Research On Coordinated Control Strategy Of Parallel Inverters System In Microgrid

The microgrid operating in the island mode puts forward much higher power quality requirements on account of outage of the large power grid.Multi-inverter module parallel system based on droop control,which can realize independent peer-topeer control of each module without interconnecting lines,has been widely applied in micro-grid in island mode in virtue of its high power supply reliability.However,harsh power equalization conditions lead to problems such as low uniformity accuracy and control coupling in traditional droop control strategy.In addition,the voltage imbalance caused by the unbalanced loads access further affects safety and stable operation of the system.In order to remedy above defects,two parallel three-phase inverters are taken as an example to carry out in-depth research on the coordinated control strategies of the inverters in the parallel system.Firstly,based on the coordinate transformation,the mathematical model of the single voltage source three-phase inverter decoupling under the dq axis is established.Meanwhile the double loop controller of the capacitor current inner loop and voltage outer loop is determined,and its PI parameters are respectively adjusted.Then the basic principle and inherent defects of the droop control are analyzed in detail based on characteristics of micro power supply.The droop coefficient is set by the dynamic phasor method and the root locus method.With the necessary conditions to achieve power equalization under droop control acquired by analyzing the circumfluence generation mechanism and power distribution mechanism,it is found that the reactive power control loop is easily affected by the inconsistent equivalent impedance due to the lack of integral link.So the Q-(35)U robust droop control is constructed to improve the reactive power equalization accuracy.In addition,because of the control coupling phenomenon caused by the resistive component in the equivalent impedance,the virtual impedance technique is introduced to adjust the resistance characteristic of the inverter output impedance,and the variable virtual impedance strategy is proposed to optimize the voltage drop caused by the fixed value virtual impedance.The results of simulation and experiment platform verify the effectiveness of the above control strategy.Finally,aiming at the output voltage imbalance caused by the unbalanced load of the three-phase inverter,the SOGI sequencing module is applied to balance the threephase voltage by voltage compensation,sequence droop control,sequencing virtual impedance and QPR controller adjustment.At the same time,the equalization of the current and power of the two inverters are realized and verified by simulation experiments.