The structural configuration of delta-connected high-voltage transformerless battery energy storage system (D-HVT-BESS) inherently introduces second-order ripple currents (SORCs) into the battery cells. Under low power factor conditions, the SORCs exhibit frequent zero-crossing, which accelerates battery aging and compromises the accuracy of state monitoring. Therefore, a high-proportion reactive power compensation control method for D-HVT-BESS via optimal zero-sequence current injection is proposed. The key to the proposed method is to use zero-sequence current to adjust the reactive-to-active power ratio of each phase leg. This adjustment enhances the power factor per phase, thereby eliminating the zero-crossing of SORCs. Furthermore, an optimization model is formulated to minimize the system power loss induced by zero-sequence current. The optimization model achieves this by optimizing the amplitude and phase angle of zero-sequence current. Additionally, to mitigate the inter-phase state of charge (SOC) imbalance caused by the zero-sequence current, an inter-phase SOC balancing control strategy based on an absolute phase selector is developed, enabling real-time optimization of the absolute phase of zero-sequence current to achieve dynamic SOC equalization among phases. Finally, a 380 V/100 kW/50 kWh experimental prototype is constructed for validation. Experimental results verify the effectiveness and operational feasibility of the proposed method.