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Analysis and Improvement of Dynamic Performance and Fault Ride-Through Capability of Wind Turbines Based on Doubly-Fed Induction Generators

Rahimi Kelishadi, Mohsen | 2011

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 41837 (05)
  4. University: Sharif University of Technology
  5. Department: Electrical Engineering
  6. Advisor(s): Parniani, Mostafa
  7. Abstract:
  8. Variable speed wind turbines (VSWTs) provide the capability of wide speed operation and independent control of active and reactive power. In contrast to the fixed speed WTs, they have higher efficiency, power quality and controllability. VSWTs are mainly divided in two categories: WTs based on doubly fed induction generator (DFIG) and WTs based on full converter with permanent magnet synchronous generator.
    At now, among the different alternatives to obtain VSWTs, DFIGs are the most commonly used. This is because the voltage source converter (VSC) in DFIG has to handle a fraction of the total power under steady state conditions, and thus the size and cost of the converter is reduced. DFIGs are very sensitive to grid voltage dips. This is because the grid voltage dips imposed at the connection point of the DFIG to the grid induce large voltages in the rotor windings, resulting in high rotor current. The high rotor current can damage the rotor-side converter (RSC), and may cause large increase in the dc-link voltage. In these conditions either the RSC maybe deactivated or disconnected. Deactivation of the RSC, on the other hand, converts the DFIG to a squirrel cage induction generator, consuming more reactive power and possibly resulting in voltage instability and collapse.
    According to high penetration of DFIG based WTs, it is important that the WTs remain connected to the grid during the voltage dips in order to improve the low voltage ride-through capability and thus to fulfill the new gird code requirements. This dissertation mainly deals with the analysis and improvement of the low voltage ride-through (LVRT) capability of the DFIG-based WTs. In the dissertation, by using small signal, modal and sensitivity analyses poorly damped electrical and mechanical modes of the system are extracted. Then the effects of different turbine-generator parameters, controllers and operating conditions on the weak modes and DFIG transient response are investigated, and required considerations for selecting the controller parameters are proposed. The above studies provide a suitable bed for analysis and improvement of the DFIG LVRT behavior. By using the obtained results, the effective role of rotor controllers and generator parameters, such as stator and rotor resistances, on the DFIG LVRT capability become clear. Then, it is dealt with the DFIG transient behavior analysis (under symmetrical/asymmetrical voltage dips) during and after clearing the fault. According to the analysis, mathematical formula and LVRT improvement approaches for the DFIG transient behavior are presented. The proposed LVRT approaches are divided into two parts. The first LVRT part is realized through the RSC control and by using additional passive hardware, called stator damping resistor (SDR). It limits the peak values and oscillations of the DFIG transient response such as rotor current, electromagnetic torque, and rotor power during the voltage dip. The second LVRT part is realized through efficient control of grid-side converter (GSC) that effectively reduces the dc-link voltage fluctuation when the DFIG is subjected to a voltage drop. In contrast to the conventional crowbar, by using the proposed LVRT approaches not only the DFIG ride-through behavior is effectively improved, but also the controllability of the DFIG is preserved. Also, in comparison with the active LVRT approaches such as series voltage compensation (by the VSC), the proposed methods are simple and economical with lower computational complexity
  9. Keywords:
  10. Doubly Fed Induction Generator (DFIG) ; Stator Flux ; Low Voltage Ride Through (LVRT) ; Crowbar ; Stator Damping Resistor (SDR)

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