Can power line communication work over both live LV (+-230V) and MV (11kV) conductors? Is there hardware available that makes this possible? Up to what voltage is power line communication possible?
MV Coupling Units come in inductive and capacitive versions. Very well know technology. You connect the PLC Unit to the coupler via a coaxial cable.
I'm just going to give a simple reason why it might be tricky to get commercial PLC working at 11 kV.
Power line comms interfaces use a transformer and a decoupling capacitor to connect the transformer across the AC supply. For 250 volt, the transformer is largely protected by the series capacitor from taking excessive 50 or 60 Hz current: -
Note the capacitor on the live side connected to the NEWPORT transformer - it says 470nF x 2 and this usually means a total capacitance of ~1uF. Note that it is rated at 250V AC. Try finding a capacitor like this suitable for 11kV and you are going to struggle.
The highest SMD or leaded part in Farnell is rated at 1kV - a long way short of the mark and this is a dc rating BUT there is another problem....
The current taken from the AC thru the Newport transformer with 1uF on a 250V 50Hz supply is 78mA. Imagine what this current would be like on an 11kV line - ~3.5 amps. This would cause a problem and probably a fire so, what if the 1uF were lowered in value to 22nF - well the data may not be transmitted correctly because it will act as a low pass filter and tend to differentiate the data.
Also, please don't expect PLC data on 250V (or lower) to pass thru the sub-station transformer onto 11kV without the data getting mangled.