This paper discusses a methodology to accurately determine the impact of using wind-energy-conversion systems, WECS, on the operation of the central power system. To do so, the power-generation-simulation code PROMIX is used to simulate the operation of the power system on an hourly basis and on a power-plant level, including the technological restrictions of every plant. The actual impact assessment of the WECS is done by comparing the output of two scenarios; a base case and an alternative scenario which includes the WECS. To demonstrate the method, several case studies for WECS in different locations in Belgium are worked out. The impact is quantified by looking at the greenhouse-gas-emission reductions that can be obtained by using WECS. In these case studies, we closely look at the influence of the variability of the WECS power output, the geographical spread of the wind farms, the capacity factor and the capacity credit of the WECS and the effect of the power-generation mix. (c) 2005 Elsevier Ltd. All rights reserved.