The Western HVDC Link

AFAIK, repairs are not carried out on the seabed. The fault must first be located using a TDR. Sufficient length of cable needs to be de-buried in order that a loop (including the fault) can be brought to the surface, where it will be cut. Hopefully, the fault is at or close to the cut. A new section is then jointed in, with the fault cut out. The cable is then lowered to the seabed, and re-buried. After testing, including a new TDR profile of the repaired cable, it can be restored to service. I can’t remember how much spare we have, but it is measured in km, as that is the likely section that will be jointed in. The trick is doing the cut, such that the cable length you have will bypass the fault. 


Regards,


Alan.

Alan, thank you. So does that mean the cables are laid with a bit of a wiggle in order to allow enough slack to bring them to the surface?


My personal interest is proximity to IF2.

Not sure about that. 


Regards,


Alan.

I am not too sure about HVDC cables as they look fatter, and historically have normally been laid in diveable depths, but submarine telco cables are always made quite a bit longer than the direct path might suggest, because the sea floor is far from flat. You do not want delicate cables stretched across a rocky valley at any point. Indeed in the early days of cable laying in the late 1800s this caught out quite a few projects in the bigger oceans. The channel and the north sea is quite shallow around 50-100m deep in most places and gently undulating, with the odd sand bank and a few shipwrecks to avoid,  picture here  so the first cable layers had a false sense of security, moving on to the trans atlantic cables on the other hand turned out to be another game altogether.


When fishing for cables at deeper than safe diving depths you may grapple and lift at several points and buoy to give support at intermediate depths, so only the middle of the lifted sections comes to the surface and is not too stressed.

If all else fails, you may end up with two joints.

I have just spotted an article in The Register (www.theregister.co.uk/.../) that would suggest that the cable fault has been located. 

“Italian cable contractor Prysmian has not clarified the issues behind the fault, raising fresh doubts over the infrastructure. The company said in a statement that the fault had been located on "pole 2 of the southern part of the link". National Grid clarified that the fault was on the "southern land cable".


Prysmian declined to comment on The Register's requests for clarification.”


Regards,


Alan.

When running at maximum power, 2,200 MW a working voltage of 600 kV gives a current of 3,666A.


Seeing that the latest fault is at the southern end of the link on pole 2 - I'm assuming that pole 2 is a physical pole or pylon rather tha HVDC speak for Positive or Negative(?) - if overhead, what sort of conductor would be used for 3,666A ? 


In my "cable collection" box, I have a sample of HT-PEX 145 kV 1 x 2000mm2 aluminium cable and from tables see that this is good for just over 2,000A in air at 30 deg C or 1,375A direct burial at 20 deg C. The insulation being about 25mm over the aluminium. For this HVDC Link, four time the voltage and just under three times the current, suggests a monster cable, if in proportion the conductor would need to be about 90mm diameter, plus goodness knows how much insulation and my guess would therefore be about 290mm overall diameter plus any armouring, but what about uninsulated overhead?


From a visit to the Dinorwig Pumped Storage facility a few years back, I asked about the underground cables - there being no overheads in the area and was told that it was an oil-cooled cable and if we went home via a particular road, the rather nice looking stone built bungalows, were actually the cooling stations for the oil.


Clive

For overhead air insulated cables, for AC only consideration of skin depth, and for both AC and DC consideration of corona, tend to be the limiting size determining factors - skin depth drives you to  split the current over multiple thinner strands, to avoid corona you really want a smooth and fat conductor free of any high spots.

intro to the field theories  typical cable sizes    catalog link aluminium and alloy cables for transmission use  Note the comedy names of the common sizes...

Largest practical strands are rated in single figures of kA,  and  that ends  up 30-50mm dia.  There maybe a steel cable in the core of the twist for strength.

Bundles of 2, 4 or very occasionally 6 such cables are commonly seen spaced by spreader bars and dangling off each pylon arm in the UK. Typically the left and right arm triplets are two independent 3 phase circuits, I am not sure how you would split up DC on a standard 6 armed tower.

I was sent this image once, to illustrate a problem with wind on long wire spans.

I think it may have been edited. Click it and see what you think..

I'm assuming that pole 2 is a physical pole or pylon rather tha HVDC speak for Positive or Negative(?)

I hadn't thought of that - I'd presumed it was referring to a 2nd d.c. conductor (there possibly being more than 2 overall?)


  - Andy.

Sorry to disappoint people, but “pole 2” refers to either the positive or negative conductor. 


Regards,


Alan.