Research On The Propagation Mechanism And Distribution Characteristics Of Subsynchronous Oscillation In Large-Scale Wind Power Grid-Connected System

In recent years,with the large-scale operation of new energy sources in China,as well as the rapid development of flexible transmission and other power electronics technology,China’s power grid has gradually formed a "double-high" power system characterized by a high proportion of renewable energy and a high proportion of power electronics.Due to the differences in China’s east-west energy structure,the west has sufficient new energy resources,while the east has a dense load,making long-distance and high-capacity power transmission has become an inevitable trend.However,the frequent occurrence of sub-synchronous oscillations after the large-scale connection of new energy wind power to the grid is a serious threat to the consumption of new energy wind power and even the stable operation of the entire grid.The repeated occurrence of sub-synchronous oscillations in wind power grid-connected systems has shown that sub-synchronous oscillation components can be propagated in multi-level grids,and the oscillation energy exhibits a wide range of propagation characteristics in the grid.Therefore,this thesis investigates the propagation mechanism and distribution characteristics of the sub-synchronous oscillations generated in the grid-connected wind power system,with the following main research elements:Firstly,a mathematical model of doubly-fed wind power grid-connected via series compensation line has established for the study of the propagation characteristics of sub synchronous oscillation in large-scale wind power grid-connected,including the wind turbine mechanical drive train model,doubly-fed induction generator model,converter and its control system model,DC bus capacitance dynamic model and power grid series capacitance compensation transmission line model.On this basis,the mechanism for the generation of sub-synchronous oscillations in doubly-fed wind power connected to the grid via a series compensation line is analyzed.The established system mathematical model will form the basis for the subsequent theoretical analysis and time-domain simulation of the propagation characteristics of sub-synchronous oscillations.Secondly,based on the instantaneous power theory,the manifestation and formation mechanism of sub-synchronous oscillation power are studied.To address the problem that the propagation characteristics of oscillations are difficult to be quantified,two quantitative analysis indexes,namely,the nodal propagation intensity of oscillation power and the branched transmission coefficient,which can characterize the propagation characteristics of oscillation power,are proposed to quantify the propagation characteristics of oscillation power,and the influencing factors affecting the propagation of oscillation power are studied.The two quantitative analysis indexes of the oscillation propagation characteristics provide an effective method for locating the key nodes and identifying the propagation paths of the oscillation power propagation,and the correctness of the above theoretical analysis and the effectiveness of the proposed quantitative analysis method are verified by time domain simulation.Finally,in order to further study the propagation and distribution characteristics of the sub synchronous oscillations generated by wind power is connected to the grid.The oscillation energy in the branch is resolved based on the transient energy function,and the oscillation propagation mechanism is studied according to the distribution of the oscillation energy in each branch in the network under the dominant oscillation mode of the system.The distribution coefficients of the branch oscillation energy are defined from the branch oscillation energy in the dominant mode,and the dominant path of oscillation propagation is thus identified.These dominant paths together form the oscillation cut-set,from which the oscillation cut-set can be further used to locate the high-risk areas of oscillation after sub-synchronous oscillations in the system.The effectiveness of the proposed method is finally verified by a time domain simulation example.