Background and Objectives: The wind turbines (WTs) with doubly fed induction generator (DFIG) have active and reactive power as well as electromagnetic torque oscillations, rotor over-current and DC-link over-voltage problems under grid faults. Solutions for these problems presented in articles can be classified into three categories: hardware protection devices, software methods, and combination of hardware and software techniques.
Methods: Conventional protection devices used for fault ride through (FRT) capability improvement of grid-connected DFIG-based WTs impose difficulty in rotor side converter (RSC) controlling, causing failure to comply with grid code requirements. Hence, the main idea in this paper is to develop a novel coordinated model predictive control (MPC) for the power converters without need to use any auxiliary hardware. Control objectives are defined to maintain DC-link voltage, rotor current as well as electromagnetic torque within permissible limits under grid fault conditions by choosing the best switching state so as to meet and exceed FRT requirements. Model predictive current and electromagnetic torque control schemes are implemented in the RSC. Also, model predictive current and DC-link voltage control schemes are applied to grid side converter (GSC).
Results: To validate the proposed control method, simulation studies are compared to conventional proportional-plus-integral (PI) controllers and sliding mode control (SMC) with pulse-width modulation (PWM) switching algorithm. In different case studies comprising variable wind speeds, single-phase fault, DFIG parameters variations, and severe voltage dip, the rotor current and DC-link voltage are respectively restricted to 2 pu and 1.2 times of DC-link rated voltage by the proposed MPC-based approach. The maximum peak values of DC-link voltage are 1783, 1463 and 1190 V by using PI control, SMC and the proposed methods, respectively. The maximum peak values of rotor current obtained by PI control, SMC and the proposed strategies are 3.23, 3.3 and 1.95 pu, respectively. Also, PI control, SMC and the proposed MPC methods present 0.8, 0.4 and 0.14 pu, respectively as the maximum peak values of electromagnetic torque.
Conclusion: The proposed control schemes are able to effectively improve the FRT capability of grid-connected DFIG-based WTs and keep the values of DC-link voltage, rotor current and electromagnetic torque within the acceptable limits. Moreover, these schemes present fast dynamic behavior during grid fault conditions due to modulator-free capability of the MPC method.
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Shahid Rajaee Teacher Training University