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Optimal Design of Brushless Doubly-Fed Induction Generator Considering Harmonic Effects for Maximizing Volumetric Power Density

Gorgin-Poor, Hamed | 2013

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 45028 (05)
  4. University: Sharif University of Technology
  5. Department: Electrical Engineering
  6. Advisor(s): Oraee Mirzamani, Hashem
  7. Abstract:
  8. In recent decades, electricity generation using renewable energy sources especially wind has gained considerable attention worldwide. Various generating systems have been proposed for wind turbines to convert wind mechanical power into electrical power. Up to 70% of the installed wind turbines incorporate Doubly-Fed Induction Generator (DFIG) and a fractionally rated power converter. Other schemes still have technological and economic penalties such as high cost of active materials, complexity of machine manufacturing and control systems and large and expensive converter, which limit their spread in wind power plants. Among these, Brushless Doubly-Fed Induction Generator (BDFIG) has attractive features to be the next generation of wind generators. Having no brush and slip rings, a robust structure, lower operating and maintenance costs as well as requiring a smaller mechanical gear box besides its advantages in grid connection issues such as better low voltage ride through capability are the benefits of BDFIG over DFIG. The disadvantages of BDFIG are related to its design, since it has slightly larger dimensions and manufacturing cost as well as lower efficiency in comparison to a DFIG with the same rating. However, its promising feature warrants further investigations on the design possibilities. It is therefore necessary to optimize the machine structure in order to enhance its performance, both technical and economic, against other options. In this research, design optimization of the BDFIG structure is performed. At the first stage, a novel rotor configuration is proposed in order to reduce spatial harmonic distortion of air-gap magnetic field as well as improving some drawbacks of the conventional structure, including unequal magnitudes of current in rotor bars, rotor teeth saturation at low average air gap magnetic fields, high core loss and inefficient magnetic material utilization. The rotor loops are proposed to be connected in series in the new scheme rather than nested arrangement of the conventional design. Then, an analytical electromagnetic-thermal model based fully on design parameters are extracted considering spatial harmonics and their effects. The proposed approach combines three equivalent circuits including electric, magnetic and thermal models. Utilizing electric equivalent circuit makes it possible to apply static form of magnetic equivalent circuit. Since, the elapsed time to reach steady-state results in the dynamic form is too long for applying in population-based design studies. The operating characteristics, which are necessary to evaluate the objective function and constraints values of the optimization problem, can be calculated using the presented approach considering iron loss, saturation and geometrical details. The objective function which is a combination of power to weight ratio, efficiency, power factor, rotor differential leakage inductance and voltage regulation, is formulated and the design algorithm is presented in the next step. The results of an optimized BDFIG are verified using 2-D Finite Element (FE) analyses and are compared to the results of the D180 frame size prototype machine. Finally, the results of optimally designed 2.5MW BDFIG are compared with the results of two similarly rated DFIGs with different pole pair numbers
  9. Keywords:
  10. Nested Loop Rotor ; Optimal Design ; Brushless Doubly Fed Induction Machine ; Magnetic Equivalent Circuit (MEC) ; Power-to-Weight Ratio

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