  Voltage control devices are employed in power distribution systems to reduce
the power consumption by operating the system closer to the lower acceptable
voltage limits; this technique is called conservation voltage reduction (CVR).
The different modes of operation for system's legacy devices (with binary
control) and new devices (e.g. smart inverters with continuous control) coupled
with variable photovoltaic (PV) generation results in voltage fluctuations
which makes it challenging to achieve CVR objective. This paper presents a
two-timescale control of feeder's voltage control devices to achieve CVR that
includes (1) a centralized controller operating in a slower time-scale to
coordinate voltage control devices across the feeder and (2) local controllers
operating in a faster timescale to mitigate voltage fluctuations due to PV
variability. The centralized controller utilizes a three-phase optimal power
flow model to obtain the decision variables for both legacy devices and smart
inverters. The local controllers operate smart inverters to minimize voltage
fluctuations and restore nodal voltages to their reference values by adjusting
the reactive power support. The proposed approach is validated using the
IEEE-123 bus (medium-size) and R3-12.47-2 (large-size) feeders. It is
demonstrated that the proposed approach is effective in achieving the CVR
objective for unbalanced distribution systems.
