In this paper, data streams are classified into four types conforming to a standardized infrastructure of communication networks for a substation automation system (SAS) based on IEC61850 system. The data exchanged on the net are demonstrated to be stochastic according to investigation on the Ethernet communication principles. Four generalized stochastic Petri nets (GSPN) based models for performance analysis of communication networks of IEC61850 system are developed based on the three-level structure of SAS, different time requirements of the four data streams and different networks topology for different voltage level. The GSPN-based model associated with immediate and exponential transitions is proven to be theoretically isomorphic with Markov chain; hence we apply the mathematic methods of performance evaluation contained in Markov chain to the GSPN models proposed. The computer simulation of the model including only sample value data streams shows that it can meet performance evaluation needs of communication networks of IEC61850 system. Further researches should be focused on the performance of the other three models to explain clear how those different data streams are interrelated to and interact on each other.