Control of a Small Wind Turbine in the High Wind Speed Region

ABSTRACT:

This paper proposes a new soft-stalling control strategy for grid-connected small wind turbines operating in the high and very high wind speed conditions. The proposed method is driven by the the rated current/torque limits of the electrical machine and/or the power converter, instead of the rated power of the connected load, which is the limiting factor in other methods. The developed strategy additionally deals with the problem of system startup preventing the generator from accelerating to an uncontrollable operating point under a high wind speed situation. This is accomplished using only voltage and current sensors, not being required direct measurements of the wind speed nor the generator speed. The proposed method is applied to a small wind turbine system consisting of a permanent magnet synchronous generator and a simple power converter topology. Simulation and experimental results are included to demonstrate the performance of the proposed method. The paper also shows the limitations of using the stator back-emf to estimate the rotor speed in permanent magnet synchronous generators connected to a rectifier, due to significant d-axis current at high load.

 SOFTWARE: MATLAB/SIMULINK

 CIRCUIT DIAGRAM:

Fig. 1. Schematic representation of the wind energy generation system: a) Wind turbine, generator and power converter; b) Block diagram of the boost converter control system; c) Block diagram of the H-bridge converter control system.

EXPECTED SIMULATION RESULTS:

 

Fig. 2. Simulation result showing the behavior of the proposed method under increasing wind conditions (10 m/s, 17 m/s from 10 s, and 33 m/s from 13s): a) rectifier voltage command (v_ r ), rectifier voltage (vr) and minimum rectifier voltage command (v_ r min); b) boost current (ib), filtered boost current (~i b), current limit (ilimit) and MPPT current target (imppt); c) turbine torque (Tt) and generator torque (Tg); d) mechanical rotor speed (!rm).

 Fig. 3. Simulation result showing the behavior of the proposed method under decreasing wind conditions (30 m/s, 21 m/s from 4.5 s, and 8.5 m/s from 7s): a) rectifier voltage command (v_ r ), rectifier voltage (vr) and minimum rectifier voltage command (v_ r min); b) boost current (ib), filtered boost current (~I b), current limit (ilimit) and MPPT current target (imppt); c) turbine torque (Tt) and generator torque (Tg); d) mechanical rotor speed (!rm).

Fig. 4. Experimental results showing the behavior of the propose method under increasing wind conditions (10 m/s, 17 m/s from 10 s, and 33 m/s from 13 s): a) rectifier voltage command (v_ r ), rectifier voltage (vr) and minimum rectifier voltage command (vr min); b) boost current (ib), filtered boost current (~I b), current limit (ilimit) and MPPT current target (imppt); c) mechanical rotor speed (!rm).

 Fig. 5. Experimental results showing the behavior of the propose method under decreasing wind conditions (30 m/s, 21 m/s from 4.5 s, and 8.5 m/s from 9 s): a) rectifier voltage command (v_ r ), rectifier voltage (vr) and minimum rectifier voltage command (vr min); b) boost current (ib),filtered boost current (~I b), current limit (ilimit) and MPPT current target (imppt); c) mechanical rotor speed (!rm).

CONCLUSION:

The operation of small wind turbines for domestic or small business use is driven by two factors: cost and almost unsupervised operation. Specially important is the turbine operation and protection under high wind speeds, where the turbine torque can exceed the rated torque of the generator. This paper proposes a soft-stall method to decrease the turbine torque if a high wind speed arises and, as a unique feature, the method is able to early detect a high wind condition at startup keeping the turbine/generator running at low rotor speed avoiding successive start and stop cycles. The proposed method uses only voltage and current sensors typically found in small turbines making it an affordable solution. Both simulation and experimental results demonstrate the validity of the proposed concepts. This paper also shows that commonly used machine and rectifier models assuming unity power factor do not provide accurate estimations of the generator speed in loaded conditions, even if the resistive and inductive voltage drop are decoupled, due to the significant circulation of d-axis current if a PMSG is used. This paper proposes using a pre-commissioned look-up table whose inputs are both the rectifier output voltage and the boost current.

REFERENCES:

[1] W. Kellogg, M. Nehrir, G. Venkataramanan, and V. Gerez, “Generation unit sizing and cost analysis for stand-alone wind, photovoltaic, and hybrid wind/PV systems,” IEEE Transactions on Energy Conversion, vol. 13, no. 1, pp. 70–75, Mar. 1998.

[2] P. Gipe, Wind Power: Renewable Energy for Home, Farm, and Business, 2nd Edition. Chelsea Green Publishing, Apr. 2004.

[3] A. C. Orrell, H. E. Rhoads-Weaver, L. T. Flowers, M. N. Gagne, B. H. Pro, and N. A. Foster, “2013 Distributed Wind Market Report,” Pacific Northwest National Laboratory (PNNL), Richland, WA (US), Tech. Rep., 2014. [Online]. Available: http://www.osti.gov/scitech/biblio/1158500

[4] J. Benjanarasut and B. Neammanee, “The d-, q- axis control technique of single phase grid connected converter for wind turbines with MPPT and anti-islanding protection,” in 2011 8th International Conference on Electrical Engineering/Electronics, Computer, Telecommunications and Information Technology (ECTI-CON). IEEE, May 2011, pp. 649–652.

[5] M. Arifujjaman, “Modeling, simulation and control of grid connected Permanent Magnet Generator (PMG)-based small wind energy conversion system,” in Electric Power and Energy Conference (EPEC), 2010 IEEE, Aug. 2010, pp. 1 –6.

 

 

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