| Three-level neutral-point-clamped (NPC) converters have been widely applied in power conversion device occasions of medium high voltage and power, being capable of generating high quality output-voltage, and, most recently, increasing in occasions of the low-voltage low-power applications. The three-level converters have the problems of low-frequency voltage oscillation that appears in the neutral-point (NP) of the DC-link under some operating conditions because of its unique structure and the limitations of the existing modulation methods. This thesis presents two modulation technique based on two modulation wave that can completely removes the low frequency voltage oscillations that appear at the NP for the three-level converter.The working principle of the converter is analyzed firstly and the switching state of each bridge arm is defined. The mathematical model of the converter is set up and the mathematical expression of the NP current is derived. The mathematical model of the NP potential of the DC-link is established and the NP potential expression is derived by the NP current.The principle of the two PWM strategy based on two modulation wave: double modulation wave PWM (DMWPWM) and double group double modulation wave PWM (DGDMWPWM) is analyzed from the midpoint of the average NP current to be zero. The intrinsic ralationship and implementation of the modulation method is studied. The relationship between the two group modulation waves of the DGDMWPWM with the boundary of carrier wave is studied by using three dimensional models and the average switching frequency is derived.The two modulation method that can eliminate NP potential low-frequency voltage oscillation is implemented by simulation and verified the problem of NP potential balancing. From the inverter output voltage waveform quality and the NP potential fluctuation amplitude of the three-level inverter with different modulation methods were compared. The various modulation algorithm voltage dynamic recovery plans is given under the condition of the initial DC-link capacitor voltage unbalance and voltage rebalance time is derived from the theory. DGDMWPWM method has the best ability of balance neutral-point potential. The modulation and the carrier wave frequency of the DGDMWPWM influence on voltage recovery capability are also studied.Finally, the SVPWM and the DMPWM method are implemented on the experimental platform. SVPWM approach has three times the output waveform of the low-frequency fluctuations, the DMWPWM method only exist high frequency fluctuations. Experimental results verified the correctness of the modulation theory. |
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