When high-voltage transformerless energy storage systems reach capacities of hundreds of megawatts

unplanned outages cause power grid deficit transients that severely impact power grids. This paper investigates high-voltage transformerless energy storage systems based on modular multilevel topology. The factors affecting reliable operation of energy storage systems are systematically analyzed and categorized. A fault isolation strategy is proposed to preserve voltage output capability when faults or warnings occur in batteries and battery management systems. Following fault isolation

a fault-tolerant operation control method is developed for battery clusters under DC-side fault isolation conditions

enabling coordinated operation between faulty and healthy submodules. Additionally

a circulating current injection strategy is proposed to compensate for power factor deviations in bridge arms caused by hybrid operation. The proposed method ensures that when any faulty battery cluster is taken offline

other energy storage sub-modules can continue to operate normally for charging and discharging

while the system power factor remains stable

effectively enhancing the system’s reliability and security.