  This paper proposes an online voltage control strategy of distributed energy
resources (DERs), based on the projected Newton method (PNM), for unbalanced
distribution networks. The optimal Volt/VAr control (VVC) problem is formulated
as an optimization program, with the goal of maintaining the voltage profile
across the network by coordinating the VAr outputs of DERs. To overcome the
slow convergence rate of conventional gradient-based methods, a PNM-based
solution algorithm is developed to solve this VVC problem. It utilizes a
non-diagonal symmetric positive definite matrix, developed from the Hessian
matrix of the objective, to scale the gradient, and thus a fast convergence
performance can be expected in this Newton-like algorithm. Moreover, taking
advantage of the instantaneous feedback of voltage measurements, the online
implementation of the PNM-based VVC is further designed to deal with fast
system variations. In this online PNM-based VVC scheme, each bus agent
communicates the instantaneous voltage measurements to the central agent, and
the central agent communicates the VAr output commands of DERs back to each bus
agent. The fast convergence performance of PNM results in a stronger capability
to track the system variations in real time. Finally, numerical case studies
are performed to validate the effectiveness, superiority, and scalability of
the proposed method.
