Three-phase Inverter Based On Virtual Inductance Parallel Strategy Research

With the fast growth of distributed power supply system and higher requirements of the power quality, the parallel power inverter technology is more widely used. In the existing parallel techniques, the droop parallel technology comes from the synchronous generator’s droop characteristic and has the advantages of no interconnect, stronger anti interference and higher redundancy. However, the internal resistance and dynamic characteristics of the inverter power system are different from the synchronous generator, which makes the traditional droop control have many problems in the power inverter parallel control. This analysis is based on these issues.First, the mathematical model of the three-phase three-wire voltage source inverter is established. Then the parallel system model structure is made based on the single model. A detailed analysis of the nature of the circulation of parallel systems is made and the conclusion that the equivalent output resistance and the output voltage vector are the two points to restrain the circulation is obtained.Then, from the perspective of the inverter output impedance, the changes of the inverter output impedance in the varied state of open-loop and closed-loop and real parallel inductive is analyzed. Based on it, the virtual inductive strategy is proposed, to design the output impedance and reduce the coupling of active and reactive power. To realize the virtual inductive in the dq rotating coordinate system, the mathematical formula and structure is derived. The theory and the stability of the traditional drooping control are researched and an improved active droop control method is also proposed, to achieve the purpose of synchronous operation and no frequency deviation in steady-state operation of the parallel inverter. This paper also gets the conclusion that the droop coefficient, the output resistance and the rated capacity must meet some conditions for the parallel inverters with different capacity.Finally, the Simulink simulation experiments are made to verify the conclusions derived in this paper and the effectiveness of control strategies. The simulation results show that this improved droop control strategy based on the virtual inductance can effectively inhibit the circulation of parallel inverters in the same capacity and different capacity parallel systems.