Research On Power Distribution Of Low-voltage Microgrid In Island Mode

With the continuous decline of fossil fuel storage and the increasingly serious environmental pollution problem,the microgrid that promotes the consumption of renewable energy has gradually become a current research hotspot,and the power balance problem that affects the stable operation of the microgrid is an important research of the microgrid content.Droop control is favored in microgrid power distribution control because it can not only meet the "plug and play" requirements of distributed generation,but also distribute load power in proportion to the rated capacity of each inverter.However,due to the characteristics of the grid-connected port of the inverter during the island operation of the microgrid,it is difficult for the traditional droop control to accurately distribute the load power.Therefore,this article first analyzes the main reasons why the traditional droop control in island microgrid cannot distribute the load power in a balanced manner,and on this basis,designs an improved droop control strategy based on suppressing circulating current and an improved droop control based on a consensus algorithm.The main content of the full text is as follows:First,the three-phase inverter LC filter model and the voltage and current double closed-loop control model in the dq coordinate system are established respectively,and the Thevenin model of the inverter is derived by combining the two.Then,according to the characteristics of the output power of the parallel inverter and the impedance of the low-voltage line,the traditional droop control was obtained,and combined with the control framework of the single inverter,the overall closed-loop control system of the parallel inverter of the microgrid was constructed.Then,it discusses in detail the reasons why the traditional droop control cannot distribute the load power in a balanced manner,and gives the simulation results to prove the correctness of the theoretical analysis.Secondly,an improved droop control strategy based on suppressing circulation is designed.This strategy uses the circulating current of rotating coordinate to construct the virtual impedance.The constructed virtual impedance can be controlled by PI without real-time detection of the line impedance value to make the line equivalent impedance continuously approach until it is equal,thereby evenly distributing the load power.In addition,the constructed virtual impedance not only effectively suppresses the circulating current,but also does not cause a large drop in the output voltage.The article builds a parallel model of two inverters with similar capacity in Matlab,and compares the improved strategy with the existing control method.The simulation results prove the effectiveness and superiority of the improved strategy.Then,an improved droop control based on consensus algorithm is proposed.A single inverter using this control strategy obtains the output active power of all adjacent inverters through a sparse communication link,and then uses the sum of the relative errors of the output power to reconstruct the output voltage reference amplitude;this control strategy that does not require characteristics of inverter grid-connected port can distribute the load power in proportion to the rated capacity of each inverter,and ensure the stability of the inverter’s output voltage.The article builds a multi-bus microgrid model containing 3 distributed power sources in parallel in Matlab,and designs four operating conditions close to the actual operation of the microgrid to simulate the improved control.The simulation results verify the effectiveness of the control strategy and performance in case of communication failure.Finally,compare the advantages and disadvantages of the two droop control improvement strategies from the applicable occasions,considering the factors affecting the droop control accuracy,and the control effect.