The concept of capability chart, traditionally related to the synchronous machines, has been extended to the whole generator-transformer unit, and has proven to be an effective tool in planning and design activities: different solutions can be compared in terms of active and reactive powers injected by the generating unit into the high-voltage busbar. The operation of the unit in points belonging to the capability chart requires to suitably choose the set-points of the unit controls: a control law is proposed which requires minimum changes to tile usual control structures, and does not need new measurements. Numerous tests on a typical generating unit highlight the effectiveness of the capability chart as a planning tool, and the features of the proposed control law.