Discrete-event system modeling and supervisory control for under-load tap-changing transformers

Discrete-event systems (DES) can be found as essential integrated subsystems 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. ULTC may be controlled either automatically or manually. The paper discusses synthesis of supervisory controller of ULTC as a discrete-event system. Different parts of the ULTC as well as its control logic (specification) have been modeled as DES. Supervisory controllers are designed for the tap-changer in Automatic and Auto/Manual modes of operation. It is shown that the specifications are controllable and the closed loop control system is non-blocking. Protective system designers in electrical powers systems can use the proposed approach to verify their control logic for ULTC.