The framework of supervisory control of discrete event systems (DESs) is applied to agent-based power protection systems for achieving high reliability and selectivity. The DES-based supervisory control uses feedback information of events occurring in systems to achieve the set of desired event sequences. The large-scale and complex power systems need systematic tools for protection and control on a more abstract level. The supervisory control technique on the event-based abstract level can be used as a formal method for protection of large-scale power systems. Specifically, a design procedure of a supervisor that coordinates the behaviour of relay agents to isolate fault areas through the minimum operations of circuit breakers is presented.