In this paper we propose a novel performance index and a multi-layered information structure for monitoring potential quasi-static voltage problems in electrical power systems. The proposed performance index is based on the sensitivity of the smallest singular value of the power flow Jacobian matrix with respect to the corresponding system load level. This performance index is computed at different levels of system aggregation/decomposition, which we define as a "multi- layered information structure". This information structure requires little or no information exchange among decomposed subsystems. Moreover, it can be seamlessly implemented in today's horizontally decomposed interconnections. Two multi- control-area systems are used to illustrate the proposed performance index and information structure.