Research On Control Strategy Of Virtual Synchronous Generator Under Unbalanced Conditions

The proposed microgrid aims to realize the flexible and efficient application of distributed power sources and solve the problem of large-scale and diverse distributed power supply grid-connected problems.In order to improve the penetration rate of new energy power generation,the virtual synchronous generator(VSG)technology has been proposed by relevant scholars.The characteristics of large moment of inertia,large damping,self-synchronization,voltage regulation,and frequency modulation make VSG technology widely favored.This paper mainly studies the VSG control strategy under the two conditions of parallel line impedance mismatch and load imbalance,and summarizes the following aspects.According to the research background of the subject,the research status of microgrid inverter and virtual synchronous generator is analyzed,and the control strategies under the two conditions of line impedance mismatch and load imbalance are reviewed.This paper introduces the basic principle and components of virtual synchronous generators,analyzes the operating characteristics of the two modes of off-grid and the effect of various parameters on the power frequency response and system stability.A voltage compensation droop control is proposed for the parallel line impedance mismatch.The principle of line impedance mismatch is analyzed.The line impedance mismatch will generate a circulating current between each VSG,which affects the reactive power distribution.According to the sagging coefficient adaptive control,a voltage compensation droop control is proposed,and the reactive power deviation compensates the line impedance.The voltage drop caused by the difference compensates for the disadvantage of the common point voltage drop while reducing the reactive power distribution deviation.The small-signal modeling of the control strategy is carried out to analyze the stability and select reasonable parameters.A voltage self-compensation and frequency following composite control strategy is proposed for load imbalance.The unbalanced principle is analyzed.Unbalanced load will cause VSG output voltage and current to be asymmetrical,and the active and reactive powers will contain twice the frequency component,causing power fluctuation;a voltage is proposed according to the relationship between positive and negative sequence voltages.The compensation control strategy compensates the positive sequence voltage reference value,reduces the negative sequence voltage component,and suppresses the unbalanced voltage;improves the frequency control,adds the frequency following,corrects the frequency error,and realizes the parallel current sharing.