Power system dynamic models are essential for the stability analysis of a perturbed electrical system. In recent years, hydro-dominant power systems have experienced the occurrence of ultra-low frequency oscillations (ULFO). While power system dynamic simulation stands as an essential tool for the research community to analyze and understand this phenomenon, such systems involve non-linearities in their dynamic model affecting the ULFO analysis’s validity. This present work investigates the impact of governor non-linearities representation on the validity of the ULFO analysis. Sinusoidal-input describing function (SIDF) is borrowed from control theory to appreciate existing governor non-linearities models’ capacity to respond accurately to ULFOs. Through rigorous test-cases, results prove the imperativeness of prior accurate measurement of those governor non-linearities before their numerical implementation to ensure the power system dynamic simulation model’s validity for ULFO analysis.