This paper gives an overview of medium-voltage (MV) multilevel converters with a focus on achieving minimum harmonic distortion and high efficiency at low switching frequency operation. Increasing the power rating by minimizing switching frequency while still maintaining reasonable power quality is an important requirement and a persistent challenge for the industry. Existing solutions are discussed and analyzed based on their topologies, limitations, and control techniques. As a preferred option for future research and application, an inverter configuration based on three-level building blocks to generate five-level voltage waveforms is suggested. This paper shows that such an inverter may be operated at a very low switching frequency to achieve minimum ON-state and dynamic device losses for highly efficient MV drive applications while maintaining low harmonic distortion.Index Terms-Dual multilevel inverter (MLI), low switching frequency, medium-voltage drives, multilevel inverters, neutralpoint-clamped inverter, synchronous optimal PWM.

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Abstract-The quasi-Z source inverter (qZSI) with battery operation can balance the stochastic fluctuations of PV power injected to the grid/load, but its existing topology has a power limitation due to the wide range of discontinuous conduction mode during battery discharge. This paper proposes a new topology of the energy stored qZSI to overcome this disadvantage. The operating characteristic of the proposed solution is analyzed in detail and compared to the existing topology. Two strategies are proposed with the related design principles to control the new energy stored qZSI when applied to the PV power system. They can control the inverter output power, track the PV panel's maximum power point, and manage the battery power, simultaneously. The voltage boost and inversion, and energy storage are integrated in a single stage inverter. An experimental prototype is built to test the proposed circuit and the two discussed control methods. The obtained results verify the theoretical analysis and prove the effectiveness of the proposed control of the inverter's input and output powers, and battery power regardless of the charging or discharging situation. A real PV panel is used in the grid-tie test of the proposed energy stored qZSI, which demonstrates three operational modes suitable for application in the PV power system.

In today's renewable energy power generation systems, inverters have become key devices connected between energy sources and power grids. The newly presented quasi-Zsource inverter (qZSI) has the unique advantages of boosting DC bus voltage without additional boost link, lower component ratings, as well as constant DC input current compared with the traditional Z-source inverter. To better design the control schemes of this novel topology, the small-signal model is deduced using state variables perturbance mathematical analysis. By this model, the voltage-current closed-loop control of shoot-through duty cycle is designed. The space vector PWM (SVPWM) technique is modified to satisfy the particularity of qZSI. Simulation results certify the correctness and effectiveness of the proposed model and control strategies and demonstrate the applicability of qZSI technology to overcome the disturbances from DC link or AC loads in renewable energy power generation systems.

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