# What frequency do long-distance undersea powerlines run at?

I recently saw this photo of the project "NATO-L", or North Atlantic Transmission One-Link. It's an under-sea powerline planned to connect Europe with North America.

Does anyone know what frequency such cables run at? I was thinking it looks like a giant coax cable, and so it must have characteristic impedance and other related parameters. I seem to recall the big over-land powerlines run at 400Hz. I searched google about this cable, but I could not find any information about the frequency.

EDIT: Thank you for the answers about the DC and 400hz. It seems I was incorrect. I do recall seeing something related to powerlines many years ago, and there was definitely something about 400hz, since I've had that number stuck in my head since then. But it seems I misunderstood.

• A lot of undersea power cables are high-voltage DC. This one probably isn't, though, as it has three conductors. Commented Jun 29 at 13:16
• Big over-land power cables usually run at whatever the local AC mains frequency is, 50 or 60 Hz. Higher frequency -> higher loss. DC is best, but historically DC to AC inversion was expensive and inefficient. Commented Jun 29 at 13:30
• That photo is a stock image of an undersea cable. That cable is AC, but it's also probably 10 or 20 years old. A cable crossing the north Atlantic would be DC. Commented Jun 29 at 13:46
• ”I seem to recall the big over-land powerlines run at 400Hz” They don’t as it would require inverters in both ends to be usable for 50/60 Hz grids (entire world). Also, the cable in your image is a three phase AC submarine cable, suitable for <100 km distance, depending on voltage level. Commented Jun 29 at 13:56
• So the photo is not related to the project "NATO-L" and should be deleted.
– Uwe
Commented Jun 29 at 21:01

The NATO-L page contains:

NATO-L will connect North America to Western Europe via Eastern Canada and UK & Ireland through a 6GW HVDC set of subsea cables.

I.e. high-voltage DC.

• Building a long distance (3300 km) HVDC overhead line above the ground is much easier and cheaper than an undersea cable (3500 km).
– Uwe
Commented Jun 29 at 16:21
• @Uwe not to mention easier to maintain. But how to get transatlantic with an overhead cable? Commented Jun 30 at 7:00

It's DC.

Any AC cable would have significant losses from skin effect.

Since the power grids at each end won't be synchronized, inverters will be required anyway - it's more straightforward to use DC for the link.

• The loss of long AC cables is not caused by only the skin effect. The major problem is the high capacitance of underground or underwater cables causing additional current to charge and discharge the cable capacitance. For very long cables its entire current carrying capability would be needed by the charging current alone. There is no current left to transport power to the other end of the cable. The capacitance of overhead power lines is much smaller. Of cause the skin effect is valid for both overhead power lines in the air and cables below the surface of the soil or the water.
– Uwe
Commented Jun 29 at 19:37

400 Hz is used within aircrafts and spacecrafts to save weight. The weight of a transformer is much smaller when designed for 400 Hz instead of 50 or 60 Hz. The transmission loss of a (long) cable used with 400 Hz is much higher than used with 50 or 60 Hz.

But for very long distance underwater cables it does not make sense to use 400 Hz instead of 50 Hz. The optimal frequency for such a cable is as low as possible, therefore there are a lot of those cables designed for HVDC (high voltage direct current with "0 Hz").

To exchange power between Europe and North America AC current is not useful anyway because the grids frequencies are different and not synchronized. Conversion from AC to DC and back to AC of another frequency could not be avoided.

Overhead power lines run at 50 Hz in Europe and 60 Hz in the USA. 400 Hz has too much loss and is restricted to planes and ships.

• There are single phase overhead power lines run at only 16.7 Hz in Europe used to power the electrified rail networks. If you see pylons with 2 or 4 conductors it is 16,7 Hz. Pylons with 3, 6 or 9 conductors are used for 50 Hz. But Japan has two separate power grids with 50 and 60 Hz. Transfer of power between both grids is possible only using conversion to DC.
– Uwe
Commented Jun 29 at 21:25
• 16 2/3 Hz, to be precise Commented Jun 30 at 6:58
• @MrGerber To be ultra precise, it once was 16 2/3 Hz but it was changed to 16.7 Hz in October 16th 1995 Read en.wikipedia.org/wiki/Railway_electrification - Low-frequency alternating current.
– Uwe
Commented Jun 30 at 8:28
• I stand corrected! Thanks, @Uwe Commented Jun 30 at 11:21

400 Hz is used for AC power distribution in US military aircraft and ships. For example, see MIL-STD-704.