This paper is concerned withthe optimal operation of reservoirs for electric generation. It reports several tests carried out on a simple system under special conditions highlighting some important characteristics of their optimal behavior. The tests were carried out in a progressive way in order to make clear the influence of several factors including water head, discount rate, inflow seasonality and system design. The main conclusions have been verified on an actual large scale system, which demonstrated the conservative nature of the optimal operation of reservoirs and the different roles they play according to their relative position in the cascade. The conclusions address the issue of adequately modelling the long term hydrothermal scheduling problem. The results suggest that the assumption of parallel operation of reservoirs, implicit in composite reservoir models and some simulation heuristics, could be a modelling oversimplification.

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This paper presents a methodology to improve the power system economical dispatch from a voltage stability margin perspective. The time horizon under discussion is the short-term operation planning. The proposed method is based on active/reactive power re-dispatch for normal operation, and also minimum load shedding strategies in case of critical contingencies. The actions are taken in the direction provided by modal participation factors computed for generator and load buses. The generators with negative impact on system margin, which are indicated by the modal index, are penalized with high costs on the objective function of the optimal power flow program used to run the re-dispatch process. Results of this work show a decrease on system losses and significant increase on voltage stability margin as well as on system reactive reserves. In addition, this work presents a study considering critical contingencies, for which is proposed an optimal load shedding strategy also based on modal participation factors to identify the most adequate buses for load shedding purposes. Finally, the proposed methodology is applied considering a typical hour-to-hour daily load curve, and the method presented very good performance since it considerably increases voltage stability margin for the insecure intervals.