Design and PLC based implementation of supervisory control for under-load tap-changing transformers

Discrete-event systems (DES) can be found as an essential integrated subsystem in electrical power systems. The continuous trajectory of the system state can be interrupted by discrete control actions and uncontrolled disturbances. Under-load tap-changing transformers (ULTC) which obviously have discrete-event behavior are widely used in transmission systems to take care of instantaneous variations in the load conditions in substations. The paper discusses synthesis and physical implementation of a DES supervisory control for ULTC system. It is shown that the specifications are controllable and the closed loop control system is non-blocking. We used a heuristic method for easier implementation of the supervisor on a Programmable-Logic-Controller (PLC), to overcome the general implementation problems as well as, the implementation problem caused by the auto/manual mode of ULTC operation. A step -by- step procedure is developed to generate a ladder diagram which implements the DES supervisor on a PLC.