  We augment existing generator-side primary frequency control with load-side
control that are local, ubiquitous, and continuous. The mechanisms on both the
generator and the load sides are decentralized in that their control decisions
are functions of locally measurable frequency deviations. These local
algorithms interact over the network through nonlinear power flows. We design
the local frequency feedback control so that any equilibrium point of the
closed-loop system is the solution to an optimization problem that minimizes
the total generation cost and user disutility subject to power balance across
entire network. With Lyapunov method we derive a sufficient condition ensuring
an equilibrium point of the closed-loop system is asymptotically stable.
Simulation demonstrates improvement in both the transient and steady-state
performance over the traditional control only on the generators, even when the
total control capacity remains the same.
