Abstract: Digital twin has become an important medium for digital-intelligent updating of electrical equipment in our country due to its powerful data integration, analysis and interaction characteristics. In this review, the architectural compositions of the digital twin operation and maintenance (O & M) systems for the power generation side, substation side, and transmission and distribution side are sorted out. The key equipment and supporting technologies for each level are analyzed in terms of the layers of perception and interaction, the data processing, the modeling and simulation, and the function and application. The development potentials of the digital twin technology combining with other intelligence technologies to realize the O & M applications are also discussed. According to the operation and maintenance characteristics of power equipment and the application requirements of digital twins, we proposed a five-dimensional assessment architecture and quantitative evaluation scheme. Finally, by reviewing the national and industrial policies on guiding the current digital twin applications, we made a discussion on their critical challenges and development suggestions in terms of the combination of the fundamental O & M technologies, cross-domain sharing, network data security and industry reform.