Optimal Operation of Multireservoir Systems Using a Composite Representation

For a large hydroelectric power system, such as that of the Pacific Northwest, an important operational decision each month is the amount of hydrogeneration. This decision is important because the inflow of the water is uncertain while hydro, with zero marginal cost, can be used not only to satisfy firm load commitments, but also to displace other firm resources or to serve secondary loads. In such a case, the tradeoff between savings at the present and expected benefits in the future is determined mainly by the total hydrogeneration. The use of a composite representation of multireservoir hydroelectric power systems to determine the optimal monthly total hydrogeneration is described. The analytical tool employed is that of stochastic dynamic programming, and the statistical model for the streamflow prediction is based on previous flows and snowpack information. For the anticipated 1975 system in the Pacific Northwest, comparison between the optimal operation introduced here and the presently used rule-curve operation indicates that substantial savings may be obtained, mainly owing to the more uniform displacement of the high marginal cost thermal resources by hydrogeneration.