Research On Modeling And Synchronization Stability Of Grid-connected Inverters For Multi-scenario Applications

With the increasing penetration of new energy power generation,the grid-connection technology needs to change from “passive adaptation” to “active support and independent operation”.The high controllability and flexibility of grid-connected inverters make it possible to realize that change,but bring in complex synchronization instability accidents endangering the safe and stable operation of power system.Focus on the static and small-signal synchronization instability of grid-connected inverters controlled under the phase-locked loop(PLL)and virtual synchronous generator(VSG),this dissertation takes the multi-scenario applied grid-connected inverter systems as the study objects,including the PLL-based current-controlled inverter(PLL-CCI)system,the VSG-based voltage-controlled inverter(VSG-VCI)system and the hybrid system combining the PLL-CCI and VSG-VCI.By establishing unified mathematical models,this dissertation will carry out profound study of the synchronization features,synchronization instability mechanism and stability improvement methods of the PLL-CCI and VSG-VCI in different application scenarios and varied grid strengths.The aim is to provide theoretical basis and practical guidance for the reasonable selection and optimization of synchronization control methods of grid connected inverters to promote the friendly grid-connection of new energy power generation.The static instability of grid-connected inverters in multi-scenario applications resulting from the loss of steady state operating point is studied.By establishing quasi-static models of the PLL and VSG in different application scenarios,the synchronization mechanisms of the PLL and VSG are analyzed and the coupling mechanisms among PLLs,VSGs as well as PLLs and VSGs are revealed,then the static synchronization stability of the PLL-CCI and VSG-VCI in different application scenarios are studied.Simultaneously,by analyzing the power transfer characteristics of grid-connected inverters in multi-scenario applications,the static power stability of the PLL-CCI and VSG-VCI in different application scenarios are studied.Based on the above researches,main factors that influence the static stability of grid-connected inverters are found out.Moreover,the conditions ensuring the existence of steady state operating point of each inverter in the PLL-CCI system,the VSG-VCI system and the hybrid system are determined.Which contributes to reasonably set the running points of grid-connected inverters in different application scenarios and varied grid strengths,thus to avoid the static synchronization instability.Based on the harmonic linearization theory,unified mathematical models for the small-signal synchronization stability analysis of grid-connected inverters in multi-scenario applications are established.According to the small-signal disturbance and response in frequency-domain,coupled sequence-admittances of the PLL-CCI,coupled sequence-impedances of the single-loop and multi-loop VSG-VCI are built.Based on the coupled sequence-admittances or impedances,the interactive process of circuit-coupling and frequency-coupling between the inverter and grid is studied,thus the equivalent sequence-admittance or impedance ratio-based stability analysis models for the single grid-connected inverter are derived.Based on the equivalent sequence-admittances,the coupling relationships among grid-connected inverters are studied,then the equivalent sequence-admittance ratio-based stability analysis models for the grid-connected multi-PLL-CCI system,multi-VSG-VCI system and the hybrid system are bulit.The small-signal synchronization instability issues resulting from the insufficient damping of synchronization control loops of grid-connected inverters in multi-scenario applications are further studied.Based on the equivalent sequence-admittance ratio models,the influences of frequency-coupling and negative damping on coupled low-frequency resonances are analyzed simultaneously.Moreover,the coupled low-frequency resonance characteristics of multi-PLL-CCI systems are studied.Based on the sequence-impedance models,the synchronous frequency resonance(SFR)of single-loop and multi-loop VSG-VCI are effectively studied,the relationships of the SFR and “negative damping” of sequence impedances are figured out.Then the influences of VSG control,virtual impedance and inner control on SFR are determined.Based on the sequence-impedance ratio or equivalent sequence-admittance ratio models,the small-signal synchronization stability of the VSG-VCI system and hybrid system is studied and compared to that of the PLL-CCI system,thus to discover the optimization directions of the synchronization control of weak-grid-connected inverters in multi-scenario applications.Regarding the serious synchronization instability of the weak-grid-connected PLL-CCI,corresponding improvement schemes are proposed.As for the static instability of the PLL-CCI,by utilizing the multi-function control advantages of the PLL-CCI itself,a reactive-power current compensation control scheme is designed to improve the static stability of the PLL-CCI,considering the grid codes that the voltage drop at the point of common coupling is within 10% and the power factor is higher than 0.9.As for the coupled low-frequency resonance of the PLL-CCI,based on the sequence-admittances,on the one hand,on the premise of retaining PLL,the feedforward control technology is adopted to design a control scheme to balance the dynamic performance and stability of the PLL,thus to improve the small-signal synchronization stability of the grid-connetced CCI;on the other hand,by removing the PLL,the filter technique is utilized to study the small-signal synchronization stability improvement method of the grid-connetced CCI without the PLL based on the instantaneous power theory.