Mechanism And Interaction Research On Sub-synchronous Oscillation Of Doubly-fed Wind Farm Integrated Into Grid Through Multi-level Modular Converter Based High Voltage Direct Current System

With the growing energy demand and the requirements for green and low-carbon development,renewable resources such as wind energy and solar energy play an increasingly important role in the "carbon neutrality",and has been vigorously developed.Due to reverse distribution of wind energy resources and loads,large-scale wind power is generally transmitted by long-distance series compensation or voltage source converter-based high voltage direct current(VSC-HVDC)transmission system.Among them,the modular multilevel converterbased high-voltage direct current(MMC-HVDC)transmission technology has become the mainstream HVDC transmission technology due to its advantages such as low losses,high waveform quality,and powerful fault handling capabilities.Doubly fed induction generator(DFIG)-based wind farm integrated through MMC-HVDC are expected to become one of the most important wind power transmission technology.However,there have been many reports on the sub-synchronous oscillation in the DFIG-based wind farm integrated through MMCHVDC.Therefore,it is of great theoretical significance and practical value to carry out related work include modeling and mechanism analysis on the subsynchronous oscillation problem of such a system.Aiming at the subsynchronous oscillation in DFIG-based wind farm integration system via MMC-HVDC,this dissertation will evaluate three aspects involveing high-fidelity modeling,the mechanism analysis of subsynchronous oscillation,and the interaction between the DFIGbased wind farm and MMC-HVDC,as listed in the following.Firstly,as for modeling research,a modeling idea that merges the electrical subsystem and the control subsystem with rotor voltage and grid-connected voltage is proposed.Then,a fullscale small-signal impedance model of DFIG considering phase-locked loop,grid-side converter and its control and DC link,as well as MMC-HVDC considering the sub-module capacitor voltage ripple and circulating current suppression control are established.Based on the theoretical research,five different simplified models are also provided.Finally,frequency scanning method is used to verify the effectiveness of the proposed model.Secondly,in order to compromise model fidelity and simulation speed,how the modeling scale meet the requirement of the concern issue is discussed.The impedance difference is defined to quantify the difference between the simplified model and the full-scale model.Based on the five simplified models,whether the different models match the concerned subsynchronous oscillation is respectively analyzed from the perspectives of influencing factors and frequency range,so as to build the basis for the mechanism analysis.Thirdly,the mechanism analysis on the subsynchronous oscillation of DFIG-based wind farm integration through MMC-HVDC is thoroughly investigated.Based on the RLC equivalent circuit of the simplified impedance model and the impedance property,how the converter control affect subsynchronous oscillation characteristics is obtained from the equivalent resistance and equivalent reactance.Furthermore,the role and interaction of all parameters on system subsynchronous oscillation are comprehensively revealed.Finally,as for the causes of subsynchronous oscillations in the interconnected systems,the interactive effect between DFIG-based wind farms and MMC-HVDC as well as the influence of key parameters are carefully revealed both qualitatively and quantitatively with time-domain simulation and relative gain array analysis.