Coupling capacitors are used to transmit communication signals to
transmission lines. Some are used to measure the voltage in transmission
lines. In signal transmission the coupling capacitor is part of a power line
carrier circuit as shown in the schematic below. A coupling capacitor is
used in this circuit in conjunction with a line trap. Line traps can be
installed at the substation or on a transmission line tower.
Figure 1. Power line carrier schematic showing use of coupling capacitors