Common-mode Voltage Suppression For Neutral-point-clamped Three-level Inverter

The neutral-point-clamped three-level inverter has been widely applied in railtransport traction systems, high-power AC speed regulation and new energy powergeneration etc because of their attractive features, such as low voltage stress on eachpower switch device, low switching frequency and small output voltage harmonics. Inthe practical application, the inverter output side will produce common-mode voltage,which causes great harm to the load and the surrounding electrical equipments, and,the performance of the system is seriously affected under the condition of highermodulation factor, low power factor and the neutral-point potential imbalance. Forthis purpose,neutral-point-clamped three-level inverter common-mode voltagesuppression and neutral-point potential balance control are studied to improve overallsystem performance in this paper.This paper proposes a new nearest-three-virtual-vector modulation strategy tosolve the high common-mode voltage problem generated by the nearest-three-vectormodulation and traditional nearest-three-virtual-vector modulation. the newnearest-three-virtual-vector modulation can effectively suppress common-modevoltage, at the same time, it can control the neutral-point voltage balance over the fullrange of inverter output voltage.The mechanism and reason of the generation ofimbalance of neutral-point potentials based on the topology and working principle ofneutral-point-clamped three-level inverter are analyzed.For the problem ofimbalanced neutral-point potential of the nearest-three-vector modulation, Thespecific implementation process of traditional nearest-three-virtual-vector modulationis studied. According to the definition of common-mode voltage, the common-modevoltage generated by27switches states of three-level inverter are deduced.As thenearest-three-vector modulation and traditional nearest-three-virtual-vectormodulation switching sequence contain positive small vectors,which create thelargest common-mode voltage, the new nearest-three-virtual-vector modulationmerges the virtual small vector by zero vector and large vector, merges the virtualmiddle vector by three middle vectors, abandons the positive small vector, which cansuppress the common-mode voltage effectively. In order to reduce the switchingfrequency of the new nearest-three-virtual-vector modulation, phase duty cyclemethod are proposed in this paper, calculating the three-level three-phase duty cycle by duty cycle of the basic vector, it reduces the switching frequency effectively.An experimental setup of neutral-point-clamped three-level inverter includingTMS320F2812and EP1C6Q240C8is used for the experimental verification,experimental results show that compared with the nearest-three-vector modulationand the traditional nearest-three-virtual-vector modulation,the newnearest-three-virtual-vector modulation in this paper can effectively suppresscommon-mode voltage, at the same time, it can control the neutral-point voltagebalance over the full range of inverter output voltage.