Research On Small Signal Stability Of AC/DC Hybrid System Under The Dynamic Effect Of Phase-Locked Loop

Under the background of China’s huge AC network,flexible DC transmission projects are continuously put into operation,which greatly promotes the development of AC/DC hybrid system.The phase tracking of the flexible DC and AC grid interconnection points is realized by the phase-locked loop.Therefore,whether the phase-locked loop can accurately track the voltage phase of the grid-connected point is particularly significant.In recent years,it has been found in the past literature that the wide bandwidth of the phase-locked loop in the weak connected grid can affect the stability of the system.Therefore,the research on the system instability caused by the dynamic characteristics of the phase-locked loop needs further study.Based on the parameter stability region of the proportional-integral controller of the phase-locked loop,the interactions between the dynamic characteristics of PLL and the power systems are analyzed,this reveals the instability problem caused by the dynamic characteristics of the phase-locked loop.Firstly,the research background of this paper is introduced.The research status at home and abroad is analyzed from three aspects:stability of AC/DC hybrid system,dynamic characteristics of phase-locked loop and parameter stability region.The conclusion that the internal mechanism of the AC/DC hybrid system instability caused by the dynamic characteristics of the phase-locked loop is still need to be further studied.Then,in order to analyze the influence of phase-locked loop dynamic characteristics on small signal stability of AC/DC hybrid system,the phase-locked loop is modeled separately,and the mathematical modeling and linearized model of the phase-locked loop,DC system and AC system unit are given in modules.Next,the application principle and related steps of eigenvalue analysis method are briefly summarized.The method of calculating the parameter stability region of phase-locked loop proportional integral controller is introduced in detail.On this basis,the peak-like bulge appearing in the parameter stability region is analyzed.The dynamic characteristics of the phase-locked loop are used as the feedback link,and the remaining grid system is the feedforward link.The closed-loop interconnected system model is constructed,in this way,the interactions between the dynamic characteristics of the phase-locked loop and the AC/DC hybrid system are analyzed.Finally,the parameter stability region of the phase-locked loop proportional integral controller is simulated and analyzed by numerical examples.The parameter stability region represents system stability is verified by considering the modeling of the generator shaft block system and the modeling of the overall generator.The results show that the parameter stability region of phase-locked loop indicates the stable value range of the phase-locked loop proportional integral controller.For the peak-like bulge in the parameter stability region,through the participation factor analysis,it is caused by the dynamic interaction between the phase-locked loop mode and the relevant mode in the system.The conclusions are further validated by the root locus variation diagram of the mode motion and the nonlinear simulation results of the relavant variables.The dynamic interactions will make the closed-loop mode move to the right half plane,resulting in the negative damping in the system,which will cause the system to lose stability.