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Multilevel converters (MLC) have emerged as standard power electronic converters in high power as well as quality demanding applications. They are classified into current-fed MLC and voltage-fed MLC. Voltage-fed MLC has widely researched whereas the current-fed MLC are the recent topic of research. Based on the principle of duality between voltage and current sources, several current-fed MLCs analogous to voltage-fed MLCs have been identified. Current-fed MLC offer several advantages in terms of high power capability, transformer-less operation, short circuit protection and excellent quality of output current waveform. The goal of this paper is (1) to present review of circuit topologies, modulation schemes and applications of current-fed MLCs and, (2) to propose an emerging low device switching frequency modulation technique known as synchronous optimal pulsewidth modulation for current-fed MLCs. The circuit configuration and advantages of each topology along with various modulation techniques are discussed in detail. Compared to voltage-fed MLCs, the operation of current-fed MLCs need to satisfy additional switching constraints. Classical methods for realization these operational constraints are surveyed and new generic method has been proposed. Finally future scope of research has been presented to encourage further development of topologies and modulation techniques for current-fed MLCs.

Large-scale photovoltaic (LSPV) energy conversion systems have been installed at many places across the world. The essential component of LSPV system is the dc-ac conversion stage. The usage of multilevel converters is one of the recent advances in the dc-ac conversion stage of LSPV system to enable transformer-less inversion. Current-fed multilevel inverter has been chosen in this paper as it provides high power inversion with inherent voltage boosting and thus avoids the usage of transformer. High power conversion necessitates low device switching frequency operation in order to satisfy thermal constraint of semiconductor devices and also to improve efficiency. However, low device switching frequency operation leads to higher harmonic distortion of inverter output currents. Synchronous optimal pulse-width modulation (SOP) technique is an emerging low device switching frequency modulation technique which has been successfully implemented for voltage source multilevel inverters. However, state-of-the-art SOP technique cannot be directly applied to modulate current source multilevel inverter topologies due to additional constraints on the switching commutations. Therefore, the purpose of our study was to propose modified SOP technique to achieve : low device switching frequency and minimal harmonic distortion of inverter output currents. The topology of current-fed five-level inverter was used for demonstrating the performance of proposed technique. A generalized conversion method was introduced in the modified SOP technique for including switching constraints of current-fed inverter. In addition, a state-sequencing machine was developed by utilizing redundant inverter states to produce equal switching commutation among all semiconductor devices at minimal switching frequency of 350 Hz. The experimental results obtained from five-level current source inverter of 1.2 kW demonstrated the effectiveness of proposed SOP technique. Index TermsSynchronous optimal pulsewidth modulation (SOP), current-fed multilevel inverter, low frequency modulation, solar power integration. Manuscript

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