Research On Multi-timescale Instability Mechanism And Passivity Enhancement Control Method Of AC Microgrid

The AC microgrid with high-permeability distributed power generation is greatly affected by the natural environment and operating conditions.Compared with traditional power grids,its stability problem with new characteristics is caused,and the system is vulnerable to the inherent nonlinear characteristics of power electronic converters and the outside disturbances,which challenges the stable operation and efficient control of microgrid.In this paper,the AC microgrid composed of single-phase LCL inverters is the research object,and its modeling,stability,and passivity enhancement control are studied.The microgrid in this paper is divided into islanded and grid-connected microgrid.In addition to the microgrid stability affected by control parameters,the stability of islanded microgrid is also related to the load conditions,while the stability of the grid-connected microgrid is mainly affected by the interaction between the grid and the inverter.In short,the main work in this paper on microgrid modeling,stability analysis and passivity enhancement control are as follows:(1)As for the islanded AC microgrid composed of two single-phase LCL-type inverters with a resistive load,the BPF-based droop control scheme is first extended to islanded single-phase microgrid in hybrid frame in this paper to achieve the deviation restoration of voltage amplitude and frequency,without any communication lines and additional control loops.In addition,based on the characteristics of multiple time scales,a reduced-order small signal model of the AC microgrid is derived,thererby the modeling process and theoretical analysis of the microgrid can be simplified by using this model.Then,the analysis method based on eigenvalues is used to analyze the influence of system parameter variations on the stability and dynamic performance of the microgrid,and the analytical results show an overdamped system with good stability and robustness against system parameters shift.Finally,the effectiveness of the eigenvalue analysis is verified by simulation results,and the experimental results verify the feasibility of the BPF-based droop control method in the islanded single-phase microgrid.(2)For the AC microgrid consisting of multiple parallel inverters connected to the grid,the passivity-based criterion can be utilized to analyze the interactive stability between the inverters and the grid.However,when using the passivity theory to evaluate the stability of multi-parallel inverter systems,the influences of grid synchronization and multisampling on the passivity of inverter output admittance are overlooked in previous studies.Hence,the impact factors in the grid-connected control,such as multisampling,capacitor current feedback active damping(CCF-AD)and various phase-locked loops(PLLs),on the passivity of inverter output admittance is investigated in this paper.Moreover,the stability of the interconnected system in the inductive and capacitive grid scenarios are also assessed.Since the size of control delay are determined by the multisampling coefficient,the passivity of inverter output admittance in the highfrequency range is mainly affected by multisampling and CCF-AD,whereas the passivity in the low-frequency range is related to the bandwidth of PLLs.In case of an identical PLL bandwidth,the non-passive region of admittance when using SOGI-PLL appears in a lower frequency band,and thus the width of non-passive region is smaller in comparison,which provides stronger robustness.Finally,the validity of the theoretical analysis is verified by simulation and experimental results.(3)Aiming at the actual demand for enhancing the stability of islanded microgrid under different load conditions,a robust passivity enhancement control method is constructed in this paper for an islanded microgrid with global stability,including linear,nonlinear and constant power loads(CPL).A hybrid control strategy is composed of the passivity enhancement control in the bottom layer of the microgrid and the enhanced droop control based on virtual impedance and BPF in the upper layer.The upper-level control is used to suppress the circulating current between the inverters and achieve power sharing,and the bottom-level control is utilized to realize the control of inverter output voltage,which is effective to ensure a stable AC bus voltage under load disturbance and parameters shift conditions.The passivity control is also adopted in a PWM rectifier as a CPL to reduce the harmonic pollution on the AC side of the rectifier and improve the voltage quality at the point of the common coupling(PCC)of inverters.In addition,the steady-state and the dynamic performance and the robustness of the passivity-baed control are simulated and analyzed under three aforesaid load conditions.The results show that the power quality and system stability of the AC microgrid under complex load conditions are improved by applying the hybrid control strategy in this paper.The effectiveness of the control strategy is further validated by the experimental results.