Ultra low frequency oscillations suppression strategy based on power support from thermostatically controlled load clusters

Ultra low frequency oscillation (ULFO) in hydro-dominated power systems can intensify governor regulation and degrade frequency stability. This paper proposes a demand-side supplementary damping strategy using bounded active-power support from thermostatically controlled load (TCL) clusters. A thermodynamic-electrical model of inverter-based air-conditioner TCLs is developed to connect compressor-frequency regulation, electrical-power response, refrigeration dynamics, building thermal constraints, comfort limits, and aggregate admissible capacity. Based on this model, a hierarchical control framework allocates TCL damping commands under capacity, communication, availability, recovery, and coordination constraints, and the resulting TCL active-power branch is embedded into the hydro-dominated small-signal state-space model. Eigenvalue analysis, Prony identification, and time-domain simulations on a hydropower single-unit system and a four-machine two-area benchmark system verify the effectiveness of the strategy under multiple complex operating scenarios. The results show that TCL clusters can provide a feasible non-mechanical demand-side damping channel for improving the frequency stability of hydro-dominated grids.