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Interconnected power systems face different contingencies which may result in instability and blackouts. Therefore, online dynamic security analysis is becoming increasingly important. A suitable online stability index shows how close or far the system is from instability; as a result would deliver on-time alarms to system operators when the system is close to instability. This paper presents a practical and efficient method to calculate online dynamic stability index for power systems. The proposed approach does not need the model of the system and uses only the data from Phasor Measurements Units (PMU)s. İn this paper, first seven different dynamic Stability indices are defined and compared to each other under diffrenet types of contingencies. Some of the defined indices require only the voltage signal from different PMUs and others require the dominant roots of the system which can be estimated using Prony analysis method. The best criterion is selected based on the simulation results. The effectiveness of the proposed stability index is tested on a complete and accurate model of the Iranian power system. Simulation results revealed that the selected index can provide a good estimation of the distance to system collapse. [ABSTRACT FROM PUBLISHER]