Protection Engineering Basics and Schemes Protection Engineering | Page 124

Basically it is called unidirectional or forward direction
relay. Let see figure and try to understand the working
phenomenon of directional Overcurrent relay.
There are two inputs of relay as voltage and current,
voltage will be considered as reference voltage here with
angle zero degree.
Commonly the impedance of line is reactive in nature,
when fault occurred at P1, then fault current flow from
bus 1 to bus 2, and as we knew that the impedance is of
reactive nature so the voltage in circuit leads the current
by 90 degree.
Let see one figure also, this figure is trip-block figure of
directional overcurrent relay.
Is the angle of current with respect voltage and is the
angle difference of 2 to 8 degree, this angle difference is
the difference between block and trip states of relay.
Operation condition is
-180 degree<
-
<0 degree (trip)
Otherwise (Block)
pg. 124