Virtual inductance self-demagnetization based LVRT control strategy for doubly fed WTs

The distinctive driving scheme makes the stator windings of doubly fed induction generator (DFIG) be coupled with the power grid directly. When the grid fault occurs, dc component will appear in stator flux, ready to induce large electromotive force (EMF) in its rotor circuit. As a result, both the rotor circuit and the rotor-side converter (RSC) would suffer from over-currents and over-voltages without proper measures being taken. Although the crowbar is widely used to protect the driving converter and keep DFIG tied to grid, the flexibility of DFIG control system has not been exploited and it's hard to meet the ever stringent requirements of grid codes in the near future. Although, today some transient flux compensation based LVRT strategies have been reported for DFIG-based WTs, the strongly dependence on stator flux linkages makes this kind of algorithms a little complicated and dependent on DFIG parameters. Following the analysis of the transient electromagnetic characteristics of DFIG on grid fault, a novel LVRT control strategy based on virtual inductance self-demagnetization algorithm is proposed. With the novel strategy not only the rotor transient voltage surge is suppressed very well, beneficial to enlarging the controllable voltage dip range, but also the transient flux observation is removed completely, decreasing the computation complexity and the dependence on DFIG parameters, therefore, the rapidity and robustness of the control system is improved. The analysis and design in the paper is validated with the simulation and experiment results on 11kW test-rig for WTs.