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The liberalization of the electricity markets causes power systems to work closer and closer to their limits. The online assessment of system security becomes a topic of paramount importance in control centers. In this context fast indices to quickly assess system security are fundamental in an on-line DSA session. This paper considers three voltage stability indices recently proposed in literature and carries out some comparisons. In the first part the authors briefly describe the theoretical background of each index. The second part is devoted to two comparisons: at first, the three indices are compared by using an IEEE test system. Secondly, the FVSI (Fast Voltage Stability Index) and the VCI (Voltage Collapse Index) are compared by adopting a model of the Italian HV transmission grid. The comparisons will show the peculiar information provided by each of the considered indices and will assess the performance of these indices also on a realistic power system. Final remarks are reported and discussed.