Coordinated Planning of Flexible Interconnection and Grid-Forming Energy Storage in Low-Voltage Distribution Networks Considering Off-Grid Operation

Severe weather events increasingly threaten the reliability of low-voltage distribution networks (LVDNs). Existing fault restoration strategies primarily focus on medium-voltage networks, leaving LVDNs highly vulnerable during prolonged grid outages. To enhance the off-grid operation capability of LVDNs, this paper proposes a coordinated planning model integrating flexible interconnection devices and grid-forming energy storage systems (GFM-ESSs). First, this study develops a hierarchical planning framework for LVDNs to jointly optimize the allocation of soft open points (SOPs) and grid-forming energy storage systems (GFM-ESSs), with the aim of reducing both interruption-related losses and equipment investment in distribution transformer areas. Second, active support capability evaluation indices are formulated for LVDNs, facilitating a quantitative assessment of how the placement and sizing of SOPs and GFM-ESSs influence the effectiveness of active support. The nonconvex model is relaxed and efficiently solved using second-order cone programming (SOCP). Case study results demonstrate that the proposed method leverages the spatial power transfer ability of flexible interconnections to reduce the required GFM-ESS capacity, thereby achieving optimal economic efficiency and enhanced active support performance. Furthermore, quantitative analysis reveals that placing the GFM-ESS at intermediate nodes yields the best active support effect. Ultimately, the coordinated planning scheme effectively mitigates voltage limit violations and ensures a highly reliable power supply during severe grid outages.