Many relaying applications like distance relays, directional overcurrent relays require measurement of voltages at a bus. This task is done by a voltage transformer (VT).
The principle of a voltage transformer is identical to the conventional transformer. Hence, its equivalent circuit can be represented as shown in fig 8.1.
Typically, the secondary voltage of the VT is standardized to 110 V (ac). Hence, as the primary voltage increases, the turns ratio N1:N2 increases and transformer becomes bulky.
To cut down the VT size and cost, a capacitance potential divider is used (fig 8.2). Thus, a reduced voltage is fed to primary of the transformer. This reduces the size of VT. This leads to development of coupling capacitor voltage transformers (CCVT).
