Existing research primarily concentrates on the impact of the energy storage unit or a single energy storage converter on the characteristics of DC transmission sending-end systems, without adequately considering the diverse characteristics of internal storage units. This study focuses on the sending-end system based on line-commutated converter high-voltage direct current (LCC-HVDC) technology integrated with grid-forming energy storage stations. Considering variations in control parameters, equipment capacities, and line parameters among different energy storage units, the equivalent aggregation method was applied to simplify the multi-unit storage model. A coherence equivalence criterion for energy storage plants was developed, and system stability was analyzed using an impedance analysis approach. The results indicate that the equivalent aggregation method effectively preserves the distinctive features of individual storage units in stability analysis. The equivalent model shows strong agreement with the detailed model in both dynamic response and impedance characteristics. This approach successfully balances model simplification with analytical precision for large-scale energy storage systems.