Coupling capacitor voltage transformers are an economical
way to obtain the potential required to operate distance (and
directional) type relays. They also provide a means to couple
communication channels to the power line for use with vari-
ous relaying schemes. Unfortunately, a CCVT may not repro-
duce the primary voltage exactly and can introduce significant
error into the distance relay measurement The transient error
that is produced by the CCVT becomes more pronounced as
the change in the voltage from prefault to fault is increased (a
fault at the end of a line with a high source to line impedance
(ZS/ZL) impedance ratio, for example). A typical CCVT tran-
sient is shown in Figure 18. If a transient like this were to
occur for a fault at the end of a line with a high ZS/ZL ratio
Distance Relay Fundamentals
