Study, analysis, and development of a small‐scale AC‐AC modular multilevel converter for solid‐state transformer applications

SummaryThe modular multilevel converter presents itself as a promising topology in medium voltage and high power applications due to its modular characteristic and flexibility in applications where DC‐AC and AC‐DC power conversion are required. Despite this, applications with electric traction present this topology in the AC‐AC conversion due to the converter at the input of a solid‐state transformer and the power system's single‐phase characteristic at the converter's input. One of the main problems characteristic of this topology is the harmonic content present in the medium frequency converter output voltage due to its square waveform. Thus, the harmonic content in this voltage is high, increasing the losses and volume in the core of the medium frequency transformer. This paper presents a multilevel modulation scheme using two frequency components in the modulating wave to reduce the harmonic content in the medium frequency converter output voltage. With the converter branch inductors, it is possible to verify the filtering of the output voltage and thus reduce the harmonic content of this voltage. A low power and voltage (1 kW/500 V) of the AC‐AC modular multilevel converter is implemented, and simulation results show some operating characteristics of the modulation scheme. Experimental results at low power and voltage (0.6 kW/208 V–125 V/1.2 kHz) present the operation of the converter and the characteristics proposed by the modulation scheme, where it is possible to verify the waveform of the sinusoidal output voltage with lower harmonic content.