Abstract
This paper proposes a Log-Power Proportional-Integral Extremum Seeking Control (LP-PIESC) framework for maximizing the power capture of a wind turbine operating at below-rated wind speeds, i.e., the so-called region-2 of a turbine’s power curve. Extremum seeking control (ESC) has emerged as a viable algorithm to maximize energy capture for a wind turbine operating in region-2. Despite the encouraging results of early ESC strategies, the basic algorithm suffers from slow and inconsistent convergence behavior under changing wind speed within region-2. It has been shown that replacing the power signal with its logarithm results in an algorithm that is robust and predictable even when the mean wind speed varies. In addition, new studies have suggested that replacing conventional ESC with proportional plus integral ESC (PIESC) results in faster convergence to optimal conditions. In the current paper, the idea of log-power feedback is merged with the PIESC scheme and is applied to tune the parameters of the region-2 torque controller for the NREL 5-MW turbine reference model. The results of this new algorithm are compared with the ESC with log-of-power feedback using NREL OpenFAST simulations. The log-power feedback PIESC is also implemented for the blade pitch set-point angle. Energy capture over the course of the simulations and damage equivalent loads calculated with MLife are used to assess the results. The simulations performed under different turbulent intensity cases demonstrate the rapid convergence of the log-power feedback PIESC.
Highlights
Wind turbines extract power in two different wind regimes: above-rated wind speed when the machine is regulated to produce its rated power while maintaining the rated rotor angular speed despite wind variations, and below-rated wind speed when the angular rotor speed is actively controlled to maximize the power extracted from the wind
The LP-Extremum Seeking Control (ESC) and the LP-plus integral ESC (PIESC) controllers with the algorithm parameters shown in Tables 2 and 3 are evaluated with OpenFAST simulations for different hub-height mean wind speeds of 4 m/s, 8 m/s, and 12 m/s, no wind shear, and under turbulence intensities (TI) of 10% and 15%, respectively
The results suggest that the LP-PIESC gives an energy output very close to the baseline controller, where the optimal tuning parameter value is known
Summary
Wind turbines extract power in two different wind regimes: above-rated wind speed when the machine is regulated to produce its rated power while maintaining the rated rotor angular speed despite wind variations, and below-rated wind speed when the angular rotor speed is actively controlled to maximize the power extracted from the wind. Extremum Seeking Control (ESC) and PIESC are gradient-based algorithms that tune parameters in real-time in order to maximize a system’s performance index. These algorithms do not require comprehensive physical models to work. The optimum is reached asymptotically, starting from a suboptimal guess of the parameter values
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