Research On Neutral-Point Voltage Balancing Algorithm Of NPC Three-level Converter Based On Hybrid Modulation Strategy

Three-level converter can output multi-step voltage wave, and has advantages of low stress across the semiconductors, low output harmonic of voltage and high efficiency.So it has been widely used in the field of high power. With its special topology,NPC three-level converter have the defect that neutral-point voltage inbalancing without the two-level converter. Studies have shown that modulation strategy can effectively improve the three-level inverter output characteristics, which can improve the neutral-point voltage inbalancing.Have a idea suggest that the hybrid modulation strategy uses three nearest vectors method in neutral-point voltage controllable area and virtual vector method in neutral-point voltage uncontrollable area. It can balance the capacitor voltage completely in any conditions. However, when the controllable area is small, the virtual vector method is excessively dependent. So the switching frequency is high and the loss is large.For this defect, this paper describes the topology of three-level NPC converter and works, briefly analysis of the distribution of each vector in the three-level space vector,and analysis the mechanism of the midpoint voltage imbalances from vector and power. On this basis, detailly analysis of the most recent three vector modulation strategy and virtual space vector modulation strategy, and comparative analysis of the two vector synthesis principles as well as the order of switching. For the problem higher switching frequencies of virtual space vector, for variation analysis redundant small vectors duty factor,the controllable regional midpoint voltage balance of the last three vector method were determined. For hybrid modulation strategy have a greater problem of switching frequency over the use of virtual space vector algorithm. On this basis, a new hybrid modulation strategy is proposed, which extends the controllable area according to the allowable neutral-point voltage fluctuation amplitude. The area of VSVPWM is contracted, so the switching frequency and loss are reduced. The results of experiment show that the new hybrid modulation strategy can balance the neutral-point voltage and reduce the switching frequency effectively compared to former methods.Then, the Matlab/Simulink simulation model of carrier modulation is bulided to get the output voltage waveform, the output current waveform and the midpoint potential waveform of the four strategies:the nearest three vectors,the virtual vector method, the hybrid modulation strategy and the new hybrid modulation strategy, finally under the same conditions depicted switching hybrid modulation strategy and a fundamental period under the new hybrid modulation algorithm within the total number of three-dimensional map to compare which verify the correctness of theoretical analysis.Finally, a 10KW three-level converter experimental platform is builted, which use the TMS320F28335 DSP as the control core and CPLD as an auxiliary control. An experimental verification of the output voltage, the harmonic content analysis and the neutral point potential balance was made for the four three-level modulation strategies.