  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.
