Misoperation and hidden failures of protections in electric power systems have been recognized as a contributing factor to power system cascading outages and catastrophic events, for that reason it is important to develop new tools to study the sequence of protections operation given a contingency in the system. This article proposes a methodology for modeling the operation sequences of protections in a loop transmission system using generalized stochastic Petri nets (GSPNs), contemplating main and back-up protections, specially how they interact. With this methodology it is derived models for transmission systems of 2, 3 and 4 buses, which could extended to larger systems.