Optimal Coordination of On-Load Tap Changers with Local Control Rules

Volt-VAR control (VVC) in distribution networks is typically performed by autonomously operating voltage regulation devices. In modern distribution systems, most of the control and coordination functions are being carried out by Distribution Management System (DMS), which is a highly digitalized and automated platform. Even though the grid equipment and infrastructure are being modernized, there still exist legacy devices that operate based on local control rules. Integration of such legacy device models into the modern DMS is essential to ensure effective VVC. Hence, this paper proposes optimal coordination of onload tap changers by solving a mixed integer linear programming (MILP)-based mathematical program such that the tap switching operations can be reduced. Unlike existing works, which often ignore the local controller mechanism of LTCs, this work models and integrates the local control rule of the bang-bang controllers of LTCs in the optimal coordination procedure. To this end, the operational principle of the controller is interpreted as a set of mixed-integer linear constraints, enabling the formation of an efficient optimal power flow (OPF) formulation based on a linearized power flow model. Extensive numerical tests on the proposed formulation suggest satisfactory and promising performance.