The merging unit consists of Electronic Current Transformer(ECT) and Electronic Voltage Transformer(EVT).The conjuction of merging unit and bay level are the key elements in realizing serial communication between the process level and bay level of the substation automation system.According to the sample value model,the EVT model is constructed in a specific and rational way from Logic Device(LD),Logic Node(LN) and Data Object,with the communication stack service mapping of this model implemented by Specific Communication Service Mapping(SCSM).It is acknowleged that the demand of ecchange information and transmit datas in subsation automation system by EVT model.Be a kind of new transformer,its application in SAS will have a promising future since its theoretical and practical advantage.