The Study Of Variable Speed Constant Frequency Doubly-fed Generator Based On Direct Torque Control

In the field of wind power generation Variable-speed constant-frequency (VSCF) operation is a new high efficient technology, which uses doubly-fed generator working principle. When wind speed changes, the constant-frequency output can be generated from the stator side, by connecting with low frequency excitation current at the rotor side , those frequency, amplitude and phase angle is controllable.Direct Torque Control (DTC) technology is a high-efficient AC motor control method which bases on space voltage vector and direct optimum control the inverter switching state to achieve the high-dynamic performance of the torque .In this paper, DTC is applied in a doubly-fed generator to resolve some problems , such as hard controlling excitation frequency nearby synchronous speed, hard keeping constant output power ,and high ripple of the current wave.Based on the analysis of the model of the doubly-fed wind generator and the coordinate transformations, the electromagnetic equation, power equation ,and torque equation can be achieved in the rotor coordinate system. Applying rotor flux orientation DTC ,this paper simplifies the doubly-fed generator model and get a new model ,which is fit for DTC system. After studying the doubly-fed generator DTC technology, a simulation based on DTC approximate circle flux control method has been done by MATLAB/Simulink . Thereby with speed sensor DTC system is structured , which verifies that the rotor flux orientation DTC strategy is feasible in theory.Lastly, one 2.8kw variable speed constant frequency doubly-fed generator test system which is controlled by 8XC196MC is set up. Approximate circle moving track of the flux under DTC strategy fitting for low speed running is adopted. The inverter uses IGBT and peripheral detection and processing circuits are designed. Sets a reference for developing control technique of VSCF doubly fed wind generation system.