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Current profile of subsea export cables

From the offshore substation, why would the current profile slope down towards the middle of the export cable then rise again at it's terminal end (onshore substation)? Is it because at both ends of the export cable, there is effectively a 'source'? One would have expected the current from one end to the other to have a linear drop off (heat and other losses).Wondering if charging current has a play into this also. 



See attached graph.



Thanks
  • how was the data for that graph collected, and is it AC, and if so what frequency ?
  • I would expect the current to be constant all the way along, even with thermal losses.  The only way I can see it would be different is if substantial currents are leaking into the sea.


    I am assuming it's DC.  AC would be more prone to "losing" current by capacitive leakage across the insulation.
  • It is AC.
  • Good morning Simon Barker,

    What is the question ?
  • AC, good, this curve may be a correct if there is VAr compensation applied at the ends, but unequally - in effect there is shunt inductance to tune out, or resonate with (i a very low Q way) the line capacitance. This can result in the voltages at the two ends not being the same, and therefore the capacitive displacement currents vary along the cable.


    In effect the core to armour capacitance of the cable is not fully compensated by the self inductance for the desired load impedance. On long cables it is possible to put lumps of compensation part way along, but undersea is not practical unless there is already a handy island part way along the route. ? . At 50Hz high power uncompensated lines get tricky after about 50km or so, but how much is tolerable is a function of the voltage in use, and the power to be transmitted - 50A into capacitance is no biggy if the load is 1000A. if the load is a few hundred Amps however, it matters that much more.

    DC lines have no such issue - a volt in at one end, comes out as a volt at the other, eventually.
  • Mike, with a DC long line, surely there would be a voltage drop due to IR?


    Clive
  • Ah, sorry, very much so. That should have been "amp" ?.

  • mapj1:

    AC, good, this curve may be a correct if there is VAr compensation applied at the ends, but unequally - in effect there is shunt inductance to tune out, or resonate with (i a very low Q way) the line capacitance. This can result in the voltages at the two ends not being the same, and therefore the capacitive displacement currents vary along the cable.


    In effect the core to armour capacitance of the cable is not fully compensated by the self inductance for the desired load impedance. On long cables it is possible to put lumps of compensation part way along, but undersea is not practical unless there is already a handy island part way along the route. ? . At 50Hz high power uncompensated lines get tricky after about 50km or so, but how much is tolerable is a function of the voltage in use, and the power to be transmitted - 50A into capacitance is no biggy if the load is 1000A. if the load is a few hundred Amps however, it matters that much more.

    DC lines have no such issue - a volt in at one end, comes out as a volt at the other, eventually.




    Yes reactors at both ends of cable for capacitive compensation and voltage stability. This cable is for an offshore windfarm. 

  • well, draw it as an L-C-L pi network, and you will see how in can be used to step the voltages up and currents down, or vice versa.


    In a much smaller scale the inductive ballast and its PF compensation cap of the old school florry tube did a similar double act to convert the voltage needed by the tube, say 100V 400mA for a 40W tube, to 230V  at a correspondingly lower current (though never as good as the  180ma or so one would like to see if the choke was lossless)


    Radio people talk about impedance matching and draw smith charts, and power people talk about reactive kVA or kVAr , but it is the same effect.

    L match - like in the florry. ,     image 2 of these back to back, sharing the cable capacitance.


  • I don't think I've seen any mention of the voltage of this arrangement. The cable lengths are also very important. For a subsea cable compensation can only be applied at the ends which probably, depending on cable length, limits the voltage to a max of 220kV. What is the total power generated by the windfarm? It sounds like there are going to be some pretty hefty cables. Finding the appropriate ship to lay them may need some attention at an early stage.