltages []. Usually, PWM conerters are analyzed and controlled considering ase quantities [5], then the line oltages ector, ll, is transormed into a ase oltages ector,, as illustrated in Fig. 3. Considering that the sum o the ase oltages is zero or three-wire systems, then: where: = T () ll ll a = b, T ll = 3, c ab ll =. (3) bc Moreoer, the ase oltages are transormed into αβ coordinates, that is, where: αβ = Tαβ (4) α αβ = ; αβ T =. β (5) A normalized synchronization ector can be obtained diiding αβ by its norm, that is: = αβn αβ αβ (6) 308

3 ll ll where the Euclidian norm o the ector is gien by: ( ) ( ) =. (7) αβ α β The two components o the ector αβn, obtained rom (6), can be the cosine and sine signals used to synchronize the PWM inerter with the oltage at the generator terminals: B. Method II cos θ T ll- T αβ ( ) = αn and sin ( θ) β =. (8) Output oltages o a SEIG can present harmonics, which can distort the synchronization signals [8] and, consequently, the oltages produced by the generation system. In order to aoid this distortion, the ase oltages ector ( ) is iltered by band-pass ilters (BPF) tuned at the undamental requency, as shown in Fig. 4. Ideally, the band-pass ilters should hae unity gain and cannot include ase-shit in the iltered signals at the undamental requency. The iltered ase oltages ector ( BPF ) is transormed into iltered αβ oltages ( αβ), by using (4) and (5). A normalized synchronization ector αβn is obtained diiding αβ by its Euclidian norm (7). The components o the ector αβn are the cosine and sine signals to synchronize the PWM inerter. Due to the iltering action o the band-pass ilters, the normalized synchronization signals contain only the undamental requency. C. Method III Unbalanced loads can produce unbalanced oltages at the induction generator terminals. These unbalanced oltages can distort the synchronization signals and, thereore, they can aect the perormance o the digital control system. In order to aoid the harmul impact o unbalanced oltages on the synchronization signals, the synchronization ector αβ is lined up with the positie sequence oltage ector. This can be carried out by using all-pass ilters (APF) and M and M matrices, as shown in Fig. 5 [8], [9]. The allpass ilters are designed to result in a unity gain and 90 ase-shit at the undamental requency. Then, according to Fig. 5, the ector αβ is computed by: = M M αβ il (9) where matrices M and M are: αβ αβ n sin( θ) cos( θ) Fig. 3. Block diagram o the synchronization method I. sin( θ ) cos( θ ) BPF αβ T ll- BPF T αβ αβ Fig. 4. Block diagram o the synchronization method II. sin( θ ) ll αβ cos( θ ) T ll- M αβ il APF M Fig. 5. Block diagram o the synchronization method III. ll T ll- LPF LPF il il αβ -M αβ -M Fig. 6. Block diagram o the synchronization method IV. sin( θ ) cos( θ ) M = (0) M = A normalized synchronization ector αβn can be obtained diiding αβ, obtained rom (9), by its Euclidian norm (7). Again, the components o the ector αβn are the cosine and sine signals to synchronize the PWM inerter. It is important to highlight that the synchronization signals are computed rom the positie sequence oltages and, thereore, the negatie sequence oltages produced by the unbalanced loads do not aect the synchronization signals. D. Method IV Another synchronization method, called hereinater o method IV, can be used to obtain low-thd synchronization signals, een with harmonic distortions and unbalanced loads [5]. In the synchronization method IV, illustrated in Fig. 6, the ase oltages ector is iltered by low-pass ilters (LPF and LPF ), and the synchronization ector ( αβ) is lined up with the positie sequence ector o the iltered ase oltages. So, using Fig. 6, the synchronization ector αβ can be expressed as: = M M. () αβ il il A normalized synchronization ector can be obtained diiding αβ by its Euclidian norm (7). The components o the normalized synchronization ector are the cosine and sine signals to synchronize the PWM-VSI. Similarly to the method III, the synchronization signals are obtained rom the positie sequence ector o the iltered ase oltages and, consequently, the negatie sequence oltages caused by unbalanced loads and harmonic distortions produced by nonlinear loads will not increase the THD o the sine and cosine signals. 309

4 IV. SIMULATION RESULTS The generation system shown in Fig. has been simulated with SimPower Systems toolbox o the Matlab/Simulink. Table I presents the main parameters o the induction machine utilized to obtain the simulation results. The magnetizing reactance is not constant and it depends on the magnetizing current. The relationship between the magnetizing reactance X M and the magnetizing current I M was obtained experimentally and is expressed as: ( ) X I = I 0.74I.73I 9.357I M M M M M M. () Firstly, the synchronization method I was used to eriy the perormance o the generation system under sudden load changes. Fig. 7 presents the line oltage waeorms at the induction generator terminals (top) and the respectie normalized d-axis oltage waeorm (bottom) or a step load change rom no-load to a balanced three-ase 3 kw resistie load. One can see that the closed-loop control system regulates the generator oltage een under this large load change. Other load conditions were also simulated. For instance, Fig. 8 shows the synchronization signal waeorms (top) and the line oltages at the induction generator terminals (bottom) when a single-ase kw resistie load is connected between two terminals o the induction generator. One can eriy that the synchronization signals and the generator oltages (THD = 3.4%) present signiicant distortions by using the synchronization method I. Neertheless, the synchronization method III uses only the positie sequence o the induction generator oltages and, thereore, the synchronization signals will not be distorted. Fig. 9 illustrates a simulation result or the same unbalanced load used in the preious simulation, but now using the method III. It can be seen that the synchronization signals present a smaller distortion than by using the method I, and the THD o the generator oltages was reduced to.53%. Seeral other simulations were carried out or distinct load conditions and their results are gien in Tables II, III and IV. Table II clearly shows that the synchronization signals generated by method I present a signiicant THD or nonlinear or unbalanced loads. On the other hand, it is possible to reduce the THD o the synchronization signals or nonlinear loads by using the methods II or IV. For unbalanced loads, it is possible to reduce the distortion o the synchronization signals by using the methods III or IV. The generation o low-distortion synchronization signals relects in generator oltages with smaller THDs, as can be seen in Table III. The THD o the line oltages produced by the generation system with single-ase loads is considerably TABLE I PARAMETERS OF THE INDUCTION GENERATOR. Rated power 5 c Rated oltage 0 V ( connected) Rated speed 730 RPM Rated requency 60 Hz Stator resistance 0.44 Ω Rotor resistance 0.43 Ω Leakage stator reactance 0.83 Ω Leakage rotor reactance 0.83 Ω Iron and mechanical losses resistance 3.3 Ω Rotor inertia 0.03 kg.m Fig. 8. Steady-state perormance o the generation system using the synchronization method I under an unbalanced load: synchronization signals (top) and line oltages at the induction generator terminals (bottom). Fig. 7. Transient perormance o the generation system using the synchronization method I: line oltages at the induction generator terminals (top) and d-axis oltage (bottom). Fig. 9. Steady-state perormance o the generation system using the synchronization method III under an unbalanced load: synchronization signals (top) and line oltages at the induction generator terminals (bottom). 30

TABLE II THD OF THE SYNCHRONIZATION SIGNALS. Method I Method II Method III Method IV sin cos sin cos sin cos sin cos No-load.6. 0.5 0.6 0.8 0.79 0.05 0.06 3Φ resistie load (3000 W). 0.98 0.8 0.9 0.6 0.55 0.

5 TABLE II THD OF THE SYNCHRONIZATION SIGNALS. Method I Method II Method III Method IV sin cos sin cos sin cos sin cos No-load Φ resistie load (3000 W) Nonlinear load Φresistie load (000 W) TABLE III THD OF THE LINE VOLTAGES AT THE GENERATOR TERMINALS. Method I Method II Method III Method IV No-load Φ resistie load (3000 W) Nonlinear load Φ resistie load (000 W) TABLE IV UNBALANCE FACTOR OF THE LINE VOLTAGES AT THE GENERATOR TERMINALS. Method I Method II Method III Method IV No-load Φ resistie load (3000 W) Nonlinear load Φ resistie load (000 W) reduced by using the methods III or IV. On the other hand, or nonlinear loads, the generator oltages are less distorted by using the method II, although this reduction is not too signiicant. An important THD reduction or nonlinear loads can be achieed by using method II and IV, and replacing the current controllers by repetitie controllers, which minimize tracking errors or periodic disturbances [6]. Table IV shows that the unbalance actor o the line oltages by applying a single-ase load at the induction generator terminals is reduced by using the synchronization method IV instead o method I. V. EXPERIMENTAL RESULTS A prototype was built in our lab to carry out the experimental tests in the generation system presented in Fig.. The parameters o the induction generator are the same utilized to obtain the simulation results. The digital control system was implemented with the DSP TMS30F8 rom Texas Instruments. Fig. 0 presents the line oltage waeorms at the standalone induction generator terminals beore connecting the PWM-VSI to regulate the output oltages. One can obsere that the oltage waeorms produced by the generator are distorted (THD =.%) and this will aect those synchronization methods that are sensitie to harmonics. This eect can be eriied in Fig., which presents the line oltage waeorms at the induction generator terminals by employing the method I. Fig. (a) shows the line oltages (THD = 3.%) by connecting a balanced three-ase 3 kw resistie load. Fig. (b) presents the line oltage waeorms (THD = 4.6%) with a single-ase kw resistie load, which increases the harmonic distortions at the generated oltages due to the unbalanced load. Fig. 0. Experimental result: Line oltages at the induction generator terminals beore connecting the PWM-VSI. (a) (b) Fig.. Experimental result: Line oltages at the induction generator terminals using the synchronization method I. (a) Balanced threease 3 kw resistie load. (b) Single-ase kw resistie load. On the other hand, Fig. shows the steady-state perormance o the generation system by using the synchronization method IV. Fig. (a) presents the output oltages (THD =.8%) with a balanced three-ase 3 kw resistie load, while Fig. (b) shows the line oltage waeorms (THD = %) with the same unbalanced load employed to obtain the results presented in Fig. (b). In both cases it is possible to see a signiicant reduction on the THD leels by using the synchronization method IV. Finally, Fig. 3 presents the line oltage waeorms at induction generator terminals or a step load change rom noload to a balanced three-ase 3 kw resistie load. One can see the satisactory transient perormance o the closed-loop control system een under this large load change. 3

ACKNOWLEDGEMENT The authors would like to thank DEMEI (Departamento Municipal de Energia de Ijuí) by the inancial support. Our special thanks to Pro. Fernando Botterón and to Pro.

6 ACKNOWLEDGEMENT The authors would like to thank DEMEI (Departamento Municipal de Energia de Ijuí) by the inancial support. Our special thanks to Pro. Fernando Botterón and to Pro. Victor Hugo Kurtz rom Uniersidad Nacional de Misiones (UNAM) or their help along this work. REFERENCES (a) (b) Fig.. Experimental result: Line oltages at the induction generator terminals using the synchronization method IV. (a) Balanced threease 3 kw resistie load. (b) Single-ase kw resistie load. Fig. 3. Experimental result: Transient perormance o the generation system using the synchronization method IV. VI. CONCLUSION This paper presented an ealuation o distinct open-loop synchronization methods to be applied in sel-excited squirrelcage induction generators under distinct load conditions. These synchronization methods were chosen because they present a considerable cost reduction i compared to those methods that use position or speed sensors. Results show that the impact o these methods on the THD and the unbalance actor o ac oltages produced by the generation system is signiicant. The synchronization methods presented in this paper dier in terms o complexity and perormance. Method I is ery simple, resulting in a reduced computation time, but the synchronization signals are distorted by unbalanced and nonlinear loads. Then, it is not possible to generate low-thd oltages, een using high-perormance controllers. On the other hand, method IV requires a higher computation time than those spent by other methods presented here, but the synchronization signals and the output oltages hae a smaller THD, een with unbalanced and nonlinear loads. [] D. E. Basset, M. F. Potter, Capacitie excitation or induction generators, AIEE Trans., ol. 54, pp , May 935. [] B. Singh, S. S. Mury, S. Gupta, Analysis and design o STATCOM-based oltage regulator or sel-excited induction generators, IEEE Trans. Energy Con., ol. 9, n. 4, pp , Dec [3] L. A. C. Lopes, R. G. Almeida, Wind-drien sel-excited induction generator with oltage and requency regulated by a reduced rating oltage source inerter, IEEE Trans. Energy Con., ol., n., pp , June 006. [4] E. G. Marra, J. A. Pomilio, Sel-excited induction generator controlled by a VS-PWM bidirectional conerter or rural applications, IEEE Trans. Ind. Applicat., ol. 35, n. 4, pp , Jul./Aug [5] S.-C Kuo, L. Wang, Analysis o oltage control or a selexcited induction generator using a current-controlled oltage source inerter (CC-VSI), IEE. Proc. Gener., Transm. Distrib., ol. 48, n. 5, pp , Sept. 00. [6] R. Leidhold, G. Garcia, M. I. Valla, Induction generator controller based on the instantaneous reactie power theory, IEEE Trans. Energy Con., ol. 7, n. 3, pp , Sept. 00. [7] T. Ahmed, K. Nishida, M. Nakaoka, Adanced control o PWM conerter with ariable-speed induction generator, IEEE Trans. Ind. Applicat., ol. 4, n. 4, pp , Jul./Aug [8] S.-J. Lee, J.-K. Kang, S.-K. Sul, A new ase detecting method or power conersion systems considering distorted conditions in power system, in Proc. o IAS, ol. 4, pp. 67-7, 999. [9] M. Karimi-Ghartemani, M. R. Iraani, A method or synchronization o power electronic conerters in polluted and ariable-requency enironments, IEEE Trans. Power Systems, ol. 9, no. 3, pp , Aug [0] S. M. Deckmann, F. P. Maraão, M. S. Pádua, Single and three-ase digital PLL structures based on instantaneous power theory, in Proc. o COBEP, pp. 5-30, 003. [] E. Sasso, G. Sotelo, A. Ferreira, E. Watanabe, M. Aredes, P. Barbosa, Inestigação dos modelos de circuitos de sincronismo triásicos baseados na teoria de potências real e imaginária instantâneas (p-pll e q-pll), in Proc. o CBA, 00. [] G. D. Marques, A comparison o actie power ilter control methods in unbalanced and non-sinusoidal conditions, in Proc. IECON'98, 998, pp [3] J. Sensson, Synchronization methods or grid-connected oltage source conerters, IEE Proc.-Gener. Transm. Distrib., ol. 48, no. 3, pp. 9-35, May 00. [4] R. Kennel, M. Linke, P. Szczupak, Sensorless control o 4- quadrant-rectiiers or oltage source inerters (VSI), in Proc. o PESC, pp , 003. [5] R. F de Camargo, H. Pinheiro, Synchronization method or three-ase PWM conerters under unbalanced and distorted grid, IEE Proc.-Electr. Power Appl., ol. 53, no. 5, pp , Sep [6] C. Rech, H. Pinheiro, H. Gründling, H. L. Hey, J. R. Pinheiro, Comparison o digital control techniques or low cost PWM inerters, IEEE Trans. Power Electr., ol. 8, n., pp , Jan

New Synchronization Method for Three-Phase PWM Rectifiers

New Synchronization Method for ThreePhase PWM Rectifiers ROBINSON FIGUEIREDO DE CAMARGO HUMBERTO PINHEIRO Grupo de Eletrônica de Potência e Controle, Federal Uniersity of Santa Maria CEP 97105900, Santa