Why Line Matching Units Are Used in Only Two Phases?

Communications is a two way process. So... one conductor to receive and another to transmit. Hence no need to use the third conductor.

PLCC from one line to ground would probably interfere with other equipment as well as having high loss in the earth return.

Two wires would make a better transmission line. Radiation from lines would be minimised if the two lines carried equal and opposite currents.

The two outer lines are probably symmetrical to ground and the hardware of the lines, making it easy to balance.

However, the third line is probably not symmetrical with the others.

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They don't have to be in pairs. PLCC systems can use any combination of phases and/or ground return, it depends upon the overall design philosophy of the communications system, the relative length of the circuit, what type of control and protective relaying is being implemented, simplex vs. duplex, redundacy, separate voice and data channels, higher data rates, age of the system, etc., etc.

From the Westinghouse T&D Handbook, Power Line Carrier Application, page 422, Chapter 36. Methods of Coupling:

"...There are a number of different ways of utilizing one or more conductors of a three-phase power line as conductors for carrier frequency currents... The simplest of these... is to use a single conductor of the power line as one leg of the carrier circuit, with ground as the return path..."

All of the above are valid answers. But, the reason is not exactly that. The need for the line matching unit is to carry a relatively low power radio transmission on a differential transmission mode on a pair of wires. It is a very efficient transmission mode.

Equally important is, that a single line works as an antenna without opposing pair. To the neighboorhood its unavoidable radiation decays at r², that is at the square of the distance from the wire.

In a differential pair most of the transmitted energy is confined between the pair itself, and only the difference of the two radiation reaches the neighborhood. Which is very small compared to a single line transmitter.

Due the low losses, the transmitter needs lower power, the neighborhood experiences much less disturbances.

In our national grid,[INDIA], i have found out that Line matching units are always on R and B phases.