The communication capability of Ethernet is a key for normal operation of digital substation automation system based on IEC 61850. Aiming at the united ring and star networks in whole substation， the simulation software OPNET is adopted to construct the mathematical models of various typical data flow in this paper. IEC 61850-9-2 protocol is used to transfer the sampling values. A variety of simulating test schemes are made， which consider following factors such as: structure and bandwidth of network， the different transferring of sampling values by the process network， the breaking failure of the ring network， the influence of file transferring on the sampling values and tripping message. The results of simulating experiments for an actual medium-size substation are given. It provides a reference for construction of a digital substation system.