Abstract
Traditional ampacity evaluation for direct-buried cables often assumes static soil thermal resistance, which obscures actual operating margins. To address this, a new electromagnetic-thermal coupled finite element model for cross-linked polyethylene (XLPE) cables was developed, incorporating the dynamic thermophysical properties of porous soil. The study quantifies how water content dynamically alters heat transfer. Native soil's thermal conductivity experiences a step mutation, jumping from 0.55 to 1.44 W/(m K) at a 15% percolation threshold. Conversely, backfill sand offers stabler heat transfer, reaching 0.85 W/(m K) at just 5% water content. Simulations of rainfall-to-drying cycles reveal a nonlinear relationship between ampacity and moisture. In supersaturated native soil, ampacity drops locally from 1250 A to 1150 A due to entrapped air. During the drying phase, hydraulic hysteresis disrupts the continuous water network, causing ampacity to plummet to 710 A. Ultimately, this model characterizes dynamic soil thermal resistance under varying hydrological conditions, providing strong theoretical support for evaluating the dynamic transmission potential of underground cable systems.