Results are presented of research and development of system dynamic equivalents for transient stability studies. The continued growth in the size of interconnections and the increased emphasis on system reliability has resulted in the assembly of large power systems models for transient stability studies. The increased size and complexity of the power system models, in conjunction with the need to perform simulations over longer time periods, requires large blocks of computer core to perform transient stability studies with resulting long computation times. As a result, the cost of running transient stability studies represents a sizable expenditure by utilities. The major accomplishment of this research has been the development of a computer program that can reduce a large-scale system model into a smaller equivalent model for use in transient stability studies. The program has been demonstrated and validated using large-scale representations of both the eastern U.S. and western U.S. interconnected systems. The computer program accepts a normal transient stability data base as input, develops an equivalent that is a fraction of the size of the full power system representation, while adequately retaining the dynamic characteristics of the full system. The reduction process requires only a fraction of the time needed formore » a transient stability simulation. Test results have shown that the reduced ''dynamic equivalent'' is two to six times more efficient than the original system in terms of computer running time and memory storage, thereby offering significant savings in computation costs. The dynamic equivalents program has the capacity for handling 2000 buses, 3000 lines and 350 generators. The program also incorporates two synchronous machine models, six excitation system models, ten prime mover governor models, three power system stabilizer models and a non-linear load representation.« less