Research On Three-phase Single-stage Full-bridge APFC Control Strategy In Unbalance Grid

Harmonic pollution is one of the main causes of various power quality problems.In order to purify the electromagnetic environment,it is necessary to control the harmonics in the power grid.Among many harmonic remediation schemes,active power factor correction(APFC)is an importan t solution that can "cure" the harmonic pollution generated by the various types of power electronic devices to achieve "green" power conversion,with its unique advantages it is avored by the majority of scholars.High-power equipments are usually powered by three-phase power system.Compared to single-phase electrical equipment,harmonic pollution generated by three-phase electrical equipment is more worthy of attention.In the three-phase APFC technology,in line with the development trend of power electronic technology,three-phase single-stage APFC technology has become a hot research object.Over the years,scholars have proposed a variety of circuit s which have been applied to three-phase single-stage APFC converter,in which the current-type full-bridge topology on the basis of according with single-stage APFC converter ’s pursuit of high efficiency,high power density and other ch aracteristics,has become a very attractive APFC implementation program with its relatively competitive work characteristics.In this paper,the operation of three-phase current-mode full bridge APFC converter is analyzed in detail.The PFC mechanism of the converter operating in the traditional constant duty cycle control is clarified.Aiming at the problem of the primary voltage spikes of the transformer in the current-type full-bridge topology,this paper uses the active clamp technique to suppress it.When the converter operates in the traditional constant duty ratio control,limited by the harmonic components of the input current,the system’s power factor is difficult to enhance.In order to make the APFC system achieve better PFC effect,this paper focuses on improving the traditional control strategy,and derives a variable duty ratio control strategy based on the maximum power factor.This strategy can significantly reduce the inherent low-frequency harmonic content of the converter,and has a positive effect on the power factor of the system.For the actual operation of the power system,the power quality is influenced by the various types of electrical equipments,the power grid’s input voltage is not ideal balance.When APFC converter works in poor power quality condition,the input and output characteristics will be significantly deteriorated.On the basis of the analysis of the working mechanism of the converter under unbalanced power grid,through the method of improving variable duty ratio control strategy,an improved variable duty ratio control strategy aimed at working under unbalanced gtid is proposed to optimize the negative influence caused by the unbalance grid.This strategy can significantly improve the operating characteristics of the converter when the grid input is unbalanced.In this paper,Psim software is used to simulate the theoretical analysis.On the basis of simulated verification,the experimental platform is designed to verify the control strategy of the variable duty ratio and the improved variable duty ratio when the input of the grid are balance and imbalance respectively.The experimental results show the correctness of the theoretical analysis.