Control of a Waveshaper-MMC With Thyristor-Based Front-End Converter for Open-End Winding Variable Speed Medium-Voltage Induction Motor Drive

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For variable frequency induction motor drive application, conventional modular multilevel converter (MMC) faces some issues, like high capacitor voltage ripple at low frequency, high circulating current, and high component count. This article proposes a novel configuration to address these issues, where a six-pulse thyristor-based controlled rectifier is combined with a waveshaper-based MMC (WS–MMC). Each phase of the converter consists of a WS and director valve, and then the phases are connected in series to create a common dc-link. The use of a thyristor-based controlled rectifier as a front-end converter increases reliability and reduces component count to a large extent. Unlike conventional MMC, circulating current does not exist in this converter, and the capacitor voltage ripple does not depend on the operating frequency. As a consequence, the current rating of the switching devices is reduced for the same power output, and capacitor voltage ripple at low frequency stays within the specified limit. Besides this, the component count of the proposed configuration is far less than the MMC-based topologies available so far. The operation principle, relevant control techniques, and analysis of the converter are discussed in detail in this article. The complete system is simulated in MATLAB/SIMULINK, and the performance of the control methods has been validated at different operating frequencies with various load torques. The results show satisfactory performance.