Wide-range high voltage input auxiliary power supply for modular multi-level converters

Abstract

The modular multilevel converters (MMCs) are composed of many sub-modules(SMs) connected in series. Module control unit is key components of the SMs, and a auxiliary power supply is needed to energize the module control unit. However, Many MMCs projects require the voltage of IGBT to be higher than 3300V, which means the input voltage of auxiliary power supply used in MMCs should be higher than 2500V. On the otherhand, the voltage of DC-link capacitor can fluctuate from several hundred volts to several thousand volts, which means the auxiliary power supply must work with wide-range input voltage. In conclusion, it is quite challenging to design a high voltage input DC-DC auxiliary power supply to operate at MMCs. In this paper, a two stage cascaded auxiliary power supply for MMCs is proposed with 300V-3000V input and 24V output, and the design scheme for auxiliary power supply is given. Besides, the transfer function of the forestage Flyback converter based on small sigpal model is derived, and the converter's stability in independent operation is analyzed. The open-loop and closed-loop output impedance of the forestage converter is calculated, and the result is compared with the closed-loop input impedance of the second stage converter to analyze the impedance matching of power supply. Finally, a 60W prototype auxiliary power supply is built, and related experiments are carried on it. The results verify that the scheme designed for the auxiliary power supply with wide-range high voltage input is practicable and correct.