Trapezoidal Current Mode for Bidirectional High Step Ratio Modular Multilevel dc-dc Converter

Abstract

High voltage direct current (HVDC) transmission systems and low voltage direct current (LVDC) networks are becoming popular due to their advantages and present feasibility, mainly pushed by the advance in power electronics. However, one of the main drawbacks of dc networks is the lack of a dc-dc converter that performs as the ac transformer in terms of efficiency, reliability and high-step voltage ratio, specially in medium and high voltage. Modular multilevel converters are an attractive alternative because they can manage medium and high dc voltages with standard semiconductor devices, and they can achieve high efficiency with soft switching modulation, such as resonant, trapezoidal or triangular currents usually used in double active bridges (DAB). This paper proposes a novel generalized trapezoidal current mode applied to high step ratio Modular Multilevel dc-dc Converters. The proposed modulation increases the efficiency and achieves bidirectional control of the power, soft-switching and a natural balance of the voltage in the capacitors. The simulation results show the bidirectional operation and the capacitor voltage balance of the converter under different operating conditions with high efficiency (98.2%).