The Research On Control Strategy Of Three-Level Neutral Point Clamped Inverter Under Low Switching Frequency

Three-level neutral point clamped(3L-NPC)topology is mature and applied to medium or high voltage and high-power occasions such as electric locomotive traction drive system and flexible DC transmission.In order to reduce power consumption,it is necessary to reduce the system switching frequency in the locomotive traction drive system.At this time,three-level NPC mostly adopts Specific Harmonic Elimination Pulse Width Modulation(SHEPWM).This paper focuses on the initial value problem of three-level SHEPWM,the digital implementation method,the self-balancing characteristics and neutral point voltage balance control of three-level NPC based on SHEPWM.The basic principle of SHEPWM applied to three-level NPC is studied.Based on the initial value requirements of solving SHEPWM nonlinear transcendental equations,a group of optimal initial value formulas are selected to make the calculated switching angle trajectory fluctuation less and the maximum modulation ratio larger by comparing and analyzing the advantages and disadvantages of different initial value formulas.The basic principle of polynomial curve fitting,a common implementation method of SHEPWM,is analyzed in detail.A method of fitting the switching angle trajectory of SHEPWM by Fourier series is proposed in view of the shortcomings of piecewise fitting and searching for the optimal piecewise points in fitting the switching angle trajectory of SHEPWM by polynomial curve fitting method.Through the fitting method based on Fourier series,the switching angle trajectory of SHEPWM can be fitted in the full modulation ratio range,which avoids the disadvantage of polynomial curve fitting method.The self-balancing characteristics of three-level NPC based on SHEPWM and the factors affecting the self-balancing rate are studied.It is deduced that the total voltage on the DC bus side,switching frequency and load impedance affect the self-balancing rate of three-level NPC.On the basis of little influence on the harmonic performance of output voltage,the neutral point voltage control method of fine-tuning switching angle is studied to control the neutral point voltage of three-level NPC.Aiming at the problem of neutral point voltage fluctuation of three-level NPC,the optimal third harmonic content to control the neutral point voltage fluctuation is calculated based on the analysis of the relationship between the third harmonic and the neutral point current,which is the optimal third harmonic content method.The optimal third harmonic content method can suppress the fluctuation of neutral point voltage,but its maximum modulation ratio is limited.Aiming at the problem of optimal third harmonic content method,a third harmonic content method which can achieve the maximum modulation ratio is proposed.This method achieves the goal of maximum output modulation ratio by sacrificing the control ability of a certain neutral point voltage fluctuation.Matlab/Simulink simulation models and three-level NPC prototype are built,and the effectiveness of the proposed method is verified by simulations and experiments.