During the revision of the technical specification for the merging unit in the smart substation published by State Grid Corporation of China

the problems existing in the engineering application of merging unit in the smart substation are investigated

and the problems in the performance indicators and application functions etc. are discussed. The key technology of communication protocol of the merging unit

the voltage parallel and switching logic

the resynchronization mechanism after signal loss and metering are analyzed. Other problems which cannot be solved in short time are also discussed and the related solutions are provided. It will promote the application of merging unit and provide guidance to the construction of smart substation in the future. This work is supported by State Grid Corporation of China.