Research On Control Strategy Of Three-phase Three-level VIENNA Rectifier

Power electronic rectifying equipment has been widely applied in social life and production.VIENNA rectifier is a unidirectional power-flow three-level topology.It needs least one power switch in each bridge-arm,no dead-time and the voltage stress of each power switch is only half of the DC voltage.It can greatly improve the power density with a high reliability.Because of its excellent performance,VIENNA rectifier has been widely used in high-power DC applications.However,there is still room for improvement in the control process of VIENNA rectifier.For example,the conventional hysteresis control method is simple and robust,but it cannot eliminate the coupling among the three-phase current.So it is rarely applied in VIENNA rectifier and its advantages cannot be exerted.The control performance of SVPWM-based vector control is good,but the three-level characteristic of VIENNA rectifier causes the implementation process of SVPWM tedious.In addition,the three-level characteristic of VIENNA rectifier determines that it has the problem of neutral-point voltage(NPV)unbalancinig.It will affect the quality of three-phase current,the current harmonics and the current THD will increase.The unit power factor rectification operation is the normal state of VIENNA rectifier.While the synchronous control of three-phase voltage and current will be greatly affected when the grid voltage is distorted or interfered,meanwhile the current distortion and overshoot phenomenon are serious.Therefore,it is of great significance to improve the control performance of VIENNA rectifier,including simplifying control algorithm,improving efficiency and expand applications,etc.In this paper,the control strategy of VIENNA rectifier is studied from three aspects:current control,NPV balancing control and phase detection(realize the synchronous control between three-phase voltage and current).(1)Current control is the key of VIENNA rectifier.The commonly used control strategies of VIENNA rectifier can effectively control the current,but all of them have some shortcomings.Such as low control accuracy,complex algorithm and high mathematical model accuracy requirement,etc.A space vector switching pattern hysteresis control(SVSP-HC)strategy is proposed based on the convention)hysteresis current control(C-HCC)and SVPWM.The hysteresis bandwidth of SVSP-HC is determined by system parameters and does not need to be adjusted repeatedly.When implemented through digital control,the theoretical current tracking error of SVSP-HC and the additional current error caused by the digital control delay can be partially offset.The simulation and experimental comparisons with other hysteresis control methods show that the proposed SVSP-HC can achieve a better control performance with a lower switching frequency.SVSP-HC is suitable to be used in the control process of VIENNA rectifier and has great potential to be used in other three-level converters.(2)Compared with C-HCC,SVSP-HC has many advantages,but it is implemented in three-phase stationary frame too.Similar to C-HCC,SVSP-HC does not eliminate the coupling among the three-phase current thoroughly.Redundant switching states during the switching state selection process still occur occasionally.Therefore,SVSP-HC is furtherly implemented and optimized in synchronous rotating dq frame,that is,SVSPdq-HC.SVSPdq-HC can eliminate the coupling among three-phase current thoroughly by the decoupling scheme between d-axis and q-axis.Obviously,compared with SVSP-HC,SVSPdq-HC adds the coordinate transformation process,while it does not affect the dynamic performance of the system.Moreover,through the simulation and experiment comparisons between SVSP-HC and SVSPdq-HC,it can be known that SVSPdq-HC can achieve a better current control performance than SVSP-HC with a similar or even lower equivalent switching frequency.In general,SVSPdq-HC is the promotion and optimization of SVSP-HC and has a better application prospect.(3)The NPV unbalancing is an inherent problem of VIENNA rectifier.When the NPV unbalancing phenomenon is serious,the converter will be damaged and the power grid will be impacted.The commonly used NPV control methods can achieve the purpose of NPV balancing.While there are also shortcomings,such as weak adjustment ability or great impact to three-phase current,etc.According to the reverse effect of the redundant small vectors to NPV in VIENNA rectifier,a method based on SVSP-HC is proposed to optimize the selection logic of the redundant small vectors.The proposed method is simple and has little effect to the three-phase current.Compared with the commonly used zero sequence voltage injection method and SVPWM-based method of adjusting the action time of redundant small vectors,the proposed method has more advantages.Therefore,the proposed method of optimizing the selection logic of redundant small vectors can be widely used in the NPV control process of three-level converters,including VIENNA rectifier.(4)Phase detection is indispensable in the control process of VIENNA rectifier and it is the premise of realizing unit power factor rectification.The commonly used phase detection methods are SRF-PLL and the various improved methods based on SRF-PLL.All the improved methods behave excellent in some specific characteristics,but there are also some common problems,such as:complex algorithm,stability reduction and difficult to be realized through digital control,etc.Therefore,in order to detect phase information quickly and accurately,a piece-linearization approximation PLL(PLA-PLL)method is proposed.By analyzing the one-to-one correspondence between the instantaneous three-phase voltage value and the phase angle,the phase information can be obtained quickly by table lookup calculation.At the same time,by filtering the phase difference between two adjacent digital control periods,the limitation of the filter bandwidth when directly filtering the phase angle is avoided.The advantages of PLA-PLL are good steady-state performance,fast dynamic response and simple implementation process,etc.It can be widely used in three-phase systems where the grid voltage phase information needs to be detected.In summary,research in this paper is devoted to the control of VIENNA rectifier.Its current control strategy,NPV balancing control and phase detection technology are studied to improve its overall control performance.At the same time,by applying the VIENNA rectifier experimental prototype,the proposed method was verified.All the work of this paper is to contribute to the wider application of VIENNA rectifier.