Research On Three-Level NPC Inverter Based On Carrier Modulation

With the rapid development of the new energy industry,photovoltaic power generation and wind power generation have become research hotspots,these new energy generation applications are generally high-voltage and high-power applications,multilevel converters provide a good solution for high power applications.Among them,the three-level Neutral Point Clamped(Neutral Point Clamped,NPC)inverter is the most widely used and has many advantages such as simple structure,low voltage stress on the switching tubes and low harmonic content of the output voltage,but its inherent midpoint voltage imbalance problem limits its application development.In this dissertation,two control methods,zero-sequence voltage injection method and asymmetric modulation wave control method,are studied for the midpoint voltage imbalance problem of the three-level NPC inverter,and the asymmetric modulation wave control method is improved.The main research contents are as follows:This dissertation first analyses in detail the topology and operating principle of the threelevel NPC inverter and establishes a mathematical model in the three-phase abc stationary coordinate system.To facilitate control of the voltage-current AC quantities,the three-phase AC component is transformed into a two-phase DC component under the dq coordinate axis by means of coordinate transformation,and PI control of the dc component in the d-axis and q-axis is used to suppress disturbances in the fundamental wave.Secondly,after theoretical derivation of the expressions between the zero-sequence voltage in carrier modulation and the distribution factor in space vector modulation,it is concluded that an equivalent relationship exists between the two modulation methods.The effect of the carrier in-phase and anti-phase stacked PWM modulation strategies on the harmonic content of the output voltage is analyzed using double Fourier series,and the correctness of the theoretical analysis is verified by simulation.Then,the method of balancing the midpoint voltage of the three-level NPC inverter is studied,and a large-signal average circuit model of the three-level NPC inverter is established.Based on this model,a control method based on asymmetric modulation waves is adopted,and the simulation results show that although this method can effectively balance the midpoint voltage,the midpoint voltage still fluctuates greatly.In the follow-up,this method is improved by using the characteristics that injecting zero-sequence voltage components can improve the utilization rate of DC side voltage,and the improved control method is verified by simulation.It can achieve a better midpoint voltage balance effect,and can ensure good output voltage waveform quality.Finally,a three-level NPC inverter experimental platform is designed and built,and based on this experimental platform,the effectiveness of the improved control method is experimentally verified.The experimental results fully demonstrate the correctness and feasibility of the improved control method.