Research On The Control Strategy Of Neutral Point Voltage Fluctuation In Three-Level Neutral-Point-Clamped Converter

Three-level neutral-point-clamped converter has the advantages of simple structure,small EMI,lower switch stress and loss,but the neutral-point voltage balancing problems limit its application in the actual industry application.This paper focuses on this problem and gives out a solution.The working principle of the three-level NPC converter is analyzed in detail,and the mechanism of the voltage balance of the DC side of the converter is studied.On this basis,the relationship between the zero sequence voltage and the neutral point current is analyzed,and the influence of the zero sequence voltage limited on the average current is discussed in three cases.Zero sequence voltage can be injected to control the neutral point voltage balance of the three-level converter,but low frequency fluctuations cannot be eliminated when the modulation index is high or power factor is low.The double signal PWM method can eliminate the neutral point fluctuation in the whole range,but will increase the switching frequency and deteriorate the output waveforms of the inverter.In this paper,a hybrid modulation scheme is adopted to achieve the goal of eliminating the low frequency fluctuation and reducing the switching frequency.Then we analyze the influence of carrier amplitude changing to the converter output voltage and come up with the conclusion that the NPC converter can still output the ideal phase voltage and keep its volt-second balance when the carrier amplitude is changing with the DC side capacitors in proportional.On the basis of this,a neutral point voltage balance method based on carrier amplitude zero voltage is derived.This method first changes the carrier amplitude with the DC side capacitors proportional and thus,keep the output voltage characteristics.Then inject zero level voltage to the three-phase reference voltage on the basis of bus neutral point voltage balance and keep the DC side voltage balance.This control strategy can not only balance the DC side neutral point voltage but also keep the output reference voltage characteristics.A three-level NPC converter for experiment is set up,and the control strategy proposed in this paper is proved by experiments.The experimental results show the validity of various control methods which are put forward in this article.