Tracking Peak Power Point

A Tracking Peak Power Points for Wind Energy Conversion System

I) ABSRACT In it is topic a method of tracking the peak power in a wind energy conversion system (WECS) is proposed, which is independent of the turbine parameters and air density. The algorithm searches for the peak power by varying the speed in the desired direction. The generator is operated in the speed control mode with the speed reference being dynamically modified in accordance with the magnitude and direction of change of active power. The peak power points in the P-w curve correspond to dP/dw = 0. This fact is made use of in the optimum point search algorithm. The generator considered is a wound rotor induction machine whose stator is connected directly to the grid and the rotor is fed through back-to-back pulse-width-modulation (PW/M) converters. Stator flux-oriented vector control is applied to control the active and reactive current loops independently. The turbine characteristics are generated by a dc motor fed from a commercial dc drive. All of the control loops are executed by a single-chip digital signal processor (DSP) controller TMS320F240. Experimental results show that the performance of the control algorithm compares well with the conventional torque control method. KEYWORDS: Peak power point tracking, rotor side control, speed control mode, turbine characteristics, wind energy convertion system, wind turbine, wound rotor induction machine.

II) INTRODUCTION In recent years, there has been a growing interest in wind energy as it is a potential source for electricity generation with minimum environmental impact. With the advancement of aerodynamic designs, wind turbines, which can capture hundreds of kilowatts of power, are readily available. When such wind energy conversion system (WECS) are integrated to the grid, they produce a substantial amount of power, which can supplement the base power generated by thermal , nuclear, or hydropower plants.

The cage rotor induction machine is the most frequently used generator for grid-connected WECS. When connected to the constant frequency network, the induction generator runs at near-synchronous speed, drawing the magnetizing current from the mains, thereby resulting in constant speed constant frequency (CSCF) operation. However, if there is flexibility in varying the shaft speed, the energy capture due in fluctuating wind velocities can be substantially

S.S.G.M.C.E., Shegaon.

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A Tracking Peak Power Points for Wind Energy Conversion System

improved [1]-[3].

The requirement for variable-speed constant frequency (VSCF) operation led to several

developments in the generator control of WECS. By using back-to-back PWM inverters between the grid and the machine and employing vector control or direct torque control (DTC) techniques, the active and reactive powers handled by the machine can be controlled independently. The algorithm is experimentally verified in a small-scale laboratory setup. The generator considered is a wound rotor induction machine whose stator is connected directly to the grid and the rotor is fed through back-to-back PWM converters (Fig.1). Stator flux-oriented vector control is applied to control the active and reactive current loops independently [4]-[6]. The operating region of the system in the power-speed plane is indicated in Fig.2.

III) PEAK POWER TRACKING METHOD The proposed algorithm is explained with the help of Fig.3, where the P-w curves corresponding to three wind velocities are shown.

The generator output power and speed are sampled at regular intervals of time. If the wind velocity is steady at v1, the difference between successive samples of active power P (i.e., ∆ P) will be very small and no action is taken. The change in speed reference ∆w* is made proportional to ∆P.

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A Tracking Peak Power Points for Wind Energy Conversion System

IV) ALGORITHM. The algorithm is implemented in the following manner. The active power is sampled at a particular rate and the incremental change is computed as ∆P (k) = P(k) – P (k – 1).

(1)

The magnitude of ∆w* (k) is given by

∆w * ( k ) = ∆P ( k ) . K i )

(2)

if (∆w * (k − 1) = 0) S = Sign [ ∆P (k )], else

S = Sign ( ∆P ( k ) ) . Sign ( ∆w * (k − 1) ∆w * (k ) = S . ∆P(k ) . K t .

if ( ∆P(k) <= Pband ) w * ( k ) = w * ( k − 1), else

w * ( k ) = w * ( k − 1) + ∆w * ( k ).

V) SELECTION OF SAMPLING FREQUENCY The choice of sampling frequency is critical for the algorithm to work properly. This is related to the speed-loop response time. The correct execution of the algorithm depends on the correct detection of the operating points on the P-w characteristics of the turbine. Hence, the sampling period for this algorithm should be more than the response time of the speed loop. The sampling period is taken as four times the speed loop time constant of the system. VI) SELECTION OF Kt Kt determines the change in speed reference for a given change in P. Therefore, it depends on the slope of the P-w characteristics. To choose a value of Kt, an approximate idea of the turbine characteristics is needed. It is obvious that the ∆w /∆P is more for lower wind velocities and vice-versa.

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A Tracking Peak Power Points for Wind Energy Conversion System

Therefore, the maximum value of Kt is limited by the lowest value of ∆w / ∆P. A large value of Kt will also result in a large transient in generator torque, which is not desirable. Hence, the value of Kt selected is substantially lower than the limit imposed by the minimum value of ∆w / ∆P. From Fig.3, it can be seen that the P-w curves are flat-topped near the peak power points. Therefore, the change in ∆P for an increment in speed would be very small in this region. So, the final operating point may not move exactly to the power point but may settle down close to it.

VII) EXPERIMENTAL RESULTS The proposed algorithm is verified on a small-scale laboratory prototype, where the principle and implementation remain identical to that of a practical WECS. Depending on the size of the WECS, the converter topology and the generator employed may change.

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A Tracking Peak Power Points for Wind Energy Conversion System

The experimental setup (Fig.4) consists of a 3-kW wound rotor induction machine with its stator connected to the 415-V, 50-Hz, 3-φ power grid, and the rotor being red by two back-to back IGB1 based PWM converters, the setup is organized for generation operation where the torque-speed characteristics of the wind turbine is generated by a 5hp dc motor driven by a commercial four-quadrant thyristor drive. A TMS320F240 DSP-based digital control platform is designed and employed for implementing the direct power algorithm. The processor runs at a clock frequency of 36 MHZ and the sampling frequency used is 56 µs. The software is assembly coded for fast real time execution. The speed loop time constant is designed to be 250 ms, and the sampling period for the active power is taken to be 1s. According to the design procedure, Kt has to be much lower than 0.5 for the generator speed to settle close to the maximum power point without any overshoot. The selected value for Kt is 0.25. With these parameters, the algorithm is run for the different wind velocities. The resulting operating points for the generator are plotted in Fig.7 along with the optimum power curve of the turbine. Due to the flat-topped nature of the P-w curves of the turbine, the error in the settling speed does not result in appreciable reduction in the generated power. VIII) CONCLUSION An algorithm for searching the optimum operating point for a WECS in speed control mode is proposed. This technique makes peak power tracking independent of the turbine characteristics and the air density. The criteria for selecting the critical control parameters are described. The algorithm is implemented on a laboratory setup using a grid-connected wound rotor induction generator controlled from the rotor side. Experimental results show that the performance of the control algorithm compares will with the conventional torque control method. APPENDIX A WIND TURBINE AND GENERATOR DATA 1) Rotor Diameter: 27 m; Swept area: 573 m2; Rotational speed, generator 1:43 r/min; Rotational speed, generator 1:33 r/min; Number of blades:3

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A Tracking Peak Power Points for Wind Energy Conversion System

Cut-in- speed: 3.5 m/s; Rated wind speed (225 kw):14 m/s; Cut off wind speed: 25 m/s Survival wind speed: 56 m/s 2) Gearbox Nominal power: 433 kW Ratio: 1:23.4; 3) Generator main winding 225 kW, 400 V, 396 A, 50Hz, 1008 r/min, 163 kVAR; 4) Generator low power winding 50kW, 400V, 101A, 50 Hz, 760 r/min, 48 kVAR; APPENNDIX B WOUND ROTOR INDUCTION MACHINE USED IN LABORTORY PROTOTYPE 3kW, 415 v, 50 Hz, four pole, three phase; Stator: 415 V, delta connected, 7.2 A; Rotor: 415, Y connected, 6.6 A.

IX) REFERENCE: IEEE TRANSACTION ON ENERGY CONVERTION, VOL18 NO 1, MARCH 2003. BY RAJIB DATTA , V.T. RANGANATHAN

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CONTENTS SR. NO. 1 2 3 4 5 6 7 8 9

PARTICULARS ABSTRACT INTRODUCTION PEAK POWER TRACKING METHOD ALGORITHM SELECTION OF SAMPLING FREQUENCY SELECTION OF KT EXPERIMENTAL RESULTS CONCLUSION REFERENCE

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ALGORITHM. The algorithm is implemented in the following manner. The active power is sampled at a particular rate and the incremental change is computed as ∆P (k) = P(k) – P (k – 1).

(1)

The magnitude of ∆w* (k) is given by

∆w * ( k ) = ∆P ( k ) . K i )

if (∆w * (k − 1) = 0) S = Sign [ ∆P (k )], else

S = Sign ( ∆P ( k ) ) . Sign ( ∆w * (k − 1) ∆w * (k ) = S . ∆P(k ) . K t .

if ( ∆P(k) <= Pband ) w * ( k ) = w * ( k − 1), else

w * ( k ) = w * ( k − 1) + ∆w * ( k ).

S.S.G.M.C.E., Shegaon.

(2)

S.S.G.M.C.E., Shegaon.