Coordinated Rotor-Side Control Strategy for Doubly-Fed Wind Turbine under Symmetrical and Asymmetrical Grid Faults

Abstract

In order to solve the problems of rotor overvoltage, overcurrent and DC side voltage rise caused by grid voltage drops, a coordinated control strategy based on symmetrical and asymmetrical low voltage ride through of rotor side converter of the doubly-fed generator is proposed. When the power grid voltage drops symmetrically, the generator approximate equation under steady-state conditions is no longer applicable. Considering the dynamic process of stator current excitation, according to the change of stator flux and the depth of voltage drop, the system can dynamically provide reactive power support for parallel nodes and suppress the rise of DC side voltage and rotor over-current. When the grid voltage drops asymmetrically, the positive and negative sequence components are separated in the rotating coordinate system. The doubly fed generator model is established to suppress the rotor positive sequence current and negative sequence current respectively. At the same time, the output voltage limit of the converter is discussed, and the reference value is adjusted within the allowable output voltage range. In order to adapt to the occurrence of different types of power grid faults and complex operating conditions, a fast switching module of fault type detection and rotor control mode is designed to detect the type of power grid faults and voltage drop depth in real time and switch the rotor side control mode dynamically. Finally, the simulation model of the doubly fed wind turbine is constructed in Matlab/Simulink. The simulation results verify that the proposed control strategy can improve the low-voltage ride through performance of the system when dealing with the symmetrical and asymmetric voltage drop of the power grid and identify the power grid fault type and provide the correct control strategy.