Synchronizing Stability Analysis And Control Technology Of Voltage Source Converters In Power System With High-Penetration Renewables

With the large-scale integration of renewables in modern power systems,power-electronic equipment represented by voltage source converters(VSCs)are penetrating sharply in power system,gradually replacing traditional electromechanical conversion equipment,and profoundly changing the modern power system.Power electronics dominated power system will deeply change the characteristics of grids physically,the applicability of long-established stability analysis theory and control technology is hindered,and many stability mechanisms and dynamic characteristics cannot be explained.Power-electronic equipment is the significant powerconversion equipment in power system,and its control technology and stability analysis are the hot topics concerned by the academia and industry.However,limited by the various types and complex controls of power-electronic equipment,the dynamic characteristics and stability mechanisms of equipment are not clear enough,which seriously threatens the stable operation of power system.To this end,this dissertation takes VSC as the research object,centring on its two typical control technologies(grid-following control and grid-forming control),based on the motion equation(from structural dynamics),aiming to explore and study the dynamic behavior and synchronization stability mechanism of power electronic equipment in a physical sense,and theoretically guiding the design of control system,so as to improve the safety and reliability of VSCs in power electronics dominated power system.The main work completed of this dissertation are drawn as follows:Firstly,the dynamic interactions between the control loops of grid-following converter are studied.Based on the motion equation,a mathematical model describing the internal dynamic interactions of a converter is proposed,by which the cognition of dynamic behaviors of VSC is deepened.With the proposed mathematical model,the dynamic interactions among AC voltage control loop,active power control loop and phase-locked loop(PLL)are mainly explored,and the negative damping effect of AC voltage control under different control bandwidths,grid strengths and voltage sag depths is explained.In order to overcome the above-mentioned negative damping effect that destroys the synchronization stability of converter,a damping enhancement strategy for AC voltage control loop is proposed,which effectively avoids the instability of converter.Based on the proposed model,the equivalent swing equation between the active power and virtual power angle is obtained,which proves the low virtual inertia characteristic of grid-following converter.Secondly,the dynamic interactions between the control loops of grid-forming converter are studied.The small-signal model of grid-forming converter with multiple control loops is deduced,then a motion equation-based mathematical model describing the dynamic interactions of an converter is proposed.Using this mathematical model,this dissertation proves the dynamic interactions between inner voltage/current control loops and the outer grid-forming control loop(power control with synchronization function),explains that the negative damping effect of inner voltage/current control loops reduces the synchronization stability of system under low control bandwidth,and analyzes the reason why the positive damping effect of low grid strength contributes to system synchronization stability.In addition,aiming at the typical grid-forming controls(doop control and virtual synchronous generator control),a simplified mathematical model of active power control loop is established,using the energy change of energy storage unit during the dynamic response of system,the physical significance of grid-forming converter's virtual inertia is illustrated,and the adjustability of virtual inertia is pointed out.Furthermore,by comparing grid-following converter and grid-forming converter,the application advantages of latter in power electronics dominated power system are clarified.Thirdly,the analysis method and parameter design strategy for grid-forming control considering electromagnetic transient characteristics are proposed.Aiming at the grid-forming control considering electromagnetic transient characteristics,a low-order state space model of converter is established.Using the eigenvalue analysis method,the dynamic interactions between control loops can be clarified,which deals with the coupling issues between control loops of gridforming converter.The dynamic interactions between virtual excitation loop and active power control loop are explored,and the reason of virtual excitation loop affecting the dynamic response and steady-state control accuracy of output powers is explained,and a guide plan for the parameter design of virtual excitation loop is correspondingly proposed.In addition,a parameter design method based on simulated annealing algorithm is proposed,which realizes the optimal damping ratio design of system,and effectively improves the dynamic performance of converter.Finally,the transient instability of grid-following converter and grid-forming converter under large disturbances is revealed.The transient models of grid-following converter and grid-forming converter are established respectively,which provide the theoretical basis for synchronization stability analysis of converter under large disturbance.Based on the relationship curve between voltage and virtual power angle,the transient instability of grid-following converter is analyzed;based on the relationship curve between active power and virtual power angle,the transient instability of grid-forming converter is explained;it is clear that the transient characteristics of two-type converters are similar to those of synchronous generators.Furthermore,the equivalent relations of transient characteristics of two-type converters are explored from four aspects,viz.,input,restoring force,damping coefficient and inertia coefficient,which strengthens the unity of converter analysis and design under different controls.Based on that,a unified selection scheme for the control parameters of two-type converters under large disturbances is given,and the largedisturbance stability of converters is improved.