In September 2000, the Brazilian system dispatch and spot prices were
calculated twice, using different inflow forecasts for that month, as in
the last 5 days of August the inflows to the reservoirs in the South and
Southeast regions changed 200%. The first run used a smaller forecasted
energy inflow and the second used a higher energy inflow. Contrary to
expectations, the spot price in the second run, with the higher energy
inflow, was higher than the one found in the first run. This paper
describes the problem, presents the special features of the
PAR(p) model that allow the described behavior, and shows the
solution taken to avoid the problem.