Transmission and Distribution Systems

Summary

Power delivery systems are divided into two general tiers: a transmission system that spans long distances at high voltages on the order of hundreds of kilovolts (kV), usually between 60 and 500 kV, and a more local distribution system at intermediate voltages in the low tens of kV. Superconductivity obviates many constraints and concerns about transmission lines, most notably thermal loading limits and line losses. Distribution lines and short to medium transmission lines are limited in their capacity to transmit power by resistive heating. It is thus the magnitude of the current, continuing over time and increasingly heating the conductor, that limits the loading; this is the thermal limit. There are two common methods for controlling or supporting voltage in transmission and distribution systems, based on very different physical mechanisms: transformer taps, and reactive power injection.