High voltage electricity transport

The benefit of using high voltage is that we can deliver the same amount of power with lower current through the transmission lines. Lower current reduces the losses due the resistance of the lines. This is true whether we use AC or DC at high voltage (and, in fact, high voltage DC transmission is becoming more common)

The benefit of using AC is that, given the technology of the late 19th and early 20th century that was present when our transmission network was developed, it is much easier to convert high voltage AC to medium or low voltage AC for delivery to the end customer. We can do this using transformers. No comparably affordable and reliable technology was available for converting between DC voltages when the power network was designed and deployed.

And if the voltage is higher how can the current be lower?

Notice I said above "to deliver the same amount of power".

A 100 W lightbulb in a 240 V country uses the same amount of power and produces the same amount of light as a 100 W lightbulb in a 120 V country. But in the 240 V country, the lightbulb is designed with higher resistance so that it draws less current than the 100 W lightbulb for use in the 120 V country.

Similarly, if we have 20 residential customers drawing 20 kW of power in aggregate, and we feed them with a 20 kV line (using a transformer to step that down to 240 or 120 V before delivering it to their homes), that line will carry less current than if we feed those customers with a 10 kV line.

The only difference I can see, is the relief on the cables because of the polarity switch, is that why AC is better to transport electricity over long distances?

This is a bit off the main focus of your question, but actually AC isn't better than DC as far as the wire losses are concerned.

First, because the AC signal spends some of its time near 0 V, the peak voltage of the AC waveform must actually be higher to deliver the same power as a given DC voltage. For example, when we say we have a "120 V AC" power source, we mean the AC voltage has a root mean square (rms) voltage of 120 V, as this is able to deliver the same power to a resistive load as a 120 V DC source. But the peak voltage of this AC source is about 170 V. This means the wire must be insulated to prevent arcing at 170 V rather than just 120 V.

Second, because of the skin effect. This means that AC currents tend to mostly flow on the outer surface of a wire, while DC currents can flow through the full cross-section of the wire. The effect is small at the fairly low frequencies we use for power transmission, but it still means that the transmission wires have effectively higher resistance when carrying AC than when carrying DC.

So again, the main reason for choosing AC power transmission is to be able to use transformers to convert between voltages rather than because AC is inherently better.

High voltage in general allows more energy to be sent down a given wire size, as lower current can be used. The lower the current, the less the wire (resistive) losses. That is:

• Power delivered is current * voltage, or W = E*I
• Power lost to heat is resistance time the square of current, or W (loss) = I^2 / R

It's that I^2 term that gets you. Minimizing current is a big win in terms of reducing losses. (I'm not considering reactance here yet. That's another discussion.)

As to why AC is used, it's easier to generate and work than DC at almost every stage, especially when you consider that most of the core technology for power generation and distribution was developed in the late 19th century:

• The source generator, essentially a rotating magnet, makes AC to begin with.
• This AC is stepped up via transformers to high AC voltage and sent down the wire. Transformers are simple and reliable: no moving parts, no electronics.
• The high-voltage transmission line does care about AC vs. DC (DC is better). More about this below.
• The HV network also cares about phase-alignment when power is moved from grid to grid (DC is better). Again, more below.
• Near the consumer side, again transformers step down the AC to a friendlier voltage for local use. Again - simple, no moving parts, no electronics.
• At the consumer, 3-phase AC is ideal for most big motors. Single-phase is easy to step down to a safe voltage, like 240/120V for appliances and lighting.

Now, let's talk about DC. High-voltage DC (HVDC) is a technology that was originally developed in Sweden (by ASEA, now ABB) to solve a problem with undersea cables: dielectric and shield loss. More here: https://mycableengineering.com/knowledge-base/dielectric-loss-in-cables

The Swedes long knew that the constantly-changing electric field in an underwater AC cable resulted in large coupling losses to the surrounding armor material. This coupling becomes heat, that is, loss. So for taking power across the fjord from one island to another, it proved worthwhile to convert to DC prior to sending down the cable, then convert back to AC for use. More here from ABB.

And a bit about the Nazi-hating Swede who brought it about: Uno Lamm.

There's another benefit to using HVDC, be it overhead lines or buried: no skin effect. AC current in a cable produces localized eddy currents in the middle of the cable which oppose current, resulting in the main current being concentrated in the cable outer perimeter. This concentration of current increases the cable resistance, so more energy is lost as heat. More here: https://www.electrical4u.com/skin-effect-in-transmission-lines/

DC current doesn't form eddies to oppose current, and so has almost no skin effect. This means all the cable is being used, allowing more current to be sent down the same size wire at lower losses.

Finally, there's the intertie problem. When moving AC power between grids, their phases and voltages need to be closely matched. This is very difficult for large-scale systems. More about this here: https://www.testandmeasurementtips.com/how-ac-power-sources-get-synchronized-faq/

DC mitigates this issue for interties - no phases to match, and it's easier to adjust the step-up voltage and add it to the network as a new source comes on line. It's used in many large power corridors as an intertie, including this one: the Pacific DC Intertie which takes power from the Bonneville Dam on the Columbia River and ships it to southern California.

Since then the use of HVDC undersea cable has been deployed not only for submarine power cables, but also for tying offshore wind power to onshore stations. (This uses a variant called HDVC Light, more from the Swedes here: https://new.abb.com/systems/hvdc/hvdc-light)

As inverter technology matures and costs come down, the trend for long lines is to migrate to HVDC, while retaining AC for local loops owing to its continued advantage for motors and other large machines.

Even then, the same electronic technology that makes HVDC practical can and does get applied to the consumption side, so we will continue to see more DC in the local side too. This is already happening with data centers, which are beginning to use 48V DC for server rack power. Even induction motors, the machines that really like AC, can move to Inverter / VFD drives for greater efficiency and flexibility, at some expense.

transformers, giant transformers, are cheaper than huge stacks of rectifiers and choppers to convert the HV DC into lower voltage DC.

And the transformers are more robust.