  This paper presents a two-stage zonal Volt/VAR control scheme for
coordinating inverter-based resources (IBR) with utility-owned voltage
regulators (VR) to regulate voltage in unbalanced 3-phase distribution systems.
First, correlations between nodal voltages are derived from nodal voltage
sensitivity studies. Then, the feeder is partitioned into non-overlapping,
weakly-coupled voltage control zones based on nodal voltage correlations. IBR
are used in the first stage to regulate voltage changes continuously and VR are
used in the second stage to regulate large voltage deviations. An online VR
voltage setpoint tuning strategy is developed to reduce excessive tap changes
and avoid large voltage fluctuations without retrofitting existing VR
controllers. In addition, the proposed algorithm uses real-time voltage
measurements only from the critical nodes (typically less than 4% of total
nodes) to reduce the sensing and communication needs. Actual distribution
feeder topologies and load and PV time-series data are used to verify the
performance of the algorithm. Because the method is a rule-based approach, it
runs extremely fast, requires fewer measurements, and requires no retrofit to
the existing VR control mechanisms. Simulation results show that the
performance of the proposed method in terms of voltage control results and
average numbers of VR tap changes are satisfactory.
