Research On Parallel Control Strategy Of Three-phase Inverters Based On Virtual Impedance

With the rapid development of the economy,microgrid technology based on distributed power generation has received widespread attention both domestically and internationally.In practical applications of microgrid,multiple micro-source inverters are often operated in parallel to construct microgrid systems.The key to efficient and stable operation of microgrid lies in reliable parallel inverter control strategies.This article mainly focuses on the research of power balance,circulating current suppression,and output voltage quality issues among parallel inverters.Starting from the perspective of single inverter control,the topology of the three-phase voltage source inverter is analyzed,and a mathematical model of the three-phase inverter is established based on this.The voltage and current dual closed-loop control system and parameters are designed;a brief description of common microgrid inverter control strategies is provided,with an introduction to the characteristics and applicable conditions of each control strategy.Droop control is chosen as the main control method for the inverter,focusing on the analysis of the working principle of droop control,obtaining the droop control expression and droop characteristic curve.The effectiveness of the control strategy is verified through the simulation model of a single three-phase inverter.Next,research on parallel inverter systems is conducted,and an equivalent circuit model of two inverters operating in parallel is established.Factors contributing to circulating currents between parallel inverters and reasons for uneven reactive power distribution among the inverters are analyzed in detail.According to the circulating current equation,the system circulating current is caused by differences in inverter output voltage and the presence of differences in equivalent output impedance.The deviation of reactive power is related to inverter line impedance differences,impedance magnitude,and reactive droop coefficients.The working principle of regulating the output impedance characteristics of inverters by introducing virtual impedance is analyzed,and the limitations of traditional virtual impedance droop control are explained.Finally,a deeper analysis is conducted regarding output voltage drops and the equal distribution of reactive power.In order to avoid output voltage drops and the inaccurate distribution of reactive power caused by introducing virtual impedance,an improved droop control strategy is combined with the virtual impedance method.This proposed an improved droop control strategy based on virtual impedance.For the output voltage drop issue caused by introducing virtual impedance,a voltage amplitude feedback and corresponding voltage compensation method are provided to maintain system voltage stability.To address the issue of parallel inverters not being able to accurately distribute reactive power,a reactive power-related item is introduced into the reactive power droop control loop.This enabled the droop coefficient to adjust adaptively with changes in power,thus achieving precise reactive power allocation and effectively suppressing circulating currents.Simulation analyses of traditional droop control with virtual impedance and improved droop control are conducted using the Matlab/Simulink.The results showed that the improved droop control strategy based on virtual impedance can maintain system voltage stability while balancing power and suppressing circulating currents,thus verifying the rationality and effectiveness of the proposed control strategy.