Low-frequency alternating-current (LFAC) transmission has attracted significant attention for medium- and long-distance offshore wind integration due to its ability to mitigate the substantial charging currents and reactive power burdens associated with long submarine cables. This paper investigates the frequency-dependent electrothermal behaviors of a 500 kV three-core XLPE submarine cable using a coupled electromagnetic–thermal finite-element model. The simulation framework evaluates the current distribution, power losses in metallic components, temperature rise, and ampacity across various frequency regimes. To validate the numerical model, a thermal-circuit approach based on the IEC 60287 standard is developed, with comparisons confirming that deviations remain within acceptable engineering margins. The study reveals that operating at lower frequencies effectively mitigates skin and proximity effects, leading to reduced conductor and sheath losses. Quantitative results demonstrate that reducing the operating frequency from 50 Hz to 5 Hz results in a 30.6% reduction in total power losses and a 14.2% increase in current-carrying capability. These findings confirm that LFAC transmission offers a viable pathway to enhance the efficiency and capacity of submarine power transmission systems.