Study On Stability Of Paralleled System Of Multi-grid-connected Converters In Microgrid

The energy shortage and environmental pollution problems the world facing are becoming more and more serious,distributed generation based on renewable energy has received increasing attention.The LCL type grid-connected converter serves as the interface between the distributed power source and the public power grid,its stable operation plays a decisive role in the grid-connected system.With the increase in the number of distributed power source,the system has a structure in which multiple LCL grid-connected inverters are connected in parallel,the coupling between inverters affects the overall performance of the parallel system.This paper takes multi-paralleled LCLfiltered converters system as the research object,focusing on the mathematical model,multi-resonance characteristics and system stability.Taking the LCL type grid-connected inverter based on grid-connected current single-closed-loop control as the research object,the parameters of the PI controller were designed under the condition of digital control characteristics,and an experimental platform was set up for experimental verification.This paper established a mathematical model of a parallel system in the form of a matrix.Based on this mathematical model,it is analyzed that the grid-side currents output by the grid-connected converters in the paralleled system not only flow into the grid,but also have interactive current components flowing between them.Based on this,the multi-resonance mechanism and the influencing factors of the parallel system are studied.In addition to the inherent LCL resonance,the paralleled system also has a resonance due to the coupling of the output impedance of the different converters to the grid impedance,and the resonance frequency is related to the number of parallel connections.Derivation of parallel system stability criteria based on the matrix model,decomposing the entire system into an internal stand-alone system and an external system formed by coupling can greatly simplify the analysis process.Both will stabilize the parallel system at the same time.A single LCL-type grid-connected inverter designed when based on the strong grid condition will cause instability in the case of multiple parallel units.Based on the multi-resonant characteristics of the parallel system,a parameter optimization design method for a single controller that can improve the stability of the parallel system is proposed.In the parallel system of LCL grid-connected inverters with single closed-loop control of grid-connected currents,system crossover frequency must be lower than all resonant frequencies at the same time to ensure system stability in parallel.After the filtering parameters are selected,the resonant frequency of the parallel system will not change.By increasing the ratio of the integral ratio and the proportional ratio of the PI controller can continuously reduce the control system crossing frequency,the stability requirements of the parallel system can be met.At last,simulations verify the effectiveness of the newly designed control parameters for parallel system.In order to prove the correctness of the theoretical analysis,two laboratory PWM inverters were used to build a grid-connected inverter parallel system experimental platform.Experiments verify the reliability and accuracy of the theoretical content of the mathematical model,multi-resonance characteristics and stability analysis of the paralleled system.