This paper describes basic principles and purpose for applying wide area protection schemes, also known as Remedial Action Schemes (RAS) or System Protection Schemes (SPS). In the areas of power system automation and substation automation, there are two different trends: centralization and decentralization. More and more dynamic functions are moving from local and regional control centers towards central or national control centers. At the same time we also observe more "intelligence" and "decision-power" moving closer towards the actual power system substations. Greater functional integration is being enclosed in substation hardware. This raises discussions concerning reliability (security and dependability). The main targets for this paper are 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; (4) Identify similarities and differences between classic equipment protection and system protection -concerning philosophy as well as concerning product and system design; and (5) Show the value of wide area protection. (6) Illustrate the great breakthrough for wide area protection, since the introduction of synchronized phasor measurements, based on PMUs (Phasor Measurement Units).
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