Consideration of the effects of symmetrical and asymmetrical voltage dips in the control and operation of a grid-connected doubly-fed induction generator.
Makhoba, Khanyisani Godgoven.
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Grid integration of Type 3 Wind Energy Conversion Systems (WECS) based on the doubly-fed induction generator (DFIG) is a great challenge since the direct connection of the stator side to the grid makes it susceptible to loss of control and rotor over-voltages during voltage dips. Without effective countermeasures in place, this can result in the fatal failure of the power electronic converter. This thesis presents a selective study of the behaviour of Type 3 WECS based on DFIG under symmetrical and asymmetrical voltage dips. As part of the mitigation strategies, a crowbar protection scheme, demagnetizing current control and dual vector control are studied in this thesis. The main findings are drawn from a MATLAB/Simulink simulation model of a 2 MW, 690 V DFIG. This software platform offers built-in power electronic device models; therefore, the study is mainly focused on the control aspects of the DFIG. The fault ride-through (FRT) capability of the 0.8 kW DFIG test bench is also analyzed in this research. It is deduced that it is possible to control a DFIG WECS during voltage dips that are less than 32 % in depth by solely using the traditional dual vector control technique. Voltage dips greater than 32 % result in the saturation of the power electronic converter and loss of control. As part of the mitigation strategies developing in this study, it was found that the combined control of the demagnetizing current and the injection of a backwards rotating flux produced excellent results.