Improvement and simulation on direct torque control system of permanent magnet synchronous motor

To solve the problems of large flux, torque ripple and inconstant inverter switching frequency in conventional direct torque control (DTC), a space vector modulation (SVM) technique is introduced to the conventional PMSM-DTC system and analyzed thoroughly based on the mathematical model. This novel control method which enjoys the advantages of VC and DTC has only one easy adjustable PI regulator and is easy to realize. The simulation verifies that this control system is capable of effectively improving the steady state performance and keeping the excellent dynamic performance of the PMSM-DTC system simultaneously and remain the switching frequency constant.     

An asymmetric fault ride-through control method forPMSG wind turbine based onsuper-capacitors

With the rapid increasing interaction between wind turbines and power systems, wind turbines have to have the fault ride-through (FRT) capability to ensure the safe operation of power grid. In order to improve the asymmetric FRT capability of permanent magnet synchronous generator (PMSG) wind turbine, a novel control FRT method based on super-capacitors is proposed. In this method, super-capacitors are connected to the DC bus of AC-DC-AC converter via a bi-directional DC-DC converter, and the power processed by the super-capacitors is controlled to limit the increasing of the DC bus voltage of the AC-DC-AC converter in grid failure condition, and to reduce the 2nd order voltage ripple in the DC bus. Meanwhile, the proposed control method employs the grid voltage feed-forward approach to reduce the negative sequence current component in the grid side converter. According to the low voltage ride through (LVRT) standard, the selection of the super-capacitor capacity is also discussed, and then a mathematical model of the super-capacitor with the bi-directional DC-DC converter is established for designing the controller. Simulation results for a 1MW PMSG wind turbine by MATLAB show that the proposed control method reduces both the negative sequence current component in grid side converter and the 2nd order voltage ripple in DC link at the same time. The results also demonstrate that the asymmetric FRT capability of PMSG wind turbine is improved, which verifies the effectiveness of the proposed control method in this paper.     

Improved control strategy of direct-driven permanent magnet synchronous generator for wind turbine under unbalanced grid faults

An improved unbalanced grid fault ride-through control strategy is proposed to meet the low voltage ride through(LVRT) requirements based on the analysis of power characteristics of a full rated converter for direct-driven permanent magnet synchronous generator(PMSG). According to the power characteristics,the relationship among dc-link capacitor voltage,power absorbed by interface reactors and grid-side power output is studied. The information of grid voltage drops is introduced into generator-side converter control to achieve coordinated control of grid-side converter and generator-side converter. The soft control strategy on dc-link capacitor voltage is adopted to suppress double-frequency active power ripple at the expense of a certain voltage fluctuations which is still within the safety range. The simulation results demonstrate that the proposed control strategy can enhance direct-driven PMSG’s unbalanced grid faults ride-through capability.     

Overall control strategy and operational performances of a doubly fed wind turbine generator system

In order to comprehensively and correctly analyze the real-time operational characteristics of a grid-connected wind turbine generator system, it is necessary to analyze the overall control strategies and operation performances of the wind turbine generator system. According to the type of a doubly fed wind turbine generator system, the mathematical models of wind turbine, drive train and doubly fed induction generator (DFIG) are presented. From the viewpoints of the maximum wind energy capture and safe operation of the wind turbine, a maximum power output control strategy is proposed by considering the DFIG loss minimization, and a variable pitch control strategy is also presented by considering generator rotational speed restriction and the output power restriction. Combined with a power decoupling strategy of DFIG, the overall operational performances of the doubly fed wind turbine generator system are simulated. By comparing the simulation, theoretical analysis and actual operational data, the results show that the presented model and overall control strategies of the doubly fed wind turbine generator system are valid.     

Comparative study of low voltage ride-through techniques for doubly-fed induction generator wind turbines under asymmetrical grid faults

Asymmetrical grid faults occur more frequently and have more adverse effects on the doubly fed induction generator (DFIG) than symmetrical grid faults in the transmission system. The transient response of DFIG under asymmetrical grid faults is analyzed firstly. Meanwhile, operation behaviors of several low voltage ride-through (LVRT) techniques under severe asymmetrical grid faults are given by simulation with the Matlab/Simulink software. Then the characteristics of these LVRT techniques are further researched and analyzed based on the simulation results. Finally, the economies of these LVRT techniques are discussed. The conclusion lays a certain foundation for engineering development of these LVRT techniques.