Research On Stability Of Voltage-Controlled Microgrid-Interfaced Inverters Under Island/Grid-Connected Dual-Mode Operation

Distributed Power Generation System(DPGS)based on renewable energies(photovoltaics,winds,etc.)and energy storage devices(batteries and supercapacitors,etc.)has been widely used in microgrids.Due to the characteristics of the geographical distribution of renewable energy in China,many DPGSs are installed at areas which are far away from the electric power backbone network.In these areas,interfaces between power grids and DPGSs are generally weak,which leads to poor reliability.Therefore,DPGS is required to have abilities to operate in grid-connected mode(system is connected with the grid)and islanded mode(system is disconnected from the grid).As the power conversion unit and the interface unit in DPGS,the voltage source microgrid inverter(VSMI)based on power electronics significantly affect the output power quality and stability of the system by its control performance in islanded mode and grid-connected mode.Compared with current control method,voltage control method can ensure seamless switching between islanded mode and gridconnected mode,thus avoiding rush current and voltage during the transient.Due to different output filter,such as LC filter in islanded mode and LCL filter in gridconnected mode,dual-mode VSMI system generally has different stability performance.To study stability of the dual-mode VSMI in islanded mode and grid-connected mode,this thesis researched around the dual-mode VSMI based on traditional outer capacitor voltage loop and inner capacitor current loop control.The specific research contents are as following:(1)This thesis derived main circuit's s-domain models of dual-mode VSMI in islanded mode and grid-connected mode,respectively.Then key parts in the digital control system of the dual-mode VSMI such as sampling,calculation,and modulation were investigated and analyzed.Corresponding mathematical models were also derived.The analysis indicated that the digital control system used in this thesis had 1.5 times of sampling period equivalent digital delay.Therefore,these models provided a basis for the discrete z-domain modeling of the subsequent system and the corresponding stability analysis.(2)This thesis established models of the dual-mode VSMI system based on the traditional outer capacitor voltage loop and the inner capacitor current active damping loop in s-domain and z domain,respectively.Then Nyquist and Routh criteria were applied to study the pole distribution and surroundings of the point(-1,j0)of the openloop transfer functions of the digital control system in two modes,further to investigate the stability of two operating modes.The analysis showed that the control system in two modes had similar stabilities,that is,the stability of the system in a single operating mode and the design range of controller parameters were affected by the actual resonant frequency of the system;The stability of the system and the design range of the controller parameters in dual mode were affected by the system parameters of the two operating modes simultaneously.Resonant frequencies in different modes are not the same and are susceptible to be affected by variations of system parameters and the grid impedance,which leads to a stability blind area of the system,i.e.,when the system resonant frequency is in the stability blind area,no matter how the controller parameters are designed,control systems of two operating modes cannot be stable at the same time.(3)To eliminate the inherent stability blind area in the traditional control method,this thesis investigated a Parallel Feedforward Compensation(PFC)control method,and proposed a blind-area-free control design method.Then the stability of the proposed method in the z-domain was analyzed through Nyquist and Routh criteria.Analysis results showed that the stability of the dual-mode digital control system which used PFC method was no longer affected by the resonant frequencies of two modes and grid impedance variation at the same time,thus eliminating the stability blind area in the traditional control method.Besides,the system robustness against the grid impedance variation was also improved.The correctness of the above analysis and the effectiveness of the proposed method were verified through Simulation and experiments.