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

Abstract

High voltage direct current (HVDC) transmissionsystems and low voltage direct current (LVDC) networks arebecoming 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 adc-dc converter that performs as the ac transformer in termsof efficiency, reliability and high-step voltage ratio, specially inmedium and high voltage. Modular multilevel converters arean attractive alternative because they can manage medium andhigh dc voltages with standard semiconductor devices, and theycan achieve high efficiency with soft switching modulation, suchas resonant, trapezoidal or triangular currents usually usedin double active bridges (DAB). This paper proposes a novelgeneralized trapezoidal current mode applied to high step ratioModular Multilevel dc-dc Converters. The proposed modulationincreases the efficiency and achieves bidirectional control of thepower, soft-switching and a natural balance of the voltage inthe capacitors. The simulation results show the bidirectionaloperation and the capacitor voltage balance of the converterunder different operating conditions with high efficiency (98.2%).