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As increases in electrical power demand and trends going toward power system deregulation are pushing power systems operating closer their security limit. Power systems are not only more complicated but also vulnerable to a disturbance. Some serious power system blackouts with severe consequence occurred in US and European countries are evidence for these statements. The causes of blackouts are not unique and one of them is related to transient stability problem. In practice, transient stability can be avoided by applying correctly preventive actions. In this paper, we introduce a new preventive method based on energy approach. The method employs a heuristic approach combination with controllability and observability gramians as a measure to choose generators which are used to reschedule power generation output in order to improve transient stability by increasing critical clearing time (CCT).