Research On Neutral-point Voltage Balancing Control For Three-level NPC Converters

Due to high power and high performance,multilevel conveters have been increasingly become the preferred option in high-and medium-voltage and high-power applications.Among them,the three-level NPC converter is the most widely studied and employed for its simple topology and fewer power devices and capacitors.In this paper,modulation strategies,neutral-point voltage-balancing methods,and some key issues for three-level NPC converters are discussed in detail.The effect of the voltage imbalance of the dc-link capacitors in the three-phase and single-phase three-level NPC converters on the converter and the load has been analyzed from the point of view of converters and loads,respectively.Then,the quantitative analysis of the cause of voltage imbalanced in the three-phase and single-phase three level NPC converters is described,and the characteristics and influence factors of the low-frequency voltage oscillation in the neutral point are presented.Additionally,the principle of the neutral-point voltage compensation for the three-phase and single-phase three-level NPC converters is discussed.Commonly,the redundant small vectors in pairs are employed to achieve the balancing control of the capacitor voltages.Because the positive and negative small vectors in a pair can generate the same line-to-line voltage,but the neutral-point currents have the opposite polarity.Two kinds of simplified two-level space vector modulation methods: zero sequence voltage injection method and unified voltage modulation method are modified,and the equivalent relationship between them is also proved.Then the improved zero sequence voltage injection is introduced into the simplified three-level space vector modulation,and a zero sequence injection method based on k for the balancing control of the neutral point voltage is proposed.With the equivalent relationship between the redandunt zero vectors in the two-level space vector diagram that is from the simplified three-level space vector diagram and the redundant small vectors in the three-level space vector diagram,the poroposed method changes the relative time of application of redundant zero vectors,which in turn adjusts the relative time of application of positive and negative small vectors in a pair,and then the neutral-point voltage balance is obtained.Finally,the feasibility and effectiveness of the proposed method are verified by simulation and experimental results.An in-depth analysis on high dv/dt of the output voltage,high voltage total harmonic distortion,high switching losses,and slow voltage-balancing dynamic in VSVPWM method is presented,and in order to explain the voltage-balancing dynamic that is provided by different modulation techniques,a compensated capability value is introduced.Based on the features of simplified three-level space vector diagram,the relationship between the average neutral-point current and zero sequence voltage is established in each small hexagon(or each zone),and a zero sequence voltage injection method with one carrier waveform is developed.A unified modulation and neutral-point voltage control method for three-phase and single-phase three-level converters is achieved,which is not only applied in three-phase three-level converter,but also in single-phase three-level converter.Moreover,it is less complex to implement,as well as having low computational effort,short execution time and fast voltage-balancing dynamic.Simulation and experimental results verify the feasibility and practicability of the developed method.The three-level space vector plane is divided into six sectors according to the polarity of three-phase reference voltages,and the relationship between the neutral-point current and zero sequence voltage is established in each sector.Then,a zero sequence voltage injection method with two carrier waveforms without modifying the original reference voltages is proposed.The strategy proposed is also extended to balance the neutral-point voltage in the single-phase three-level NPC converter.The proposed scheme achieve a unified modulation and voltage-balancing method for three-phase and single-phase three-level NPC converters.Simulation and experimentation validates the correctness and effectiveness of the proposed method.