I was watching a YT video recently where they said something along the lines of “AC current is too inefficient for long distance power transmission, so xyz project needs DC power” and then moved on. I (from my vague memories 20 years ago of school) thought that AC had less loss and inefficiencies. What am I missing/what did I tune out in school?
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AC is not more efficient for long-distance power distribution from just an energy transmission point of view. For the same voltage and wire DC are more efficient. This is called the skin effect result in high wire resistance for AC. There is also no capacitive and inductive effect with the environment that requires current flow in the wire that results in resistive losses, this is especially large for an undersea cable crossing saltwater.
The Wikipedia article has the following line https://en.wikipedia.org/wiki/High-voltage_direct_current
>Depending on voltage level and construction details, HVDC transmission losses are quoted at 3.5% per 1,000 km, about 50% less than AC (6.7%) lines at the same voltage
The problem with DC is the same voltage part. Voltage conversion of AC is simpler with just transformer. DC requires a lot more complex and expensive electronics.
For DC you need fewer wires as there is no three-phase requirement. You can also use a single wire and the earth has the current return, which results in even fewer wire costs.
So DC is more efficient if you just look at energy transmission. But endpoint equipment cost more, wires cost less.
The result is the longer the point-to-point connection is the less is the cost difference between DC and AC and the advantage from lower losses is higher the longer the connection is.
If it is not just a point to point but multiple points you need to connect to the wire then you need to have expensive equipment at each connection point
The long the connection is the larger is the transmission advantage for high voltage DC.
If you build powerlines you will look at transmission losses as a financial cost. So the design is a tradeoff between construction cost vs operation cost from power losses. You also need to add the maintenance cost of the system but I do not know if AC or DC has the advantage there.
When it is cost-efficient changers over time because the conversion electronics has in cost over time
High voltage DC is quite common in Europe for undersea cables because the advantages for them are the highest.
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