With the development of large-scale complicated modern power systems, the requirement for the associsted protection scheme tends to be more stringent and its combination more complex. However, it is very difficult to figure out the.factors of failure of such systems. This paper proposes a Petri net model of a transmission line protection relaying system, including three types of relays as well as an automatic re-closing device, and shows how to diagnose serial failure of the system by analyzing invariant sets of the model. Furthermore, it gives four basic types of failure sequences and its execution is much more intuitive and effective than the traditional method. With the development of large-scale complicated modern power systems, the requirement for the associsted protection scheme tends to be more stringent and its combination more complex. However, it is very difficult to figure out the.factors of failure of such systems. This paper proposes a Petri net model of a transmission line protection relaying system, including three types of relays as well as an automatic re-closing device, and shows how to diagnose serial failure of the system by analyzing invariant sets of the model. Furthermore, it gives four basic types of failure sequences and its execution is much more intuitive and effective than the traditional method.