1. The Application of Three-level NPC Converter to WES By: Amir Yazdani yazdani@power.ele.utoronto.ca PhD candidate University of Toronto Feb. 20, 2004

2. Abbreviations NPC Neutral-Point diode Clamped converter WES Wind Energy System VSC Voltage-Sourced Converter BTB Back-To-Back (AC/DC/AC) converter system PWM Pulse Width Modulation THD Total Harmonic Distortion

3. Outline Wind turbine characteristics Variable-Speed technology Motivations for using the Three-Level NPC in WES Three-Level NPC circuit and operation Current research Conclusions

4. Wind Turbine Characteristics

5. Variable-Speed WES (General) 1.Adjusts the generator speed to obtain the maximum power Optimized power capture 2.Connects the generator to the grid via a two-level VSC BTB  Decoupling of the grid and turbine  Reactive-power/AC-voltage control  Good power quality

6. Variable-Speed WES (Induction machine and Two-Level BTB)  Gearbox Wear

7. Variable-Speed WES (Synch. machine and Two-Level BTB) No Gearbox

8. Why Three-Level NPC? Higher DC voltage by use of ordinary switches (lower current, higher efficiency) Faster dynamic response Superior power quality (lower THD, lower torque pulsations) Comparable switching losses with respect to Two-Level BTB Demand for higher power WES

9. Three-Level NPC vs. Two- Level VSC Two-Level VSCThree-Level NPC

10. PWM Spectra Three-Level NPCTwo-Level VSC

11. Waveforms of Three-Level NPC

12. Variable-Speed WES (Synch. machine and Three-Level NPC BTB)

13. Demerits Higher component count Two capacitor banks More complicated PWM and control (e.g. C-voltage equalization issue) Mid-point current

14. Current research Mathematical modeling Control DC-voltage balancing DC-link voltage regulation of the BTB system Fault analysis

15. Conclusion The use of Three-Level NPC technology in WES offers: More economical designs Lower THD waveforms Superior dynamic performance