Investigation On Topolgoy And Control Strategy Of Three-phase Buck Rectifier With Low THD And High Efficiency

Three-phase pulse width modulation(PWM)rectifier can draw well-balanced and low-distortion sinusoidal currents from each phase of three-phase ac input,and it can also provide stable and reliable dc voltage for the later stage electrical equipment.Therefore,three-phase PWM rectifiers are widely used in AC-DC applications with strict requirements on input current harmonics,power factor(PF)and system efficiency,such as data and communication center,more electric aircraft(MEA),electric vehicle(EV)chargers,smart grid and other fields.The three-phase boost rectifier and the three-phase buck rectifier(3ph-BR)are two basic three-phase PWM rectifiers.The three-phase boost rectifier has the advantages of low cost,simple control,high efficiency and low input current total harmonic distortion(THD),which has been widely used in industry.The 3ph-BR has the advantages of wide output voltage range,small starting current,reliable short-circuit current limiting ability and so on.It also has a wide application prospect in the industry.However,compared with three-phase boost rectifier,the 3ph-BR has lower efficiency,higher input current THD and more power switching devices,which limits the application of the 3ph-BR.This paper aims to improve the system efficiency and reduce the input current THD of the 3ph-BR.The rectifier topology,PWM modulation strategy and control technology are studies in this paper to further improve the theoretical system of the 3ph-BR.With the improvement of power electronic devices and control technology,the power density of the 3ph-BR is getting higher,and the effect of parasitic capacitance on the input current quality of the the 3ph-BR is becoming more serious.It is shown in this paper that the input current has obvious high frequency noise and low frequency distortion due to the effect of common mode(CM)voltage and parasitic capacitances.A modified 3ph-BR topology with low input current THD is proposed in this paper.This topology can suppress the CM current flow to the input side,so as to avoid the influence of CM current on the input current quality,making the 3ph-BR with high power density have lower input current THD.In this paper,the effects of parasitic capacitances on the operation modes,input current and dc-link currents of the conventional 3ph-BR are discussed in detail.The operaiton modes of the modified 3ph-BR are described and the expressions of the dc-link current ripple and output capacitance voltage ripple are derived.The theoretical analysis in this paper provides technical guidance for the practical application of this modified topology.In order to further improve the efficiency of the 3ph-BR,many different topologies and PWM modulation strategies of the 3ph-BR have been proposed.In the existing topologies,the MOSFETs in the bridge legs withstand the input line voltage.The high voltage stress limits the selection of MOSFETs and is not conducive to the optimization of system efficiency.In this paper,a new 3ph-BR with low voltage stress on switching devices and its PWM strategy are proposed.In this topology,the voltage stress on the MOSFETs is lower than the peak value of input phase voltage,so the selection of MOSFETs is easier.Moreover,the MOSFETs with low rated voltage have lower cost and lower on-resistance and superior switching characteristics,which is easier to improve the system efficiency.In this paper,the operation mode of the proposed topology is described,and the switching loss of the conventional 3ph-BR and the new topology is analyzed and compared.It is conducive to realize the characteristics of low-cost and high-power density of the 3ph-BR by adopted the proposed topology.The space vector PWM(SVPWM)strategy directly affects the input current THD,the switching loss and the dc-link current ripple of the 3ph-BR.At present,the research on SVPWM modulation strategy is mainly focused on the condition that the input is grid power supply system,while less attention have been paid on SVPWM modulation strategy in the case of aviation power supply system.The input voltage frequency of ground grid power supply system is fixed and low,i.e.,50 Hz or 60 Hz.The input frequency of aviation power supply system fluctuates greatly,and the input frequency is relatively high,i.e.,360 Hz-800 Hz.In this paper,the phenomenon of sector update delay(SUD)in SVPWM modulation strategy of digital controlled the 3ph-BR under wide input frequency is revealed,and its effect on the input current THD and system efficiency is studied.Morevoer,a sector prediction algorithm is proposed to effectively suppress the phenomenon of SUD.After using the proposed prediction algorithm,the input current THD is significantly reduced and the efficiency is further improved under the condition of wide input frequency.The SUD and its solution method studied in this paper are a supplement to the existing SVPWM modulation strategy,which further promote the application of the 3ph-BR in the EMA applications.When the there-phase input voltage is balanced,the topological architecture and SVPWM modulation strategy are the main factors affecting the input current THD and system efficiency of the 3ph-BR.However,in practical application,the three-phase input voltage is often in the unbalanced states,which will have a serious impact on the input current quality and system loss of the 3ph-BR.The existing control strategies usually need three-phase phase phase-locked loop(PLL),positive and negative sequence voltage or current separation technology,which makes the control system complex and difficult to implement.In this paper,a simple input voltage feed-forward control technique based on switch function matrix is proposed,which does not require three-phase PLL and complex coordinate transformation,and does not require high bandwidth of the feedback loop.Therefore,the low harmonics of input currents and output voltage can be effectively suppressed by using low cost and small volume digital controller.The control technology proposed in this paper provides a simple solution for the reliable operation of the 3ph-BR under unbalanced input condition.According to the theoretical analysis of the topological structure,modulation strategy and control technology of the 3ph-BR,the corresponding simulation verification are carried out,and the corresponding experimental prototypes are set up.Simulation and experimental results verify the correctness and feasibility of the theoretical analysis in this paper.