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.
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