under-sea power transmission cables [Telecom]

.......

That may seem like an application for a massive Hydrogen generating plant where the power is, and shipping it across the gap to where the people actually are.

They call aluminium "frozen electricity" (except you can't unfreeze it back into electricity), maybe in this century hydrogen may one day be named "bottled electricity"?

I'm wondering how safe it is to transport high voltages through bodies of water. I realize it happens in the Bay Area (the cable that replaced the Hunters Point power plant, for instance), but does anybody know how safe this practice is?

I know that rats are drawn to electric cables, which is apparently why there are so many electrocuted rats who have eaten through cables. I'm wondering if there is any other danger from running high voltages through water.

Anyone?

Not good if the power "leaks" - but [isn't ] that ... about getting the cable design right?

Less flippantly, good insulation should translate into lower loss and higher transmission effectiveness.

These are getting more common now that offshore wind farms are being built in many parts of the world, but power feeds to islands or across big rivers have been around for a while.

50 MW to Jersey in the english Channel from France Many subsea cables are DC - usually since they connect different grids which are not running in sync. So you probably dont like the idea of using the sea as the return path for the power?

The cable has to be shielded from the water otherwise the power doesnt appear at the far end......

The main thing likely to be sensed by fish and so on is the electric and magnetic fields, although the electric field may be suppressed by some cable constructions.

There have been similar issues with telecomms cables, with recent design needing to be resistant to bites from big fish like sharks.

The theory is they are attracted to the fields from the original signals in electrical cables. Nowadays such cables are fibre optic so that isnt an issue, but there will usually be a power feed for a long cable for the embedded amplifiers.

Current state of the art is around 350 to 400 Km before you need an amp.

Regards,

stephen snipped-for-privacy@xyzworld.com - replace xyz with ntl

Well, if the cable springs a leak, it'll short out, but that's what circuit breakers are for.

There are some fairly large undersea power cables in use, such as the

330MW one between Connecticut and Long Island. There's a 104km 40MW AC cable from England to the Isle of Man and a 580km 700MW DC cable between the Netherlands and Norway. There are also cables in Japan, which has earthquake issues.

Kona to Honolulu is under 300km so at first glance it should be doable, or it could be three much shorter hops via Maui and Molokai.

R's, John

Underwater power cables are always shielded with the shield effectively grounded to the earth (although it may be used as a neutral in AC circuits or a return in the more common DC circuits). This is done for two reasons, one to control the voltage across the insulation to prevent corona effects and insulation degradation, the other to provide safety for trawlers who will snag the cable . A side result saves the lives of sharks and other sea life.

An interesting thing happened with the first fiber optic undersea cable. Previous electronic cables were coaxial, with the outer conductor grounded and repeaters were in series feed by DC power on the inner conductor in a constant current mode. Fiber still needs power for repeaters (especially for long ocean crossings) and the first fiber cable used separate conductors for the power feed. On a test (I believe it was off of the Azores...) the cable failed catastrophically within a couple of weeks due to shark bites. The problem was the electric field due to the separate power conductors attracting the sharks, who thought the cable was dinner. The solution was to shield the cable and ground the shield, at least on the continental shelves and in shallow water.

With that adaptation, the major problem with undersea communications cables these days is fishing trawlers snagging the cable.

E. Tappert

The DC cable between Victoria and Tasmania was originally going to use the sea as the "return" conductor, but there were oil rigs in the vicinity and they kicked up a (justified) fuss about electrolysis corroding the rig structures, so the line ended up being dual conductor with both insulated from the environment (AFAIK).

I'm assuming all these long cable runs are now DC to maximise the overall power that you can pipe down these things - all built on the back of super-efficient AC to DC conversions at either end. That might keep "bitey" things away from the cable rather than an AC field.

Or, newer fiber optics cables and technologies coming along which have so much more capacity that it no longer makes sense to continue using the only slightly older versions, and they're simply abandoned in place on the ocean floor, long before the end of their useful life (or are converted into serving undersea observatories).

(At least it's my understanding that this has happened with at least some transoceanic cables.)

There is another reason to use DC. With AC, some of the transmitted energy is in the fields between the conductors, leading to losses due to the conductive sea water. With DC, any fields are static, and do not lose energy.

Yep, not having any reactive losses would also be a big plus. The proponents of DC in the very early years would be having a laugh now as the limitations of AC are being circumvented with the aid of modern technology.

It is not the dielectric losses that are at fault. Even if perfect insulating materials are used (the imaginary component of the dielectric constant is zero), the capacitance between the conductors will create a very low power factor increasing system losses. Years ago, I think it was in the late 1960's, Scientific American had an article about an underground high tension line that was being tested by GE. The line was 26 miles long, three phase 60 Hz. From the generator end of the line, the current into the line was the same with the load end open-circuited as it was when the load end was short circuited! At that time, conversion of DC back to AC required expensive silicon controlled rectifier circuitry. Today this is less of a problem. With DC, capacitance between the conductors is beneficial as it serves to lessen the requirements of the end inverters circuitry.

In article ,

"Would you believe..."

Today!

see:

or, maybe that's not the 'Concorde' you meant.

I take it you are saying -- without reluctance, obviously -- "no mho!"

*GRIN*

Note: it's _not_ a good idea to start those kinds of puns in this group. A number of us have a more than AMPle supply of them, some of which are real joules; we get a charge out of telling them, too, despite the static that often accompanies it. When confinement to a single cell was not sufficient deterrence, battery -has- been threatened.

Ah well, fame at this _is_ fleeting -- last time I held first place for only 24 hours -- at midnight, they threw me back in my cage. But recognition was given -- I was King Faraday!.

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