Static Var Compensator(SVC) is a recent power electronics device that can provide reactive power to control power system voltage. The response speed of SVC, detailed operation characteristics of the SVC must be taken into consideration to improve steady-state stability and steady-state performance. In this paper, the system strength to voltage control is represented as linear contribution of slow-response Var devices to the change of shunt susceptance, which is applied to operation point control while keeping desirable voltage profile. The controllable voltage variation and the feasible slope setting avoiding violation of control limit are quantified based on available control margin at current operation point. The quantitative analysis provides an effective control method of SVC that improves the utilization of its control margin. The paper also discusses coordination among multiple controls of local SVCs.