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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Traditionally, AC power has been used for long distance transmission because you can use transformers to step up the voltage. Since the power losses in a wire are proportional to the current (stepping up voltage steps down current), this minimizes the power loss over a long haul.
However, in recent decades, we’ve invented solid state devices capable of stepping up DC power in the same fashion.
There are some significant benefits with this approach.
First, AC power needs to be frequency coupled. If I run my AC power system at 50 Hz and you run yours at 60 Hz, we can’t directly connect them. However, all DC power runs at the same frequency, so we can tie them together easily.
AC power also has what is termed a ‘skin effect’. The constantly fluctuating power only flows on the outer edges of the conductor. In contrast, DC power flows through the entire conductor. This means you need smaller wires for the same power with DC – and when you’re talking laying down metal conductors for most of a continent, that material savings adds up.
AC power also involves inductive and capacitive losses that aren’t an issue with DC power. When you have a time-varying current, you create magnetic fields. Those magnetic fields interact with the surrounding environment and cause power losses over long distances.
That being said, while we’ve invented ways to step up DC voltage, it’s normally very expensive compared to a traditional transformer. So you’re not going to use it to transmit power a mile from the plant to the local distribution station since you need those expensive installations at both ends. When you’re transmitting power a thousand miles and you still only need two such devices vs. a thousand miles of cable? DC is the way to go.
It’s not the AC vs DC itself that’s the reason, but the fact that AC power is very easily converted between different voltages. When it comes to losing power over wires, it’s the current (amps) that actually affect how much power is lost to heat. To mitigate this we can raise the voltage. More volts means less amps for the same amount of power (watts).
Of course, high voltages are dangerous and we don’t want them in our homes. So we need a way to change the voltage fairly easily. *That* is why AC power is used. The way to change voltage with AC is as simple as coiling some wires.
Times changed and tech advanced
AC is easy to change between voltages. Take a hunk of iron, coil two wires around it. Congratulations you now have a transformer! This was feasible in the early 1900s
Converting DC voltages to other DC voltages requires either a DC motor feeding a DC generator, or *semiconductors*
The motors and generators would be insanely large and when deciding on AC or DC they didn’t yet know about semiconductors so they opted for AC so they could convert voltages
Modern times let us use large semiconductor DC-DC converters and inverters at the end to get the AC our devices want
AC has losses on transmission lines due to real wires having capacitance and inductance which while being apparent power, they suck additional real current from the generators which gets impacted by the wire resistance. The skin effect also comes into play on really big power lines, AC current only travels down to a certain depth so really high current connections have multiple (3/4/5) wires in a shape because a single super thick wire wouldn’t carry any current in the middle. DC doesn’t have this problem, you’re only worried about resistance, no other weird effects
It depends on the situation. DC doesn’t experience as much voltage drop over very long distances, but it’s more expensive to transmit.
AC is very cheap and easy to transmit over moderately long distances, but unlike DC, it does experience significant voltage drop along the way. Power companies boost the voltage at various points to compensate for this, so despite the tradeoff, AC still ends up being far more cost effective for power grids.
However, if one needs to transmit power over very long distances (over 1,000km), DC actually overtakes AC in terms of economics and efficiency. In those applications, DC is the winner.
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.
The most important thing for the efficiency of a transmission line is the voltage, because most of the losses are proportional to current, and higher voltage means you can transmit the same amount of power with less current.
AC is easier to step up to high voltage, but it has higher loss once it’s on the wire. DC only really cares about resistance, where AC is impacted by inductance and capacitance as well. AC also uses the conductors less efficiently due to skin effect, where DC uses more of the wire’s cross-section.
Basically, if you use AC, you’ve basically built a very low frequency radio transmitter, and that energy has to come from somewhere.
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.
Transmitting electricity at higher voltages is desirable, because it reduces losses due to electrical resistance. AC can have voltage converted with very efficient, very reliable and old technology (transformers).
AC has some disadvantages though.
AC connections have to be synchronised, and have to be kept synchronised otherwise any power lines connecting them will disconnect or be damaged. This requires a lot of planning and organisation and lots of powerlines. If you just need 1 or 2 power lines going from 1 grid region to another, DC has the advantage that it doesn’t have to be synchronised.
AC voltage is constantly changing – it goes up, peaks, goes down again, reverses, increases up to a reverse peak, goes back down again, reverses, etc. So, on average, the voltage is less than the peak. The same is true of the current. However, the wiring, transformers, insulators, etc. all have to be designed to be able to cope with the peak voltage. Because DC always operates at the peak, the resistive losses for the same maximum voltage are reduced by 50%.
AC power lines suffer from inductive and capacitative losses. These require expensive compensation equipment. Underground/underwater cables experience extreme capacitative losses – these losses increase as voltage increases. At 500 kV, even a 10 mile underground cable requires extreme amounts of compensation – which makes it just about viable under a city where you can tap off and install compensators every 2 or 3 miles. If you want a 20 mile underwater cable – that’s just not happening, unless you drop the voltage a lot (which drastically increases resistive losses).
The problem with DC is that the conversion to DC is incredibly expensive and also suffers some losses. However, if you have a very long line (e.g. 1000 miles) then the cost and losses of the converter stations is outweighed by the cost and efficiency savings of the line itself. If you have an underwater connection, then the breakeven point is much shorter (10-20 miles). If you need to connect to unsynchronised power grids, then a DC connection is a practical choice. You just build two converter stations in the same building and connect them with a 6 foot DC power line .
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