Conditions for theoretical coherency in multimachine power systems

The problem of electromechanic coherency in multimachine power systems is considered. Properties of theoretical coherency are defined in a rigorous way and related to necessary and sufficient structure conditions to be satisfied by a suitable set of electromechanical parameters and by the load-flow values in the initial equilibrium point before the start of a disturbance. The relations between coherency based nonlinear dynamic equivalents and modal reduced linearized models are illustrated. The structural conditions for theoretical coherency can be given the meaning of uncontrollability conditions. The analysis of the theoretical coherency represents the starting point for a following analysis of the practical coherency.