Research On Control Strategy Of Grid-connected And Islanded Three-phase Four-wire Microgrid Inverters Based On Impedance Model

The widespread application of renewable energy has made the research on microgrid technology being one of the hotspots in recent years.As an important part of microgrid,the microgrid inverters need to keep high reliability and high stability,while they are also required to have output power quality that meets the relevant standards.Therefore,the research on control strategy of microgrid inverters is of great significance.For this reason,this paper focuses on the control strategy of three-phase four-wire microgrid inverters,which includes studying the basic principles,building the impedance model based on the improved droop control strategy,and designing the control strategy under unbalanced and harmonic condition in grid-connected and island mode.The details are as following:Firstly,the small-signal model is built for the split-capacitor-type three-phase four-wire three-level topology,and the overall control structure of the system is given.The midpoint voltage fluctuation model is calculated by the fundamental period average method.Based on this model,the midpoint voltage self-balancing effect is analyzed,and the midpoint voltage balance method using voltage deviation feedback is given.Meanwhile,the influence of voltage feedback and current feedback on the output impedance of the inverter is analyzed in single-phase system.And the transformation between dq-domain impedance,αβ-domain impedance and sequence-domain impedance is deduced in three-phase system.Besides,three kinds of stability criteria for the impedance model are introduced.Secondly,the basic principles,steady-state and power distribution characteristics of traditional droop control scheme are analyzed.Based on the result,an improved droop control scheme which uses reference value switching and amplitude integrator is given and analyzed.Subsequently,the impedance model of voltage-current double-loop control is built,the control parameters are designed.after which the impedance model of the improved droop control scheme is calculated.In addition,the differences in impedance model and pole distribution between the improved P-ω,Q-V droop control scheme and P-V and Q-ω droop control scheme are discussed,indicating that the latter has better stability in the application of this paper.The droop coefficient and virtual impedance parameters are designed later.Thirdly,an unbalanced control strategy based on negative and zero-sequence current controllers and virtual impedances is designed for unbalanced grid and unbalanced load conditions,while a harmonic control strategy based on multi-harmonic voltage controller is designed for background harmonics and nonlinear load conditions.Subsequently,a hybrid islanding detection method is designed for the control algorithm switching in grid-connected and island mode,which uses frequency,amplitude,unbalance voltage and harmonic voltage as the detection variable.The slide-mode frequency shift method is combined to increase the speed of islanding detection under power matching conditions.Finally,a three-phase four-wire microgrid inverter simulation model is built on the MATLAB/Simulink simulation platform.Moreover,a three-phase four-wire microgrid inverter experimental platform centered on two 50 k VA microgrid inverter is set up.The reliability and effectiveness of the control strategy designed in this paper are verified by the simulation and experiment results.