  Increasing penetration of inverter-based distributed energy resources (DERs)
opens up interesting opportunities for the transmission systems. We present a
hypothesis that the numerous DERs in var control mode can be seen as
geographically distributed var devices (mini-SVCs) and if controlled properly,
can be exploited to increase system flexibility by providing local var support
to the grid as an ancillary service. Based on this premise, a var support
framework is proposed in this paper. It utilizes a novel D-OPF formulation for
unbalanced three-phase feeders enabling the estimation of the maximum var
support that can be provided by the DERs to the grid at different operating
points without compromising the distribution network performance. Further, a
co-simulation method is developed to investigate the true impact of the
proposed DER var support on the grid in an integrated Transmission-Distribution
(T-D) system.
