School of Electrical and Electronic Engineering，North China Electric Power University，Baoding071003，China

The principles of wide area protection and the technical requirements on protection implementation are becoming increasingly mature.However，the reliability is still the key factor constraining further improvement and application of wide area protection.This paper presents a reliability assessment method for the wide area protection principle scheme based on the protection principle failure probability model and Markov state space.Firstly，the no-action probability of major functional modules are analyzed based on the principle failure probability model，and the overall reliability of the protection principle scheme is then assessed through rational division of Markov state space while considering the factors of data transfer and programming.Finally，a case study is analyzed，the results showing that the proposed method can provide reference for designing and optimizing the wide area protection scheme as well as for finding the weak links.