Novel hybrid control of high-speed flywheel energy storage using modular multilevel converter in a DC power system

• In order to improve voltage utilization, flywheel energy storage is controlled using an MMC-based structure and is being explored for transportation applications. • Firstly, the topology of a traction power supply system with modular multilevel converter-flywheel energy storage systems (MMC-FESS) is proposed. • Secondly, the bubble sort and carrier phase-shifted method is proposed to control the low-frequency and high-frequency operation of high speed flywheels. • Finally, combined simulation and experimental results fully validate the effectiveness of this topology and control strategy. Flywheel energy storage (FES) is characterized by fast energy storage and release, which is suitable for use in DC power systems. In particular, it has unique advantages in the applications to DC rail transit. This paper explores the integration of modular multilevel converters (MMC) with high-speed FES on the DC traction network side of urban rail transit. To solve the problem of voltage balance control of MMC’s sub modules under high-frequency fundamental waves, a novel bubble sorting carrier-phase shift modulation method is proposed to ensure high-precision voltage balance control of MMC under low modulation ratio and high modulation ratio conditions. The algorithm achieves enhanced operational efficiency by optimising the sorting process, which involves minimising unnecessary steps and implementing separate sorting for charging and discharging submodules. In addition, the use of MMC enables stable control of the high-speed flywheel, thereby maintaining the voltage stability of the DC traction network and limiting voltage fluctuations to a reasonable range. Finally, a high-speed simulation platform was built to verify the feasibility of the proposed control strategy under subway braking and starting conditions. Further, to verify the effectiveness of the proposed bubble sort-carrier phase shifted modulation method, an experimental platform was built for validation.