Coordinated Planning of Energy Storage Systems and Soft Open Points for Flexibility Enhancement in Distribution Networks

The large-scale integration of distributed generation (DG), particularly renewable energy sources such as photovoltaics, into distribution networks introduces significant variability and intermittency. These characteristics pose challenges to the stable and economic operation of power systems by creating dynamic flexibility demands across both temporal and spatial dimensions. To maintain reliable operations and ensure effective utilization of renewable energy, distribution networks must be equipped with sufficient flexibility to respond to fluctuations in supply and demand. One effective approach to enhancing flexibility is the deployment of flexibility resources. Energy storage systems (ESS), known for their rapid response and controllability, have been widely utilized to mitigate net load variability and support grid stability. In parallel, Soft Open Points (SOP)—advanced power electronic devices capable of flexible power flow control—have emerged as promising tools for managing power distribution and relieving network congestion. In this context, this paper proposes a coordinated planning methodology that jointly considers the deployment of ESS and SOP to enhance the operational flexibility of distribution systems. First, a flexibility demand model and a flexibility supply model are constructed to quantify system needs and resource capabilities. Then, an evaluation index system is established to assess overall system flexibility as well as the transmission margin of critical branches. Finally, a bi-level optimization model is formulated. The upper-level planning layer minimizes the annualized total cost by determining optimal locations and capacities for ESS and SOP, while the lower-level operational layer verifies the results based on system operation costs and power losses. Simulation results on the IEEE 33-bus test system validate the effectiveness of the proposed method, demonstrating improvements in renewable energy utilization, reductions in power losses, and better balance between flexibility supply and demand across the network.