The Application of Three-level NPC Converter to WES By: Amir Yazdani PhD candidate University of Toronto Feb. 20, 2004

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

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

Wind Turbine Characteristics

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

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

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

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

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

PWM Spectra Three-Level NPCTwo-Level VSC

Waveforms of Three-Level NPC

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

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

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

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