Contents I. Introduction
In general, renewable energy sources, energy storages and distributed power generation devices such as photovoltaic power systems, batteries and fuel cells basically supply DC power; hence, the generated surplus power is usually fed into the utility power grid through DC/AC power converters (inverters). On the other hand, various global standards like IEEE-1547, IEEE-929 and EN-61000–3-2 impose strict requirements on the output power quality of the grid-connected inverters, i.e., harmonics and total harmonic distortion (THD) of the output currents [1]. Current-source inverters incorporating multilevel techniques are likely to be an effective solution to satisfy such requirements in medium-and high-power applications. The most significant feature of the current-source inverters is inherent simplicity of their configuration and control algorithm because they can directly inject the desired currents to the power grid without AC current feedback control, and can achieve a high power factor operation with ease. Therefore, the current-source inverters behave like buffers between the DC power sources and the AC power grid because of their robustness against the grid voltage fluctuation. In addition, the multilevel approach of the inverters is an indispensable key technology to reduce the output waveform distortion [2] [3]. However, researchers and engineers have explored few topologies of the multilevel current-source inverters so far. One of the traditional methods to generate the multilevel current waveform is a parallel connection of H-bridge inverters, which is a dual circuit of a cascade multilevel voltage-source inverter [4]. This configuration requires, however, multiple isolated DC current sources, and a large number of isolated gate drive circuits as well as switching devices. Another approach is a multi-cell topology based multilevel current-source inverter, which is a dual circuit of a full-bridge voltage-source inverter with flying capacitors [5] [6]. Complexity of the intermediate-current-Level balancing control is a fatal drawback of this topology. Another different multilevel current-source inverter, which employs a single-rating inductor-cell topology, is also discussed in the past work, which is a dual circuit of the improved diode-clamped multilevel voltage-source converter. However, both of the multi-cell and the single-rating inductor-cell topologies require large inductors to obtain the stable intermediate level currents. These intermediate inductors cause extra losses in the multilevel current-source inverter, resulting in lower efficiency of power conversion.
