Why do transmission lines carry such high voltages?

Line Construction is a Big Ecological Footprint!

Consider the following hypothetical (and non-physical) situation:

• Electricity is generated at the generating plant at 120 Volts and then delivered to the households over conductors.

• There are 10 households and each needs 1000 Watts (for their toasters)

• The electric company must therefore supply 10x1000 = 10,000 Watts.

• Power = P = I x V I = P/V I = 10000/120 = 83 amps would therefore flow as current through this 120 V distribution system.
  
  
  
  
• Ohms Law: V = I x R; but V also can be expressed as P/I therefore P/I = I x R or P =I²R

• So electrical power which is transmitted is dissipated as heat according to P = I²R (this is how your stove element works)

• Lets set R= 1: We now have heat dissipation = (83.3)*(83.3)(1) = 6944 watts. Heat dissipation is energy lost by the system. This loss is unavoidable!

  
  

• To deliver the 10,000 watts that the consumer needs requires that we generate 16,944 watts and hence have an overall efficiency of 10,000/16,944 = 59% which the consumer would pay for

How to solve the loss problem:

Current = Power/Voltage; If we increase V by a factor of 10, then I lowers by a factor of 10 (at constant power) and the power dissipated as heat lowers by a factor of 10².

Hence if we increase 120 Volts to 1200 Volts we have only 69.4 watts of energy loss and a 99% energy efficient delivery system This is why high voltage (typically 115-230 thousand Volts or kiloVolts) transmission lines are required to delivery electricity from central generating sources (e.g. a hydroelectric dam) to consumers/grids hundreds of miles away.

But you don't want 115 KV coming into your house:

To step down the Voltage Use a Transformer

	A transformer uses alternating current in one coil to induce alternating current in another. The induced voltage is given by: Vout = Vin x N2/N1 where N1 = Number of coils in the Primary and N2= Number of coils in the secondary. When N2 is less than N1, we reduce Vout. This is why there are transformers on power lines to step the voltage down to 120 Volts by the time it reaches your house.

Energy conservation tells us that Power In = Power Out

so

V_out x I_out = V_in x I_in

Since V_in is very high, I_in is low and (to prevent transmission loss); when V_in is stepped down to produce V_out (what you get at your house), I_out increases so you can run your stuff.

This is why a typical electricity substation is large:

The substation in my neighborhood.

Distrubution of Power Plants in the US
All have to be grid connected

Google Earth Emissions Tool

The economics of power transmission why the move to 765 KV has serious cost reductions and reduced losses.

So what's the problem.

Higher voltage means stronger localized electromagentic fields (EMF) which would adversely affect local biology - if there is any. Currently the most at risk population would be bats.

Note, 765 KV lines do exist already in the world and in some parts of the US.

The proposal is to build the equivalent of the interstate highway system with KV transmission because a) its cost effective and b) its needed to export wind and solar based electricity from remote areas.