  A modular and scalable converter for medium voltage (MV) AC to low voltage
(LV) DC power conversion is proposed; single-phase-modules (SPMs), each
consisting of an active-front-end (AFE) stage and an isolated DC-DC stage, are
connected in input-series-output-parallel (ISOP) configuration to reach desired
voltage and power capacity. In prior art, high-speed bidirectional
communication among modules and a centralized controller is required to ensure
module-level voltage and power balancing, which severely limits the scalability
and practical realization of higher voltage and higher power systems. Moreover,
large capacitors are used to suppress double-line-frequency voltage variations
on the common MV DC bus shared by the AFE and the DC-DC stage originating from
AC power pulsations through the SPMs. We propose a comprehensive controller
which achieves voltage and power balancing using complete decentralized control
of the DC-DC stages based on only local sensor feedback and the AFE stages are
controlled using feedback of only the LV DC output. Furthermore, reduced
capacitor requirement on the MV DC bus is achieved through design and control.
The proposed method is validated through simulation and experimental results.
