Voltage balancing control in 3-level neutral-point clamped inverters using triangular carrier PWM modulation for facts applications

In this paper, a novel technique to balance the voltage of the two split dc capacitors of a 3-Level neutral-point-clamped inverter using triangular carrier pulse width modulation is presented. This technique, suitable for reactive power compensation and for inverters operating with a relatively low switching frequency, consists in adding a square wave at six times the output frequency. Subsequently, this paper presents a comparison with two already known strategies in which a sinusoidal waveform at two and six times the output frequency are injected. The current contribution to the midpoint of the dc bus is then analyzed for different modulation indexes and operating conditions. Based on this analysis, a small-signal averaged model, suitable for control design purposes is presented. Finally, simulation and experimental results on a 690-Vac, 120-kVA test bench that validate the theory are shown.