Research On Midpoint Balancing For NPC Three-level Inverter

Because of large capacity, high pressure rating, less harmonic contents ofoutput current, small switching loss, and many other advantages, thethree-level converter has been used widely and it is the ideal choice forhigh-voltage high-power situations. Compared to other three-level topology，the diode-clamped three-level topology is the most extensively studiedtopology. But the three-level converter has a defects that two-level converterdon’t have, that’s the midpoint potential imbalances. In this paper mainlystudy the mid-point potential balance of the three-level converter.In this paper, background, research status, meaning of three-levelconverter and the key issues of three-level converter are introduced firstly.From the vector and power factor angle to introduce the cause of the of themidpoint potential imbalance for Three-level converter. In order to introduceThree-level neutral point potential balancing control mechanism, diodeclamped three-level converter system as the research object in this paper.Before introducing the algorithm of three-level simplified SVPWM control,two-level vector control algorithm are been introduced firstly. In elucidatingthe three-level SVPWM control algorithm, the detailed description is a precisemidpoint potential control algorithm--Balancing factor method. The controlof the midpoint potential fluctuations it is very difficult in some conditions, sothis leads to the three-level control algorithms-virtual vector algorithm. Thisalgorithm separation of sometime in mid-vector as small vector time, so asincrease the duration of action of small vector in a control period and muchmore conducive to achieving the balance of the midpoint potential. In someconditions the use of software to control the midpoint potential is difficult toachieve the desired result, so in this paper the midpoint potential of thehardware control method is analyzed. This method needs much more powerdevices and drive circuit in the DC side, so that the control system becomescomplex. But relative to the use of software to control the midpoint potentialbalance, this method can be more effective to achieve the balance of themid-point potential, applies to the midpoint potential that’s difficult to control in the three-level converter.In order to verify the midpoint potential balance control, on the basisof theoretical analysis build a simulation model of three-level converter basedon MATLAB, and a experimental platform of DSP as control core.