Improved Virtual Space Vector Modulation Strategy For NPC Type Three-level Inverter

With the development of the modern power industry,three-level inverters with the advantages of high output voltage waveform quality,low switching device voltage stress,and high efficiency have begun to gradually replace the traditional two-level inverters.As one of the most widely used multi-level converters,the Neutral-Point Clamped（NPC）three-level inverter has received extensive attention.However,this type of inverter has the problems of neutral-point potential fluctuation and large common-mode voltage,which seriously affects its working performance and its scope of application is also limited.Aiming at the problems of neutral-point potential fluctuation and large common-mode voltage,and based on the influence of neutral-point potential fluctuation on the space vector,an improved virtual space vector modulation method is proposed.This method ensures that the average neutral-point current generated by each virtual vector is zero.A new virtual small vector is constructed by selecting a small vector and a medium vector with a small common-mode voltage(amplitude is less than U_dc/6),and using the three adjacent sectors The neutral-vector constructs a new virtual medium vector,which eliminates the position offset of the virtual space vector caused by neutral-point potential fluctuations,suppresses neutral-point potential fluctuations and common-mode voltage,realizes smooth switching between switching sequences,and reduces output distortion.On this basis,in order to eliminate the neutral-point potential fluctuations caused by factors such as switching delays,dead zone settings,component parameter asymmetry,etc.,this paper proposes a vector conversion compensation method.Without introducing a new space vector,the original There are vector relationships in the switching sequence,and the vector convers ion principle is followed to achieve isochronous equivalent co nversion between vectors and generate corresponding neutral-point compensation currents,thereby achieving suppression of neutral-point potential fluctuations in the full range.Through the above measures,the overall performance of the NPC three-level inverter is improved.In addition,this article introduces the traditional virtual space vector modulation method,analyzes its principle of suppressing neutral point potential fluctuations,and the reasons for the large common-mode voltage.In the MATLAB/Simulink simulation platform,a simulation model of the NPC three-level inverter is established.By comparing and analyzing the simulation res ults of the traditional method and the improved method proposed in thi s paper,the effectiveness and superiority of the proposed method in suppressing the neutral point potential fluctuation and common-mode voltage are verified.