Abstract: Interprovincial power grid interconnections serve multiple functions, including power transmission, the integration of clean energy, and mutual support for power supply reliability. Their investment returns are jointly influenced by operational scenarios, planning years, and the functional positioning of the interprovincial connections. Current evaluations of power grid investment returns focus on single economic indicators, such as electricity consumption growth, and lack a comprehensive consideration of benefits such as reliability and environmental sustainability, making it difficult to provide effective support for investment decisions regarding large-scale cross-regional power transmission. This paper takes the H regional power grid as its research subject. Based on the GenX multi-area power planning model, it constructs a comprehensive benefit evaluation system covering economic, environmental, and reliability aspects. Four operational scenarios are established—typical load, peak load, large-scale renewable energy generation, and off-peak mutual support—to compare the differences between the current investment plan and the optimal investment plan, and further analyze the temporal evolution and spatial distribution characteristics of the comprehensive benefits. The study indicates that: (1) From the perspective of overall benefits, investment in interconnectors can improve the system's comprehensive benefits, but its impact is significantly scenario-dependent; under the optimal investment scenario, the system's total cost decreases by 0.18%–2.22% across the four scenarios. (2) In terms of temporal evolution, the long-term benefits of interconnector investment will change as load grows and the scale of renewable energy expands. Under the "large-scale renewable energy" scenario, the economic benefits of interconnector investment gradually increase, but constraints remain in the "off-peak mutual support" and "peak load" scenarios. (3) In terms of spatial distribution and investment scale boundaries, the benefits of interconnector investment exhibit a differentiated distribution pattern across provinces. Under the "large-scale renewable energy" scenario, provinces rich in renewable energy experience the most significant increase in environmental benefits, while receiving provinces primarily gain economic and environmental benefits.