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Purpose: The purpose of this paper is to present a new system frequency response model with participation of wind-hydro-thermal units to overcome frequency deviations. Design/methodology/approach: The extracted minimum frequency equation is considered as a constraint in security-constrained unit commitment calculations. Because of high-order polynomials in the frequency transfer function and high degree of nonlinearity of minimum frequency constraint, Routh stability criterion method and piecewise linearization technique are used to reduce system order and linearize the system frequency response model, respectively. Findings: The results of this paper indicate that by using this model, the hourly minimum frequency is improved and is kept within defined range. Originality/value: This combined model can be used to evaluate the frequency of the power system following unexpected load increase or generation disturbances. It also can be used to investigate the system frequency performance and ensure power system security which are caused by peak load or loss of generation in presence of renewable energies. [ABSTRACT FROM AUTHOR]