Control of Parallel ULTC Transformers in Active Distribution Systems

This paper proposes a two-level control scheme for controllable devices in a smart grid, enabled with various types of distributed energy resources (DERs) and the other sources of flexibility provision. The main focus is on under-load tap changer (ULTC) control of parallel transformers in active distribution systems. The first-level control that can be viewed as a function of distribution management system, minimizes the system operation cost, considering the voltage limits, maximum line current, maximum circulating current in parallel transformers, and controllers' capacity as the optimization constraints. The second-level control holds the voltage stability in emergency conditions. The control rules in this level are intended to restore the system voltages as soon as possible, minimize the total number of costly control actions, and prevent conflicts between different controllable devices. In the second-level voltage control, the solution speed is of utmost importance. Therefore, the formulation of the control problem is first linearized using node analysis technique and capacitor banks, ULTC, DERs and voltage regulators are coordinated based on their priority and their effect on the bus voltages.