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This paper describes basic principles and philosophy for wide-area protection schemes, also known as remedial action schemes (RAS) or system protection schemes (SPS This raises discussions concerning reliability (security and dependability). The main targets for this paper is therefore to: 1) sort out the terminology used in this area; 2) describe different application areas and related requirements; 3) illustrate different design principles -"top-down," "bottom-up," hierarchy, flat, etc., for different applications; and 4) identify similarities and differences between classic equipment protection and system protection-concerning philosophy as well as concerning product and system design.The theme of the paper is on the use of information technology to obtain more flexibility and smartness in power-system controls.

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.