Modulation and control of a single-stage HVDC/AC solid state transformer using modular multilevel converter

At high voltage and power levels in utility-scale applications, transformers are needed to step-down grid voltages for integrating renewables and motors. A novel single-stage solid state transformer (SST) interface using modular multilevel converter (MMC) is proposed to interface a low voltage machine with a high voltage DC grid. Due to its modular structure, the high voltage side is easily scalable. This interface has sinusoidal voltages with low dV/dt and sinusoidal currents resulting in low magnetic losses in the high frequency transformer. The machine connected converter is a single-stage matrix converter. At low machine speeds, an advanced control approach is proposed which shifts the control of generating adjustable magnitude voltages at the machine terminals to the MMC on the high voltage side of the transformer. This results in reduced voltage stress across the transformer, matrix converter and the machine and further reduction in losses to boost system efficiency at light load conditions. Simulation and experimental results validate the proposed modulation and control.