  Electromechanical transients in power networks are mostly caused by mismatch
between power consumption and production, causing generators to deviate from
the nominal frequency. To that end, feedback control algorithms have been
designed to perform frequency and load/renewables-following control. In
particular, the literature addressed a plethora of grid- and frequency-control
challenges with a focus on linearized, differential equation models whereby
algebraic constraints (i.e., power flows) are eliminated. This is in contrast
with the more realistic nonlinear differential algebraic equation (NDAE)
models. Yet, as grids are increasingly pushed to their limits via intermittent
renewables and varying loads, their physical states risk escaping operating
regions due to either a poor prediction or sudden changes in renewables or
demands -- deeming a feedback controller based on a linearization point
virtually unusable. In lieu of linearized differential equation models, the
objective of this paper is to design a simple, purely decentralized,
linearization-free, feedback control law for NDAE models of power networks. The
objective of such controller is to primarily stabilize frequency oscillations
after a large, unknown disturbance in renewables or loads. Although the
controller design involves advanced NDAE system theory, the controller itself
is as simple as a decentralized proportional or linear quadratic regulator in
its implementation. Case studies demonstrate that the proposed controller is
able to stabilize dynamic and algebraic states under significant disturbances.
