5 ms·

The highest capacity sub sea transmission cables are DC now I think. There is an interconnect between Ireland and France that is DC if memory serves. I think i

by roomey 5y ago

The highest capacity sub sea transmission cables are DC now I think. There is an interconnect between Ireland and France that is DC if memory serves. I think it's only efficient for the largest capacity lines tho

7952 5y ago
AC cables are just more expensive per km, I think due to skin effects.
- Johnythree 5y ago
  That and the enormous capacitance between a long cable and the sea.
XorNot 5y ago
DC is also used because you avoid grid synchronization issues, and this is generally the trend for long distance connects now in general. It means the grids can have frequency divergence without affecting the transmission line itself.
- Lascaille 5y ago
  The issue with DC transmission in subsea cables is that the XLPE insulation can become polarized and - from what I remember - have water drawn into it due to the consistency of the field.
  - spookthesunset 5y ago
    You could probably flip the current at periodic intervals to deal with this.
    - Lascaille 5y ago
      You're talking about flipping the polarity, going occasionally from conductor-positive to conductor-negative (the shield must remain at ground potential.) If I remember correctly that actually makes things much worse if you've operated at one potential for a considerable amount of time. You could operate the cable at AC ~ 0.1Hz but your ability to actually transmit MWH would suck.
- pfdietz 5y ago
  And DC avoids problems caused by induced currents due to geomagnetic storms (you get a voltage shift, but that's no big deal, compared to what happens to transformers.)
  - aftbit 5y ago
    Seems unlikely to be a problem undersea, no?
    - Johnythree 5y ago
      The induced currents flow in the ground (and water) which cause big voltage differences at either end. Whatever, the ground currents penetrate very deep into the earths crust thus equally effecting buried cables.
- idiotsecant 5y ago
  grid synchronization 'issues' are a feature, not a bug - by very, very slightly varying how hard to you try to 'push' out of synch you can manipulate power flows at will. This is dramatically more difficult with DC. That, and ease of voltage level manipulation were the reasons so much transmission was AC in the first place!
  - XorNot 5y ago
    No they're issues: if it was easy, then no one would bother with HVDC - HVDC exists because it reduces losses and vastly eases grid coordination because conversion to and from AC at either end is handled under local control and can be totally independent of the source/destination. Its the difference between just shifting the frequency or phase on your local control clock at the substation, and trying to coordinate with another state/territory/nation's entire AC generation infrastructure to keep everything in phase.

by roomey 5y ago

7952 5y ago

AC cables are just more expensive per km, I think due to skin effects.

Johnythree 5y ago
That and the enormous capacitance between a long cable and the sea.

XorNot 5y ago

DC is also used because you avoid grid synchronization issues, and this is generally the trend for long distance connects now in general. It means the grids can have frequency divergence without affecting the transmission line itself.

Lascaille 5y ago
The issue with DC transmission in subsea cables is that the XLPE insulation can become polarized and - from what I remember - have water drawn into it due to the consistency of the field.
- spookthesunset 5y ago
  You could probably flip the current at periodic intervals to deal with this.
  - Lascaille 5y ago
    You're talking about flipping the polarity, going occasionally from conductor-positive to conductor-negative (the shield must remain at ground potential.) If I remember correctly that actually makes things much worse if you've operated at one potential for a considerable amount of time. You could operate the cable at AC ~ 0.1Hz but your ability to actually transmit MWH would suck.
pfdietz 5y ago
And DC avoids problems caused by induced currents due to geomagnetic storms (you get a voltage shift, but that's no big deal, compared to what happens to transformers.)
- aftbit 5y ago
  Seems unlikely to be a problem undersea, no?
  - Johnythree 5y ago
    The induced currents flow in the ground (and water) which cause big voltage differences at either end. Whatever, the ground currents penetrate very deep into the earths crust thus equally effecting buried cables.
idiotsecant 5y ago
grid synchronization 'issues' are a feature, not a bug - by very, very slightly varying how hard to you try to 'push' out of synch you can manipulate power flows at will. This is dramatically more difficult with DC. That, and ease of voltage level manipulation were the reasons so much transmission was AC in the first place!
- XorNot 5y ago
  No they're issues: if it was easy, then no one would bother with HVDC - HVDC exists because it reduces losses and vastly eases grid coordination because conversion to and from AC at either end is handled under local control and can be totally independent of the source/destination. Its the difference between just shifting the frequency or phase on your local control clock at the substation, and trying to coordinate with another state/territory/nation's entire AC generation infrastructure to keep everything in phase.