Power transmission systems are being operated closer to their stability limits compared to twenty years ago. As a result, an increasing risk of blackouts combined with extremely high costs economically and socially is observed. To reduce this risk the personnel in the control rooms of the transmission system operators have to be assisted by a system, which assesses the dynamic state of security online. The system presented in this paper is designed to investigate the security with regards to voltage stability, transient stability and small signal oscillatory stability. The performance of the system is sufficiently high to be employed in the online environment of the control room.