  With the increasing penetration of renewable energy resources, power systems
face new challenges in balancing power supply and demand and maintaining the
nominal frequency. This paper studies load control to handle these challenges.
In particular, a fully distributed automatic load control (ALC) algorithm,
which only needs local measurement and local communication, is proposed. We
prove that the load control algorithm globally converges to an optimal
operating point which minimizes the total disutility of users, restores the
nominal frequency and the scheduled tie-line power flows, and respects the load
capacity limits and the thermal constraints of transmission lines. It is
further shown that the asymptotic convergence still holds even when inaccurate
system parameters are used in the control algorithm. In addition, the global
exponential convergence of the reduced ALC algorithm without considering the
capacity limits is proved and leveraged to study the dynamical tracking
performance and robustness of the algorithm. Lastly, the effectiveness,
optimality, and robustness of the proposed algorithm are demonstrated via
numerical simulations.
