Outline

  • Abstract
  • I. Introduction
  • II. Dfig System Description, Modeling, and Verification
  • A. System Description
  • B. Symmetrical and Unsymmetrical Voltage Dip
  • C. Modeling and Validation
  • III. Reactive Power Theory and Signal Extraction
  • A. Instantaneous Reactive Power Theory
  • B. Real-Time Signal Processing
  • IV. Dual Sequences—positive and Negative Sequences Field-Oriented Current Controller
  • A. Control of Rotor Vsc
  • B. Control of Grid Vsc
  • C. Simulation Results of Dual Sequences Controller
  • V. Conclusion and Recommendation
  • References

رئوس مطالب

  • خلاصه
  • مقدمه
  • توصیف سیستم DFIG,مدل و تایید
  • توصیف سیستم
  • شکل 1: شکل ساده شده سیستم DFIG با کنترل کننده پیشنهادی
  • کاهش ولتاژ متقارن و نامتقارن
  • مدلسازی و اعتبار سنجی
  • جدول I :پارامترهای DFIG,kw 850
  • تئوری قدرت راکتیو و استخراج سیگنال
  • تئوری قدرت اکتیو لحظه ای
  • زمان ایده ال پردازش سیگنال
  • شکل 6 : جدا کردن توالی مثبت و منفی توسط فیلتر پایین گذر
  • شکل 7 :جدا کردن توالی مثبت و منفی به روش توسط لغو تاخیر سیگنال
  • دو توالی-توالی مثبت و منفی,کنترل کننده جریان میدان جهت دار
  • کنترل VSC روتور
  • کنترل شبکه VSC
  • نتیجه شبیه سازی توالی دو کنترل کننده
  • نتیجه گیری و پیشنهاد

Abstract

Doubly fed induction generator (DFIG) still shares a large part in today’s wind power market. It provides the benefits of variable speed operation cost-effectively, and can control its active and reactive power independently. Crowbar protection is often adopted to protect the rotor-side voltage source converter (VSC) from transient overcurrent during grid voltage dip. But under unbalanced grid voltage condition, the severe problems are not the transient overcurrent, but the electric torque pulsation and dc voltage ripple in the back-to-back VSCs. This paper develops dynamic models in MATLAB/Simulink, validates it through experiments, investigates the behavior of DFIG during unbalanced grid voltage condition, and proposes new controllers in separated positive and negative sequence. Methods to separate positive and negative sequence components in real time are also developed, and their responses to unsymmetrical voltage dip are compared. Simulation results prove that the separated positive and negative sequence controllers limit the torque pulsation and dc voltage ripple effectively.

Keywords: - - -

Conclusions

The LVRT capability of DFIG under symmetrical voltage dip has been thoroughly investigated in many researches, while DFIG’s behavior under unsymmetrical voltage dip is seldom studied. Instead of the large transient rotor current caused by the symmetrical voltage dip, the large electric torque pulsation and dc voltage ripple in back-to-back VSCs are identified as the most severe problems of DFIG under unsymmetrical voltage dip. In this paper, the DFIG is proposed to be controlled in dual sequence—positive and negative sequences independently. In order to implement the separated positive and negative sequence controllers of DFIG, two methods to separate positive and negative sequences in real time are compared. The “signal delay cancellation” is much faster than the “low-pass filter,” and is chosen in this study. Equations of instantaneous active p and reactive power q, and voltage equations of DFIG and grid VSC in positive dq and negative dq sequence are derived. The complete DFIG system with the proposed controller is modeled in MATLAB/Simulink. The simulation results prove that the independent positive and negative sequence controllers of rotor VSC and grid VSC effectively limit the electric torque pulsation and dc voltage ripple.

In this paper, the control objective is focused on how to improve the performance of DFIG itself under unsymmetrical voltage, integrated with a strong power grid. In future, the control objective can be focused on how to use DFIG to improve grid performance when it is connected with a weak power grid, such as to limit the grid voltage unbalance, etc.

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