Why is the total of inductive losses, capacitive losses and resistive losses higher in AC cables underwater compared to DC cables?
Thanks so much!
In: Physics
Capacitive and inductive effects happen when voltage changes.
AC flips the polarity of the conductors multiple time per second, 50 or 60 depending on where you live. DC will only change voltage when you start energy transfer. So there is no capacitive or inductive losses for DC.
Look at a schematic of a common model for a transmission line [https://www.researchgate.net/profile/M-Popov-2/publication/297013850/figure/fig1/AS:337313000378370@1457433094828/Schematic-representation-of-a-transmission-line.png](https://www.researchgate.net/profile/M-Popov-2/publication/297013850/figure/fig1/AS:337313000378370@1457433094828/Schematic-representation-of-a-transmission-line.png) The capacitive effect is like if there is capacitors connected to the ground and the inductive effect are like inductors along the wire. There are also resistors around the wire because the wire will have some resistance even if it is low.
The change the voltage on the other end all those capacitors need to be changed up to the voltage, which requires a current that passes trough the resistors and that means energy is lost and heat is produce. For inductive effect, the magnetic field in the inductor needs to be created. Then the voltage will go down to zero and the to a negative value. The same thing happens but with elections removed from capacitors and magnetic fields in the other direction.
The change and discharge happen 100 or 120 times per second because the voltage goes from 0+, 0,-,0 each cycle, both + and – require a change in capacitor charge and magnetic field.
The energy loss in wire resistance because you need a current to change all this is wasted energy.
The wire capacitance depends on what is around the conductor. A wire up in the air is quite far away from the ground that can hold a charge so the capacitive effect is quite low. Undersea cables have water especially salt water that can hold change so the capacitive effect gets quite large.
Add to this a smaller skin effect where AC what to travel on the surface of a wire and DC uses the whole wire. So wire resistance is lower for DC. The effect depends on the frequency at 50 hz is at a depth of 9 mm the current is only 37% of the value at the surface. So thick AC wires need to be thicker the a DC wire for the same resistance. So you need to pay more for equal wire resistance.
The reason DC is not used in the power grid more is voltage conversion is a lot harder. DC at the same voltage has lower losses the AC on land too. The difference is less underwater because of water vs air around the wire. You need a very long point-to-point connection on land at high power level for DC to be an economic advantage. It is a tradeoff between more energy loss in the system versus a more expensive system to build.
DC current doesn’t have inductive or capacitive losses. Both of these losses come from changing the voltage of the line, something that DC by definition does not do. DC is one fixed voltage on the wire at all times, so no energy is spent raising and lowering that voltage over and over like in AC.
AC turns the flow of electricity around and again and again … the magnetic field induced around the cable needs to dissolve and turn around with that (the round magnetic lines need to go the other way around when the flow turns). A changing magnetic field however induces an electric current in the cable, which is always counter to the current we want to have in the new state. That is one source. In undersea or overland cables with moist weather around, the very slightly magnetic water molecules around the cable are also turned according to the mag fields and the change in it. This obviously needs energy which is siphoned from the mag field change i.e. your current again.
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