Advanced controls of three-phase grid-connected power converters for variable speed wind turbines

This thesis focuses on advanced controls of power converters to improve the performance of wind power generation systems (WPGSs) in two aspects: one is power "quality" optimization by current control of three-phase grid-connected voltage-source inverters (VSIs) and another is power "quantity" maximization by maximum power point tracking (MPPT) control. For WPGS safety and protection, the system protection innovation is also of concern.To enable a WPGS to extract maximal wind power at variable wind speeds, this thesis proposes a fuzzy-logic-based MPPT control strategy which is independent to the system characteristics and demands a low hardware requirement. The strategy can alleviate the effect of turbine inertia and accommodate the system with wind speed variations. System-level simulations on a 10 kW WPGS and a 50 kW WPGS have verified the satisfying performance of the strategy. For comparative studies, other MPPT strategies are also simulated in the two WPGSs. The field tests on a 3 kW WPGS show good performance with the proposed MPPT controller.System protection is a critical safety issue with a WPGS. This thesis proposes a new method for three-phase grid voltage detection and protection to offer a fast and accurate response to grid voltage faults. The method has been verified by both simulations and experimental tests. On the converter-side, a hardware-based cycle-by-cycle over-current protection method is proposed to offer an immediate and highly reliable current restriction within each PWM cycle for boost chopper, which has been verified by experimental tests. A dual-timer sampling scheme for the current control system also improves the inverter ac and dc over-current protections.To improve the power quality, a space vector pulse width modulation-based (SVPWM-based) proportional-integral (PI) current controller and an SVPWM-based predictive current controller are proposed in this thesis for three-phase grid-connected VSIs. Including a traditional hysteresis current controller, the three current controllers have been simulated, tested and comparatively studied. Both simulation and test results confirm the high performances of the two proposed current controllers. Current control systems with the two proposed current controllers have been comprehensively analyzed in theory. Simulation and test results show a good consistence with theoretical analysis.