Study On Operation And Control Strategy Of DFIG-based Wind Power Generation System With Series Grid-Side Converter

Doubly fed induction generators (DFIGs) have been widely used for large-scale variable-speed constant-frequency wind power generation systems due to their excellent operational performance. However, as the DFIG's stator is directly connected to the grid, wind turbines based on the DFIG are very sensitive to grid disturbances, especially to grid faults, and have aggravated operational performance under unbalanced grid voltage condition as well as poor low voltage ride-through (LVRT) capability during grid faults. How to improve the capabilities of the DFIG systems under such abnormal operating conditions has attracted special attention during the last few years. Recently, some research efforts have been devoted to overcome these issues with modified DFIG system topologies. A new DFIG system configuration with an additional grid-side converter, referred to as the series grid-side converter (SGSC), in series with the stator windings of the DFIG and grid connection is proposed where the inspiration for the SGSC is derived from the dynamic voltage restorer (DVR). This topology has been demonstrated to be an advanced low voltage ride-through technology because the DFIG system with this configuration can achieve zero voltage ride-through and has excellent LVRT potential. However, the research on this new topology is still in its early days and there has been little published literature worldwide. This dissertation intends to study the operation and control strategy of the new DFIG system with SGSC under abnormal operating conditions, such as grid voltage unbalance and grid faults. Concrete research work is as follows:1) The unified mathematical model of the DFIG system with SGSC is built by means of space-vector method. Based on this model, a steady-state control strategy suitable for this new topology during normal grid condition is further proposed and verified by simulation results. These works lay the foundation for the operation and control of the DFIG system with SGSC under abnormal operating conditions.2) The impact of grid voltage unbalance on the DFIG and the behaviors of the DFIG system with SGSC under unbalanced grid voltage conditions are analyzed. On this basis, three selective enhanced control strategies are proposed during network unbalance to restrain the adverse effects of voltage unbalance on the DFIG and improve the operational performance of the whole system. Research results show that compared with the existing unbalanced control methods, the main advantage of the proposed method is that while achieving the goals of zero oscillations in electromagnetic torque and total active or reactive power, or total current unbalance, the stator and rotor currents in the three phase windings are also balanced. Thus, the sustained localized heating on the stator and rotor windings caused by the unbalanced stator and rotor currents can be avoided and the life spans of the windings insulation materials would be extended effectively. The operational performance of the whole DFIG system and the stability of the connected grid can also be dramatically improved.3) Based on the analysis of the transient components in the stator and rotor during grid faults and the mechanism for the DFIG system with SGSC suppressing the rotor over current, a LVRT control strategy allowing DFIG to ride through the grid faults is proposed. By controlling the output voltage of SGSC to suppress the transient DC flux and negative sequence flux components in the stator flux and also by controlling the rotor-side converter (RSC) to further restrain the stator and rotor currents, a successful ride-through during grid faults is achieved. Research results show that the proposed control strategy can make the DFIG system with SGSC achieve zero voltage ride-through and improve the LVRT performance of the whole DFIG system and the stability of the connected grid.4) The reason for rotor over current during grid faults is explained from the perspective of electrical circuit. The criterion for determining the value of active Crowbar resistor is given. Then the characteristics of some common LVRT techniques are further researched and analyzed based on the simulation results under severe symmetrical and asymmetrical grid faults. Finally, the economies of these LVRT techniques are discussed. These works lay a certain foundation for engineering development of these LVRT techniques.5) An experimental platform of the DFIG system with SGSC is developed based on the TMS320F2812 digital signal processings (DSPs) and the intelligent power modules (IPMs). On the experimental platform, detailed experiments are carried out during normal and unbalanced grid voltage conditions, respectively. These experimental results further verify the validity and efficiency of the aforementioned analysis and research.