Research On The Method Of Circulating Current Suppression For Resistive Inverter Parallel System Of Microgrid

Microgrid is a key way to consume distributed energy,and the construction of microgrid provides a feasible solution to realize active distribution network,and also prepares for the transition from traditional grid to smart grid.With the introduction of the "double carbon" strategy,the microgrid sector has received much attention and its key technologies have been further developed.As the interface unit between load-side and common bus energy conversion,inverter plays an important role in microgrid.However,due to the differences in geographic factors(connection impedance)and their own factors(rated capacity,output impedance,and controller parameters),the parallel operation causes the problem of inaccurate load power distribution and loop current,which seriously threatens the stable operation of the system,especially during the transient process of load switching and parallel operation.To address these drawbacks,this paper researches the inverter parallel connection technology,microgrid power quality control technology and circulating current suppression technology,and proposes a resistive inverter parallel operation control strategy to solve the technical problems such as unreasonable power distribution,power quality degradation and circulating current.First,this paper constructs a mathematical model of inverters and derives the loop current and power characteristics of a multi-inverter parallel control system.On this basis,for the characteristic that the resistive component in the feeder impedance of low-voltage microgrid is dominant,this paper adopts a resistive sag control loop,and introduces an adaptive virtual complex impedance with negative inductive resistance component in the double closed-loop feedforward loop to offset the inherent inductive component of the system and weaken the feeder impedance difference.The simulation results prove that the method can solve the problem of inaccurate power distribution and significantly reduce the circulating current amplitude by more than 60%.Secondly,for the inherent contradiction of impedance rise and voltage dip and the difficulty of power calculation hysteresis in traditional control,this thesis improves the sag control loop from two aspects,on the one hand,the compensation amount of voltage and frequency is increased in the input port to solve the problem of abnormal fluctuation of voltage and frequency;on the other hand,the hysteresis problem is solved by introducing the differential term in the feedforward control.Finally,simulation and experimental cases with capacity ratios of 1:1 and 3:2:1 are built based on the PSCAD/EMTDC simulation platform and the new energy generation experimental platform to verify the proposed control scheme.To be close to the actual working conditions,the simulation and experimental procedures take into account the disturbance factors such as load-side fluctuation and inverter random throwing and retiring.The results show that the proposed control strategy can overcome the influence of feeder impedance difference,effectively reduce the loop current amplitude,eliminate the power distribution deviation,and quickly compensate the shortage when the frequency and voltage fluctuate,and achieve the expected control requirements.