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The hydraulic turbine regulating system (HTRS), which is a core part of hydropower station, is essential and significant for the safe and stabilization operation of the hydropower station. The HTRS is a highly nonlinear, strongly coupled non-minimum phase system, which is difficult and significant to design controller to maintain the frequency stability of the HTRS. In this paper, a fixed-time synergetic controller (FTSC) is proposed to make the frequency of the HTRS stable for small and medium-size power stations in an upper bound time. The input/output feedback linearization is implemented to establish the relationship between the frequency of HTRS and the control input. An FTSC is presented based on the Lyapunov stability theory, synergetic control theory, and fixed-time stability theory. Besides, the proposed FTSC can ensure the frequency stability of HTRS in a limited and quantifiable convergence time, which is not dependent on the initial operating condition and thus overcoming the shortcoming of finite-time stability. Finally, comparative simulations between the proposed FTSC, PID, and sliding mode control (SMC) are presented. Meanwhile, the effects of different hydraulic-mechanical parameters are considered. The results validate the effectiveness and the superior performance of the proposed FTSC over existing PID and SMC under various operating conditions. • A FTSC is proposed for stabilization and chattering-free of the HTRS in small and medium power stations. • The Proof for the frequency stability of non-linear HTRS with the proposed FTSC is presented. • The convergence time which is not dependent on initial operating conditions can be evaluated. • Superior performances of the FTSC are presented by comparing with PID control and SMC. [ABSTRACT FROM AUTHOR]