 |
The transmission system is the central trunk of the electricity grid. Thousands of distribution systems branch off from this central trunk and fork and diverge into tens of thousands of feeder lines reaching into homes, buildings, and industries. The power flow to the distribution systems is largely determined by the power flow through the transmission systems. In fact, when most people talk about the power "grid," they're referring to the transmission system.
The transmission system truly is a grid. Transmission lines run not only from power plants to load centers but also from transmission line to transmission line, providing a redundant system that helps ensure the smooth flow of power. If a transmission line is taken out of service in one part of the power grid, the power can usually be rerouted through other power lines to continue delivering power to customers.
In essence, the power from many power plants is "pooled" in the transmission system, and each distribution system draws from this pool. This networked system helps achieve a high reliability for power delivery because any one power plant that shuts down will only constitute a fraction of the power being delivered by the grid.
One result of power pooling is that the electricity drawn off the grid always comes from a diversity of power sources, which may include coal, nuclear, natural gas, oil, and renewable energy sources such as hydropower, biomass, wind, and solar power. This is often referred to as "system power" because it is the standard power mixture that supplies the transmission system. There are financial and contractual means of tying an individual generating source, such as a wind farm, with an individual user in a meaningful way, but it is still true that the electricity one draws from the grid is always system power.
Read more about the way the U.S. power grid is structured and controlled.
|
