Two-Stage Volt-VAr Optimization of Distribution Grids With Smart Inverters and Legacy Devices

This article develops a two-stage volt-VAr control strategy coordinating the discrete controls of legacy grid devices and the operation of smart inverters (SIs) in power distribution grids with high penetration of photovoltaic generation. The first stage dispatch problem optimally coordinates the tap settings of on-load tap changers, on/off status of shunt capacitor banks, and SIs’ active power output. The second stage incorporates the SI volt-VAr mode, i.e., $\mathbf{Q(V)}$ as per IEEE-1547, and an adaptive volt-VAr droop function, $\mathbf{Q(\Delta V)}$, to dispatch the SI reactive power generation. Both dispatch problems are formulated as mixed-integer linear programming problems to maintain tractable formulations and reduce the complexity and computational burden that usually arise when solving large-scale optimal power flow problems. The proposed two-stage strategy is tested on a modified IEEE 123-bus system considering different droop settings. The dispatch strategy augmented with $\mathbf{Q(\Delta V)}$ droop outperforms the model with only $\mathbf{Q(V)}$ droop in stabilizing the PCC voltage while maintaining less active power curtailment.