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This paper presents the results of a study for an on-line voltage security monitoring system in AEP. It is designed to work inside the control center with phasor measurement units (PMUS) providing the synchronized real-time measurements. Classification type decision tree (DT) models are utilized to predict voltage security status. Fast and direct measurement from PMUS combined with decision trees quickly assessesvoltage security, leaving more time for corrective or preventive actions. Two attributes are considered in the decision tree models: generator vars and angular difference. Both are observed to be good indicators of voltage security status. The paper focuses on the angle difference atb-ibutes because bus voltage angle is directly measured by the PMUS. A stressed power system is characterized by widerring angolar separation of bus voltage angles as it moves towards voltage insecurity.DTs exploit the complex non-li.new relationship between voltage security status and generator varsfangutar difference in terms hierarchical roles extracted tlom a large number of off-line loadflow simulations. Several DT models were analyzed using the existing PMUS, and in the future when additional PMUS are incrementally installed. A PMU placement technique is discussed to determine the critical locations of new PMUS. The voltage security monitoring system was tested on a 360-bus subsystem inside AEP.

In previous work concerning the Voltage Instability Predictor (VIP), the proximity to voltage collapse (or instability) was expressed in terms of distance between two voltage curves or between two impedance curves. In this paper, a new measure, power margin, Is introduced to describe the proximity to collapse in terms of power. The results of recent work on the effects of contingencies and system dynamics on the VIP are also presented, extending the prior work that assessed the effectiveness of the VIP under conditions of increased power transfers, using power flow simulations to examine voltage collapse conditions. These results show that the VIP algorithm successfully predicted voltage instability where conventional protection devices, using only voltage Inputs, did not.

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