Study On Generation Mechanism And Suppression Strategy Of Multi-inverter Parallel Circulation

Photovoltaic power generation has become one of the most common and important forms of distributed power generation,and it has become a hot spot for experts at home and abroad.Due to the increasing requirements for inverters,a single single group of inverters can no longer meet people's needs for the power grid.Parallel operation of inverters is one of the important means to solve the expansion of renewable energy power generation such as large-scale photovoltaics.The resulting increased circulation between inverters is becoming increasingly prominent,leading to even damage to the inverters and the collapse of large power grids.Solving the circulating current problem caused by the parallel operation of multiple inverters is the core to ensure the efficient and stable operation of the microgrid.This article takes 2 inverters as research objects,and studies the circulating current problem of isolated grid-connected parallel inverters.First of all,this paper performs mathematical modeling based on the main circuit topology of a single three-phase inverter,and based on this mathematical model,an equivalent model of single inverter dual closed-loop control is established.The whole process of the model is analyzed in detail,and the output frequency complex frequency domain equation and the inner and outer loop governing equations are derived.According to the droop control methods commonly used in inverters,equivalent models using traditional droop control methods and virtual impedance droop control methods are established.The principle and power characteristics of the droop control method are analyzed,and active and reactive power and voltage and frequency are obtained.The relationship between the active and reactive power of the parallel system is derived,that is,the corresponding impedance and power relationship.The double closed-loop control model is verified by simulation.Secondly,the research and analysis of a multi-inverter parallel system are carried out.The method of inverter circulating current generation and the different circulating current definitions are analyzed.Based on this,the generation mechanism of different circulating currents including power circulating currents and zero-sequence circulating currents under the same capacity are analyzed.And its characteristics,and vector analysis of the circulation in different situations.According to the equivalent model of two parallel inverters,the definition of circulating current and the circulating current equation are obtained.The power characteristics of the inverter are analyzed,and the output power equation is derived.The power circulating current is obtained by the inverter output voltage being different and equivalent.Caused by the difference in impedance.For the parallel inverter of common DC bus,the zero-sequence circulating current formula is defined according to the zero-sequence voltage generated by it,and the generation of the zero-sequence circulating current is analyzed.According to the traditional droop control method and the virtual impedance-based droop control,the limitations and defects of these two methods are verified through simulation.At the same time,it can be seen that both of them have a large circulation.Finally,by analyzing the traditional robust droop controller's method for suppressing circulating current and studying its principle,an equivalent model is established for the difference from traditional droop control,and the corresponding output reactive power equation is derived.Based on the premise of the power microgrid,an enhanced robust voltage droop control strategy is proposed.The proportional integral(PI)link is added to the active power and reactive power control,and the virtual complex impedance is designed to make the line resistive.The combination of the two optimizes the power consumption of the line,improves the robustness of the system,and reduces the bus voltage loss.The ultimate goal is to eliminate power circulation.Finally,Matlab / Simulink simulation results show that the proposed control strategy is reasonable and effective,and has theoretical guidance significance for engineering practice.