Research On Neutral-Point Voltage Balance Of The NPC Three-Level Inverter

The neutral-point clamped three-level inverter(NPC)has been widely used in high-voltage and high-power transmission,new energy power generation,power grid quality control and other fields.Two improved methods based on space vector pulse width modulation(SVPWM)are proposed to solve the problem of the inverter’s neutral-point voltage imbalance.Firstly,the nearest three vectors modulation(NTV)is introduced,which has the advantages of easy implementation and simple control.However,the action time of this modulation basic vector is fixed and cannot be adjusted dynamically.The ability to control the neutral-point voltage balance is weak.Therefore,an improved NTV modulation strategy is proposed,that is,the output K is obtained by comparing the voltage difference between the upper and lower capacitors,and then the action time of the redundant small vector is dynamically adjusted.The simulation results show that the improved NTV modulation improves the ability to control the neutral point voltage under static and dynamic conditions.Secondly,in order to solve the problem of the dead zone of NTV modulation control,the control without dead zone is realized by introducing the nearest three virtual vector modulation(NTV~2).However,this modulation will make the common mode voltage(CMV)generated by the NPC three-level inverter larger.To this end,an improved NTV~2modulation is proposed,that is,the basic vector with a smaller common-mode voltage is selected for synthesizing a virtual vector,which reduces the system common-mode voltage and also reduces the number of switching states.The simulation results show that the common-mode voltage generated by the improved NTV~2modulation strategy is only half of that of the traditional modulation strategy,and it can also improve the neutral-point voltage balance speed by 50%.Finally,the two improved modulations proposed are verified experimentally on the RT-LAB platform.The experimental results show that:(1)The control accuracy and control ability of the improved NTV modulation on the neutral-point voltage are better than those of the traditional modulation.(2)The improved NTV~2modulation is superior to the conventional modulation in suppressing the common mode voltage and improving the speed of the neutral-point voltage balance.