A Modified DC Chopper for Limiting the Fault Current and Controlling the DC-Link Voltage to Enhance Fault Ride-Through Capability of Doubly-Fed Induction-Generator-Based Wind Turbine
A simple conventional DC chopper is employed to protect the Doubly-Fed Induction Generator (DFIG) from over-voltage; however, it is not capable to keep transient over-current in an acceptable level in stator and rotor sides. 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 chopper is proposed not only to keep the DC link voltage in an acceptable range but also to limit the high current level in the stator and the rotor sides in a permissible level without incorporating any extra fault current limiting strategy. Unlike the conventional DC chopper, in the proposed DC chopper, it is not required to stop 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 of the modified DC chopper, three extra semiconductor switches are included, which are triggered to insert DC chopper resistance either in parallel or series connections with the DC link regarding the DC link voltage level and the DC link current level, respectively. Calculation of the DC chopper resistance is discussed. To prove effectiveness and robustness of the proposed modified DC chopper in terms of both limiting the fault current and controlling the DC link voltage of the DFIG, symmetrical and asymmetrical grid faults are applied in a power system including the DFIG based wind turbine modelled in PSCAD/EMTDC software.