With a large-scale introduction of wind power the power system operation planning including keeping of spinning reserves will be influenced since extra uncertainties are introduced. To study this a more detailed analysis of system operation is required. This analysis has to consider wind speed forecast uncertainties, system load forecast uncertainties, ramp rates of thermal units and spinning reserves etc. This paper shows a new methodology to analyze the operation planning with an emphasis on keeping reserves. The result form the calculations include instantaneous, fast and slow reserve margins at each hour of the planning period.