Amplitude control of the neutral-point voltage oscillations in the three-level converter

This paper presents a control method to regulate the amplitude of the voltage oscillations that appear in the neutral-point of the three-level diode-clamped converter for some operating conditions. The control is applied to the hybrid modulation technique, which is based on combining two modulation techniques. One of them is based on two modulation signals per phase, and can remove completely the low-frequency voltage oscillations on the neutral point. However, such a modulation strategy has an important drawback; it increases significantly switching losses on the converter. The hybrid modulation used here combines such a modulation with sinusoidal PWM. The main characteristic of this hybrid modulation is the capacity to reduce switching losses at the cost of having some low-frequency voltage oscillations on the neutral point. The amplitude of these oscillations can be regulated thanks to the proposed controller, which defines the exact degree of mixture between the two modulation strategies. Some simulation and experimental results are presented in this paper.