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This paper presents a novel three-level voltage source converter for AC-DC-AC conversion. The proposed converter based on H-bridge structure is studied in detail. The control method with traditional double-closed-loop control strategy and voltage balancing algorithm is applied to the rectifier side. Correspondingly, a simplified modulation algorithm is applied to the inverter side, and the voltage balancing of inverter side is realized through the optimal selection of switching combination. Then, the application of the proposed topology is assessed in general and ideal operation conditions. Furthermore, the proposed topology with a variable voltage variable frequency (VVVF) is verified in experimental conditions. The performance of the proposed converter and control strategy is evaluated by experimental and simulation results.Energies 2018, 11, 1147 2 of 17 different sub-modules [19] or dc-link voltages [20]. It has the advantage of flexible extending of the output levels and power rating. However, the bulky and expensive phase-shifting transformers for isolated dc sources make it hard to increase the power density. A back-to-back CHB converter without any isolating device [21] can avoid these problems. However, short-circuits caused by the hardware topology are difficult to solve and the proposed topology cannot be expanded to a three phase system.In this paper, a new three-level voltage source converter for AC-DC-AC conversion is proposed. It can be used in three-phase system and more easily to extend to higher voltage level than a back-to-back NPC converter. Compared to the back-to-back CHB converter proposed in [21], a half H-bridge cell used in the new topology provides more redundant vectors and makes it overcome the short-circuit problem, which simplifies the control method. In addition, the proposed topology utilizes fewer switches at the cost of increasing the number of dc-link capacitors, the separated dc links will decrease the total dc voltage of the system, which is beneficial for the insulation design in many fields [22].The rest of the paper is organized as follows. In Section 2, the circuit configuration, characteristics and working principles of the proposed topology are studied in detail. The overall control strategy and pulse-width modulation strategy considering the voltage balance control is given in Section 3. In Section 4, two operation conditions are analyzed, and the simulation and experimental results demonstrate the effectiveness of the proposed control strategies. Section 5 concludes the paper.

Cascaded H-Bridge (CHB) converters characterized with high scalability have been widely used in photovoltaic (PV) and energy storage systems. However, the three-phase power and the voltages are often unequal due to various reasons, making the system less efficient. Therefore, the ability to deal with unequal power is necessarily required in CHB. This paper proposes three inter-phase power control methods based on simplified PWM strategy to improve the efficiency of the system. A control method with better performance both in dynamic and steady states is adopted to adjust power distribution. The relationship between duration times and DC power is analyzed. On the basis of controlling the inter-phase power, an inner-phase power control method is proposed by redistributing the duration time of each submodule, to further enhance the power control ability of the system. The simulation and experimental results show that under the condition of balanced or unbalanced voltage, the interphase and inner-phase power can be controlled, which verifies the feasibility and practicability of several modulation strategies proposed in this paper.