LAUSR

The optimal relay coordination (ORC) problem for directional overcurrent relays (DOCRs) has been solved by many conventional and modern optimization algorithms. All these studies were based on the common assumption that all DOCRs are numerical, digital “hardware-based,” static, or electromechanical. Unfortunately, the transition between these technologies does not happen instantaneously, so protection engineers could see different models of these protective relays in some real electric power systems. Moreover, when old electromechanical, static, and digital relays are replaced with the latest state-of-the-art numerical relays, the older relays are kept as backup protective devices. Some called them “primary” and “local-backup” relays, whereas others called them “main-1” and “main-2” relays. The reason behind the second terminology is the chance that the old relays could act ahead of the numerical relays. In this paper, a realistic mathematical model of the ORC problem is formulated and solved using a new hybrid evolutionary algorithm. To judge whether this realistic ORC problem is completely/partially solvable or not, the IEEE 6-bus, 15-bus, and 42-bus test systems are simulated. The results prove that the technique is an effective tool to indicate which relay sets accept/do not accept this double primary relay strategy.