Research On Neutral-Point Potential And Losses Of Three-level Neutral Point Clamped Inverter Based On Virtual Vector Space Algorithm

Three-level Neutral Point Clamped Inverter, mentioned by Japanese scholars, has been applied rapidly in high voltage and high power fields since 1981.Compared with two-level topology, three-level topology has obvious advantages, such as the voltage each device withstands is only half of the two-level topology, and the output waveform is also improved. But in three-level inverter, the number of power devices is more than two-level. With the number increase of power devices, the loss also increases. Therefore, the designation of heat dissipation system becomes very important. At the same time, the neutral point potential unbalancing is one of the inherent problems of three-level-neutral point clamped inverter. Based the two problems mentioned above, the main research contents are as follows:(1)In this paper, basic principles of modulation strategy of three-level NPC inverter are introduced, such as SPWM, SVPWM and other modulation methods, and focuses on the research of SVPWM modulation strategy.(2)Based on the problem of imbalance of the neutral point potential in the DC side of the NPC three-level inverter, this paper introduces the topology and working principle of the NPC three-level inverter. Strategies of Nearest-Three-Vector and Nearest-Three Virtual-Space-Vector are analyzed.(3)Based on the problem of the increasing losses, this paper analysis the source of losses, a new modulation method is proposed which improved based on the Virtual-Space-Vector. The proposed method solves the problem of the traditional modulation method, where short vectors can not be paired in most triangles and the effect of the neutral-point potential control has limitation. The traditional virtual space vector algorithm achieved the balance of the neutral-point potential complete control over full range, but it increased the switching frequency of devices, and the device junction temperature is arise because of the increasing loss of the system. Based on the occurred questions, an improved virtual space vector algorithm is proposed, which not only makes the fluctuation of neutral-point voltage small, but also gets lower losses and higher system efficiency than the traditional virtual space vector modulation.(4)The main circuit of the three-level NPC inverter and the power device driving circuit are built. Analyzed the simulation and experiment results of balancing problem of neutral-point potential under SVPWM and NTV2. The simulation and experiment results of system losses under NTV and NTV2 are also analyzed. The simulation and the experimental results have verified the feasibility and effectiveness of the method.