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 PWM 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, the 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 mid-point of the dc bus is then analysed 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 V AC, 120 kVA test bench that validate the theory are shown.