Energy 101: 02 - The Grid

No doubt, this is one of the most overlooked elements of the entire electric system by both the general public and utility providers alike despite the fact that the power lines are usually the most visible aspect of the power industry.  Most people would consider the grid as being nothing more complex than running wire from a power plant to the various customers and maybe this is partially why it is almost ignored except when the power goes out.  As for the utilities, the transmission lines aren't actually generating revenue like a power plant would.  So it can be easy to see how a utility company would prefer to maintain, and invest in power generation instead of upgrading power lines and other grid elements. 

Above is the substation at the natural gas plant in Morro Bay CA.  Just Past the substation, you can see the the large transmission line towers, and more towers carrying the transmission lines off over the hills.

So what is "the Grid"? 

The power grid is the power lines and associated equipment that forms an interconnected network of suppliers and customers.  This definition does not accurately portray the complexity of the system nor does it express how improving the grid can reduce energy consumption, improve electricity quality and reliability.  One complicating aspect of the electri grid is that electricity cannot be stored in sufficient quantities to meet demand, which means that electricity must be produced at the same rate that is it consumed.    This means that utilities must be accurately predicting demand, and have some excess capacity available for sudden spikes in demand. 

How it works:

Power is generated at a supplier such as a coal plant, a wind turbine, or such. To most efficiently transport that electricity, voltage is increased or “stepped up.”  For instance, the voltage coming out of the steam generator at a coal power plant may be 33,000 volts (abbreviated as 33 KV, where kV = kilo-Volts), and a transformer in an on site sub station will bump up that voltage to 500,000 - 750,000 volts (abbreviated 750 kV). Transmission lines carry this electricity over long distances to various customers.  Some industrial customers may need high voltage, so they will have their own substation that will be tapped directly into the transmission lines and this substation will reduce the voltage to whatever the customer requires. This reduction in voltage is called “stepping down.”

Different cities, communities and residences may have different electrical needs, so how the power gets from the transmission lines to a house may not necessarily follow these steps in every instance, but it’s a good approximation of the process.  High voltage transmission lines will feed a step down substation that will take the 500 – 750 kV and reduce the voltage to 69 kV.  A distribution substation, which is a substation close to a residential neighborhood, will take the 69 KV and step that down to 2,400-19,000 volts that can service a comerical building or small industrial customers.  However, this is still way to high of a voltage for a residential home, so somewhere near the house, usually on a telephone pole, there is one more transformer that is usually described as a garbage can that steps the power down to 110/240V used in the home. 


Different Elements of the Grid: 

Transmission lines:  As mentioned earlier, are high voltage power lines that are transmitting electricity over long distances.  The farther the electricity is traveling the higher the voltage is used to prevent loss, typically 500kV to 750kV.  In 2002, there were 157,810 miles of transmission lines stitched across the US.  Some of you may have noticed that transmission lines have three or six wires.  This is because the electricity is generated in three phases (which is explained in the fundamentals of electricity article), and each wire is carrying a different phase.

Distribution Lines:  These are lower voltage lines that are feeding customers.  These are the ones you would see in the neighborhood around your house and would be the ones that are 2400 volts - 19000 volts.  An example of a distribution power line can be seen to the right.  The metal cylinder on this power pole is a transformer that will step the power down to what the customer requires. 


Substation:  A substation can fill many roles.  Typically its main function is to step voltage up or down and to protect the grid from power failures by isolating sections with the uses of switches.  In 2004, there were 10,287 transmission substations and 2179 distribution stations within the North American power grid.  


Transmission Loss

When electrons flow through a conductor (aka a power line or wire), the metal has some resistance to the flow of these electrons.  Like friction in a mechanical device, this resistance converts energy, in this case electrical energy, into heat.  The more electrons that flow the more that are lost when passed through the conductor.  There are several ways that this loss can be mitigated.  One way is to reduce the amount of current (current = electrons) that is transmitted across the power line by reduce the current and increasing the voltage.  That is why long distance power lines are also very high voltage, since the higher the voltage the less current needs to travel.  Other possible solutions, which aren't as economically feasible, would be to increase the size of the power lines, use new materials that are more conductive, and/or to use more power lines. 





The NPR website has a great map of the North American grid here.



Reference Links:
http://www.npr.org/templates/story/story.php?storyId=110997398

http://sites.energetics.com/gridworks/pdfs/factsheet.pdf
http://arxiv.org/PS_cache/cond-mat/pdf/0401/0401084v1.pdf
http://sites.energetics.com/gridworks/index.html
http://tonto.eia.doe.gov/energyexplained/?featureclicked=2&