Current source converters for megawatt wind energy conversion systems
The rapid growth of wind energy market has propelled the research and development of high-power wind turbines in the megawatt range. At this power level, current source converter (CSC) topologies possess favorable features such as simple structure, grid friendly waveforms, controllable power factor, and reliable grid short-circuit protection. This dissertation proposes the use of current source converters for permanent magnet synchronous generator based megawatt wind energy conversion systems (WECS). Related research in terms of converter topology, modulation scheme, control strategy and grid integration are carried out to adapt the proposed configuration for megawatt wind applications. Various current source converter topologies are compared for wind applications. Detailed feasibility study and performance evaluation are conducted based on theoretical analysis and simulation results. Among all, the back-to-back pulse-width modulated (PWM) current source converter is identified as the most promising converter configuration for megawatt WECS due to its high performance, control flexibility and compliance with grid connection codes.A novel multi-sampling space vector modulation (MS-SVM) scheme with superior harmonic performance and controllability is proposed to operate the PWM CSC. The device switching frequency under MS-SVM is investigated and methods to eliminate additional switching are presented. The proposed scheme is compared with the conventional modulation schemes. It is demonstrated that the MS-SVM scheme provides superior performance at low switching frequency. It not only offers high control flexibility but also substantially reduces the low-order harmonics existing in the conventional schemes. System modeling and controller design for the current source converter based WECS are then presented. Dynamic, steady-state and small-signal models are developed for analysis and controller design. An optimum de-link current control scheme is developed to achieve the best dynamic performance and maximize the system overall efficiency. Control strategies such as decoupled active and reactive power control and power feed-forward control are also proposed to further improve the system dynamic performance. Grid integration issues, especially the low-voltage ride-through capability of the current source converter based WECS, are addressed. Challenges for the grid-connected current source converter are identified based on grid code requirements. A unified de-link current control scheme is proposed to assist the system to ride through grid low-voltage faults while maintaining the control capability of active and reactive power during and after the fault. The unified de-link controller can be well embedded in the system control structure. Smooth transitions between normal and fault operations are achieved. Simulation and experimental verifications for various objectives are provided throughout the dissertation. The results validate the proposed solutions for the main challenges of using current source converter in a megawatt WECS.