Research On LVRT Control Methods Of Grid-connected DFIG System Under Asymmetrical Voltage Dips

Due to the increasingly serious energy crisis and environmental degradation problems, wind power, as one of the most important alternative energy sources, has attracted more and more attention. Doubly fed induction generator(DFIG) has become the mainstream model in wind power system because of its low cost, small inverter capacity and so on. But under asymmetrical grid voltage dips, the control strategy based on ideal grid conditions can not meet the requirements of increasingly stringent wind power grid. This paper focuses on research on low voltage ride through(LVRT) control of grid-connected DFIG system under asymmetrical voltage dips, and the main research content is summarized as follows:This paper is based on grid-connected DFIG system’s model under ideal grid voltage, and the correctness of which and the proportional integral(PI) controller are verified by simulations and experiments.Under asymmetrical grid voltage dips, the mathematical model of DFIG is rebuilt, and the transient flux is analysed; This paper analyses the theory that respectively through the hardware protection in the rotor side, stator side and the dc voltage side, the capability of the DFIG system’s low voltage ride through can be enhanced obviously, and introduce a method of resistance selection in the Crowbar of rotor side.Under lightly asymmetrical grid voltage dips, the technology of both the power grid voltage locking phase and the positive and negative sequence separation is introduced firstly in this paper, which is based on double second-order generalized integrator(DSOGI). The grid side converter(GSC) model and the rotor side converter(RSC) model are re-established, and three different types of current controller based on GSC and RSC whose limited current capacity are considered are studied, which are respectively including double dq, proportional integral(PI) current controller based on positive and negative synchronous speed rotating coordinate system, proportional resonant(PR) current controller based on the two-phase static coordinate system αβ, and proportional integral resonance(PIR) current controller based on forward synchronous speed rotating coordinate system. Under lightly asymmetrical grid voltage dips, to enhance DFIG system’s ability of LVRT according to the actual situation, the corresponding control targets of the two converters are deeply studied.Finally, the results of both simulations in MATLAB/Simulink and experiments in d SPASE platform validate the correctness and feasibility of these studied control strategies.