A Hybrid Modular Multilevel Converter Family With Higher Power Density and Efficiency

Modular multilevel converter (MMC) has many advantages of scalability, reliability and easy implementation, but suffers from large number of switching devices and capacitors. To improve the power density and efficiency for medium voltage applications, this article introduces a hybrid modular multilevel converter (HMMC) family through the combination of active neutral-point-clamped converter and chain-link arm. Three topologies in HMMC family can effectively reduce chain-link voltage by half compared with traditional MMC. Single- and three-phase operation principles are presented to highlight this important features. Several critical metrics, such as component number, capacitor sizes, and semiconductor losses, are compared between the conventional MMC and HMMC topologies at three different medium voltage levels. The result indicates the best topology from the HMMC family can save around 30% devices, 50% total capacitor and 32% power losses compared with traditional MMC. A simple single-phase control method is also mentioned to balance capacitor voltage. Finally, a single-phase HMMC experimental prototype is built to verify the effectiveness of three topologies.