Research On Neutral Point Voltage Fluctua-tion Control For Three-Level Power Converter

The emergence of three-level converter provides the solutions for the realization of high-voltage high-power system with large-capacity and high-performance, therefore, the three-level technology has become a very active area in power electronics technology. As the advantages of simple structure and low cost, the diode clamped three-level inverter has been widely studied and applied.Sinusoidal pulse width modulation strategy for diode clamped three-level inverter is detailedly studied in this dissertation. And the neutral point current model is established, the variation and offset of neutral point voltage are analyzed. The causes of neutral point voltage fluctuation are discussed, and the research provides a theoretical basis and guid-ance for the filter capacitors design and the neutral point voltage control. Based on the analysis, the impacts of different adjustments on neutral point voltage are studied from the angle of the carrier wave and modulated wave, respectively.The impact of the carrier’s amplitude on the neutral point voltage based on neutral point current, and derives the mathematical expression between neutral point voltage and scale of the carrier’s amplitude is studied in this dissertation. With the relationship between neutral point voltage and carrier’s amplitude, a neutral point voltage control scheme based on carrier amplitude scaling is proposed. The same effect can also be achieved by an equivalent transformation of the carrier amplitude scaling to modulation wave. In order to simplify the calculation, facilitate the realization, the modulation wave control mode for neutral point voltage control is proposed. It also explores new ideas of the conversion be-tween carriers and modulation wave in three-level systems. The relationship between neu-tral point voltage and carrier overlap is studied in this dissertation. With analyzing the im-pact of the carrier overlap method based on changing the vertical position of the carriers on the neutral point voltage, the neutral point voltage control scheme based on carrier overlap is proposed. In order to avoid the adverse effects of the over modulation in linear modulation area, a hybrid carrier overlap method based on carrier amplitude scaling is proposed, the neutral point voltage control can be achieved by changing only one carrier’s amplitude. In the full study of the carrier’s overlap, a SPWM modulation strategy based on the carrier overlapping is proposed. The overlap displays a symmetrical distribution on the horizontal axis for the strategy, it does not make the neutral point voltage offset, but can effectively improve the output waveform quality (particularly in the low modulation area).There are two main forms of modulation wave changes, one is by adding DC offset, and the other is by adding harmonic components. The impact of the changed modulation wave on the neutral point voltage is analyzed. The DC offset can make the neutral point voltage shift is proved, and the range mapping relationship between SPWM and SVPWM for three-level systems is given, in this dissertation. Therefore, adding DC offset can also be introduced into the three-level SVPWM method. And the neutral point voltage control scheme by adding DC offset is proposed. The relationship between adding harmonic com-ponents and neutral point voltage is further analyzed, it shows that the neutral point volt-age will shift by adding6n times harmonic components, the direction of shift is deter-mined by the phase of injected harmonic components. The specific relationship between neutral point voltage and injected harmonic amplitude is given. With the relationship, the closed-loop neutral point voltage control method by injecting sixth-order harmonics is proposed.In order to validate the above theoretical studies,a three-level inverter is built based on the DSP+CPLD platform. All proposed strategies and analysis, three-level neutral point voltage fluctuation characteristics, the proposed neutral point voltage control strategy based on carriers and modulation wave, are studied on the experimental platform. Simula-tion and experimental results fully verify the correctness of theoretical research in this dis-sertation.