The Western HVDC Link

I looked thru some links about the project and it's a LOT  more complicated than you would think  certainly more than a few rectifiers and a couple of old transformers. I can clearly see why they don't just change the fuses and switch on!

Is the insulation material something new and "revolutionary" here?

Is there any concern about its performance?


Terence.

The problem here is we won’t know the answer until the fault has been located, brought to the surface and examined. The voltage is new - I think it is the first cable running at 600kV dc. The fault could be a manufacturing fault, a loading / installation fault, or something that has been caused by ground conditions, or it could be damage from excess current caused by the previous fault. Until the fault is located and examined, we won’t know. A “footprint” will have been taken of the cable after installation. A scan of the faulty cable should show the fault when compared to the initial footprint. 


An actual fault should be fairly easy to fix during favourable weather conditions. The worst case scenario would be if nothing can be found - it would take a lot of courage to switch it on and see what happens - almost like a “bang” test with unlimited energy. 


Reagrds,


Alan.

No such thing as a silly question! With a long transmission line, you get a phase shift along its length due to its reactance. This exact phase shift is a function of distance, as well as load. If youHave a second cable of a different length, it is likely that the difference in phase (even though the frequency is the same) will prevent you from being able to close the second end. The solution is a dc link, with the “inverter” in synchronism with the local grid connection point. 

Thanks Alan - I did wonder if it might be something along the lines of 'propagation delay' over such a long length, but figured that it must be a similar length to the existing England - Scotland lines of the grid so any delay would be similar for both and so everything should 'come out in the wash' as it were. Perhaps undersea cables have a different reactance (per m) from overhead lines and that makes a difference?

   - Andy.

The long AC links on land usually have intermediate loading or phase correction components.

Under water this is not so sensible.

The water does increase the AC losses if the cables are spaced with water between.

Long land links with nothing part way along can also be DC, there is a trade off of costs

and losses at the inverters vs loading/tuning of the reactance.

Out of interest, how do they go about the testing a HVDC cable of such length? There are surprisingly ambitious subsea HVDC cables planned in the North Sea (100s of miles), presumably with some level of multiple redundancy. I'm not so much interested in the transformer side - I'm sure it's incredibly complicated but it's also reasonably accessible!


In my early career I worked in an industry involving a variety of cable diagnostics, so I have a passing interest. For example they had TDR (time domain reflectometry, basically spamming an AC signal down the line and looking at the backscatter/delay from impedance changes), multi-hub integrity measurements, while some rather fancy options put a dual-frequency AC signal along the cable and measured the current at various distances in order to find insulation problems/shorts (it worked surprisingly well over 30-60km gas pipelines, depending on what you were looking for). I think there were also fibre-optic acoustic noise solutions that could detect noise/likely impacts.

As far as I know, TDR is still the preferred method, although with a precision pulse generator and a precision oscilloscope rather than an “all in one” unit with just a “go” button. The idea is that you take (and save) the profile of the good cable, then after the fault, the “new” trace can be compared to a previously taken good trace. In theory at least, the fault should show up as a difference between the two images.


Other methods include continuous temperature measurement using an integral fibre optic core, and Marine AIS systems that allow you to set alarms based on the proximity of ships to the cable. 


Regards,


Alan.

How are these cables protected against trawlers/anchors etc?

Power cables / pipelines are often buried, but most of the time their protection relies on them being shown on shipping charts for vessels to avoid. The use of ship tracking software helps too, as vessels know there is a reasonable chance of them being identified if they catch something with their anchor. 


Regards,


Alan.

AFAIK, trawlers should not be scraping the bottom. But what happens if a cable is damaged? How is it fixed?