  Electromechanical transients in power networks are mostly caused by a
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
aim of such a controller is to primarily stabilize frequency oscillations after
a significant, 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.
