The Research On The Neutral Point Potential Oscillation And The Output Harmonic Voltage Issue Of The Three-level NPC Inverter

Accompanied by the consuming of the limited fossil energy and the rising consideration of the environment protection,renewable energy attracts people’s attention.Besides that,the electric vehicle is becoming more and more prevalent.They all need converters to act as the interface apparatuses,and converters are important in these fields.Take the cost and material issues into consideration,the multilevel converter is the best choice for medium-voltage and high-power applications.Among them the three-level neutral point clamped(NPC)converters are prevalent,and the converter in this topology is researched in this paper.To keep the performance of the three-level NPC inverter,it is essential to maintain the neutral point potential equals to half of the direct voltage.The redundant small voltage vectors are utilized in conventional strategies to adjust the neutral point potential.However,the ability to regulate the neutral point potential is decreased when the modulation index is high,or the power factor is low.And the regulation range is limited.At the same time,the oscillation of the neutral point potential appears in this method.The drifted neutral point potential and the oscillation in neutral point potential both increase the harmonic elements in the output voltage.The multi-stage medium vector strategy is proposed after investigating the relationship between the voltage vectors and the neutral point potential.This strategy is combined with deadbeat control strategy and the model predictive control strategy,in this way the converter can eliminate the drifted neutral point potential and the oscillation of the neutral point potential in all modulation index range with fast response.Besides that,several voltage vectors are selected and arranged to cope with the high and uncertain switching freqncy issue of the model predictive control.However,the algorithms mentioned above still have to face the uneasy requirement of calculation ability to the microcontroller,which makes it can hardly be used in highly dynamic demand situation.As previous researches have revealed that the space vector modulation and the sinusoidal carrier-based modulation are the same.The combination of dual-carrier-based modulation and the dual-modulation wave carrier-based method is proposed in this paper.The secondary compensation of the duty and the swiching modulation mode make this algorithm achieve the same performance as the multi-stage medium vector modulation in a simpler way.The output voltage of three-level inverter is also affected by the load.Considering the non-linear load existing in the distributed generaion system,the research to maintain the quality of the output voltage in these situations makes sense.To make the control strategy a general one,the algorithm to reduce output harmonic voltage in the droop control frame is proposed.The proposed algorithem utilizes the harmonic power droop,and it utilizes the virtual power to construct the control loop.It reduces the output harmonic voltage without complex calculations.To verify the algorithms proposed in this paper,the three-level prototype inverter is built in laboratory.The experimental results reveal that the multi-stage medium vector works and the output harmonic voltage is reduced with the harmonic power droop control.