A hydroelectric power plant real-time monitoring system (1;1") comprising i) a real-time data acquisition system (11; 11") connected to dedicated sensors positioned in a hydroelectric power plant and configured to acquire in real-time real-time measured power plant data, said real-time measured power plant data comprising real-time measured simulation inputs (111;111") ii) a preventive simulation system (16;16"), iii) and a preventive diagnostic system (17;17"), characterized - in that said preventive simulation system (16;16") is configured to receive said real-time measured simulation inputs (111;111") from said real-time data acquisition system (11;11") - in that said real-time measured simulation inputs (111;111") comprise a real value of each of at least one power plant control parameter, - in that at least one predetermined virtual value of said power plant control parameter and at least one predetermined variation rate of said power plant control parameter are stored in a computer comprised in said preventive simulation system (16;16"), - in that at least one preventive simulation software (161;161") is stored in the computer comprised in said preventive simulation system (16;16") and is configured to simulate periodically a virtual transient behavior of the hydroelectric power plant from said real value to said predetermined virtual value of said power plant control parameter following said predetermined variation rate of power plant control parameter and configured to provide periodically preventive simulated power plant data related to said virtual transient behavior of the hydroelectric power plant comprising at least one preventive simulated monitored quantity (162;162") related to said virtual transient behavior of the hydroelectric power plant, - and in that said preventive diagnostic system (17;17") is configured - to compare said preventive simulated monitored quantity (162;162") with a predetermined threshold to trigger, when said preventive simulated monitored quantity (162;162") exceeds said predetermined threshold, an alarm identifying a power plant transient behavior problem in a preventive manner - and/or to compute at least one output related to said preventive simulated monitored quantity (162;162") and compare said output with a predetermined threshold to trigger, when said output exceeds said predetermined threshold, an alarm identifying a power plant transient behavior problem in a preventive manner. i) a real-time data acquisition system (11; 11") connected to dedicated sensors positioned in a hydroelectric power plant and configured to acquire in real-time real-time measured power plant data, said real-time measured power plant data comprising real-time measured simulation inputs (111;111") ii) a preventive simulation system (16;16"), iii) and a preventive diagnostic system (17;17"), - in that said preventive simulation system (16;16") is configured to receive said real-time measured simulation inputs (111;111") from said real-time data acquisition system (11;11") - in that said real-time measured simulation inputs (111;111") comprise a real value of each of at least one power plant control parameter, - in that at least one predetermined virtual value of said power plant control parameter and at least one predetermined variation rate of said power plant control parameter are stored in a computer comprised in said preventive simulation system (16;16"), - in that at least one preventive simulation software (161;161") is stored in the computer comprised in said preventive simulation system (16;16") and is configured to simulate periodically a virtual transient behavior of the hydroelectric power plant from said real value to said predetermined virtual value of said power plant control parameter following said predetermined variation rate of power plant control parameter and configured to provide periodically preventive simulated power plant data related to said virtual transient behavior of the hydroelectric power plant comprising at least one preventive simulated monitored quantity (162;162") related to said virtual transient behavior of the hydroelectric power plant, - and in that said preventive diagnostic system (17;17") is configured - to compare said preventive simulated monitored quantity (162;162") with a predetermined threshold to trigger, when said preventive simulated monitored quantity (162;162") exceeds said predetermined threshold, an alarm identifying a power plant transient behavior problem in a preventive manner - and/or to compute at least one output related to said preventive simulated monitored quantity (162;162") and compare said output with a predetermined threshold to trigger, when said output exceeds said predetermined threshold, an alarm identifying a power plant transient behavior problem in a preventive manner. - to compare said preventive simulated monitored quantity (162;162") with a predetermined threshold to trigger, when said preventive simulated monitored quantity (162;162") exceeds said predetermined threshold, an alarm identifying a power plant transient behavior problem in a preventive manner - and/or to compute at least one output related to said preventive simulated monitored quantity (162;162") and compare said output with a predetermined threshold to trigger, when said output exceeds said predetermined threshold, an alarm identifying a power plant transient behavior problem in a preventive manner.