Neutral-Point Voltage Balancing Control of Active NPC Converter for The High-Power Flywheel Energy Storage System

Renewable energy is connected to the grid on a large scale, and the frequency stability of the grid is reduced. The high-power flywheel energy storage system (FESS) can reduce the power fluctuation of renewable energy and improve the frequency stability of power grid. The three-level (3L) active neutral point clamped (ANPC) voltage-source converter is widely used in high-power medium-voltage applications. But, the neutral point (NP) voltage unbalance has always been a disadvantage of this topology. This paper analyzes the principle of the back-to-back ANPC converter in the high-power flywheel energy storage system, studied the NP voltage fluctuation problem of the back-to-back ANPC converter. Then aiming at the problem of the NP voltage fluctuation, an algorithm based on small vector coefficient adjustment of capacitor voltage difference is designed. Finally, the effectiveness of the NP voltage balancing algorithm is verified in the simulation and experiments.