Research On Control Method Of Microgrid Parallel Inverters Under Unbalanced Condition

With the rapid development of distributed generation system,microgrid technology has become one of the research hotspots in recent years.The micro sources in microgrid are mostly connected to the AC network side through distributed inverters.Distributed inverters are an important part of the energy exchange in microgrid.Therefore,the control strategy of inverters in microgrid is the key to the stable and reliable operation of microgrid.Grid connected inverters are often under complex conditions such as unbalanced grid voltage,grid impedance and filter inductance.At present,droop control and virtual impedance method has been widely used in grid connected inverter to improve system capacity and stability,which however exists potential unbalanced load voltage and stability problems under unbalance conditions.The control strategy of the voltage source inverter in the isolated island parallel state in the microgrid is studied in this thesis.The principle and output impedance of inverter droop control in microgrid are analyzed in detail,and the design of virtual impedance is further carried out.The stability of microgrid inverters under unbalanced line impedance is analyzed with the the new independent reciprocal basises modeling method.A fully selfadjusted virtual impedance method is proposed in this thesis with the modeling method of the new full-dq transformation.The arbitrary unbalanced information is identified by the pseudo-inverse method.Based on the pseudo-inverse parameter identification method,a self-adjusted virtual impedance is achieved for accurate power sharing and reliable voltage support with adaptive unbalanced voltage compensation.A parallel experimental platform consisting of two three-phase inverters is built,and the experimental platform is used to study the control strategy in this thesis.The simulations and experiment verify the accurate identification of unbalanced line impedance and the effectiveness of proposed adaptive virtual impedance method on the unbalanced PCC voltage support.