| Microgrid is a new technology based on distributed generation,which is an important supplement to smart grid.It can effectively promote the access and local absorption of distributed energy,and further improve the efficiency of renewable energy.Most distributed micro-sources belong to inverted power supply,these micro-sources operate in parallel with AC bus through inverters,which constitute the parallel operation environment of multiinverters in microgrid.Therefore,the reliable and efficient control technology of parallel inverters is the key to ensure the stable operation of microgrid.In this paper,the problem of circulating current restraining and accurate load power sharing control between parallel inverters in islanded microgrid are studied integrally.Firstly,the three-phase voltage source inverters mathematical model was established based on the corresponding topology from the view of single inverter control and the voltage and current double closed-loop control equations of three-phase inverters were obtained.Constant power control and droop control model of three-phase inverters were established and the respective control characteristics and applicable conditions were analyzed.A presynchronization control method for parallel inverters based on rotating frame was presented and above methods were verified by three parallel inverters simulation case.Secondly,the relation between circulating current and load power sharing was analyzed according to the general definition of circulating current,on this basis,the problem of circulating current restraining was transformed into the problem of accurate power sharing and the characteristics of circulating current were further analyzed.The power transfer equation of inverters was derived and theoretical basis of droop control was explained.The control mechanism of virtual impedance was analyzed and the limitations of traditional droop control and virtual impedance droop control were analyzed detailedly.The control effects of these two methods were verified by simulation case in PSCAD/EMTDC software.Finally,the control principle and shortcoming of traditional robust droop control were analyzed in detail,and an improved robust droop control method with bus voltage and frequency self-recovery function was proposed.The load power accurate sharing and steadystate circulating current were achieved in improved method and it can reduce the dynamic circulating current simultaneously.The bus voltage quality was improved greatly through closed-loop control of bus voltage and frequency.The stability of the improved controller was analyzed by using small signal modeling method and the effectiveness of the improved robust droop control method was verified by a simulation case. |
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