In recent years, renewable energy (RE) penetration has become an important target in power systems. However, RE power is affected by climate change and has strong randomness and volatility. Adequate transmission capacity and energy storage systems (ESSs) are conducive to the integration of RE. Therefore, coordinated transmission renewable–storage expansion planning (TRSEP) is an effective decision-making approach to cope with the impacts of climate change and achieve the development target of RE penetration. Electricity trading between different systems is common; therefore, in addition to the penetration of RE into the internal loads of the system, the proportion of RE generation in tie lines is gaining attention, making analyses of the RE transmission path necessary. Referring to the flow of carbon emissions, this paper defines the RE power flow density to track the transmission path of RE. Next, a TRSEP model is proposed that can clearly distinguish the RE transmission path into internal loads, external loads, and energy losses. To address the presence of bilinear terms in the proposed model, the McCormick method is applied, and a customized feasibility correction strategy is designed to obtain a good feasible solution. Numerical results from case studies are provided to verify the rationality and effectiveness of the approach proposed in this paper.