  The increasing penetration of renewable and distributed energy resources in
distribution networks calls for real-time and distributed voltage control. In
this paper we investigate local Volt/VAR control with a general class of
control functions, and show that the power system dynamics with non-incremental
local voltage control can be seen as distributed algorithm for solving a
well-defined optimization problem (reverse engineering). The reverse
engineering further reveals a fundamental limitation of the non-incremental
voltage control: the convergence condition is restrictive and prevents better
voltage regulation at equilibrium. This motivates us to design two incremental
local voltage control schemes based on the subgradient and pseudo-gradient
algorithms respectively for solving the same optimization problem (forward
engineering). The new control schemes decouple the dynamical property from the
equilibrium property, and have much less restrictive convergence conditions.
This work presents another step towards developing a new foundation -- network
dynamics as optimization algorithms -- for distributed realtime control and
optimization of future power networks.
