Transmission
From Thermal-FluidsPedia
After electricity is generated, it must be sent through transmission wires. The transport of electricity across an electrical network is fundamentally different from transport of a good along a transportation network. In transportation networks, cargo moves from one location to another along specific routes, usually the most direct route. Any interruption along a particular cargo route will affect only that route and possibly a few routes in its vicinity. Unlike cargo, electrical signals travel along the paths of least resistance, not necessarily along the shortest line connecting the two geographical locations; sometimes many thousands of miles are covered before a signal reaches its destination. An interruption along an electrical transmission line can affect power delivery at a point that is a long distance away from the original failure. In addition, as different generating stations go on and off the grid, either by choice or due to an equipment malfunction, loads continuously change, possibly affecting the stability of the system and causing fluctuations in the frequencies and voltages at various points on the grid. Because wires link everything, all generators must spin at exactly the same rate and in complete sync with each other.
To prevent the overload of power lines, a reliable control strategy and power conditioning system must keep current in each line within safe limits. Also, it must allow for additional capacity to absorb extra flow in case of a sudden failure somewhere else on the network. A generator located in the wrong place will not be able to meet an increase in demand because to do so would push the flow on some lines over their safe limits (1).
For the efficient transmission of power, electricity must be in the form of an alternating current at very high voltages. Early power stations generated and sold DC electricity to their customers. The major problem with DC power is its high transmission loss, which limits its application to short distances. As time and technology progressed, power stations switched to AC power, which could be transmitted long distances with little loss of power. Today, almost all electric power generated in the United States is in AC form (a).
To minimize energy losses through the network, current must be reduced as much as possible. Resistance losses depend only on the type of material and the current passing through it, but not on the voltage. As we saw, for the same power, voltage and current are inversely related; therefore, to lower the electric current we must raise the voltage. This is commonly done using a step up transformer which raises voltages to 300 - 500 kV before power is transmitted through the grid. At the city of destination, voltages are dropped back to convenient levels (around 12-30 kV) using a reduction transformer and further reduced to 120V (220V in Europe) before being distributed among various end users.
Superconductors may also play a role in reducing transmission losses. Superconductors are materials that lose their electrical resistances at temperatures below a certain limit. Most materials exhibit this behavior at temperatures very close to absolute zero, although some alloys of yttrium, barium, and copper become super conductors at much higher temperatures. If room temperature superconductor materials are found, much of the losses due to long-distance transmission would be eliminated.
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References
(1) Blumstein, C., Friedman, L. S., and Green, R. J., “The History of Electricity Restructuring in California,” Center for the Study of Energy Market (CSEM), UCEI, Berkeley, CA. 2002.
(2) Toossi Reza, "Energy and the Environment:Sources, technologies, and impacts", Verve Publishers, 2005
Additional Comments
(a) The conversion from DC to AC was not a simple process. Until the middle 1880s, Thomas Edison’s power company supplying DC power dominated the utility market over his arch competitor George Westinghouse. Westinghouse used the newer and superior AC technology invented by Nikola Tesla which could be transmitted across long distances with very little losses. Rather than challenging him on technical merits, Edison started a smear campaign against Westinghouse using the argument that AC is dangerous and that its use should be avoided at all costs. He even set up stations using Westinghouse’s AC generator, burned and killed animals in public, and later lobbied the New York legislature to use AC power in designing the electric chair. Edison’s plan to bring about the demise of Westinghouse was eventually unfolded and AC technology was adopted in power transmission.
Further Reading
Bureau of Naval Personnel, Basic Electricity, Dover Publishing Company.
The Environmental Effects of Electricity Generation, IEEE, 1995.
The Electricity Journal, Direct Science Elsevier Publishing Company, This journal addresses issues related to generating power from natural gas-fired cogeneration and renewable energy plants (wind power, biomass, hydro and solar).
International Journal of Electrical Power and Energy Systems, Direct Science Elsevier Publishing Company.
Home Power Magazine (http://www.homepower.com).
External Links
Federal Energy Regulatory Commission (http://www.ferc.gov).
Energy Information Agency, Department of Energy (http://www.eia.doe.gov/fuelelectric.htm).
California Energy Commission (http://www.energy.ca.gov/electricity).
National Council on Electricity Policy (http://www.ncouncil.org).
Southern California Edison (http://www.sce.com).
Pacific Gas and Electric (http://www.pge.com).