A predispatch model that minimizes generation and transmission losses on hydroelectric power systems is presented and applied to the hydroelectric power system of COPEL, a Brazilian utility located in southern Brazil. Power loss in the generation and transmission systems is minimized on an hourly basis throughout a day. Load demand as well as generation, transmission and interchange operational constraints are satisfied. Hydroelectric generation characteristics are described in detail. Power loss in hydroelectric generation is associated with a reduction in turbine-generator efficiency and effective water head. The latter is due to an increase in tailrace elevation and penstock head loss. Power loss upon transmission is calculated as a quadratic function of active power flow, as represented in a DC load flow model. The predispatch model was evaluated under typical operational conditions, achieving significant savings with respect to actual operation. © 1996 IEEE.12312201228Yang, J.S., Chen, N., Short Term Hydrothermal Coordination Using Multi-Pass Dynamic Programming IEEE Transactions on Power Systems, , vol. 4, n° 3, pp. 1050-1056, August 1989Chang, S., Chen, C., Fong, I., Luh, P.B., Hydroelectric Generation Scheduling with an Effective Differential Dynamic Programming Algorithm IEEE Transactions on Power Systems, , vol. 5, n° 3, pp. 737-743, August 1990Turgeon, A., Optimal Short Term Hydro Scheduling from the Principle of Progressive Optimality Water Resources Research, , vol. 17, n° 3, pp. 481-486, June 1981Amado, S.M., Ribeiro, C.C., Short Term Generation Scheduling of Hydraulic Multi-Reservoir Multi-Area, Interconnected Systems IEEE Transactions on Power Systems, , vol. 2, n° 3, pp. 758-762, August 1987Merlin, A., Louzanne, R., Maurras, J.F., Auge, J., Ziglioli, M., Optimization of Short Term Scheduling of EOF Hydraulic Valley with Coupling Constraints: The OVIDE model Proceedings of Power System Computational Conference, , pp. 345-354, Lausanne, 1981Wakamori, F., Masui, S., Morita, K., Sugiyama, T., Layered Network Model Approach to Optimal Daily Hydro Scheduling IEEE Transactions on Power Apparatus and Systems, , vol. 101, n° 9, pp. 3310-3314, September 1982Brannlund, H., Sjelvgren, D., Bubenko, J.A., Short Term Generation Scheduling with Security Constraints IEEE Transactions on Power Systems, , vol. 3, n° 1, pp. 310-316, February 1988Nanda, J., Bijwe, P.R., Optimal Hydrothermal Scheduling with Cascaded Plants Using Progressive Optimality Algorithm IEEE Transactions on Power Apparatus and Systems, , vol. 100, n° 4, pp. 2093-2099, April 1981Luo, G.X., Habibollahzadeh, H., Semlyen, A., Short Term Hydrothermal Dispatch Detailed Model and Solution IEEE Transactions on Power Systems, , vol. 4, n° 4, pp. 1452-1462, November 1989Pereira, M.F.V., Pinto, L.M., A Decomposition Approach to the Economic Dispatch of Hydrothermal Systems IEEE Transactions on Power Apparatus and Systems, , vol. 101, n° 10, April 1981Habibollahzadeh, H.H., Bubenko, J.A., Application of Decomposition Techniques to Short Term Operation Planning of Hydrothermal Power Systems IEEE Transactions on Power Systems, , vol. 1, n° 1, pp. 41-47, February 1986Ohishi, T., Soares, S., Carvalho, M.F., Short Term Hydrothermal Scheduling Approach for Dominantly Hydro Systems IEEE Transactions on Power Systems, , vol. 6, n° 2, pp. 637-643, May 1991Carvalho, M.F., Soares, S., Ohishi, T., Optimal Active Power Dispatch by Network Flow Approach IEEE Transactions on Power Systems, , vol. 3, n° 3, pp. 16401647, August 1988Philadelphia Electric Company, Superposition Factors program, User's Guide, 1972Lasdon, L., Optimization Theory for Large Systems Macmillan, New York, 1970.Murtagh, B.A., Saunders, M.A., Minos 5. 4 User's Guide, Technical Report SOL 83-20R, Stanford University, 1992