Study On The Excitation Control Strategy Of Doubly-fed Induction Generator Under Special Operation Mode

The Doubly-fed Induction Generator (DFIG), which has features of variable speed operation, superior steady-state and transient-state operation characteristics and relatively small excitation rating, is especially suitable for the renewable energy generation such as wind power generation, hydropower generation with variable water head and pumping storage generation. As a key to achieve good performance during the DFIG operating, the excitation control strategies of the DFIG have being the focus of the research all over the world. Therefore, the excitation control strategies of the DFIG under some special operation modes were deeply studied in this dissertation, including the ride-through control of the DFIG under a grid fault, the coordinated control between a HVDC and the DFIGs in order to connect a wind farm into grid and the control methods of the DFIG under cut in or cut off operation.Firstly, the mathematic model of the DFIG expressed with space-vector was build. Then the transient electromagnetic relationship was derived and the transient physical process was explained combined with the analytic and simulation analyses. Research results show that the stator and rotor currents and air-gap torque are subjected to the stator and rotor flux fields in the generator. Where, the stator flux field is mainly constrained by the stator voltage which is decided by the outer operation condition of the machine. However the rotor flux field can be controlled by rotor voltage. Therefore, it's possible to control the operation state of the DFIG with proper excitation regulation. Based on the deeply understanding of the DFIG transient under a grid fault, the ride-through excitation control strategy was proposed. To guarantee the safety of rotor side converter during the grid fault, the flux linkage produced by the rotor current, which goes through leakage flux path during the transient, is controlled to counteract the effect of the harmful components in the stator flux (including the transient DC flux and negative sequence flux components). At the same time, the stator side resistance is used to weaken the harmful stator flux field. The validity work has been done by experiments and system simulations, while the control performance influenced by different factors was analyzed in detail. The research work carried out in this dissertation is important for the large-scale application of the DFIG in power system.Currently, the offshore wind power is one of the mainstream developing trends of wind power technology. Based on the features of the offshore electrical power...