Optimal Pulse Width Modulation And Neutral Point Potential Balancing Control Algorithm Of Three-level Traction Inverter

The three-level neutral-point-clamped(NPC)inverter has been widely applied in the field of medium-voltage high-power transmission since its first application in AC drive system in 1981.Limited by switching losses,inverter volume and cooling capacity,the maximum switching frequency of power device is generally not more than 1kHz.In order to reduce the stator current distortion,harmonic torque,harmonic loss,heat and noise during the motor operating in the wide speed range,multi-mode modulation strategy that taking different modulation algorithm in different speed sections were adopt.How to optimize the PWM algorithms of three-level traction inverter with the lowest current harmonic distortion and torque ripple under the limited carrier ratio has important theoretical and engineering application value.Therefore,the PWM algorithms with different types and different carrier ratios,and its harmonic performance,and neutral point potential balancing control algorithms of three-level traction inverter is investigated.Firstly,the main circuit structure of three-level traction inverter and the basic principles of asynchronous space vector pulse width modulation(SVPWM)algorithms are introduced,and an asynchronous SVPWM simplified algorithm based on the 60° coordinate system to calculate the vector action time are also studied.The harmonic performances of two different asynchronous SVPWM simplification algorithms with different synthetic vector sequences are comparative analyzed under low-modulation index.Secondly,the basic principles of three-level synchronous PWM is introduced.Aiming at the synchronous SVPWM which calculating vector action angle on-line based on space voltage vector,eight kinds of three-vector synchronous SVPWM with half-wave symmetry,three-phase symmetry and quarter waveform symmetry constraints are designed to eliminate even harmonic components.To further reduce the line voltage weighted total harmonic distortion(WTHD),three kinds of four-vector synchronous SVPWM which considering the influence of vector assignment coefficient are analyzed and designed.Then,the quantitative relationship between the vector assignment coefficient and the harmonic performance in specific vector sequences are obtained.Aiming at the optimized synchronous PWM which calculating the switching angle off-line based on the single-phase bridge-arm voltage waveform,the optimal mathematical model of the selective harmonic elimination PWM,current harmonics minimization PWM,and torque harmonics minimization PWM are established,respectively.Then,the optimal switching angle of each kinds of optimized synchronous PWM under different carrier ratios are solved by combining the initial value empirical formula and the method of estimating the initial value based on the trajectory differential,and their current harmonic performance is analyzed theoretically.Aiming at the problem of how to design the full-speed PWM technique of three-level traction inverter,the above two types of synchronous PWM are comparative analyzed from the aspects of carrier ratio,maximum switching frequency,current total harmonic distortion,torque ripple,DSP digital implementation.Then the validity of each kinds of synchronous PWM and its harmonic performance are verified by simulation.Thirdly,aiming at the neutral point potential imbalance problem of three-level traction inverter,a three-level hysteresis control method based on SHEPWM is proposed.Three kinds of redundant vector adjustment schemes are designed by analyzing the switching vector sequence and its vector action angle corresponding to the three-phase bridge-arm voltage waveform.This method directly adjusts the three-phase PWM signal on-line and can be applies to any PWM algorithm.The simulation results show that the proposed method can achieve effective control of neutral point potential under the two kinds of asynchronous SVPWM and the two types of synchronous PWM.Finally,based on the TMS320F28335 DSP controller and RT-LAB hardware-in-the-loop real-time simulation platform,the DSP program of three-level inverter optimized pulse width modulation and neutral point potential balance hysteresis control algorithms are developed,as well as the validation of the correctness and effectiveness of the algorithms.