A new direct method for transient security assessment of multimachine power systems is presented. A local approximation of the stability boundary is made by tangent hypersurfaces which are developed from Taylor series expansion of the transient energy function in the state space nearby a certain class of unstable equilibrium points (UEP). Two approaches for an estimation of the stability region are proposed by taking into account the second order coefficients or alternatively, the second and third order coefficients of the hypersurfaces. Results for two representative power systems are described and a comparison is made with the hyperplane method, demonstrating the superiority of the proposed approach and its potential in real power system applications. Artificial neural networks are used to determine the unknown coefficients of the hypersurfaces independently of operating conditions.
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