The Scheme For Co-inhibition On Common Voltage And Neutral Point Fluctuation Of NPC Three-level Inverter

With the increasing output voltage class of the inverter,the voltage stress of the switching device in the traditional two-level inverter increases,the higher voltage stress will increase switching loss of the inverter and make it difficult for parts selection.However,the multi-level inverter has the outstanding capability to decrease voltage stress.In the same time,the multi-level inverter has low total harmonic distortion of the output current and low electromagnetic interference.Neutral point clamped(NPC)three-level inverter has many advantages such as needing less devices and controlled more simply and used in industrial fields widely.Moreover,there are two disadvantages in the NPC three-level inverter.On the one hand,the common voltage will cause the generation of the leakage current because of the existing of the parasitic capacitance to the ground when connected to PV arrays,which will lower the quality of the grid-connected current,or even cause personal injury.When connected to the motor,the higher common voltage will lead to the bearing failure,and the work time shortened.In the other hand,neutral point fluctuation in a large scale will lead to more low-order harmonic included in output current and the voltage stress unbalancing.NPC three-level inverter has some inherent problems,which include common voltage and neutral point fluctuation.Based on the influence brought by small and big vectors on these two problems,this paper proposed a new phase opposition disposition(POD)modulation method with injecting different zero-sequence signal while in different divided sectors,in order to realize the co-inhibition on the common voltage and the neutral point fluctuation.Compared with traditional PD method and POD method both based on sinusoidal pulse width modulation(SPWM)and PR controlling with a capacitor-voltage loop,the common voltage and neutral point fluctuation can both be suppressed,the quality of line to line current can also be improved at high modulation index.Under the same circumstance,at low modulation index,the common voltage can be suppressed,the neutral point fluctuation will not be influence greatly,the quality of line to line current can also be improved.The MATLAB/Simulink simulation and experimental results verify the feasibility and validity.