A modified DC chopper for limiting the fault currentand controlling the DC link voltage to enhance ride-through capability of doubly-fed induction generator based wind turbine
According to the grid code requirements, doubly-fed induction generator (DFIG) based wind turbines should remain connected to gird during fault conditions for specific time frame depending voltage sag level. A simple DC chopper is employed in DC-link to protect the DFIG from over-voltage; however it is not capable to keep high transient over-currents in an acceptable level in stator side and rotor side of the DFIG. Therefore, an effective current limiting strategy should be incorporated with the DC chopper to improve fault ride-through (FRT) capability of the DFIG. In this paper, a modified DC-link chopper is proposed to keep both the DC-link voltage and the high current level in stator and rotor sides in a permissible level without incorporating any extra fault current limiting strategy. Unlike the general DC chopper configuration, in the proposed DC chopper, it is not required to cease rotor side converter (RSC) switching and employ high rated current antiparallel diodes. The proposed modified DC chopper is placed between the DC-link capacitor and the RSC. In the proposed switching strategy, three extra semiconductor switches are included, which are trigged to insert DC chopper resistance either in parallel or series connections with the DC-link. To prove effectiveness and robustness of the proposed modified DC-link chopper, symmetrical and asymmetrical faults are applied in a power system containing DFIG based wind turbine modelled in PSCAD/EMTDC software. The results are promising in terms of both limiting the fault current and controlling the DC-link voltage of the DFIG.