Voltage Control Strategy For The Neutral-Point-Clamped Three Level Converter

The neutral-point-clamped (NPC) three-level converter is a widely used multilevel converter for industry application. This topology has much advantages compared to the two level converters, such as lower voltage pressure on the power device, lower total harmonic distortion of the output line voltage, etc. Nevertheless, an inherent drawback of the NPC converter has hindered its extensively application, that is the DC-link capacitor voltage will be unbalanced in the practical application without effective control.Space vector PWM and carrier based PWM has been developed for voltage balance control. The consistency between the two method for two-level converter is analyzed first, which comes down to that the variation of zero-sequence voltage in carrier based PWM method is corresponding to the time distribution of the redundant vectors in the space vector PWM method. Based on the above analysis, the consistency has been promoted to the three level converter, and the carrier based PWM method has been proved to realize voltage balance control with the same effect as SVPWM.The relationship between the average neutral current and zero-sequence voltage in neutral-point-clamped converters is discussed, a theoretically optimal method for neutral point voltage control is presented. The detected neutral current for the voltage balance is compared with the neutral currents related to the five significant zero-sequence voltages, and then the interpolation method is used to calculate the optimal zero-sequence voltage. For further analysis, the oscillation of the neutral point voltage with different modulation indexes and power factors is studied, the neutral potential oscillation will be severer when the modulation index is closer to one, and power factor angle is closer to one.With the optimized zero-sequence method sensitive to the modulation index and power factors, another carrier based PWM method is provided in the fourth chapter, the double signal PWM method. Two new modulation waves is reconstructed,with the sum of the three phase load currents is always zero, the neutral current is kept to zero, and the neutral voltage oscillation is eliminated. But the double signal PWM method can not be used for the voltage balance control although the capacitor voltage can be kept to constant, to address this issue, based on the general consideration of the load currents, switching frequency, difference of the capacitor voltages, etc, an optimized voltage compensator is proposed and its value is adjusted according to the modulation index and power factors, which make the process of voltage balance steady and effective.The validity and practicability of the proposed algorithm is verified by simulation and experimental results.