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Abstract
The doubly fed induction generator (DFIG)-based wind energy conversion system (WECS) currently dominates the wind energy market due to its advantages over other WECSs. In it, the rotor is interfaced with the AC network through a rotor side voltage source converter (RSC) and a grid side voltage source converter (GSC) which are coupled with a DC-link capacitor that helps to maintain the voltage at the point of common coupling within permissible limits.
A DFIG's sensitivity to external faults has motivated researchers to investigate the impacts of various grid disturbances, such as voltage sag, voltage swell and short-circuit faults, on the grid side of its low-voltage ride-through (LVRT) capability. However, no attention has been paid to examining the impacts of RSC and GSC internal faults on the LVRT capability of a DFIG-based WECS.
This thesis investigates a new DFIG problem that may frequently occur and proposes a solution to it. In this study, the impacts of various intermittent and sustained internal GSC and RSC faults on the overall performance of a DFIG-based WECS are compared. These faults include a fire-through, misfire, flashover and short circuit across the DC-link capacitor that can be caused by various malfunctions in the control and firing equipment of the converter station. In this context, the effects of the previously mentioned faults on various variables of the DFIG-based WECS are investigated using PSCAD/EMTDC software package. Compliance of the DFIG performance under such faults with the recent LVRT grid codes of the USA, Spain, Mexico, Denmark, Ireland, Germany, Quebec and the UK are also investigated.
Two proposed solutions to these problems are introduced; a conventional STATCOM; and a STATCOM with the application of a high temperature superconducting (HTS) coil. The key objectives of the proposed solutions are to improve the LVRT capability of the DFIG and maintain its unity power factor operation during various voltage source converter (VSC) faults. To introduce a general and robust controller that helps to enhance the system’s performance under various faults, the controller parameters are tuned using the Nelder and Mead optimisation technique.
This study determines that the effect of internal VSC faults should be considered in the design of a DFIG control system and reactive power compensation must be available to enable the wind turbine to comply with the stringent LVRT specifications enforced by international grid codes. Moreover, a reliable technique for monitoring conditions is necessary to detect these faults in advance in order to prevent their severe consequences.