Do we still use DC power lines after Thomas Edison lost the war of the currents?

Thomas Edison and his DC (Direct Current) powered grid completely lost out to Nikolas Teslas AC (Alternating Current) power… or did it.  Tesla’s AC power won hands down because it is safer and cheaper to deliver. But…. new solid-state technology, not available to Edison, is making DC a better choice in cases of long distance or underwater high voltage transmission. Grid planners are taking another look at HVDC (High Voltage Direct Current) lines as we push for greater energy efficiency, join energy markets like EIM, include more renewables and have advances in technology. Is Edison getting his revenge?

Breaking it down

The cost of transmission in either system depends on 4 main factors. Who is the winner, AC or DC?

1)    Cost of transformers:                   Winner – AC

• AC’s main advantage is its ability to change its voltage with the use of cheap iron core transformers. 
• DC uses expensive rectifiers to change its voltage. Modern systems use solid-state semiconductor technologies 1 that Edison didn’t have available. 

2)    Cost of cable and towers:            Winner – DC

• DC lines can carry at least twice as much power as an AC line of the same voltage and require less insulation or clearance.
• DC towers are smaller - AC transmissions towers are 1.5 larger
• DC only requires 2 cables compared to an equivalent double AC 3-phase circuit with 6 conductors needed.

3)    Cost of land:                                   Winner – DC

• The right of way for an AC line is 70% wider than that of a DC line with equivalent capacity 2

4)    Cost of line losses:                        Winner – DC

• DC cables can be thicker and thus more efficient 3 since they experience no skin effect 4
• DC lines generally have a loss rate of around 3%, as compared to the 6% rate for AC lines 5

Summary

There is a breakeven point of distance where DC is the better choice to use over AC.

Facts

• The first High Voltage DC link was in Sweden in 1954.
• The connection points between the Eastern, Western, Texas and Quebec interconnections use DC Converter links because they are not synchronized together.
• DC is particularly suited to undersea transmission where the losses from AC are large. 
• Japan has two different frequencies and uses DC Converters between them.

Bonus

Consider that wind and solar produce DC current which is then converted to AC current. Its voltage is stepped up and down as it travels through the grid to your laptop where it is rectified back to DC power. Each time it is converted between AC and DC it loses between 5-20 percent 6 of its energy as heat. This is why your laptops brick warms up when it is being charged. Likewise, the rise of data centers is illustrating this inefficiency.  Variable speed wind turbine generators produce AC which is converted to DC to regulate the voltage then back to AC for the grid. This AC power is delivered to the data center, converted to DC to charge the backup batteries, and then outputted as AC which is fed into each server which converts it back to DC. That means that from the time that our wind farms produce the electricity and deliver it to the data center server it has gone through the following conversations: AC to DC to AC to DC to AC to DC…. I can’t make this stuff up! This energy loss at each conversion is why data centers get so hot and why new DC lines might be the answer.