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Analysis and Improvement of Dynamic Behavior of Brushless Doubly-Fed Induction Generator in Wind Turbines

Tohidi, Sajjad | 2012

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 43538 (05)
  4. University: Sharif University of Technology
  5. Department: Electrical Engineering
  6. Advisor(s): Oraee Mirzamani, Hashem; Zolghadri, Mohammad Reza
  7. Abstract:
  8. Recently, the brushless doubly fed induction generator (BDFIG) has been proposed to be used in wind turbines due to variable speed operation with fractionally size power electronic converter and without brushes and slip rings. In this thesis, for the first time, LVRT capability of BDFIG has been assessed regarding the grid codes and several approaches have been suggested for improving it. Firstly, the steady-state performance of the BDFIG in different operating modes has been investigated and required tests have been performed. In addition, a new operating mode named as "double cascade mode" has been introduced. The fault-on performance of BDFIG has also been studied by means of torque-speed curve.The possibility of dynamic model order reduction for LVRT studies has been investigated through small signal analysis. Then, the variations of fluxes, currents and EMFs have been analyzed under balanced three phase voltage dip conditions at the generator terminals. The analysis shows that due to appearing frozen part of the power winding flux linkage, high EMFs can be induced in the control winding during severe voltage dips, especially at high rotor speeds. They can be higher than maximum voltage can be supplied by the power electronic converter. Consequently, high currents may flow in the control winding which can destroy the converter switches. Three different approaches have been suggested to reduce the aforementioned current peak: parallel resistors called "crowbar", series dynamic resistors (SDR) in three different locations, and modifying the controllers. The analytical results and effectiveness of proposed approaches have been validated using time-domain simulations. Furthermore, the dynamic test results have verified the used coupled-circuit model.Since the BDFIG has been introduced as an alternative for the DFIG in wind turbines, the LVRT capability of these two generators has been investigated and compared. The analytical and simulation results show that the despite more complexity of the BDFIG structure, dynamic behavior of two generators is too similar. But, in the case of the same voltage dip levels, the peak of current flowing through power electronic converter will be lower for the BDFIG. Hence, the wind turbine with BDFIG exhibits better LVRT than that with DFIG. Here, the parameters of a 250kVA BDFIG has been used which has the most rating between the manufactured generators of this kind. In addition, the LVRT experiment has been performed on this BDFIG to check the validity of results.

  9. Keywords:
  10. Doubly Fed Induction Generator (DFIG) ; Wind Turbine ; Brushless Doubly Fed Induction Machine ; Low Voltage Ride Through (LVRT) ; Small Signal Analysis ; Voltage Dip ; Steady State Model

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