| The modular parallel operation of three-phase inverters can improve the power capacity and reliability,reduce the volume and weight of system,and reduce the system design and the production cost,which has a wide application foreground.However,the circulation current between the parallel inverters will reduce the quality and efficiency of power supply,so the circulation current control is becoming a hot issue to be solved urgently.This thesis researches on circulation current based on a parallel system consisting of two three-phase inverters.The main contents are as follows:Based on the parallel three-phase three-leg inverters system,the generation principle and operation mechanism of circulation current between parallel inverters were analyzed.The mathematical model of parallel system was deduced,the causes of circulation current in parallel system were analyzed,the mathematical model of zero sequence circulation current was established and the quantitative analysis of circulation current was analyzed.Proposed a distributed logic circulation current control strategy of parallel off-grid three-phase inverters system based on load power and current equalization to control the circulation current.The output power and the circulation current power of parallel three-phase inverters were analyzed,via the relationship between inverter's output impedance and output active and reactive power,used output reactive power calculated to adjust the reference amplitude of output voltage to realize load power sharing and circulation current control.Proposed the zero sequence circulation current control strategy of parallel grid-tied three-phase inverters system based on the distribution of zero-vector.The double zero-vector distributed method was proposed to control the zero sequence circulation current based on mathematical model of zero sequence circulation current.Within a sampling period in SVPWM modulation technology,controlled the zero sequence circulation current by changing the operating time of zero-vector once time to control zero sequence switch value to be zero,which called zero-vector distributed method;for double zero-vector distributed method,by changing the operating time of zero-vector twice time within a sampling period,could improve the dynamic performance of zero sequence circulation current control,the bandwidth of zero sequence current control loop and the performance of real-time control.And the control performance of circulation current between zero-vector distributed method and double zero-vector distributed method were comparative analysis.Researched the circulation current control of parallel system under unbalanced three-phase load based on double zero-vector distributed method.The factors that caused three-phase inverter unbalanced operation were analyzed and the model of three-phase load based on instantaneous symmetrical component was established.The variation of circulation current and its control effect under unbalanced three-phase load was discussed.By symmetric component decomposition of output unbalanced three-phase voltage,a PI controller was designed to inhibit the negative sequence component,thus realizing the inhibition of output unbalanced three-phase voltage.Established a simulation platform of 100 kW parallel three-phase inverters system on MATLAB / Simulink,and the corresponding simulation experiments were conducted to verify the effectiveness of the control strategies above-mentioned. |
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