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This paper addresses the problem of changing the operating point of a power system in order to keep voltage security margins with respect to contingencies above some minimal value. Margins take on the form of maximum pre-contingency power transfers either between a generation and a load area or between two generation areas. They are determined by means of a fast time-domain method. We first discuss the use of a general optimal power flow, in which linear voltage security constraints are added. The simultaneous control of several (possibly conflicting) contingencies is considered. Then, we focus on the minimal control change objective. Among the possible controls, emphasis is put on generation rescheduling and load curtailment. Examples are presented on an 80-bus test system as well as on a real system.