2013 International Conference on Circuits, Power and Computing Technologies [ICCPCT-2013]
Predictive Power Control of Grid-Connected Four-Level Inverters in Stationary Reference Frame B. Wu
V. Yaramasu
Department
/ Electrical
0
Department
and Computer Engineering
Ryerson University, Toronto, ON, M5B 2K3 CANADA
[email protected]
Abstract-This
paper
proposes
a
four-Ievel
and Computer Engineering
[email protected]
diode-c1amped
inverter for the grid connection of high power, medium voltage wind energy conversion systems. A predictive control scheme is presented in stationary reference frame to regulate the active and reactive powers along with the DC-link capacitor voltages. The proposed method doesn't require any PI controller, phased lock loop and the complex modulation. The discrete-time model of the inverter is used to predict the future behavior of the control variables for all possible switching states generated by the inverter. These predictions are evaluated using a cost function, and then, the switching state which minimizes the cost function is chosen and applied to the inverter. The feasibility of the proposed method is verified by MATLAB/Simulink software.
Index Terms-DC-AC power conversion, DC-link voltages bal ancing, Digital control, Diode-c1amped converters, Discrete-time modeling, Finite-states model predictive control, Grid-connected, Medium voltage, Multilevel inverters, Phased lock loop, Power control, Stationary frame, Wind energy
I. INTRODUCTION The multi-megawatt (MW) wind turbines use full-scale power converters to increase the wind energy conversion efficiency, especially at low wind speeds, and to meet the grid codes [1], [2]. The two-Ievel voltage source converters are used in many commercial wind energy conversion systems (WECS) at low voltage (LV) « 1000V) level and power ratings up to 6MW has been achieved through parallel connection of con verters [3]. As the power rating increases, the semiconductor device count, cost and weight increases, and on the other hand the reliability and efficiency decreases [4]. The medium voltage (MV), multilevel diode-clamped con verters operating in the range of 3000-4000V can overcome the above technical challenges [5]. They offer cost effective, compact, reliable and efficient solution compared to the cas caded two-Ievel converters at higher power levels. In order to reduce the device voltage rating and grid filter size, a four-Ievel diode-clamped inverter is proposed for the grid-connection of MW-MV-WECS. The equivalent inverter switching frequency for the same device switching frequency is higher compared to the two and three-Ievel inverters, and thus a better power quality and grid code compliance can be achieved. Traditionally, the grid-connected inverters are controlled by the virtual flux-oriented control [6], direct power control with space vector modulation [7], voltage-oriented control (VOC) [1], and direct power control [8]. The PI controllers are used in the internal current control loop to eliminate the steady state errors, however, they are sluggish in nature. To apply the
978-1-4673-4922-2/13/$31.00 ©2013 IEEE
/ Electrical
0
Ryerson University, Toronto, ON, M5B 2K3 CANADA
above mentioned controls to four-level inverters, a complex modulation (pulse width modulation (PWM) [9] or space vector modulation (SVM) [10]) along with the neutral-point balancing control [11] is necessary. The finite-control set model predictive control (FCS-MPC) has emerged as a simple and powerful tool to control the power converters and drives [12], [13]. Due to the discrete-nature of the inverter, the predictive control offers a promising approach. A preliminary simulation study has been done in [14] for the predictive current control of four-level inverters with resistive load. In this paper, the FCS-MPC concept is extended for the high power MV-WECS. The active and reactive powers and DC-link capacitor voltages are controlled without using any current control loops and modulation stage. To eliminate the need for PLL, the algorithm has been developed in the stationary (aß) reference frame. The proposed control scheme predicts the future behavior of the grid active and reactive powers and DC-link capacitor voltages for each valid switching state of the inverter using the measured grid voltages and currents, and DC-link voltages. These predictions are evaluated with a cost function and the switching state which minimizes the cost function is selected and applied as inverter gating signal. To validate the proposed method, simulations are carried out on a high power (4MVA/4000V) system using MATLAB/Simulink software. 11.
GRID-CONNECTED FOUR-LEVEL INVERTER MODEL
A permanent magnet synchronous generator (PMSG) based MV-WECS is shown in Fig. 1. The front-end rectifier can be realized with active or passive switches [15]. The DC link consists of three capacitors, and ideally they will share equal voltages. The four-level inverter is connected to the grid through an L filter. The regulation of net DC-link voltage or active power can be performed by the rectifier or inverter. In this work, the variables active power P;, reactive power Q; and DC-link capacitors balancing, Vbal are considered to be controlled by the inverter. The topology of the four level inverter is shown in Fig. 2. It is composed of 18 active switches and 18 clamping diodes. The clamping diodes withstand different reverse voltages and thus series connection of two diodes is necessary [16]. The switching states and the corresponding inverter terminal voltages are shown in Table I [14], [16]. In topology, only three switches conduct at any time and the switch pairs (Slx,
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Figure 6. Simulation results during step change to positive reactive power reference: (a) phase-a grid voltage and current, (b) grid active and reactive powers and their references, and (c) De-link capacitor voltages.
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2013 International Conference on Circuits, Power and Computing Technologies [ICCPCT-2013]
RE FERENCES [I] B. Wu, Y. Lang, N. Zargari, and S. Kouro, Power Conversion and Contro! oJ Wind Energy Systems, 1st ed., ser. IEEE Press Series on
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Figure 7. Simulation results during step change to negative reactive power reference: (a) phase-a grid voltage and current, (b) grid active and reactive powers and their references, and (c) DC-link capacitor voltages. Table 11 GRID-CONNECTED FOUR-LEVEL INVERTER PARAMETERS
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ACKNOWLE DGMENTS
The authors wish to thank the financial support from the Natural Sciences and Engineering Research Council of Canada (NSERC) through Wind Energy Strategie Network (WESNet) Project 3.1.
Power Engineering. Hoboken, NJ: John Wiley & Sons, Inc., Jul. 2011. [2] Z. Chen, J. Guerrero, and F. Blaabjerg, "A review of the state of the art of power electronics for wind turbines," IEEE Trans. Power E!eelron., vol. 24, no. 8, pp. 1859-1875, Aug. 2009. [3] M. Liserre, R. Cardenas, M. Molinas, and 1. Rodriguez, "Overview of multi-MW wind turbines and wind parks," IEEE Trans. ind. E!eetron., vol. 58, no. 4, pp. 1081-1095, Apr. 2011. [4] J. Dai, "Current source converters for megawatt wind energy conversion systems," Ph.D. dissertation, Ryerson University, Toronto, ON, 2010. [5] A. Faulstich, J. Stinke, and F. Wittwer, "Medium voltage converter for permanent magnet wind power generators up to 5MW," in Proe. /EEE EPE Conf, 2005, pp. 9-P.9. [6] M. Malinowski, M. Kazmierkowski, and A. Trzynadlowski, "A compar ative study of control techniques for PWM rectifiers in AC adjustable speed drives," IEEE Trans. Power E!eelron., vol. 18, no. 6, pp. 13901396, Nov. 2003. [7] M. Malinowski, S. Stynski, W. Kolomyjski, and M. Kazmierkowski, "Control of three-Ievel PWM converter applied to variable-speed-type turbines," iEEE Trans. ind. E!eetron., vol. 56, no. I, pp. 69-77, Jan. 2009. [8] R. Teodorescu, M. Liserre, P. Rodriguez, and F. Blaabjerg, Grid Con verlers Jor Pholovo!laie and Wind Power Syslems. Chichester, UK: Wiley-IEEE Press, Jan. 201 L [9] B. McGrath and D. Holmes, "Multicarrier pwm strategies for multilevel inverters," IEEE Trans. Ind. E!eelron., vol. 49, no. 4, pp. 858-867, Aug. 2002. [10] S. Busquets-Monge, 1. Bordonau, and 1. Rocabert, "Extension of the nearest-three virtual-space-vector PWM to the four-Ievel diode-clamped dc-ac converter," in Proe. IEEE-PESC Co1if., Jun. 2007, pp. 1892-1898. [11] M. Marchesoni and P. Tenca, "Diode-clamped multilevel converters: a practicable way to balance DC-link voltages," IEEE Trans. ind. Eleelron., vol. 49, no. 4, pp. 752-765, Aug. 2002. [12] J. Rodriguez and P. Cortes, Predielive Conlro! oJ Power Converlers and Eleetrieal Drives, 1st ed. Chichester, UK: IEEE Wiley press, Mar. 2012. [13] S. Kouro, P. Cortes, R. Vargas, U. Ammann, and J. Rodriguez, "Model predictive control-A simple and powerful method to control power converters," IEEE Trans. ind. E!eetron., vol. 56, no. 6, pp. 1826-1838, Jun. 2009. [14] P. Cortes, J. Rodriguez, S. Alepuz, S. Busquets-Monge, and J. Bordonau, "Finite-states model predictive control of a four-Ievel diode-clamped in verter," in Proe. IEEE-PESC Conf, Jun. 2008, pp. 2203-2208, Rhodes, Greece. [15] V. Yaramasu and B. Wu, "Three-Ievel boost converter based medium voltage megawatt PMSG wind energy conversion systems," in Proe. IEEE-ECCE Co1if., Sep. 2011, pp. 561-567. [16] B. Wu, High-power eonverlers and AC drives, Ist ed., sero Wiley-IEEE Press. John Wiley & Sons, Inc., 2006. [17] J. Rodriguez, B. Wu, M. Rivera, A. Wilson, V. Yaramasu, and C. Rojas, "Model predictive control of three-phase four-Ieg neutral-point-clamped inverters," in hoc. iEEE-iPEC Conf, Jun. 2010, pp. 3112-3116, Sapporo, Japan. [18] P. Cortes, F. Quiroz, and J. Rodriguez, "Predictive control of a grid connected cascaded H-bridge multilevel converter," in Proe. IEEE-EPE Conf, Sep. 2011, pp. 1-7, Birmingham, UK. [19] V. Yaramasu, M. Rivera, B. Wu, and 1. Rodriguez, "Model predictive current control of !wo-level four-Ieg inverters - Part I: Concept, algorithm and simulation analysis," IEEE Trans. Power E!eetron., vol. 28, no. 7, pp. 3459-3468, Jul. 20I3. [20] M. Rivera, V. Yaramasu, J. Rodriguez, and B. Wu, "Model predictive current control of !wo-level four-Ieg inverters - Part 11: Experimental implementation and validation," iEEE Trans. Power Eleetron., vol. 28, no. 7, pp. 3469-3478, Jul. 2013.
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