| The paper presents a detailed analysis of the charge balance mechanism using five-level back-to-back system as an example. The analysis starts from the fundamental frequency switching. Based on the switching angles in the system, the charge flowing into and out of each voltage junction are calculated and the charge balancing constraints are given. The optimized switching angles with minimum THD are chosen.; Although the voltage balancing control in fundamental frequency switching can successfully balance the do bus voltage, the input and output current have relatively high harmonic components. Two methods are proposed to reduce the harmonic in the current waveforms. For the higher modulation index operation, selective harmonics elimination PWM was implemented by adding more switching pulses. For the lower modulation index operation, voltage shifting method is proposed, which utilizes the line-to-line redundancy. Simulation results showed that these two methods can effectively reduce the harmonics.; The voltage balancing control is also extended to carried-based SPWM technique, which is capable of eliminating all lower harmonics. A triplen harmonic offset voltage is added to the sinusoidal reference voltage, which changes the average current flowing into or out of the inner junctions while keeps the output line-to-line voltage the same. The effects and limits of the offset voltages are analyzed in detail. A novel carried-based SPWM with voltage balancing control is proposed.; A new multilevel converter topology is also proposed. The proposed topology can significantly reduce the number of clamping diodes with a slightly increase in the total devices rating of the switching devices. The rating ratio of the reduced diodes and increased switching devices is 4:1. It will be the higher in the topology with higher number of voltage levels.; In the rectifier side, the number of switching devices can also be reduced. A novel simplified five-level rectifier which only requires two switching devices per phase leg is proposed. The total devices rating required in the proposed topology is only one fourth of those in the fully active rectifier. Furthermore, the total devices rating of diodes is also reduced by one third. The control strategy for new topology is also discussed. Although the new topology introduces some harmonics and doses not have regenerative capability, it could be used as a cost-effective alternate for some applications.; A five-level, three-phase back-to-back 10-kW converter prototype based on the topology with reduced number of clamping diode is fabricated and experimentally tested. The proposed voltage balancing control strategies are implemented. Experimental results are shown to demonstrate the proposed control strategy. |
