In the paper, a fuzzy decision making methodology is presented to decide the generation schedule of long-term hydrothermal problems with explicit recognition of statistical uncertainties in system production cost data, NOx emission data, system load demand and hydro
reservoir water inflows. In deciding the optimal operation, three objectives operating cost, NOx emission and unsatisfied load demand over the whole of the planning period are simultaneously minimised. Specific technique is put forth to convert the stochastic models into their
deterministic equivalents. The weighted minimax method is used to simulate the tradeoff relation between the conflicting objectives in the non-inferior domain. The fuzzy set theory is exploited to choose the best operating point over the tradeoff curve. An efficient decomposition technique is applied to reduce the complexity of the problem. In each subproblem, thermal generations are obtained by using simplified
method and water discharges are adjusted using Conjugate gradient method. The validity and effectiveness of the method has been demonstrated by analysing a power system consisting of 3-hydro and 4-thermal plants.