Generation of an Isolated Auxiliary Voltage within a High Voltage Application of an MMC Cell

The novel modular multilevel converter (MMC) topology, named Double Zero Submodule in Double Connection (DZDCSM) [1], capable of generating four levels of terminal voltage, is one of the most suitable candidates for MMC applications in the future. During operation of these MMC submodules (SM), isolated auxiliary voltages are needed to drive associated consumers like gate drivers, DC-DC converters, controllers etc. They can be generated from the available high DC-link voltage during operation of the SM, and used to charge a battery bank, which can meet the auxiliary load demand, in addition to precharging the SM capacitors. The objective of this research is to obtain such an isolated auxiliary low DC voltage to power a typical Li-ion battery charger. For this, it is proposed in this work to energize the corresponding battery charging circuit by utilizing transients in the drain-source voltage appearing across one SM power semiconductor. In this paper, such a charging circuit is investigated to achieve an isolated 5 V (DC) from 1.5 kV (AC) over two stages of voltage step down, and a hardware realization as proof-of-concept is presented. The power converter consisting of several SMs is designed to generate 20 MW. Therefore, during its operation, achieving a high efficiency of the investigated battery charging circuit delivering only a few tens of watts per SM is not important. Calculations of a simplified equivalent circuit are performed, verified by simulation, and implemented in hardware.