Coordination of OLTC and smart inverters for optimal voltage regulation of unbalanced distribution networks

•A framework is proposed for optimal coordination of on-load tap changer (OLTC) tap operations and Photovoltaic (PV) smart inverter reactive power injection for multi-phase unbalanced distribution feeders.•Convexity is achieved using proposed linearizations.•Feeder voltage deviations are minimized with minimal tap operations.•Effectiveness and scalability are demonstrated on the IEEE 37 bus test feeder and a real utility feeder with 2844 nodes and 340 PVs.•Robustness of the framework is validated against up to 30% forecast errors. Photovoltaic (PV) smart inverters can improve the voltage profile of distribution networks. A multi-objective optimization framework for coordination of reactive power injection of smart inverters and tap operations of on-load tap changers (OLTCs) for multi-phase unbalanced distribution systems is proposed. The optimization objective is to minimize voltage deviations and the number of tap operations simultaneously. A novel linearization method is proposed to linearize power flow equations and to convexify the problem, which guarantees convergence of the optimization and less computation costs. The optimization is modeled and solved using mixed-integer linear programming (MILP). The proposed method is validated against conventional rule-based autonomous voltage regulation (AVR) on the highly-unbalanced modified IEEE 37 bus test system and a large California utility feeder. Simulation results show that the proposed method accurately estimates feeder voltage, significantly reduces voltage deviations, mitigates over-voltage problems, and reduces voltage unbalance while eliminating unnecessary tap operations. The robustness of the method is validated against various levels of forecast error. The computational efficiency and scalability of the proposed approach are also demonstrated through the simulations on the large utility feeder.