Research On Control Strategy Of Combined Three-phase Full-bridge PFC Converter Under Grid Voltage Unbalance

The combined three-phase full-bridge PFC converter is a converter composed of three identical single-phase single-stage isolated full-bridge PFC single-unit modules,which combines the advantages of the bridge converter.It is an excellent topology for three-phase high-power applications.However,there are some problems in this type of converter that need to be solved,such as the lack of corresponding effective control strategies,the large ripple voltage on the DC output side and the large THD of the input current.This article will do a detailed analysis and research on the above issues.Firstly,combined with the structure and principle of the combined converter single module,the topology and working principle of the combined three-phase PFC converter are analyzed.At the same time,in order to meet the needs of combined converter control,the traditional double loop based on single-phase converter Control,the current loops of the three modules are independent and the voltage loops are shared,and the double loop control strategy applied to the combined converter is obtained.Aiming at the input overcurrent problem of each module caused by the increase of the amplitude of the three-phase power grid,a composite control strategy based on the traditional double-loop control to increase the voltage feedforward link can be effectively solved.Secondly,based on the composite control strategy,the single-phase single-stage PFC output voltage ripple component and the cancellation or superposition of the power frequency ripple in the output voltage of the three modules are analyzed.Analyze the ripple component on the output side of the combined converter under the condition of grid voltage imbalance,analyze the influence of the main parameters and unbalance of the converter on the ripple,and the influence of the output ripple component on the input current THD of the module through the control loop.In order to effectively suppress the influence of output voltage ripple on the input current THD of each module of the converter,an improved digital band rejection filter used after voltage loop output is proposed,which can not only eliminate the influence of output secondary ripple,but also obtain faster response speed.Finally,in order to completely solve the output voltage ripple problem of the combined three-phase PFC converter,the output characteristics and power quality of the converter are improved under the condition of grid voltage imbalance.A variable sampling coefficient control strategy based on traditional double loop control is proposed.Based on the analysis of the working principle of the proposed control strategy,the relationship between the sampling coefficient and the voltage imbalance of the three-phase power grid is derived,and the influence of the variable sampling coefficient control on the input current and output dynamic response of each module isanalyzed.On the basis of detailed theoretical research and simulation,a combined converter experimental device based on full-bridge PFC structure is built to verify the control strategy and theoretical analysis proposed in this paper.