The electrical grid is the system of power transmission and distribution components that delivers electrical power from its source, i.e., an electrical power generating station, to the end user, i.e., a home or business. The grid has evolved over time from a simple system that connected a single power source to a small number of users to the large and complex system of today that interconnects thousands of power sources to millions of users by more than 450,000 miles of high voltage transmission lines. In the continental United States there are three primary, and essentially independent, grids: 1) Eastern grid that covers primarily east of the Rocky Mountains, 2) Western grid that covers west of the Rockies, and 3) Texas grid that primarily serves the state of Texas. Although these grids are independent of one another, there are interconnection points to allow emergency sharing of power. In addition, there are connections to Canada and Mexico.
A grid consists of the following primary components:
- Power Plant: Power can be generated from a wide variety of sources, but ultimately the electricity output from the plant is three-phase alternating current (AC) at a frequency of 60 Hz (Herz, or cycles per second). Alternating current means that the voltage of the power is oscillating over time from a negative peak to a positive peak in the form of a sine wave at the current frequency. This is different than direct current (DC) such as is produced by a battery which consists of electrons all moving in one direction down the wire. AC power is more efficient for transmission over long distances and was selected for the country's grid in the very early days of electricity. Three-phase power refers to the fact that the generator produces three separate streams of oscillating electrons at the same frequency, but whose phases are offset by 120 degrees. The use of three synchronized but offset phases results in a very constant peak voltage when you add the three together.
- Transmission Substation: The three-phase power leaves the generator and enters a transmission substation located at the power plant. This substation uses large transformers to convert the generator's voltage (thousands of volts) up to extremely high voltages for long-distance transmission (hundreds of thousands of volts).
- Distribution Grid: Power plants are generally located long distances from power users, either because of the remote nature of the power source, e.g., hydropower from a dam in the mountains or solar power from a plant in the desert, or to provide separation from urban areas, e.g., a coal plant that emits noise or particulates to the atmosphere. Produced power is transmitted via high voltage power lines, often over hundreds of miles.
- Distribution Substation: For power to be useful in a home or business, another substation will "step-down" the voltage to distribution level (typically less than 10,000 volts). Power will also be split into multiple directions to serve multiple users and/or geographic areas.
- Local power transmission lines: From the substation power can be distributed to the home or business either by underground wire in conduit or above ground wire on power poles.
- Local transformer: At each end use location a transformer reduces the voltage down to the 240 volts that makes up normal electrical service.
In order to minimize power outages to the end users, the grids have multiple layers of redundancy. For example, if a power plant goes offline, either for maintenance, an accident, or a natural disaster, power from another plant on the grid can be rerouted via redundant transmission lines. In general, the US power distribution system is very reliable; problems anywhere within the distribution system can be addressed very quickly, if not immediately. However, the grids are patchwork systems that have evolved and developed sporadically over time to address the most pressing needs of the moment. The system does not have much redundancy from an overall capacity standpoint, for example, if several major power plants were to go down simultaneously and without warning. Similarly, there are choke points in the transmission system that would case major problems if they were to be damaged. Such system vulnerabilities have been the subject of much debate in light of the threat of terrorism.
One of the most fascinating and challenging aspects of power generation and distribution is that all electricity that is produced must either be used or stored. The electrons don't just disappear if they're not needed. What this means is that if too much electricity is generated and pushed into the grid, something must happen to compensate, which is an increase in frequency. Similarly, if too little electricity is generated for the amount needed, frequency will decrease. Too great a change either way will damage sensitive electrical equipment. To avoid such problems the grid frequency is managed continuously. This involves close communication between the grid operator, the utilities, and the plant operators. Automated system controls allow correction of frequency within a fairly narrow range; larger changes in frequency require active management. For example, natural gas plants, which can increase or decrease power generation relatively quickly, are kept in reserve in case of spikes in demand or unexpected problems with the grid. As renewables such as wind and solar, which depend on weather and are much more variable and less predictable than traditional power sources, become a larger component of total power generation, the challenge of managing that fine balance between supply and demand will increase significantly.