Research On Suppression Of Midpoint Voltage Fluctuations In Three-level NPC Converter Based On Space Vector

Nowadays,the power electronics industry pays more and more attention to applications in the high-voltage and large-capacity fields.Multilevel converters are suitable for high-voltage and large-capacity fields with low output harmonics and low voltage on a single switch.The advantages stand out in all kinds of converters,so multilevel converters have good research value and application prospects.Among multi-level converters,three-level NPC converters have been studied more,because it have their unique advantages-topology.Compared with other multilevel converters,the structure is simple,the DC side voltage utilization is high,the output harmonics are low.Although it has many advantages,due to the fluctuation of the three-phase load current and the topology of the converter itself The reason is that the midpoint voltages of the upper and lower capacitors on the DC side are not equal.If the loss of components increases,the life of the device decreases,the power switching device is easily broken down,the output harmonic content increases,and the output voltage is distorted.Aiming at the problem that the midpoint voltages of the upper and lower capacitors on the DC side of the diode-clamped three-level converter are not equal,this paper proposes an improved control based on VSVPWM.Different from the traditional VSVPWM control algorithm,this paper redefines the virtual voltage vector to make it the same length and direction as SVPWM,which greatly simplifies the partitioning rules of traditional VSVPWM.The control of the point voltage enables the improved control algorithm to combine the advantages of SVPWM and VSVPWM,and achieves the purpose of neutral point potential balance under the condition that the capacitance of the two capacitors on the DC side are not equal and the modulation ratio is large.Finally,a simulation model of the neutral potential balance of the diode-clamped three-level converter is set up,and the parameters of each device of the main circuit are determined based on theoretical analysis and operation.The simulation results show that the control scheme in this paper can keep the fluctuate within ± 2V,which is equivalent to the traditional method which has improved the suppression effect by 67%.The THD is3.63%,which has good feasibility and effectiveness.