Wind power is the fastest growing renewable energy and is promising to be the number one source of clean energy in the near future. Among various generators used to convert wind energy, the Doubly-Fed Induction Generator (DFIG) has attracted more attention due to its variable speed, higher energy capture efficiency and improved power quality. The DFIG system has back-to-back converters, one on the rotor side and one on the stator side. The two converters act as an optimal operation tracking interface between the generator and the grid or other loads. To achieve the desirable output power, field oriented control (FOC) or vector control is applied to both the rotor- and the stator-side converters.
To achieve high efficiency in wind power systems, the maximum power point tracking (MPPT) of the variable-speed operation has attracted a lot of attention. Most MPPT methods either rely on wind speed measurement or on complicated estimations and online calculations. As a result, these methods are either expensive due to the need of wind speed sensors or suffer from inaccuracy due to variations of wind turbine system models.
This dissertation proposes a novel self-tuning reference power MPPT curve. This reference power curve is simply updated by incrementally updating the curve coefficients. The method is robust and is independent of the wind turbine model. Also, regarding MPPT stability, this dissertation presents the steady state and dynamic analyses of this MPPT method. A single-pole transfer function that describes the effect of variations of wind speed on the rotor speed is obtained by applying small signal analysis on the turbine-rotor mechanical system.
To verify the wind power control system and the proposed MPPT analysis, both simulation and experimental platforms are developed. First, the DFIG system, including a wind turbine, a generator, power electronic converters and power system grid are modeled. All of the control algorithms and operation modes are simulated. The active power is always controlled at maximum output while the reactive power is controlled to achieve a particular power factor or line voltage. Second, a complete DFIG wind power test bench including a wind turbine emulator is designed and built. The back-to-back converters are designed to control the DFIG. A Microchip dsPIC33 is used to control both turbine emulator and the converters. The utility grid integration is implemented with assistance of a Phase Locked Loop (PLL) and the rotor currents of DFIG are controlled to vary the operation point. Then, the proposed MPPT is simulated and incremental current control is implemented for optimal power tracking. The incremental change is not stopped until the optimal output power is reached. Also, both simulation and experimental results confirm the dynamic behavior of rotor speed predicted by the proposed transfer function.
The real-time close-loop power control and Fault Ride Through (FRT) could be studied in the future based on the developed simulation and experimental systems. While the test bench is designed to flexibly fitting future wind research, more programming efforts are required to implement more complicated algorithms.
|Advisor:||Elbuluk, Malik, Sozer, Yilmaz|
|Commitee:||Kreider, Kevin, Quinn, D. Dane, Sastry, Shivakumar|
|School:||The University of Akron|
|School Location:||United States -- Ohio|
|Source:||DAI-B 78/11(E), Dissertation Abstracts International|
|Keywords:||Doubly-fed induction generator, Mppt, Wind power|
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