Stability Analysis And Research On Control Strategy Of Grid-connected Doubly-fed Wind Turbines

The doubly-fed wind turbine is based on a three-phase converter as the basic unit and uses a phase-locked loop to synchronize with the AC grid.When large-scale doubly-fed wind farms are integrated into the weak grid,the coupling between units and units,and between units and the AC grid deepens,and their interaction may lead to phase locking failure and small disturbance synchronization instability.For the local grid containing large-scale doubly-fed wind farms,it is crucial to accurately establish the impedance conductance model of doubly-fed wind turbines,reveal the mechanism of small disturbance synchronous instability of doubly-fed wind turbines and improve the stability margin of grid-connected doubly-fed wind turbines to improve the wind power consumption capacity.Based on the modal analysis and impedance analysis,this paper focuses on the small disturbance synchronous stability of grid-connected doubly-fed wind turbines,and conducts research in three aspects,namely,the admittance coordinate transformation and simplification of impedance criterion,the analysis of small disturbance synchronous stability of converter unit,and the improvement of grid-connected stability margin of doubly-fed wind turbines.The main work and innovations of this paper are as follows:1.To address the uniqueness of the impedance admittance modeling of the threephase converter system,the admittance equivalence relationship and the equivalent circuit of the three-phase AC system in each coordinate system are studied.Firstly,the admittance models in each coordinate system are developed,and the equivalence of the admittance models in the same rotational velocity coordinate system is explained by the admittance eigenvalues,and the stability margin of the asymmetric system in the rotational coordinate system is lower than that in the stationary coordinate system by the total damping of the system.Secondly,the equivalent circuit of the three-phase symmetric converter system is established by the nodal admittance matrix and extended to the multi-machine system.Finally,the small disturbance stability of 3-phase asymmetric converter systems with frequency coupling phenomena is mathematically demonstrated by the impedance analysis method,which can also explain the small disturbance stability with circuit resonance-like conditions.2.For the small disturbance stability of 3-phase grid-connected converter system,the simplified impedance criterion of 3-phase converter system and the influence mechanism of small disturbance stability of converter unit under weak grid are studied.Firstly,the admittance model in each coordinate system is established,and the amplitude-frequency characteristics of the admittance matrix in the complex coordinate system satisfy the diagonal conjugate symmetry.Second,based on the diagonal conjugate symmetry,it is explained that the three-phase converter system can be derived as a single-input singleoutput system in the complex coordinate,and the respective stability margins in the positive and negative frequency domains are defined.Finally,the contribution of converter unit controller parameters to the small disturbance stability of the system under constant DC voltage control is analyzed in detail.3.For the small disturbance stability of grid-connected doubly-fed wind turbine,based on the simplified impedance criterion in the complex coordinate,the influence law of the controller parameters on the small disturbance stability of the system under the weak grid and the improvement of the stability margin by using the parallel virtual admittance method are studied.Firstly,the admittance model of the doubly-fed wind turbine under symmetric control is established,and the independent double loops in the complex coordinate are derived based on the node admittance matrix to reveal the small disturbance instability mechanism of the grid-connected doubly-fed wind turbine.Second,the factors affecting its small disturbance stability under weak grid are analyzed.Finally,based on the parallel virtual admittance method,the current loop control structure is improved to enhance the system stability margin.