We provide a review of nonlinear instabilities occurring in networks of rotatory generators based on our previous papers. The instability phenomena, which we term the collective instabilities, are of emergent type and observed in networks of synchronous and induction generators. With both the cases, we provide a unified definition of collective instabilities using a coupled dynamical system model. The unified definition is expected to help inquiry to the causes of cascading failure in nation-wide grids and of sudden disconnection of wind turbines following a local disturbance.