1 Abstract— For most of the peak power extraction methods in wind turbine generation system described in the current literature, it is necessary to know the wind turbine’s maximum power curve and the wind speed measurement. These methods used the maximum power curve obtained via simulations or tests for individual wind turbines. This makes these methods difficult and expensive to implement in practice. In addition, the use of wind speed sensor to measure the wind speed adds to a system a cost and presents some difficulties in practical implementation. This paper describes the design of a buck-boost converter circuit used to achieve the maximum power control of wind turbine driven permanent magnet synchronous generator (PMSG). The PMSG is suitably controlled according to the generator speed and thus the power from a wind turbine settles down on the maximum power point by the proposed MPPT control method, where the wind turbine’s maximum power curve and the information on wind velocity are not required. Index Terms— Wind turbine, Variable-Speed, synchronous generator, Maximum power, Buck-boost converter I. I NTRODUCTION Wind energy is a clean energy which had a fast development for the two last decades. Wind energy seems to be an interesting alternative that makes possible to control the energy injected into the network and to produce not polluting fuel usable in the buildings, and thus to diversify the energy markets. To be used on wide applications and to satisfy the economic constraints, the conversion chain of this energy must be robust and reliable. It must also present a better efficiency and be realized at low cost. For that, it is necessary to extract the maximum power from wind turbine. The operating power of wind turbine depends on the wind speed intensity and especially the turbine speed [1]. If the transfer of power between wind turbine and the load is not optimal, the total efficiency of wind energy system will be largely affected [2]. The wind turbine can be operated at the maximum power operating point for various wind speeds by adjusting the turbine speed optimally [3],[4]. In previously published works, most of the MPPT methods described require the knowledge of the wind turbine’s maximum power curve and the wind speed measurement [5-7]. The maximum power curve needs to be obtained via simulations or tests for individual wind turbines, which makes these methods difficult and expensive to implement in practice [8]-[9]. In addition, the use of wind speed sensor to measure the wind speed adds to a system a cost and presents some difficulties in practical implementation. These MPPT methods described in the current literature are too expensive compared with generator whose rated capacity is small [10] This paper describes the design of a buck-boost converter circuit used to achieve the maximum power control of wind turbine driven PMSG. The PMSG is suitably controlled according to the generator speed and thus the power from a wind turbine settles down on the maximum power point by the proposed MPPT control method, where the wind turbine’s maximum power curve and the information on wind velocity are not required. II. W IND TURBINE C HARACTERISTICS The conversion from wind speed to mechanical power (Pm) can be described in steady state by [11] 3 ( ) *0.647 * *Pm Cp A v λ = (1) Where r = radius of the rotor [m] ρ = air density [Kgm-3] A = wind turbine rotor swept area [m 2 ] v = wind speed [m/s] The power coefficient expresses the conversion efficiency of the turbine as a function of the tip-speed ratio (λ) given by [12]: . m r v λ Ω = (2) Where m Ω is mechanical angular velocity of the generator (rad/s). A typical relationship between Cp and λ is shown in Fig.1. For constant-speed operation, the turbine speed ( m Ω ) is forced to remain constant. Thus, as the wind speed changes, the tip speed ratio and the power coefficient will vary. Since the Cp characteristic has a single maximum at a specific value of it is apparent that, when operating at a constant speed, the power coefficient will be maximum at only one wind speed. DC Bus Control of Variable Speed Wind Turbine Using a Buck-Boost Converter T. Tafticht, K. Agbossou and A. Chériti Hydrogen Research Institute Université du Québec à Trois-Rivières, C.P. 500, Trois-Rivières, (QC), G9A 5H7, Canada. 1-4244-0493-2/06/$20.00 ©2006 IEEE. 2 Unlike constant-speed control, a variable-speed control can adjust the speed of the turbine as the wind speed changes, so as to operate at the peak of the Cp curve. This will maximize the power generated for a particular wind speed, as it shown on Fig.2. In this figure, curves for the power generated at various wind speeds are given. The curve connecting the peaks of these curves will generate the maximum power for a given wind speed and follow the path for maximum Cp operation . III. C ONVERTER DESIGN The figure 3 gives the schematic diagram of the stand-alone wind energy system under consideration. A three-phase PMSG is connected to a DC battery bank via a rectifier. The DC/DC converter is connected with rectifier circuits like fig.3. It is assumed that the power generated from the generator is converted into DC power through diode bridge rectifier circuits with a unity power factor and the load current is continuous [13] 3 tdcdc PVIVI== (3) Where, V dc , I dc are DC side voltage and current, respectively. The mean value of DC voltage is shown like the following. 6 max 6 3 cos dc LL VVd π π θθ π − = ∫ (4) where V LLmax is the maximum value of line-to-line voltage . max 3 dc LL VV π = (5) From this, the relationship between V dc , and line to line voltage V LL and phase voltage V, is shown as following, 3 2 dc LL VV π = (6) 3 6 dc VV π = (7) From (3) and (7), the equation of I dc and I, concisely expressed is obtained. 6 dc II π = (8) The PMSG is connected with rectifier circuits like fig.4. It is assumed that the AC power generated from the generator is converted into DC power through diode bridge rectifier circuits. In continuous conduction mode, the buck-boost converter assumes two states per switching cycle. The ON State is when IGBT is ON and diode is OFF. The OFF State is when IGBT is OFF and diode is ON. A simple linear circuit can represent each of the two states where the switches in the circuit are replaced by their equivalent circuit during each state. The circuit diagram for each of the two states are shown in Fig.4-a and Fig.4-b. If the output load current is reduced below the critical current level, the inductor current will be zero for a portion of the switching cycle. In a buck-boost power stage, if the inductor current attempts to fall below zero, it just stops at zero and remains there until the beginning of the next switching cycle. This operating mode is called discontinuous conduction mode (see Fig.4-c). A power stage operating in discontinuous conduction mode has three unique states during each switching cycle as opposed to two states for continuous conduction mode. The inductor current condition where the power stage is at the boundary between continuous and discontinuous mode is shown in Figure 4. This is where the inductor current just falls to zero and the next switching cycle begins immediately after the current reaches zero. In continuous conduction mode, the ratio between the output and input voltages turns out to be: 1 bat dc V V α α = − (10) Where α is the duty cycle and V bat is the battery voltage. Fig. 2. Typical wind power versus turbine rotor speed Fig. 1. Power coefficient versus tip-speed ratio. Fig.3 : Connection diode rectifier circuits to the generator. 3 If the converter’s losses are negligible, its power transfer equation becomes: 2 2 bat dc ch dc V V P RR == (11) Which R ch represent the electric load. and that gives: 2 1 dc ch RR α α − ⎛⎞ = ⎜⎟ ⎝⎠ (12) From (7),(8) and (11), the following equation is obtained. 2 18 dc dc dc V V R II π == (13) Resistance value Rg, per phase of rectifier circuits is shown like the following by generator terminal voltage V, and line current I, g V R I = (14) The following equation is obtained, when (14) is substituted in (13). 2 18 gdc RR π = (15) From (21) and (24), the following equation is obtained. 2 2 1 18 gch RR πα α − ⎛⎞ = ⎜⎟ ⎝⎠ (16) When the reactive impedance of the PMSG will be high and the impedance of the battery load will be low (case of high wind speeds), the poor impedance matching will limit power transfer to the load. In this case, the duty ratio of the buck- boost converter must be controlled in order to effectively take out the electric power. IV. S ENSORLESS MPPT CONTROL METHOD In order to extract the maximum wind power, an analysis was provided to understand the probable displacement of the operating point in the two operating zones of wind turbine. Fig. 5, represents the typical curve of wind power variation according to the operating voltage. This figure shows that there are two operating zones: the first is located on the right side of the MPP where dp/dΩ < 0 and the second on the left side of the MPP where dp/dΩ > 0. Fig. 6 gives the algorithm of the proposed MPPT control method, where the information on wind velocity is not required. For searching the maximum wind power operating point and tracking this point in order to reduce the error Fig.5. Probable displacement of the operating point between the operating power and the maximum power, in the event of change of the wind speed, the control of the buck- boost converter perturbs periodically the operating point of the wind turbine. By acquiring the output voltage and current of PMSG, the control used this information to increase or decrease the duty cycle of the buck-boost converter to change the operating point of the wind turbine. After the perturbation, there is a displacement of the operating point from (k-1) to (k), Four cases of perturbation from operating point are distinguished: If P(k)>P(k-1) and Ω (k)> Ω (k-1), The power increases after perturbation. This indicates that the MPP research is oriented to the good direction. So, the search of the MPP continues in the same direction and reaches the operating point (k+1) by increasing the duty cycle by ∆ α . If P(k)<P(k-1) and Ω (k) < Ω (k-1), The power decreases after perturbation. This indicates that the MPP search is oriented to the bad direction. The MPP search direction must be changed and the duty cycle is increased by two ∆ α to reach the operating point (k+1). If P(k)>P(k-1) and Ω (k)< Ω (k-1), The power increases after perturbation. This indicates that the MPP search is oriented to the good direction. Then, the MPP search direction must be maintained and the duty cycle is decreased by one ∆ α d to reach the operating point (k+1). If P(k)<P(k-1) and Ω (k)> Ω (k-1), The power decreased. This indicates that the MPP search is oriented to bad direction. The MPP search direction must be changed and the duty cycle is increased by two ∆ α d to reach the operating point (k+1). Search rules of the various cases of operation are summarized in the table I. Fi g.4. Operation modes of buck boost converter 4 TABLE 1 SUMMARY OF CONTROL ACTION FOR VARIOUS OPERATING POINTS Fig. 7 gives the wind turbine output power without MPPT control method at low wind speed. In this case, one notices that the turbine does not produce energy, because the induced voltage in the PMSG will not be high enough to overcome the reverse bias in the diode bridge. Fig. 8 gives the wind turbine output power with MPPT control method at low wind speed. The use of the MPPT converter imposes a low DC bus voltage to recover the wind energy at the low winds speeds. PWM : d ‹— d 0 Measure I, V N yes ∆P/∆d>0 N First measure P i+ 1 = V.I d ‹— d + ∆d P i ‹― P i+1 d ‹— d - ∆d d ‹— d + ∆d P i = V.I yes Measure I, V N (∆p/ ∆d) i >0 & (∆p/∆d) i+3 >0 d opt ‹— d i+2 N yes yes (∆p/∆d) i+1 <0 & (∆p/∆d) i+2 <0 (∆p/∆d) i <0 & (∆p/∆d) i+3 <0 N yes (∆p/ ∆d) i+1 >0 & (∆p/ ∆d) i+2 >0 N yes Start Fig.6 : Algorithm of the proposed MPPT control method. Fig.7: Wind turbine output power without MPPT control method for low wind speed. Fig.8: Wind turbine output power with MPPT control method for low wind speed. Fig. 9 and 10 give the wind turbine output power without and with MPPT control method for high wind speed. The impact of the proposed MPPT control method used to generate power at high wind speed can be clearly seen in Fig. 13. The energy efficiency is improved on average by 24%. The MPPT converter is designed to be efficient in high and low wind speeds. Fig.9 : Wind turbine output power without MPPT control method for high wind speed. Fig.10 : Wind turbine output power with MPPT control method for high wind speed. ∆ α i ∆ α > 0 ∆ α < 0 ∆ P > 0 < 0 > 0 < 0 ∆P/∆ α > 0 < 0 < 0 > 0 region I II II I ∆Ω - – + + ∆ α i+1 + - - + 5 V. CONCLUSION The maximum power transfer from variable wind speed turbine was realized by a sensorless algorithm implemented in a buck boost converter inserted between the rectifier output and the DC bus. The control relationship between generator speed and the DC bus voltage is used to change the apparent DC bus voltage seen by the generator and thus the power from a wind turbine settles down on the maximum power point. There is a need to control the duty cycle of the buck-boost converter to implement maximum power tracking in wind turbine application. Therefore, we have proposed an optimal control method, where the information on wind velocity is not required. The use of buck-boost converter to control the dc bus voltage shows that it is possible to operate at high efficiency in the high and low wind speed region. The energy efficiency is improved on average by 24%. VI. ACKNOWLEDGEMENTS This work has been supported by the LTE Hydro-Québec, Natural Resources Canada and the Natural Sciences and Engineering Research Council of Canada. VII. R EFERENCES [1] C.L. Kana; M. Thamodharan and A. Wolf; “System management of a wind-energy converter”, IEEE Tran. Power Elec, Vol.16, Issue 3, pp. 375 – 381, May 2001. [2] D. S. Zinger and E. Muljadi, “Annualized wind energy improvement using variable speeds,” IEEE Trans. Ind. Applicat., vol. 33, pp. 1444– 1447, Nov./Dec. 1997. [3] A. Bouscayrol, Ph. Delarue and X. Guillaud, “Power strategies for maximum control structure of a wind energy conversion system with a synchronous machine “Renewable Energy, Vol. 30, pp.2273-2288, Dec 2005. [4] M. Machmoum,; F. Poitiers; C. Darengosse and A. Queric “Dynamic performances of a doubly-fed induction machine for a variable-speed wind energy generation Power System Technology”, Proceedings International PowerCon Conference vol. 4, pp. 2431 - 2436 , 13-17 Oct. 2002 [5] M. Ermis, H. B. Ertan, E. Akpinar, and F. Ulgut, “Autonomous wind energy conversion systems with a simple controller for maximum- power transfer,” Proc. Inst. Elect. Eng. B, vol. 139, pp. 421–428, Sept. 1992. [6] R. Hilloowala and A. M. Sharaf, “A rule-based fuzzy logic controller for a PWM inverter in a stand alone wind energy conversion scheme,” IEEE Trans. Ind. Applicat., vol. IA-32, pp. 57–65, Jan. 1996. [7] R. Chedid, F. Mrad, and M. Basma, “Intelligent control of a class of wind energy conversion systems,” IEEE Trans. Energy Conv., vol. EC-14, pp.1597–1604, Dec. 1999. [8] M. G. Simoes, B. K. Bose, and R. J. Spiegal, “Fuzzy logic-based intelligent control of a variable speed cage machine wind generation system,” IEEE Trans. Power Electron., vol. PE-12, pp. 87–94, Jan. 1997. [9] M. G. Simoes, B. K. Bose, and R. J. Spiegal, “Design and performance evaluation of a fuzzy-logic-based variable-speed wind generation system,” IEEE Trans. Ind. Applicat., vol. IA-33, pp. 956– 964, July/Aug. 1997. [10] W. Quincy and C. Liuchen; “An Intelligent Maximum Power Extraction Algorithm for Inverter-Based Variable Speed Wind Turbine Systems” IEEE Tran. Power Elec, Vol.19, Issue 5, pp. 1242– 1249, Sept. 2004 [11] E. Muljadi and C. P. Butterfield, “Pitch-controlled variable-speed wind turbine generation,” IEEE Trans. Ind. Applicat., vol. 37, pp. 240–246, Jan./Feb. 2001. [12] E.S. Abdin and W. Xu; “Control design and dynamic performance analysis of a wind turbine-induction generator unit”, IEEE Tran. on Energy Conversion, Vol. 15, Issue 1, pp. 91 – 96, March/Apr 2000. [13] N. Mohan, T. M. Undeland, andW. P. Robbins, Power Electronics, Converts, Applications and Design, 2nd ed. New York: Wiley, 1995. . value of it is apparent that, when operating at a constant speed, the power coefficient will be maximum at only one wind speed. DC Bus Control of Variable Speed. G 9A 5H7, Canada. 1-4244-0493-2/06/$20.00 ©2006 IEEE. 2 Unlike constant -speed control, a variable- speed control can adjust the speed of the turbine as