Fig. 2a shows the characteristics of the over current relay with
the functions of inverse and instantaneous time limits. Instantaneous
time limit relay operates without intentional time delay.
They are used for the high current faults occurred close to the
source. The operating time is about 10 ms. Inverse time limit relay
operates with a time delay which is adjustable. In general, higher
current will cause a faster operation of the relay. The minimum
current at which the relay operates is also adjustable. Fig. 2b shows
the operating characteristics of differential relay. A current differential
relay system uses Kirchhoff’s current law, currents on both
sides of the equipment are compared. Under normal conditions,
or for a fault outside of the protected zone, as shown in Fig. 2, current
I1 (sending end) is equal to current I2 (receiving end). Therefore
the induced currents of the current transformers are also
equal, and no current flows through the relay. If a fault occurs inside
of the protected zone, currents I1 and I2 are no longer equal,
and the difference between the I1 and I2 current flows through
the relay