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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For more advanced and physics based answers, there are plenty of good ones in the thread. I recommend reading them, I’ll only summarise the history and basics of it.
Nothing has changed in the physics of it all. We have just developed better methods for taking the current from the transmission lines.
In the past we had to basically run a DC motor to spin an AC generator. A setup invented by René “King of DC” Thury in late 1800’s. There obviously are some problems here, turning DC in to mechanical power then to AC.
However in the 1970’s with mercury arc valves allowed turning DC in to AC much more efficient, then soon after in late 70’s heavy duty thyristors made this even easier. Basically what they did was act as switches that synchronised with the DC grid, only feeding power to the correct phase at the correct time. Basically what you use is an inverter, it can turn DC to AC by manipulating the current by switching it on and off.
Basically you can imagine that you have a barrel with a hose on it. You want to ensure that the pressure at the end of the hose remains stable. So you fill the barrel with water from buckets at the correct times so the water level in the barrel doesn’t get too high or too low because this would affect the pressure of the hose.
Ok lets move forward from the strange times that was the 70’s, and move on to the time the soviet union fell and go to Sweden where the fish smells rancid and goats made of straw burn. IN the 90’s thanks to the developments in higher insulated solid state components, namely transistors, thyristors and diodes. That were before only used in motor drives. You would able to make simple and efficient inverter systems on the scale of a electrical grid. This was tested in 1997 in Sweden in a project by ABB, after the success of which the conquest of HVDC around the world started.
DC has always been the king of high voltage transmission, problem is that tapping in to the power was quite difficult. However, now you can just basically tap in parallel to it at any point and get the correct voltage. While with AC you have to have a huge electrical station with transformers to step down the voltage to desired level. There are some other benefits to this such as how the conductors behave, capacitance and inductive losses which are explained well on other posts here.
Basically you can have a big water tower that is supplying lots of water, and you can branch it off as much as you want to houses around the tower. Each house having their own valve that ensures that only certain amount of water pressure gets taken from the grid, and it doesn’t matter from what part of the grid it is taken from.
But why is HVDC so popular for transmission between major grids? Well for the simple reason I have explained few times. These big grids can be in different frequencies or timings of the phases, and synchronising them would be impossible. However with DC transmission you don’t have to. Whenever the phase is in the state it can be fed power in to, you turn the valve on your DC feed to the correct direction and open the circuit so current flows. You can also exactly adjust how much power you give to the system.
It really isn’t any different than what we do with solar or windpower, they make DC and we transform that in to AC and feed it to the grid. Doesn’t matter how fast the turbine spins or how much or little sun light there is, we can always feed it thanks to this system.
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