Research On The Control Strategy For Paralleled Inverters With Different Capacities In Distributed Generation Systems

With the rapid development of economy and the excessive consumption of energy in our country, the energy crisis and environmental problems is gradually emerging. It becomes the inevitable trend to vigorously develop the technology of distributed generation based on renewable energy. In distributed generation system, many inverters of different capacity operate in parallel to meet the different demand of load. The key to the inverter parallel operation is the reasonable distribution of the load. Therefore, the study of power sharing strategy for paralleled inverters with different capacity in distributed generation system is of great value in theoretical reference and engineering application.The influence of the connection impedance between the distributed inverter and the load to power sharing is studied in this paper. Virtual resistance is introduced to make the connection impedance resistive. Then the PV/Qf droop control strategy can be used in low voltage distributed generation system to share the load. In addition, the condition to meet for accurate load sharing in proportion to their power ratings among different capacity inverters is analyzed. And then it is pointed out that some limitations exist in traditional PV/Qf droop control strategy due to the dispersion of the DG units and the uncertain characteristic of line impedance in actual distributed generation system. A power sharing strategy based on the load voltage feedback is then proposed. By the improved method, the influence of the line impedance to power sharing is eliminated and the accurate load sharing in proportion to their power ratings is realized. The load voltage droop produced by droop control, virtual resistance and load change is reduced to make the load voltage maintained within the desired range around the rated value. And then the stability and reliability of the system can be proved. A simulation model of three inverters operated in parallel is established in Matlab/Simulink simulation environment. By comparing the traditional PV/Qf droop control and the improved power sharing strategy to prove the accuracy and validity of the improved method.The experimental platform with small distributed inverters in different capacity paralleled operation system based on digital control by DSP TMS320F28335 is designed and built in the laboratory. The experiment of PV/Qf droop control and the improved power control strategy was completed to be compared and analyzed in conditions of different line impedance, load suddenly changing, line impedance suddenly changing and paralleled inverters of different capacities. The experimental results show that the influence of the line impedance to power sharing can be eliminated by the improved method, and the security and stability of the system can be enhanced. It has better adaptability in distributed generation systems.