Interaction Analysis And Control Of The Multipe Grid-connected Inverters

With the depletion of traditional fossil energy,the deterioration of the ecological environment,the issue of energy crisis and sustainable development of the ecological environment have attracted more and more attention.At the same time,renewable energy such as solar energy and wind energy are receiving attention from all countries in the world because of their clean and recyclable energy.Among them,grid-connected inverter is the key interface for new energy power generation and grid connection.How to ensure the safe and reliable operation of grid-connected inverter system in a more complex power grid environment has become a hot research topic at home and abroad.With the large-scale development and application of new energy power generation,new energy power generation is increasingly showing a large-scale centralized grid connection trend.The traditional single inverter grid-connected method cannot meet the requirements of the grid-connected power level,and parallel connection of multiple machines becomes the normal state.When multiple inverters are connected in parallel,the system will have a series of problems,and its safe,reliable and stable operation also faces new challenges.To this end,this paper will study the stability and resonance circulation occurring in multi-machine parallel systems.First,establish a grid-connected inverter output impedance model.In the ?? coordinate system,the output impedance models of single-phase grid-connected inverters and three-phase grid-connected inverters are established respectively,and the two are unified.Then the model is extended to the multi-grid inverter to obtain the output impedance model of the multi-grid inverter.According to the obtained impedance model,the output current expression of each inverter is derived,and it is pointed out that when N parallel inverters are connected in parallel,it can be equivalent to a single inverter connected to the grid.At this time,the grid impedance becomes the original.N times,which simplifies the analysis.By constructing the simulation model in MATLAB/Simulink,the correctness of the model is verified by comparing the theoretical value of the output impedance with the simulated measurement value,which lays a foundation for the subsequent research.Secondly,analyze the stability of the grid-connected inverter system.The existing literature indicates that the negative damping characteristics of the inverter can cause instability in multi-machine parallel systems.In view of this,this paper based on the concept of passivity,frequency domain analysis of the inverter output impedance,pointing out that the existence of digital control delay and system parameter mismatch will lead to negative damping characteristics of the inverter output impedance,that is,the emergence of active.According to the derivation,the optimal active damping feedback coefficient is given to ensure the passiveness of the inverter output impedance to the greatest extent,so that the grid-connected inverter operates stably.It is proposed to increase the stability margin of the system and increase the robustness of the system by increasing the lead compensation link in the capacitive current feedback channel.The system was transformed into a discrete z domain for analysis.Extending the above results to the multi-grid inverter system,the influence of the change of the number of parallel inverters on the stability is studied,which verifies the correctness of the above analysis.Finally,the influence of the impedance interaction between the inverters on the resonant circulation is studied for the problem of the internal resonant circulation of the multi-grid inverter.In order to simplify the analysis process of the multi-grid inverter,the output current of the inverter needs to be separated,one part flows into the grid,and some flows into other inverters.Comparing the two parts of the current,it is pointed out that due to the quasi-resonance between the output impedance of the inverter,the resonant circulation between the inverters is generated,and the generated resonant circulation cancels the total grid-connected current when flowing into the grid.Without effect,the generation of the resonant circulating current can be suppressed by increasing the phase of the inverter's own output impedance.