The paper presents a security algorithm which consists of the superdecoupled loadflow technique for loadflow solution, outage studies, contingency selection and line overload alleviation. A superdecoupled loadflow method with a distributed slack busbar has been applied to line and generator outage studies as well as for line overload alleviation. The accuracy and reliability of the line and generator outage contingency selection method, as previously developed by comparison of its ranking results with the correct ranking of AC outage simulation, has been investigated. A fast computation technique in real power line overload alleviation by generation scheduling and/or load shedding has been proposed by using injections at the terminating busbars of the overloaded line. These injections are determined by the amount of overload only without having to compute the sensitivity and Jacobian line flow matrices and pseudoinverse of the product of these matrices. The proposed algorithm has been validated in tests on the IEEE 24 busbar test system.