Grid Integration Of Large Doubly-Fed Inuduction Generator-based Wind Energy Conversion Systems

Currently, wind energy conversion systems are on the progress of evolvingfrom small ones to large ones, from single wind turbines to large wind farms andfrom onshore ones to offshore ones. Among them, the DFIG-based (Doubly-fedInduction Generator) wind energy conversion system has taken a major marketshare, thanks to its technical and economic benefits. However, the gridintegration of large DFIG-based systems faces many challenges both in singleturbine level and wind farm level, so this dissertation intends to discuss andaddress some of the key issues. Regarding the grid integration of single windturbines, this dissertation first evaluates the performance of crowbar andcrowbar-less LVRT (Low Voltage Ride-through) methods used in typicalDFIG-based wind turbines, intending to find out the bottleneck that limits theirperformance. According to the obtained result, a new kind of RSC (Rotor-sideConverter) is proposed so as to breakout the hardware limit of conventional RSC.Regarding the grid integration of wind farms, this dissertation proposes a newhybrid HVDC (High Voltage dc) system for large DFIG-based wind farms,which can overcome many deficiencies of conventional HVDC technologies.The main contributions of this dissertation includes:1. For typical DFIG-based wind turbines, it is pointed out that the dc-linkvoltage of RSC dominants the performance of crowbar and crowbar-less LVRTmethods. For crowbar LVRT, a crowbar resistance design method based on thedc-link clamp effect is proposed, which provides a reasonable foundation forselecting the crowbar resistance. For crowbar-less LVRT, by introducing thePontryagin's minimum principle, the theoretic performance limit of a class ofcrowbar-less control methods is derived. The obtained limit can be utilized bythe equipment manufacturers to assess and improve existing control algorithmsand optimize the converter capacity on a quantitative basis.2. A new bi-directional buck-boost cascade converter, as well as itsmodeling and control methods is proposed. The proposed converter can improvethe dc-link voltage modulation index during LVRT and thus breaking out the hardware limit of conventional RSCs. It is superior to conventionalbi-directional buck-boost converters not only in terms of cost, volume andweight, but also in terms of control. Thanks to its extra control freedom, asimple controller design can achieve very satisfactory steady-state and dynamicperformance.3. A new hybrid HVDC topology for large DFIG-based offshore windfarms is proposed. The mathematical model of the overall system is establishedand the state variable model of the rectifier subsystem on the dq reference frameis derived. Based on this model, the dynamic of the rectifier subsystem isquantitatively analyzed according to different control modes of the invertersubsystem. These results provide theoretic foundations for the control design ofthe hybrid HVDC system.4. The control strategies of the hybrid HVDC system for both normal andLVRT operations is proposed. The normal operation control strategy can fulfillthe requirements of system startup, fast active power tracking and independentreactive power support to the grid. The LVRT strategy can effectively reducethe system's sensitivity to communication delay, alleviate the overload of theSTATCOM (Static Synchronous Compensator), and thus keeping the gridconnection during onshore grid fault.