In recent years the energy consumption, decentralised feed-in, storage of renewable energies [1] and the power system complexity have grown rapidly worldwide. Hence, some electrical networks are already working at their stability limits because they were not originally designed for today's load. Consequently, the risk of power system instability has greatly increased. Therefore, protecting electrical networks from critical situations such as blackouts has become a very relevant issue in the network operation. The fast evaluation of the current network state, particularly its dynamic state, and the use of appropriate remedial measures in the control rooms play an important role in solving this problem. This paper describes the online Dynamic Security Assessment system for the prevention of power system instabilities and improvement of the electrical network state by applying remedial measures.