Extended Frequency Range Operation for Cascaded MMC and Cycloconverter-Based Machine Drive System

The cascaded modular multilevel converter (MMC) and cycloconverter (CCV)-based machine drive system is a promising solution for high-power machine drive applications. However, its output frequency is limited due to the characteristics of CCV. In this article, an extended frequency range operation for the cascaded MMC- and CCV-based drive system is proposed. The proposed operation scheme includes the low-speed mode, the high-speed mode, and the switching of operation modes. In the low-speed mode, the MMC provides the medium-frequency voltage for the CCV, and the CCV drives the machine. In the high-speed mode, the thyristors of CCV are triggered by specific gate signals to ensure that the MMC directly drives the machine. The proposed switching of the low-speed mode and the high-speed mode is achieved based on the operation analysis of thyristors to ensure the smooth transition of machine current. The submodule capacitance, boundary frequency, and device current stress are discussed in this article. The proposed operation scheme extends the output frequency range of the cascaded MMC- and CCV-based drive system, and the device current stress is much lower than other MMC-based methods. Simulation and experiment are conducted, and the results validate the effectiveness of the proposed operation scheme.