Transmission line limits impede power transfers and cause congestion greatly reducing the effectiveness of systems and increasing the cost of power transmissions. Through several methods, congestion can be effectively eliminated either by building a new transmission line or by increasing the capacity of the original line between congested zones. Both methods cause susceptance change and line capacity increase between nodes. Therefore, the sensitivity of a system to variations of its parameters becomes important in terms of operation and planning. In this paper, congestion relief as a function of line susceptances and line capacities is investigated. Mathematical derivations for calculating sensitivities based on line susceptances are given systematically, and numerical studies were performed by using sequential quadratic optimization programming to determine congestion in terms of line susceptances. It is shown that the optimum susceptance range becomes critical when the line parameters are changed dynamically by using thyristor-controlled series-compensators. To relieve congestion in the operation of a network may require tracking this optimal parameter range whenever system states change. (C) 2008 Elsevier B.V. All rights reserved.