Multisource looped district heating systems (MSL-D HSs) have the potential to provide power flexibility to electric power systems (EPSs) through coupling units. An MSL-DHS has multiple coupling units

unlike a single-source DHS. By coordinating coupling units and heating networks

the thermal supply sides of these units can compensate for each other

leading to numerous combinations that provide flexibility. Quantifying the flexibility of an MSL-DHS requires characterizing the inter-coupling feature of multiple units and addressing the nonlinear hydraulic model. In this context

this paper proposes a power flexibility region (PFR) method to quantify the power flexibility of an MSL-DHS. The feasible region of an MSL-DHS model is projected onto the variable space of power output/input of the coupling units to construct the PFR model. A nonconvex nonlinear programming (NLP) boundary point searching model is established to calculate the PFR. A global optimization algorithm based on the reformulated piecewise McCormick envelope is proposed to improve the computational efficiency and ensure the convergence of the boundary point searching model. Case studies are conducted on a modified 32-node MSL-DHS from Barry Island to verify the proposed method. The impacts of supply temperature

mass flow direction and heat load level on power flexibility are analyzed.