Magnetic fields surrounding undersea cables.

[waross]

Greetings;
There is some discussion in thread238-163316 regarding high voltage Direct Current electrical transmission lines.
Such lines are often used to transmit electric power in undersea cables.
It has been suggested that the magnetic fields emanating from such cables may cause problems with magnetic compasses and/or other navigational equipment.
This seems to be the appropriate forum to ask if any mariners have experienced any effects on navigational equipment when crossing undersea power cables, or is this a non issue.
Thank You

 

[Skogsgurra]

I mentioned in the thread about HVDC systems that "The first HVDC lines used only one conductor and used ground for return. They were built to bridge waters (Gotland, Pas de Calais, Sicily etc). Environmentalists then made this arrangement more or less impossible. The wide DC current loop also disturbed magnetic compasses on ships."

This was a serious problem in the years before GPS and plotters. Larger vessels used Decca and Loran for navigation so they didn't bother. But smaller boats relied entirely on magnetic compasses and dead reckoning (Engel logs etc) and they had problems with the magnetic fields from HVDC lines with return through through water or ground. There is a short discussion in library.abb.com/.../VerityDisplay/591D08FE788CCA54C125702700599F86/$File/Project%20Directlink-.pdf where it is shown that the magnetic field is down to 0.2 uT at 10 metres distance from a modern bipolar transmission cable. The older ones had way more stray field. Often equal to Earth's magnetic field (abt 50 uT).

Gunnar Englund

http://www.gke.org

 

[Warpspeed]

I would be very surprised if a dc earth return system could even be made to work. The problem of ion migration into and out of the earth systems at each end, where the current concentrates, would be huge. Grounding systems would be rapidly eaten away before your eyes.

Single wire high voltage ac earth return systems are fairly common out here in rural outback Australia, and they work reasonably well. But I cannot imagine a dc system having any chance of working at all.

 

[GregLocock]

Isn't Basslink earth return?



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Warpspeed]

Basslink is definitely dc, so I very much doubt it could be designed as an earth return system.

The advantage of dc is that the individual current carrying conductors can be made much larger in diameter, as large as you desire to make them. They will always have equal current density throughout the whole cross section of the copper conductor, and that leads to higher operating efficiency for a given copper cross section.

With ac, the current always tends to crowd towards the outer "skin" of any conductor, and beyond a certain physical conductor diameter, little or no current will travel through the geometric centre. So there will always be a maximum practical conductor diameter for any given ac operating frequency. At 50/60Hz operating frequencies, maximum practical conductor diameter would be in the region of (roughy) about 30mm diameter. Far too small for serious power transmission over very long distances, especially underwater without requiring inconveniently high voltages.

That is why big fat copper cables used with dc are an excellent solution for efficient underwater power transmission. But earth return is not really possible.

 

[stevenal]

Warpspeed,

Are you really arguing that systems in place and running can't work? And bumble bees can't possibly fly. Monopolar earth return systems exist. The problems you mention may have been a challenge, but evidently dealt with. Even bipolar systems are built to run monopole during emergencies or maintenance.

http://en.wikipedia.org/wiki/HVDC#Monopole_and_earth_return

 

[Warpspeed]

All I can say is that I have first hand experience of corrosion problems that ground currents can cause in long distance steel natural gas pipelines. The electrified dc suburban rail network and the underground telecomunication cable system operators are fanatical about avoiding any dc ground currents, and the subsequent very rapid corrosion it would create in their own and other system.

I am not saying it cannot possibly work, only that it is EXTREMELY bad engineering to deliberately induce dc ground currents anywhere. While your own ground return system may work, it can be extremely destructive to other buried metal structures because the current always seeks the easiest path.

Sure it is possible, but I am still surprised anyone would even contemplate doing such a thing.