Research On Fundamental And Harmonic Current Sharing Strategy Of Parallel Inverters

Microgrid can realize the work of high reliability of each distributed power supply(DG).It can not only solve the problem of the large number and variety of DGs being integrated into the power grid,but also promote the access of a large number of micro-power sources,so as to realize the high reliability of power supply to the load.It can work in both grid-connected mode and island mode.When microgrid operates in island mode,the local load power needs to be shared by the parallel DG inverters.Due to the limited geographical position of each DG,the feeder impedance connected to the common AC bus mismatch,and the local load contains a large number of nonlinear loads.When the traditional droop control is adopted,large fundamental and harmonic circulation current will be generated between parallel inverters,which will affect the stable operation of the system.At this time,how to effectively suppress the fundamental and harmonic circulation current,balance the power distribution among the inverter units,and make the parallel inverter reliable operation is an important issue in the operation control of the microgrid,which is also the research focus of this paper.In this paper,the parallel system of combined three-phase inverter in low-voltage microgrid is taken as the research object.Firstly,the mathematical equation of the inverter is established,and the control block diagram of the parallel inverter is briefly analyzed.Based on the equivalent models of parallel inverters with nonlinear load in fundamental domain and harmonic domain,the causes of fundamental and harmonic circulation between parallel inverters are analyzed deeply.It is concluded that the mismatch between the feeder impedance and the rated capacity of the parallel inverters connected to the common AC bus in the microgrid is the main cause of the fundarmental and harmonic circulation in the system.If the virtual impedance is added to the whole frequency domain,the fundamental and harmonic circulation becomes smaller and smaller with the increase of the virtual impedance,but the voltage amplitude of the common AC bus will decrease and the voltage quality will become worse.Based on this,the different power sharing strategies in the fundamental domain and harmonic domain are designed in this paper.An improved droop controller with active power integral method is proposed in the fundamental domain.The integral method is introduced to remove the balance between feeder impedance and active power,so that the average active power is not affected by line impedance,so that the fundamental active power can be distributed according to the ratio of capacity.At the same time,the adaptive virtual harmonic impedance controller is introduced in the harmonic domain to restructure the output impedance of the system in the harmonic domain to match the feeder impedance,to adjust the harmonic current distribution dynamically,and to restrain the harmonic circulation current effectively.Finally,the fundamental active power and harmonic current are divided equally.After superposition of reference voltage in fundamental domain and harmonic domain,modulated signal by voltage-current double loop drive inverter to operate.And the fundamental power and harmonic power of parallel inverter can be divided equally.It also ensures the quality of common AC bus voltage.In order to verify the effectiveness of the design control strategy in the paper,a parallel three-phase inverter parallel system simulation model was established in the software,and two combined three-phase inverter parallel system experimental platforms based on the control chip were built.The control strategy proposed in this paper is simulated and verified by experiments.The results show that the effectiveness and feasibility of the system control strategy designed in this paper.