In substation there is Line Matching Unit (LMU) where the equipment impedance and line impedance matching is done, it is connected below coupling capacitor CC, and we have studied in network theory maximum power transfer thermo where impedance matching term is used. It is used for maximum communication signal can pass to other end with reduced loss.
This is the first part of the theory which is maximum power transfer. This is basically figuring out what should be the line impedance value to transfer maximum power to the load. Typically the line reactance is the negative of the load reactance and the line resistance should equal load resistance to maximize real power transfer.
The second part of this theory involves voltage stability. Typically MW transfer capability is compared to bus voltage values and since the power is a squared function of the voltage the curve makes a parabolic shape. At the apex of the curve is what referred to the "knee of the curve" which is considered the line of voltage stability and instability. In the real world MW transfer is limited by two things first the surge impedance loading (SIL) of the transmission line and second reactive resources reserves. Doesn't work theoretically as taught in network circuitry class. Basically as loading is increased voltage drops and if there isn't enough dynamic and static reactive resources then the system may enter voltage instability (where one bus voltage may not recover within normal operating criteria. In US it is between 0.95 pu to 1.05 pu).