The new technique of Frobenius norm calculation of gramians, subgramians and transfer functions of electric power systems (EPS) is suggested. It is based on spectral expansion of gramian by combinational modes of dynamic matrix of EPS predicting model. It is more effective comparing to modal analysis as it allows to prescribe the criterion of EPS stability loss risk, and also to evaluate the interaction of ill-stable system modes. The suggested approach has a transparent physical interpretation, i.e., stability loss risk corresponds to the energy of ill-stable modes group. The predicting model is formed basing on associative search technique for nonlinear dynamic plants.