For some reason HVDC (specifically HVDC, not just DC, most DC devices use low voltages, far below the voltages that would be efficient for transmission), is apparently more power efficient than AC. I’m not sure why, though.
> there is no need to support three phases and there is no skin effect. AC systems use a higher peak voltage for the same power, increasing insulator costs
> This is because direct current transfers only active power and thus causes lower losses than alternating current, which transfers both active and reactive power.
> Nevertheless, for a long AC overhead transmission line, the current flowing just to charge the line capacitance can be significant, and this reduces the capability of the line to carry useful current to the load at the remote end. Another factor that reduces the useful current-carrying ability of AC lines is the skin effect, which causes a nonuniform distribution of current over the cross-sectional area of the conductor. Transmission line conductors operating with direct current suffer from neither constraint. Therefore, for the same conductor losses (or heating effect), a given conductor can carry more power to the load when operating with HVDC than AC
Basically, while heat losses are the same, the AC system requires extra power to constantly be switching the electron flow whereas in a DC system there’s only active power to move the electrons and the only loss is heat loss. Additionally, for a given conductor, HVDC can be transferring at the peak rated voltage for the wire whereas AC can only transfer that voltage at the peak which means it’s 71% less power (although that’s more a cost savings thing).
In a DC circuit, a capacitor looks like an open circuit because after it fills up it just sits still.
In an AC circuit, a capacitor looks like a resistor, it takes work to fill and empty it every cycle.
Transmission lines over the earth behave not just like wires, but also like capacitors. Higher voltage reduces resistive losses, but in AC they’re penalized by these “fill and empty a capacitor” ones, that DC doesn’t.
At certain power levels, reactive effects take over. Takes more energy to charge up the transmission lines 50 or 60 times a second, than it actually can transmit. Also, no skin effect.
For some reason HVDC (specifically HVDC, not just DC, most DC devices use low voltages, far below the voltages that would be efficient for transmission), is apparently more power efficient than AC. I’m not sure why, though.