Investigation On The Grid Voltage Fault Ride-Through Technology Of DFIG-based Wind Turbines

As the penetration of wind power in electrical power system increases and the grid codes of wind energy gradually develops, it is quite necessary to have grid fault ride-through ability for wind turbines(WTs). The key requirements of the modern grid codes can be summarized as follows:WTs are not only required to "withstand" all kinds of grid faults, but also should have the ability to "support" the recovery of the faulted power. From the perspective of the common types of grid failure, the most directly and significantly harmful ones to the grid-connected WTs are the voltage symmetry/asymmetry fault (dip or swell), especially for the widely applied Doubly Fed Induction Generator (DFIG) system compared to system based on full power converters. From both theoretical research and engineering application points of view, thus, it is valuable to investigate the operational characteristics of grid-connected WTs under such typical grid faults, as well as putting forward the corresponding control strategies.In this background, this thesis, detailedly and systematically, study the operational characteristics of the DFIG based WTs under these common symmetry or asymmetry grid faults. And theoretical analysis, simulation and experimental validations have been carried out. Moreover, a series of improved control strategies for the grid-friendly WTs are proposed, which can satisfy the latest grid codes. The main research of the paper is:1) Modeling analysis and a series of improved control strategies under grid voltage symmetrical dip or swell slightly conditions, including flux linkage damping control, reactive power support theory, nonlinear backstepping control algorithm and variable dc link voltage reference control strategy;2) Modeling analysis and a series of improved control strategies under grid voltage asymmetrical fault conditions. In order to develop the operation performance of the overall DFIG generation system, a coordinated control strategy with direct resonant feedback control method has been presented, the rotor and GSC currents can be regulated without sequential separations to simplify the control structure, and also enhance the system parameter robustness.3) In severe faulty conditions, combining the protection device of active crowbar and variable pitch control technology with above-mentioned improved control strategies. Under the premise of ensure the safety of the WTs to face the surge current and voltage, the WTs are able to output reactive power support to the grid, which is comply with the grid code requirements.Finally, make a brief introduction of the constitution of the experiment hardware platform of a new programmable grid fault emulating power and a3kVA DFIG system. An necessary experiment research of the proposed control strategies is developed, the results of which verifies the correctness and effectiveness of control strategies. Which also show that the proposed grid fault ride-through strategies can enhance the ability of WTs’uninterruptable operation, and have good potential for the wind power industrial applications.