Why do transmission lines carry such high voltages?

Line Construction is a Big Ecological Footprint!

Electricity Generation

Necessarily Power Plants operate at high voltages 10-20 Kilo Volts

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 is a simple device that works by the process of magnetic induction between two coils of wire; a primary coil (P) and a secondary coil (S). Transformers must therefore contain an Iron core for this magnetic induction to work and this is why they are big and heavy. The total voltage reduction is just related to the ratio of the number of "coils" in the Primary to the number of coils in the Secondary. When the number of primary coils exceeds that of the number of secondary coils, the voltages are stepped down. In general this step down factor is around 100 to 1000, which which requires fairly large individual transformers.

This is why a typical electricity substation is large:

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

Locally, BPA is transmission line limited in terms of customer service and New Federal Stimulus money is targetted at some new Transmission:

McNary-John Day Project

• Ordered Feb 2009; Work started in July 2009
• 150 Jobs created
• Completion some time in 2012
• Length = 79 Miles
• Capacity = 500 KV (very high)
• Cost = 246 Million or just about 3 MIllion per mile! (because of needed Sky Crane)

• Designed to host 575 MegaWatts of wind energy

The Western Grid:

Now transmission line costs are 1-1.5 million dollars per mile! this costs must be passed on to the consumer for any regional utility to stay in business. There is now a serious congressional effort to reduce transmission line costs.

But installing large scale transmission lines remains environmentally controversial. Below is an example of a current controversy that would involve the delivery of 500 - 1000 MW of solar power (that's a lot) directly to San Diego.

But conservationists contend there are other ways to boost the region's power supply, and that those alternatives must be explored because of the deep wound ---- as wide as a football field ---- that the line would cut into oak-covered mountains and wildflower-carpeted desert canyons.
More Proposed Projects



5 Billion in Texas



Sunrise Powerlink (Controversy Resolved)