Abstract: To address the transformer capacity verification challenges induced by pulse-width modulation (PWM) broadband harmonics in flexible excitation systems, this paper establishes a three-level neutral-point-clamped (NPC) simulation model to analyze the characteristics of harmonic propagation. By introducing a harmonic additional loss factor, an equivalent thermal current model is constructed to specifically evaluate the thermal load limit under over-excitation conditions. The results indicate that while filtering components and transformer connection configurations can suppress high-frequency harmonic transmission, the superposition of the fundamental current and high-frequency additional losses under over-excitation significantly increases the equivalent thermal current. This phenomenon constitutes the thermal steady-state selection boundary for flexible excitation transformers. The findings offer a valuable reference for the capacity selection of excitation transformers.