Research On Modulation Algorithm And Midpoint Balance Control Simplification Of NPC Three-level Rectifier

In the field of medium and high voltage and high power,compared with the two-level topology,the three-level topology has the advantages of low harmonic content of output current,low voltage stress of devices and high output voltage level,so it has been widely used in high-power PWM rectifier.The three-level rectifier has three topologies: capacitor clamped,cascaded H-bridge and diode clamped.Among them,the neutral point clamped（NPC）three-level rectifier has attracted more attention because of its simple control mode,low cost and high economic benefits.However,NPC three-level PWM rectifier has many output voltage vectors at the AC side,which makes the design and control more complex.Moreover,NPC three-level topology has inherent neutral point potential imbalance.If this imbalance exists for a long time,it will have a serious impact on the rectifier system.The method to solve this problem mainly focuses on the control algorithm of PWM rectifier.In addition,the excellent modulation algorithm can make the control of three-level PWM rectifier simpler and the output effect better.In this paper,the diode clamped NPC rectifier will be deeply studied.The main work is as follows:1.The working principle,implementation process and complexity of traditional space vector PWM（SVPWM）algorithm and SVPWM algorithm based on coordinate translation of NPC threelevel rectifier are compared and analyzed,and verified by Matlab/Simulink simulation.2.Aiming at the inherent imbalance of neutral point potential in NPC three-level rectifier,a sector switching control algorithm based on coordinate translation SVPWM modulation is proposed,and the criterion of using sector switching to control neutral point potential is given.The control algorithm can effectively suppress the fluctuation of neutral point potential,overcome the problems of large fluctuation of neutral point potential in hysteresis control method and complex calculation of accurate control algorithm of balance factor,and greatly reduce the amount of calculation.The simulation models of the above three neutral point potential control algorithms are established in Matlab/Simulink,and the comparison results are given.3.Build NPC three-level rectifier experimental platform based on Rtu—BOX real-time digital control system,including the design and commissioning of main power circuit,driving circuit,voltage and current sampling circuit,as well as the systematic construction process of the platform.Finally,the effectiveness of the proposed algorithm is verified by an experimental platform.