Investigation on the Discontinuous Modulation Method of Full-Bridge Modular Multilevel Converters

Abstract

As a new type of multilevel converter topology, the Modular Multilevel Converter (MMC) has the advantages of high equivalent switching frequency, strong scalability and flexible control in recent years, thus has attracted more and more attention. This paper mainly studies the discontinuous modulation method of full-bridge MMC and proposes an implementation method of discontinuous modulation strategy for full-bridge MMC with a mathematical model established based on segmental function, taking advantage of the segmental characteristics of modulated wave of MMC under discontinuous modulation. Discontinuous Pulse Width Modulation (DPWM) makes the switches remain inactive in a specific modulation period, which can improve the equivalent switching frequency, optimize harmonic characteristics and reduce switching loss. The effects of discontinuous modulation strategy on device power losses and modular capacitor voltage is analyzed on the basis of the mathematical model. Then, the effects of discontinuous circulation on device power losses and modular voltage is analyzed by combing the discontinuous modulation strategy and circulation injection strategy. Simulation is conducted and verified that the MMC loss and modular voltage fluctuation can be reduced by the circulation injection strategy under discontinuous modulation.