E.C. Obinabo  , 
    Scalar Stability Criteria for Interconnected Nonlinear Power Systems,
    International Journal of Electrical and Power Engineering,
    Volume 2,Issue 4,
    2008,
    Pages 235-240,
    ISSN 1990-7958,
    ijepe.2008.235.240,
    (https://makhillpublications.co/view-article.php?doi=ijepe.2008.235.240)
    Abstract: This study establishes criteria for stability of nonlinear zero-input interconnected power systems using an arrangement of scalar blocks where the input-output pair for each block is strictly proper rational real or transfer function. The structure of the multivariable system as an interconnection of scalar feedback blocks provides a basis for obtaining new conditions for stability of zero-input interconnected nonlinear power systems.
    Keywords: Scalar feedback blocks;nonlinear stability;zero-input interconnected power systems;liapunov function