Output Control And Parallel Operation Control Of Single-Phase 400Hz Medium Frequency Voltage-Source Inverters

400Hz medium frequency power supply is widely used in military and civil systems.Output Waveform control and parallel operation control are the main problems. Thepaper focuses the research work on the two problems through theory analysis,simulation and experiments.After constructing the mathematical model of single-phase full-bridge inverter, thispaper deduces its open-loop and closed-loop transfer functions. From the analysis, itcan be concluded that the voltage and current regulator can be pure proportional(P)control in the situation that the proportional parameters are rather high. Then thepaper presents and analyses a closed-loop control strategy with voltage magnitudefeedback and current feed-forward of single-phase medium frequency inverters, inorder to improve the control precision of output voltage magnitude and phase.The paper develops the parallel model of two single-phase medium frequencyinverters, and analyses the mechanism of the circulating current, and points out thatthere will exist not only AC circulating current but also DC ones between the parallelinverters. From the further theoretic analysis and simulation results, it can beconcluded that: (1) the phase error between the parallel inverters mainly affects thechange of output active power of inverters, and the magnitude error mainly affectsthe change of output reactive power of inverters;(2) phase error may leads to severercirculating current situation than magnitude error does;(3) the exist of DC offseterror of inverter output voltage will result in the DC circulating current.This paper presents a DC-offset voltage droop method without the communicationline between the parallel inverters in order to eliminate the DC circulating currentbetween them. A novel droop method sharing the synchronal signal and by means ofactive and reactive current is presented to eliminate the AC circulating current. Theinstantaneous power theory is used to verify the rationality of the new droop methodand the calculation method of active and reactive current. Then the small signalmathematics model of the system with the new droop method is established, and thestability of the system and the selecting method of droop coefficient and filterparameter are analyzed.Finally, the paper points out that using the constant droop coefficient will result inuseless drop of output voltage, and puts forward a scheme of adjusting thecoefficient dynamically in order to make each parallel connected inverter achievesimilar sharing currents and increase the control precision of voltage. On the otherhand, a method of resuming the magnitude and phase of inverter output voltagewhich add compensating values to magnitude and phase of output voltage in steadystate is presented to further decrease the useless drop of output voltage.