Analysis And Control Of Doubly-fed Induction Generators During Grid Fault

Doubly-fed induction generator has recently received much attention as one of preferred technology for wind power generation.The structure of doubly-fed induction generators for wind turbine systems is similar to that of typical asynchronous machines. The rotor is connected to the grid through a bi-directional power converter, which only deals with slip power in double direction. It's not only characterized by its small cubage,light weight and low cost but also realizes flexible connection of mechanical-electric system.This paper analyses the 5th model of DFIG. Then a new rotor current controller based on active–disturbance-rejection control theory is proposed to improve the dynamic response of the system under grid disturbances. This paper carries out simulation of the controller under normal condition and grid disturbances. The results show that the proposed controller can not only tune system power output precisely in normal condition but also ensure prominent reduction of rotor current ripple in order not to be disconnected, which contributes to power system stability. Moreover, the controller takes on good dynamic performances. Besides, for the implementation of sensorless control of doubly-fed induction geneartors, a speed identification algorithm is introduced on the basis of ESO. By means of the existing structure of ADRC system, speed information is picked up from the observation results of the unknown model in ESO. As demonstrated by simulation results, precise identification of speed is achieved.Moreover, taking into account the main flux saturation and deep-bar effect, this paper concentrates on transient responses and stability of the DFIG system under symmetrical grid faults. With increasing wind power penetration, transient responses of doubly-fed-induction-generator based wind turbines gain attentive focus. Accurate prediction of transient performances of DFIG under grid faults is required with increasing wind power penetration. The present paper illustrates the influences of main flux saturation and of deep-bar effect on behaviors of DFIG during voltage dips respectively, and furthermore, clarifies their roles played in the enhancement of system transient stability. Simulation results using Matlab/Simulink are presented for a 1.5MW DFIG wind turbine system. Theoretical and small-signal analyses are also provided. The analyses proposed contribute greatly to proper selection, design and coordination of protection devices and control strategies as well as stability studies.In addition, this paper illustrates the impact of a grid fault on the mechanical loads of a wind turbine. New grid codes require wind turbines to ride-through grid faults. This poses great challenges for the design of both electrical system and mechanical structure of wind turbines. Grid faults generate transients in the generator electromagnetic torque, which are propagated in the wind turbine, stressing its mechanical components. To study the structural loads of wind turbine under grid fault, a complete model including both mechanical and electrical parts must be constructed. A drawback of wind turbine simulation is that either a simple mechanical components is used with a detailed electrical model, or a simple electrical components with a detailed mechanical model is used, which could not provide a throughout insight on the structural loads caused by sudden disturbances on the grid. In this research, a proper combination of different simulation packages, namely FAST (Fatigue, Aerodynamics, Structures and Turbulence) and Matlab, is used to model the electrical and mechanical aspects of a wind turbine respectively. The effect of a grid fault on the wind turbine flexible structure is assessed for a typical wind turbine. A set of simulations reflecting the structural dynamic response of a wind turbine to a grid fault are presented and analyzed.