Research On Stability Of Parallel Multi-inverters Based On Impedance Analysis In The Island Microgrid

With the continuous increase of energy demand and the increasingly serious environmental pollution,distributed generation has received widespread attention,and the micro-grid structure including multiple distributed power sources has emerged.The distributed sources in the microgrid mainly include energy storage,wind power,photovoltaics,micro gas turbines etc.,all of which are integrated into the microgrid by the inverter as an interface,thus constituting a parallel operation environment of multiple inverters.Because the microgrid contains a large number of power electronic converters,the microgrid exhibits low inertia,low damping,strong time-varying and other nonlinear characteristics,which will make the system more susceptible to oscillation instability.Even if each distributed power supply is stable,the interaction between the distributed power supplies can cause stability problems in the system.In this paper,several micro-source inverters with droop control and micro-source inverter parallel system with power control are taken as the research object.The mathematical models and equivalent circuits of two kinds of micro-source inverters are established.The influence of control parameters on the stability of each type of inverter system is analyzed.The micro-source inverter based on the droop control is regarded as a voltage-controlled inverter(VCI),and the micro-source inverter based on the power control is regarded as a current-controlled inverter(CCI).On this basis,a unified mathematical model of inverter parallel system with different control modes is established,and the system is equivalent to two parts according to the equivalent circuit model.The stability is determined by Nyquist criterion respectively.The influence of the number of control type inverters on the stability of the system.For the resonance of parallel system between the VCI and CCI,a virtual harmonic impedance strategy at the resonance frequency is proposed,the resonance problem is solved by impedance reshaping,and the number of mircosource based on power control is increased.Under the premise of ensuring system stability,the utilization rate of renewable energy in the island microgrid is improved.In order to verify the correctness of the above system control method,several simulation models of inverters parallel system with different control types were established by MATLAB simulation software,and experimental platforms with parallel two type inverters were built,and the software program preparation and debugging were completed.The control strategy designed in this paper is verified.The experimental results show the feasibility and effectiveness of the proposed control strategy.