| With the development of wind power generation system and demand of large-scale and long-distance electric power transmission, the grid connection technology for wind farms becomes more and more important. High voltage DC grid transmission with high performance for wind farm is a suitable choice. Due to the advantageous characteristics such as flexible regulation of active and reactive power and lower requirement for converter capacity, the doubly-fed induction generator (DFIG) has been widely used for wind turbine, whose DC grid connection technology has been widely concerned. The traditional DC grid connection system for DFIG-based wind farms remains the original mode of connection and the control strategy, leading to higher system complexity, lower transmission efficiency and lower operation reliability. Furthermore, the conventional control strategy for grid-connected DFIG-base wind power generation system usually extremely relies on the steady grid voltage, which cannot satisfy the DC grid connected system. This paper investigated a novel DC grid connection topology and two effective control strategies for DFIG based wind power generation system by theoretical analysis, system modeling, simulation analysis and experimental validation.Firstly, this paper introduces the novel DC grid connection topology and its comparison with the traditional topology. The proposed topology implements the wind power conversion from wind turbine to DC grid by using the stator side converter (SSC) and the rotor side converter (RSC). Compared with the traditional system, the proposed system has a lot of advantages such as simpler structure, lower requirement for the converter capacity, wider speed range and higher system transmission efficiency. Moreover, the stator frequency of DFIG under the proposed topology can be adjusted flexibly, allowing the generator to operate under full speed range generation mode and the power distribution generation mechanism by SSC and RSC.Secondly, a power-current double loop control method based on the air-gap orientation was proposed, in which the RSC is used to ensure the DFIG establishes the3-phase sinusoidal air-gap electromotive force (EMF) and the-SSC is used to realize the decoupling control of the electromagnetic active and reactive power. The operation performance of the proposed control method under different working conditions has been validated by Matlab/Simulink simulations platform and the1.6kw prototype experiments. Thirdly, in order to control the stator and rotor power output, a coordinated control strategy for SSC and RSC is proposed based on current references calculation. This control strategy set the electromagnetic torque, air-gap flux, stator and rotor power output as control objects, their setting value is used to calculate the stator and rotor current references via the DFIG’s mathematical model. Compared with the power-current double loop control method mentioned above, the coordinated control strategy employs better steady state performance and faster dynamic response, and is more flexible to work under both full speed generation mode and power distribution mode. Finally, the Matlab/Simulink simulations and experimental system based on1.6kW DFIG is developed to validate the availability of the proposed control strategy.Finally, a modified DC grid connected topology for DFIG based on reduced matrix converter (RMC) and its control strategy have been investigated. The modified topology employs RMC working in voltage source mode instead of the conventional two-level VSC converter in the proposed DC-grid connected system mentioned above. The use of RMC eliminates the large capacity electrolytic capacitor, contributing to lower system volume, higher integration level and lower cost. The operation performance of the proposed topology has been analyzed by Matlab/Simulink simulations platform. |
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