When the parallel battery inverters work under the micro-grid island mode

the parallel system needs to provide a stable voltage and frequency support for the micro-grid

but the differences of inverters’ equivalent output impedance and the line impedance may cause the problem of power sharing and circulating current. In order to solve these problems

on the basis of the conventional P-U and Q-f droop control strategy

the virtual impedance is proposed to redesign the converters’ equivalent output impedance as pure resistance and then the power sharing performance can be improved. Beginning with the multi-inverter parallel system’s topology

we analyzed the power flow characteristics of the system and established its output impedance model; The entire system’s control strategy was introduced in detail including the droop control with the introduction of virtual impedance and the inverter’s double-loop control; The influence of the filter and controller’s parameter was analyzed according to the impedance stability analysis method. Based on the working condition in the article

we can find that when the filter inductor increases to 5 m H

the inverter parallel system tends to be unstable; When the virtual impedance coefficient kL increases to 3

the nyquist curve of system’s impedance ratio will go cross the rejection region

and the system’s equivalent output impedance turns into inductive one against high-frequency harmonic suppression; The increase of virtual impedance coefficient RD can strengthen the effect of power split and slightly affects the system’s stability. Based on the working condition in the article

the simulation results indicate that the proposed control strategy can decrease the maximum circulation current between the multiple inverter from 30 A to 3 A

which can ensure the stability of the PV/battery micro-grid system.