Definition
The combined opposition to AC current flow consisting of both resistance (R) and reactance (X), expressed as a complex number Z = R + jX and measured in ohms (Ω). Transmission line impedance determines power flow patterns on the grid and is the fundamental parameter used in power flow studies, fault current calculations, ATC assessments, and relay coordination. The impedance of a transmission line depends on conductor geometry, tower configuration, ground resistivity, and line length; it is typically represented per unit length (Ω/mile or Ω/km).
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Topic Deck
Electrical Engineering Fundamentals
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Source
FERC Pro Forma OATT / LGIP