Abstract
DC-AC Modular Multilevel Converter (MMC) is a promising candidate for high-power AC drive applications. The main challenge of operating the MMC in AC drive applications is its performance during low-frequency conditions, as high voltage ripple is associated with the low-frequency operation, which results in voltage stresses on the involved semiconductor devices. To keep capacitor voltages balanced and bounded in low-frequency operation, different techniques have been proposed in literature, which can be classified into software and hardware approaches. In this paper, a hybrid MMC with arm-interchange concept is proposed to ensure operating successfully during low-frequency conditions. The hybrid MMC consists of two-stages. The first stage is a front-end Integrated Gate Commutated Thyristor (IGCT)-based H-bridge converter, while the second stage is a Full-Bridge Sub-Module (FBSM)-based MMC, where FBSMs is able to generate positive, zero, and negative voltage states. Detailed illustration and design of the hybrid MMC are introduced. Simulation results of the proposed converter are presented to show the effectiveness during low-frequency and normal frequency conditions in AC drive applications. Finally, a scaled-down prototype is employed for experimental validation.
Highlights
Medium-voltage AC drives are commonly used in different applications, where different converter topologies can be employed
This means high voltage ripple is associated with the low-frequency operation, which results in voltage stresses
The results show that the effectiveness of the proposed arm-interchange concept in reducing the capacitor voltage ripple during low-frequency operation
Summary
Medium-voltage AC drives are commonly used in different applications, where different converter topologies can be employed. Unlike two-level converters, multilevel converters such as neutral point clamped converter [1] and flying capacitor converter [2] can be operated to generate stepped output voltage with lower dv/dt stresses and lower harmonic content. Challenge of employing MMC-based AC drives is the lowfrequency operation at the rated torque, since the voltage ripple of the SMs capacitance is inversely proportional to the converter output frequency. This means high voltage ripple is associated with the low-frequency operation, which results in voltage stresses
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