In a doubly-fed induction generator (DFIG)-based wind energy conversion system (WECS), when the grid voltage sags severely, the rotor side converter (RSC) suffers from overvoltage and overcurrent owing to the large electromotive force (EMF). To ensure that the converter operates within a safe range when grid faults occur, this paper proposes a double impedance-substitution control (DISC) strategy to improve the system’s low-voltage ride through (LVRT) capability. When the grid voltage sag is detected, the grid side converter (GSC) and RSC are connected in parallel to the rotor circuit by changing the topology of the system. In the new topology, GSC and RSC are equivalent to inductive impedance by controlling, which can not only suppress the overvoltage and overcurrent at the rotor side, but also effectively reduce the torque ripple. Additionally, the DISC strategy can provide reactive power support to the grid during LVRT. Finally, the simulation results show that the DISC strategy can maintain the current flowing through the RSC within ±1 p.u., and compared with the existing control strategy, the effectiveness of the proposed method was further demonstrated.
The parameter matching of composite energy storage systems will affect the realization of control strategy. In this study, the effective energy and power utilizations of an energy storage source were defined. With the miniaturization of a composite energy storage system as the optimization goal, the linear programming simplex method was employed to obtain the optimized masses of Li batteries and supercapacitors under the constraints of maximum speed, climbing gradient, acceleration time and cost-effectiveness. As the module numbers shall be integers, the matching results were modified in combination with the graphical method. Owing to the influences of parameter matching schemes on the overall performance and battery life, the critical points of constraints were analyzed and the most appropriate matching numerical points of the composite energy storage system were identified. Simulation and experimental analysis were conducted under practical urban road conditions in China. The results show that the proposed matching method delivers accurate results. Compared with conventional electric buses, the mileage and overall performance of the prototype bus are improved.
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