Research On Synchronization Stability Of Grid-Connected Inverter

Since the "carbon emission peak,carbon emission neutral" strategy was put forward,promoting the construction of the modern power system with a high proportion of renewable energy has become the current key development direction.As the installed capacity of grid-connected renewable energy such as wind energy and photovoltaics continues to increase,voltage source inverters(VSIs)are widely used as grid-connected interfaces,whose control is related to the safe and stable operation of modern power system.In order to ensure the reliability of the modern power system in extreme scenarios,in-depth research has been conducted on the synchronization stability of gridconnected inverters.The main contents and achievements of this paper are as follows:Aiming at the loss of synchronization(LOS)of grid-following inverter under severe grid faults,this paper uses the principle of time-scale decoupling to reduce the order of the high-order system.Then,the reduced-order nonlinear model has been established and the mechanism of synchronization instability has been analyzed.To clarify the main factors of the LOS,the influence of the bandwidth and damping ratio of the phase-locked loop(PLL)are quantitatively analyzed by the phase-plane method.It is revealed that the decrease of the damping ratio caused by the voltage drop at the point of common coupling(PCC)is the key factor of the LOS.Based on this,a new PLL structure with PCC voltage normalization control is designed to improve synchronization stability.Then,the phase-plane analysis and the simulation results in MATLAB/Simulink verify the effectiveness of the proposed strategy.To analyze the LOS of the grid-forming inverter during grid faults,the reducedorder nonlinear model has been derived.The parameters on active power control loop(APCL)have significant influence on synchronization stability,which are analyzed by the phase-plane method.Then,it is found that the LOS is always accompanied by a positive frequency deviation,which is verified by the nonlinear theory and the simulation results in MATLAB/Simulink.Then,analogous to the second-order transient model of the synchronous generator’s rotor,the nonlinear model of the grid-forming control and the grid-following control is compared,and the connection and difference between the two control strategies are deeply revealed.To remove the LOS of the grid-forming inverter,based on the phenomenon that LOS is always accompanied by a positive frequency deviation,two stabilization strategies are designed.The corresponding nonlinear reduced-order models are established,which are used to revealing the stabilization mechanism.The frequency responses are deeply analyzed by the linearization method,and it is proved that neither of the two stabilization strategies would deteriorate the frequency stability.Then,the synchronization stability is quantitatively analyzed by the phase-plane method and the guidelines for parameters design are given.Moreover,the theoretical analysis and the effectiveness of the stabilization strategies are verified by hardware-in-loop results.Finally,the advantages of the two stabilization strategies are synthesized,which are merged into a unified stabilization strategy,so that the appropriate stabilization strategy can be adopted according to whether the equilibrium point exists.Therefore,this unified stabilization strategy can reduce the impact on the steady-state performance of the inverter as much as possible and improve the synchronization stability.