Research On Strengthen Control For Low-Voltage Ride-Through (LVRT) Of Doubly Fed Wind Turbines

As a kind of renewable green energy, wind energy is attracting much attention with the immense potential for development. In the widely used variable speed veriable frequency wind thrbines, the doubly fed induction generator (DFIG) has the higher ratio of performance to price on the converters and a mature industry chain, which makes it has become the dominant model of the wind turbines especially for the megawatt power with the advantages of low cost and flexible control.Since the stator windings of doubly fed induction generator (DFIG) are usually coupled with power grid directly, the transient components will happen in stator fluxes upon grid fault, and as a result, the surge will be seen in rotor-side currents and voltages. The flexibility of DFIG control system has not been exploited very well in the conventional crowbar circuit based low-voltage ride-through (LVRT) strategies and it's hard for them to meet the ever stringent requirements of grid codes for integration in the near future. Therefore, there are a lot of researches on the software control strategies in recent years, the most representative of which is transient flux compensation based algorithm. However, today some transient flux compensation based LVRT strategies have been reported for DFIG-based WTs, the critical dependence on stator flux linkages makes this kind of algorithms much complicated and very sensitive to DFIG parameters. Firstly, this paper established the general mathematical models of DFIG under different coordinate system, and use it as the basis for further analysis and research on the vector control and transient process of DFIG on grid fault. Following the analysis of the transient process of DFIG on grid fault, a novel LVRT control strategy based on virtual inductance self-demagnetization algorithm is proposed in the paper. With the proposed strategy not only the rotor transient voltage surge is suppressed very well, beneficial to enlarging the controllable grid fault range to the utmost, but also the transient flux observation is removed completely, decreasing the computation load and the dependence on DFIG parameters, and improving the rapidity and robustness of the control system. What's more, corresponding researches on the SEF-DFIG control system are analyzed in the paper. And on this basis, it is found that if the grid-side converter is flexible switched to the rotor side and drived in parallel collaborative with the rotor-side converter in the LVRT transient process, sharing the required transient rotor control current of DFIG in the LVRT process, and expanding the capacity of the inverter control, then combined with the virtual inductance self-demagnetization algorithm,the controllability of DFIG can achieve full scope through in principle without crowbar.which can improve the friendliness between DFIG and the power grid. Finally, based on the simulation platform of doubly-fed wind power system in the laboratory conditions, the vector control of DFIG system is researched with experiments. And on this basis, the analysis and design of the virtual inductance self-demagnetization algorithm and the SEF-DFIG system control in the paper is validated with the simulation and experiment results on 11kW test-rig for WTs. The results verify validate the correctness and feasibility of the analysis and design in the paper.