To realize the substation-area backup protective relay which adopts current differential principle in smart substation, it is necessary to obtain the sampled values (SV) and generic object oriented substation event (GOOSE) from adjacent substations. Therefore the communication schemes between smart substations are researched. There are three schemes suitable for SV transmission with high bandwidth and real-time characteristic, as follows: the solution by means of optical differential protection device, the solution by dedicated data forwarding device and the solution of extending process layer network by wide-area Ethernet. After a comparative analysis, the third solution with good interoperability and high bandwidth is the most suitable scheme for practical application of substation-area backup protection. The paper makes a preliminary theoretical analysis from several aspects, such as SV synchronization method, data channel bandwidth, communication distance, communication reliability and transmission delay. So it draws a conclusion that transmitting SV transparently between smart substations by the third solution is feasible. In the end, a few possible issues need further study is dedicated.