Abstract: High-voltage transformerless energy storage can be directly connected to the medium and high-voltage power grid without the need for a step-up transformer. It has the advantages of high efficiency and large single-unit power, and has become a key technology for large-scale energy storage. At present, most high-voltage transformerless energy storage systems adopt the cascaded H-bridge multilevel topology based on the star connection, which limits the improvement of the single-unit capacity. A design scheme for ahigh-voltage transformerless energy storage system based on the delta connection topology is proposed. The upper limit of the installed battery capacity of a single unit can reach hundreds of megawatt-hours. A power control strategy based on sequence and phase separation and the corresponding inter-phase state of charge (SOC) balancing strategy under the delta connection topology are also put forward. The rationality of the design scheme and control strategy has been verified based on engineering examples and simulation models.