| Large Power Transformer |
Large power transformers are used in electric power systems. Industrial and residential large power tranformers that operate at the line frequency (60 Hz in the U.S.), may be single phase or three-phase, and are designed to handle high voltages and currents. Efficient power transmission requires a step-up large power transformer at the power-generating station to raise voltages, with a corresponding decrease in current. Line power losses are proportional to the square of the current times the resistance of the power line, so that very high voltages and low currents are used for long-distance transmission lines to reduce losses. At the receiving end, step-down large power transformers reduce the voltage, and increase the current, to the residential or industrial voltage levels, usually 115 to 600 V.
Large power transformers must be efficient and should dissipate as little power as possible in the form of heat during the transformation process. Efficiencies are normally above 99 percent and are obtained by using special steel alloys to couple the induced magnetic fields between the primary and secondary windings. The dissipation of even 0.5 percent of the power transmitted in a large power transformer generates large amounts of heat, which requires special cooling provisions. Typical large power transformers are installed in sealed containers that have oil or another substance circulating through the coils to transfer the heat to external radiatorlike surfaces, where it can be discharged to the surrounding atmosphere.
