What is the function of coupling capacitor in substation?

A coupling capacitor in substation is used for power line communication purposes. It used after the wave trap. The capacitance ranges from 2200pf to 10,000pf. It offers very low impedance to high frequency carrier signal and allows them to enter the line matching unit and offers a very high impedance path to low frequency signal or wave and blocks it for reaching the line matching unit.

In short, it allows only those frequencies needed for communication purposes.