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Power generation and grid stability have become key issues in the last decade. The high penetration of large capacity wind generation into the electric power grid has led to serious concerns about their influence on the dynamic behavior of power systems. The Low-Voltage Ride-Through (LVRT) capability of wind turbines during grid faults is one of the core requirements to ensure stability in the power grid during transient conditions. The doubly-fed induction generators (DFIGs) offer several advantages when utilized in wind turbines, but discussions about their LVRT capabilities are limited. This paper presents a comprehensive study of the LVRT of gridconnected DFIG-based wind turbines. It provides a detailed investigation of the transient characteristics and the dynamic behavior of DFIGs during symmetrical and asymmetrical grid voltage sags. A detailed theoretical study supported by computer simulations is provided. This paper also provides a new rotorside control scheme for DFIG-based wind turbines to enhance its LVRT capability during severe grid voltage sags. The proposed control strategy focuses on mitigating the rotor-side voltage and current shock during abnormal grid conditions, without any additional cost or reliability issues. As a result, the DFIG performance is improved and utility company standards are fulfilled. Computer simulations are used to verify the expanded ride-through capability of the novel strategy and its effective performance compared to the conventional control schemes.

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Permanent magnet synchronous motor (PMSM) is a typical nonlinear and multi-variable coupled system, which is sensitive to the uncertainty or variations in the motor parameters and the external disturbance which lead to the deterioration of the steady-state performance. Therefore, it is necessary to introduce effective control methods to improve the drive quality. In this paper, the DQ-axes inductances considering the magnetic saturation of the PMSM which has a significant influence on the operating characteristics are obtained, and the variable parameters are substituted into the control system. Then, a novel sliding mode control strategy is analyzed and adopted. An improved exponential reaching law is proposed in order to solve the chattering problem and improve the performance, which is verified by the theory and simulation. It demonstrates the smooth and quick responses using the new approach can be achieved to enhance the robustness of the drive system compared with conventional sliding mode control method.INDEX TERMS Permanent magnet synchronous motor (PMSM), variable parameters, sliding mode control, improved exponential reaching law, drive performance.

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