A Circulating-Current Suppression Method For Paralleled Inverters

The rapid development of Internet and communication technology has brought tremendous changes to the way of people work and live.The three phase voltage source inverters are widely used in microgirds and distributed generations.However,circulating current is caused between the paralleled inverters systems.Circulating current is consist of zero-sequence circulating current and nonzero-sequnence current.In general circulating current between the inverters with zero-sequence current is given priority to,but when the inverters are under unbalanced working conditions,the zero-sequence current cannot be ignored.With the circulating current,the zero-sequence current between the inverter on circulating current is the main component,and zero-sequence current in parallel inverter bridge arms of the switch tube caused the unbalanced current stress,increasing the loss of inverters parallel system,reducing the efficiency of the system,etc.,so the suppression the circulating current in inverters paralleled systems is of great significance.This thesis introduces the wireless paralleled inverter control methods firstly,mainly analyzing the using of traditional droop control strategy,and the main work for inverter respectively in current control method of grid mode and in an island mode,in which the traditional voltage and current double loop control strategy are analyzed theoretically and with the simulation verification.Secondly,this thesis analyzes the paralleled inverters in parallel operation mode of mathematical model,especially the mathematical model according to the different coordinates of inverters for the circulating current between the parallel inverter system composition,circulating current's channel and the mathematical model has carried on the concrete analysis,and emphatically analyzes the zero-sequence current between the paralleled inverters,which is given the traditional droop control cannot very effectively restrain circulating current,this thesis on the basis of traditional droop control,is used by a virtual resistance and inductance synthetic to a virtual impedance used in loop control,and the superposition of the power flow and the circulation feedback compensation according to the output voltage of circulation,in the end using the simulation to verify the correctness and feasibility of the proposed control method.Finally,this thesis in view of the inverter under unbalanced conditions is also analyzed,and the condition of the first,by introducing the instantaneous symmetrical components method,we can give the relevant mathematical model under the condition of unbalanced inverter and specific expression of circulating current,and through the concrete analysis of the cause of the inverter under the condition of unbalanced work,according to the analysis of the nonzero-sequence circulating current channel exists and its specific expressions,and then the frequency characteristics wtih the nonzero-sequence current is obtained,and then designed a proportional integral quasi-resonant(PIR),through the quasi-resonant device can effectively restrain the zero sequence current,then analyzing to the design of the amplitude frequency characteristics of quasi-resonant controller analyzed control methods,in the end,according to the proposed controller method has carried on the simulation analysis and verification.