Transmission and Distribution

Jump to: Interconnection; or Reliability

“The humble wall outlet has become a gateway to one of the largest and most complex of man-made objects. The grid in most of North America is just one big electric circuit. It encompasses billions of components, tens of millions of kilometers of transmission line, and thousands of generators with power outputs ranging from less than 100 kW to 1000 MW and beyond.”

 

-- Overbye & Weber, IEEE Spectrum, February 2001 [1]

The US electric system is comprised of an interconnected network of generating plants, transmission lines, and distribution systems – commonly known as the transmission and distribution system [2].  The main components of a transmission and distribution system include the switchyard, transmission lines, a substation, and distribution lines [3].  This system “provides the means by which large amounts of power are delivered from the generating stations where it is produced to other companies or to locations where voltage is reduced, to supply subtransmission systems or substations where it is distributed to consumers” [4]. 

The first major component of the transmission system is the switchyard, which “steps up” the voltage of the electricity from the power plant and directs it to the transmission lines.  The transmission lines provide the path for electricity to be delivered to end-users.  As such, reliable electric service and regional electricity markets are primarily dependent on strong and functioning transmission systems [5].

Transmission in the US is mostly by three phase, 60 Hz (cycles per second) alternating current (AC) at voltages between 115,000 and 765,000 V [4]. Operating the transmission lines at high voltage reduces the losses of electricity from conductor heating and allows power to be shipped economically over long distances [6]. Transmission lines consist of three major components: conductors, structures, and insulators: conductors are the electricity-carrying wires; the structures are the towers that hold the wires; and the insulators are the devices used to hand the wires from the towers [3].  These components together form the system which carries the electricity from the switchyard to substations.

Substations are devices which fulfill a number of functions, including directing power from different generating facilities to the main grid, providing interconnection between various systems, and providing a means by which to protect, control, and meter electricity flows [4].  Substations receive the electricity from the transmission lines, passing the electricity though a bus, and adjusting the voltage to the necessary strength before passing it on to the distribution lines [3].  Thus, transmission lines carry high voltage while distribution lines carry lower voltage. 

The distribution system delivers power along millions of miles of distribution wires to neighborhoods, businesses, and consumers [7].  Distribution wires are typically strung on poles along streets – this distribution method is the least expensive to build and maintain.  In other cases, such as along city streets, the distribution wires are located underground, buried or run though conduits or ducts [3, 4].  Although underground cables are more reliable than overhead lines, they are more expensive to build and maintain and thus only serve select areas [4].

Distribution wires carry electricity that is higher voltage than that which reaches households and businesses.  The electricity must first pass through a distribution transformer to step-down the voltage to suitable levels. 

Interconnection

While the power system in North America is commonly referred to the as “the grid,” there are actually three distinct power grids or “interconnections” [6] (Map).  The Eastern Interconnection includes the eastern two-thirds of the continental United States and Canada from Saskatchewan east to the Maritime Provinces. The Western Interconnection includes the western third of the continental Unites States, the Canadian provinces of Alberta and British Columbia, and a portion of Baja California Norte, Mexico. The third interconnection comprises most of the state of Texas [2, 6].

The interconnection of the national grid allows for the realization of numerous benefits unlikely to be realized in a distributed generation system.  These benefits include the fact that the transmission and distribution system:

1. Interconnects systems and generating plants to reduce overall generating capacity requirements;
2. Minimizes fuel costs in the production of electricity by allowing production from the sources having the lowest production costs;
3. Allows for the use of the lowest cost additional generating units available;
4. Allows for buying and selling of electricity in the marketplace; and,
5. Allows for efficient management of emergencies such as hurricanes, tornadoes, floods, and fuel supply disruptions [4].

Reliability

See full entry on Reliability.

Transmission and distribution lines and capacity are increasing every year.  Between 1992 and 2002, 9,600 miles of transmission were added; between 2002 and 2012, an additional 10,400 miles are expected to be added [8].  Despite this added capacity, a number of challenges face the future of the US transmission and distribution system.  Smaller challenges include effects on the system by the development of distributed generation (including solar, wind, geothermal, wave/tidal, and biomass plants), the development of new nuclear power plants, and the development of hydrogen energy systems [4].  Yet, the greatest challenge for the transmission and distribution system is to maintain and enhance reliability and to reduce the potential causes or occurrences of a major blackout or power failure.  Further challenges include enticing investors to invest in grid enhancements and research and development on technologies – a challenge due to speculation that returns on transmission investment may be too low to be worth investor attention [2]. 

References

1. Overbye, T. and J. Weber. Visualizing the Electric Grid.  2001  [cited 2007 12 April].

2. EEI, Energy Infrastructure: Electricity Transmission Lines. 2002, Edison Electric Institute: Washington, DC.

3. Chambers, A., Power Primer: A Nontechnical Guide from Generation to End Use. 1999, Tulsa, Oklahoma: PennWell Publishing Company.

4. Casazza, J., Electric Power Transmission Systems, in Encyclopedia of Energy Engineering and Technology B.L. Capehart, Editor. 2007, Taylor & Francis/Dekker.

5. EEI. Transmission.  2007  [cited 2007 11 April].

6.  Final Report on the August 14, 2003 Blackout in the United States and Canada: Causes and Recommendations. 2004, U.S.-Canada Power System Outage Task Force.

7. EEI. Distribution.  2007  [cited 2007 11 April].

8. Hirst, E., US Transmission Capacity: Present Status and Future Prospects. 2004, Edison Electric Institute: Washington, DC.