Research On Constant-amplitude Oscillation Of DFIG Under Dynamic Influence Of PLL

The increasing energy crisis and environmental pollution are driving the large-scale development and utilization of renewable clean energy represented by wind power generation.While continuously providing renewable clean power for the sustainable development of society,wind power generation also brings new threats and challenges to the safe and stable operation of the power system due to the distinctive characteristics of the operation characteristics of traditional synchronous generators.Due to unreasonable wind turbine(WT)controller parameter settings or interaction with other equipment in the power grid,new types of oscillation problems will arise,such as subsynchronous control interaction and constant amplitude oscillation(CAO).Although the WT system did not continuously diverge to instability during constant amplitude oscillation but remained in a stable oscillation operating state,this oscillation still poses a major threat to the safe and stable operation of the power system and is unacceptable for the normal operation of the power system.According to the existing literature,there are few research results for the problem of constant amplitude oscillation of doubly-fed induction generaor(DFIG)WTs.Therefore,in order to understand the problem of CAO of DFIG WTs more comprehensively and improve the safety and stability of large-scale wind power grid connection,this paper starts with the important nonlinear part in the controller of DFIG WTs,i.e.the phase-locked loop(PLL),and studies the causes,evolution rules and behavior characteristics of CAO of DFIG WTs under the dynamic influence of the PLL.The main research content of this dissertation are as follows,Firstly,the working principle and control strategy of the DFIG are analyzed and summarized.Based on this,a mathematical model suitable for analyzing the problem of CAO is built and the corresponding time-domain simulation model is built in the MATLAB / Simulink environment,which lays the foundation for the time-domain simulation analysis in the following chapters.Secondly,combined with several analysis methods and tools commonly used in the study of subsynchronous oscillation in power systems,modal analysis method and time domain simulation method and Hilbert-Huang transform were selected as the main methods and tools for analysis CAO problem.Combined with the built model,the modal analysis method is used to predict the occurrence of CAO.Simulink’s simulation results are used to verify the prediction.This prediction-verification process proves that the selected analysis method is suitable for analyzing the CAO problem of DFIG.Finally,combining FFT and HHT,the frequency characteristics during the period of CAO are preliminarily explored.Furthermore,when considering the dynamics of the nonlinear PLL,the law of the CAO of the DFIG is analyzed as follows: with the increase of the bandwidth of the PLL,the oscillation frequency increases continuously,the time for the steady-state evolution to the CAO decreases,and the maximum / minimum value during the oscillation decreases slightly.It is found that there are two divergent forms of the angle of PLL when the bandwidth of PLL changes.That is,the amplitude of oscillation monotonically changes from steady state to CAO,and the amplitude of oscillation increases first and then decreases to CAO.Further,the causes of the two forms of CAO,the evolution law of the stage of oscillation divergence and the characteristics of the stage of CAO are studied.It is found that the oscillation law of the stage of equal amplitude oscillation is as follows: 1.At the beginning of divergence,the oscillation frequency of the instantaneous amplitude is the dominant mode obtained from the modal analysis.With the continuous divergence of the system,the oscillation frequency drops smoothly,and finally the oscillation frequency will oscillate periodically around the value slightly lower than the initial oscillation frequency;2.The frequency of each state quantity frequency also changes continuously,from the power frequency value under the steady state to the periodic change,the changing period is the same as that of the amplitude in the CAO stage,and as the bandwidth continues to increase,the instantaneous frequency revolves around the stable and the range of floating state values also continues to increase.