Method for controlling a hydraulic power plant, comprising a variable-speed turbine driving an electrical generator, a conversion structure (220, 230, 240) connected to said generator on the one hand and to an electrical network on the other hand, said conversion structure being configured to convert an AC voltage generated by the generator into an AC voltage of a frequency different from that generated by the generator or into a DC voltage, comprising the steps of: - having a law governing the minimum characteristic rotational speed according to a characteristic mechanical power delivered by the turbine, said minimum characteristic rotational speed ensuring the stability of the turbine; - operating the hydraulic power plant with an initial setpoint value for the electrical power to be supplied to the electrical network; - obtaining a water height value at the turbine inlet; - a frequency drop is determined on said electrical network; - defining a new electrical power setpoint value (Pes) higher than the initial value so that the rotational speed of the turbine decreases in proportion to the determined frequency drop when the new electrical power value to be supplied to the electricity network is received; - converting said new electrical power value (Pes) to be supplied into a new mechanical power setpoint value P ch; - transforming said new mechanical power setpoint value P ch into a characteristic mechanical power to be delivered by transforming said new electrical power setpoint value (Pes) into said characteristic mechanical power to be delivered; - on the basis of said law, for said characteristic mechanical power to be delivered, determining the minimum rotational speed n crit of the turbine; - controlling the hydraulic power plant to deliver an electrical power equal to the new electrical power setpoint value (Pec) and to maintain the rotational speed of the turbine above the determined minimum rotational speed n crit. - having a law governing the minimum characteristic rotational speed according to a characteristic mechanical power delivered by the turbine, said minimum characteristic rotational speed ensuring the stability of the turbine; - operating the hydraulic power plant with an initial setpoint value for the electrical power to be supplied to the electrical network; - obtaining a water height value at the turbine inlet; - a frequency drop is determined on said electrical network; - defining a new electrical power setpoint value (Pes) higher than the initial value so that the rotational speed of the turbine decreases in proportion to the determined frequency drop when the new electrical power value to be supplied to the electricity network is received; - converting said new electrical power value (Pes) to be supplied into a new mechanical power setpoint value P ch; - transforming said new mechanical power setpoint value P ch into a characteristic mechanical power to be delivered by transforming said new electrical power setpoint value (Pes) into said characteristic mechanical power to be delivered; - on the basis of said law, for said characteristic mechanical power to be delivered, determining the minimum rotational speed n crit of the turbine; - controlling the hydraulic power plant to deliver an electrical power equal to the new electrical power setpoint value (Pec) and to maintain the rotational speed of the turbine above the determined minimum rotational speed n crit.