Choice of SWA cable for burial below the water table

I am building myself a small off-grid micro-hydro system.  The turbine hut will be connected to the house by 560m of buried cable (SWA, 10mm^2 chosen for allowable voltage drop) and there is a second 340m leg of thinner cable above the house taking power up to the stream level sensors.

The ground is very boggy moorland and the cable will be below the water table for at least 10 months of the year.  

Having read previous discussions it appears that neither PVC nor LSZH sheathing is really suitable for continuous immersion. (see also https://uk.prysmiangroup.com/media/news/underground

and https://www.molexces.com/webfoo/wp-content/uploads/Water-Ingress-In-Structured-Cabling-Systems-2013-1.pdf).

Which would be better, PVC or LSZH sheathing?  It seems very strange that there is no British Standard for installations like this.

Talking to companies in this field, they say "don't worry about it".  They just use standard SWA cable, directly buried because "that's all that is available" and "the armouring may rust but each core has XLPE insulation that will continue to insulate even when wet".  Sometimes they make a nick in the outer sheath before it enters the turbine hut so that if there is any sheath damage higher up, the water running down past the armouring can drip outdoors rather than inside the switchgear.

I don't want to spend a fortune on submarine-rated cable; equally, it has taken weeks with an 8-ton digger to carve the trench and I really do not want to be replacing the cable in 10 or 20 years' time when the rest of the groundworks should be good for a century or more.

In principle I could thread 50m sections of 63/50 twinwall ducting along the cable before dropping it in the trench.  This would give better protection against sharp stones when back-filling.  If the joints were taped and sealed the downhill sections could then avoid being water-logged but they would probably still be damp.   Part of the cable though runs down a hill, under a stream and back up again, like a giant u-bend, and this would inevitably fill with water.  Ducting would be expensive, both to purchase and install.

One supplier says they could add an outer MDPE or HDPE sheath to improve the water resistance; again, this is an expensive option.  It would terminate with the last 4m inside an outhouse, so it does not run into the house itself.  It seems a bit illogical that the BS standards (I think) prohibit domestic use of PE-sheathed cables on fire-safety grounds but allow the use of PE cable ducting.

Any suggestions?

  • 560 metres of 10mm 3 core? Seems very much on the small side - what will be your operating voltage and final current demand at the destination end?

  • I presume an overhead line is out of the question.  What voltage are you shipping at - 500 metres is a long way at 230V.

    In practice PVC will be fine for years in a wet duct, but it is slightly porous, and the life of the armour will be determined by the clock starting to tick after the first scratch to the outer jacket. The advantage of a duct is lost on a very long run as it cannot be re-threaded - friction gets to you after a certain length - you have to install manhole covers part way along. Is there perhaps merit in piping only the highest risk U section and providing access covers on either side of that so that it could be replaced and jointed as a short section if required?

    Mike.

  • Having removed (in about 2015ish) PVC/PVC SWA installed below the water table in a marshy area in the early 1970s and found both cores and copper to be in near-perfect condition when the cable was shortened for re-use (the armour galvanising was slightly white and fluffy, no rust, copper still bright, IR test results good), I am inclined not to worry too much. 

  • I am using a 3-phase permanent magnet alternator (50 Hz, 230v star phase-neutral) which will power a collection of electric radiators, some on each phase, maximum total 9 kW.   So it is a 4-core cable.   560m of 10mm^2 is 0.95 Ohm per conductor and 3 kW per phase at 230 V takes 13 Amps, giving a voltage drop of 12.4 Volts in each live conductor (and zero drop in the neutral, if the loads are balanced).  I'm happy with that.

    More sensitive loads will be driven by a pair of inverters.  A transformer with 400V delta primaries and three 230 secondaries will drop the voltage enough to use full-wave bridge rectifiers without getting too close to the DC input limit on my inverter (Aurora Uno wind inverter, using a synthesised signal into the frequency input to limit the output power depending on available flow rate).  That will connect to a Multiplus inverter/charger acting as a micro-grid, with batteries for peak loads beyond the immediate turbine power.

    That's in Winter when there is plenty of water to drive a substantial (40 kg) alternator. 

    In Summer when the stream is low and I may be taking 1 litre/second or less, a changeover switch will connect the cable to a second turbine with a small alternator from a suitcase generator (max 1700W 230V phase-phase delta at 400 Hz).  I'm unsure how much the cable inductance will drop the voltage at this frequency: I asked the suppliers and they said the inductance was 0.26 hm (mH??)/km, which seems surprisingly low.   This one won't power any radiators, just the wind inverter (without needing a transformer as it's a lower phase-phase voltage than the big alternator).    At 1700W the resistive voltage drop in the cable will be about 3%, though I am more interested in the performance when it is only making 200W.

  • Thanks - see previous reply about voltages.  Yes, the land is owned by the National Trust.  We (and they) are very keen to keep it unspoilt by poles and overhead wires.

    In principle I could replace sections of the SWA cable following water ingress (how would one know?) but I'm also laying an optical fibre so that control systems at the extraction point, cottage and turbine hut can talk to each other, and I don't think that can have sections replaced.  For the fibre I am tempted to pick corrugated steel tube reinforcing as I can have it over-sheathed with HDPE to make it properly waterproof (comparable in price to standard SWA fibre with a PVC sheath) - if successful it should never need replacing.

    One thing I have wondered is whether any obvious damage to the sheath during installation could be patched-up with amalgamating tape to water-proof small nicks.  I'm wary though of using anything that might cause chemical damage to the insulation.

  • The inductance is low because we assume a balanced load and the magnetic fields of the flow and return currents more or less cancel,  if you split the cores and measured as the inductance of a single wire with no return current in the bundle it would be more serious.

    For single wire, I'd expect 1-2 nano-henries per mm per single core (1-2mH per metre = 1-2 henries  per km ) and better than 90% cancellation at 50Hz so yes, credible.

    Beware of skin depth at 400Hz the currents will not be uniform accross the full 10mm2

    That said at 50 Hz with 13 a per core and no neutral current to speak of it will be fine and at 400Hz you have  only 6 or 7A single phase  so the VD/ core will be similar

    I'll have a ponder and come back.

    Optic fibre works fine when wet - glass has a higher refractive index than water and does not dissolve or corrode so the light stays inside even when immersed..  There is 'SWA' fibre  example  which can be knocked about a bit more than the normal stuff.

    mike

  • Really interesting post. Iwas just wondering how the economics stack up. It sounds as if the capital costs are significant for a 9kW load.

    Or am I missing something as usual?