Conservation voltage reduction (CVR) via two-timescale control in unbalanced power distribution systems

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 study presents a two-timescale control of feeder's voltage control devices to achieve CVR that includes: (i) a centralised controller operating in a slower time-scale to coordinate voltage control devices across the feeder; and (ii) local controllers operating in a faster timescale to mitigate voltage fluctuations due to PV variability. The centralised controller utilises 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 minimise 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.