When electricity was invented more than 100 years ago, there was a fierce debate,
whether to use alternating current (AC) or direct current (DC).
Edison favoured DC current.
Tesla promoted AC current.
Ultimately, AC current prevailed for technical and economic reasons.
Now DC is making a come back.
The main reason electricity is now produced from renewal technology,
like solar power and wind power.
Several companies are planning to build high voltage DC lines.
Technology for power transmission advanced in the 1970's.
DC lines are now viable option for distances like 500 to 800km.
For this kind of distances, DC transmission is superior to AC.
After a certain distance, AC systems become more expensive to build,
then DC, and have larger power losses along the line,
due to issues such as higher resistance.
In such cases DC becomes the preferred option,
to transmit from big remote solar and wind farms.