Multilevel converters are well known to be suitable for high voltage and high power applications due to the fact that they can synthesize the higher voltage with low harmonics without transformers by their unique structures [l]-[3]. However, it is very difficult and impractical to construct a multilevel converter as a single entity capable of generating a very high number of levels. On the other hand, multi-modular systems are recognised as a potential solution to increase the power handling capability, reliability and flexibility of the system [4]-[ll]. These multi-modular systems when built with multilevel converters combine all the advantages of the multilevel converter technology [12]-[15] and have been proposed and studied for the high voltage and high power applications such as motor drives, Uninterruptible Power Supplies (UPS), Distributed Power Systems (DPS), and STATic COMpensators (STATCOMs) in power transmission systems to name a few. Power loss is the most challenging issue for these systems in high power applications. Usually, high power Gate-Turn-Off (GTO) thyristors with low switching frequency are widely used to reduce the power dissipation. To minimize the switching losses, the Fundamental Frequency Sinusoidal PWM (FF-SPWM) control method was proposed on the multilevel converters [16] and the multi-modular multilevel converter system [12], [15]. However, such systems with the lower switching frequency or the fundamental frequency present large circulating current when converter modules are connected in parallel directly. The reason causing the current sharing problem together with the solution is of much concern. Additionally, it is interesting to confirm which circuit of the multi-modular multilevel converters with FF-SPWM and a single multilevel converter with a higher switching frequency are potentially more advantageous to use. This paper contributes to the understanding of the performance of the multi-modular system with multilevel converters connected in parallel with the FF-SPWM control technique. The current sharing problem is analysed and the cause of such problem is explained. A controller is proposed and designed in order to improve the harmonic performance of the system at the maximum power output. A design technique and process specifically suitable for FF-SPWM will be studied in detail. Switching losses characteristics among multi-modular system based on multilevel converters with FF-SPWM, a single multilevel converter with a higher switching frequency and a single two-level converter with a higher switching frequency will be compared with the assumption that the three systems have the same output power and waveform quality.
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