The bulk power produced at a generating plant is moved to where it is needed over bulk power transmission lines. Each transmission line operates at relatively high voltages, somewhere between 35,000 volts (35 kV) and 750,000 volts (750 KV) depending on design. Higher voltage lines cost more, require bigger towers and equipment and thus have a greater negative aesthetic impact, but carry much more power: A line with twice the voltage carries four times as much power.
Thus, utilities prefer to use high voltage when they can: It costs less and avoids the need for a greater number of lines.
A large electric utility will own many transmission lines – perhaps several thousand if it serves a very populated or multistate region. These are linked together in a transmission power grid or what is often called a transmission network that crisscrosses its service territory. This transmission network permits the utility to route power from its many generating locations to the many locations (cities and towns) where it is needed, and to re-route power instantly if a particular generator breaks down or a transmission line has to be withdrawn from service for maintenance, etc.
Large utility power grids usually consist of two distinct levels, or sets of lines, as depicted in Figure 1.1. A set of very high voltage lines, each rated somewhere between 750 KV and 230 KV, simply referred to as transmission, crises crosses the service territory and connects all the major generating plants together. At certain key locations, two to four of these lines intersect at a switching substation. Equipment at this substation controls power flow on the lines and reduces the voltage of the incoming power to route it out onto a number of lower-voltage sub-transmission lines.
These sub-transmission lines operate at what are still quite high voltages – anywhere from 161 KV to 35 KV. A typical switching substation might have two 230 KV transmission lines and four 138 KV sub transmission lines. Power is routed into it on the transmission lines, lowered in voltage, and channeled out on the four sub-transmission lines. In many cases, some of the power coming in on the transmission will pass through the substation: perhaps 500 MW flows in on one of the 230 KV lines, with 150 MW passing on through, and the remaining 350 MW being lowered in voltage and routed onto the four 138 KV lines.
The sub-transmission lines lead to different distribution substations, each one routing through about two to six. A large utility will have several hundred or more distribution substations scattered throughout its system, typically two to ten miles apart. At these, power is taken off the sub-transmission line, further lowered in voltage, to primary distribution voltage (somewhere between 2,100 to 25,000 volts) and routed onto the distribution system.
