Page 228 - Practical Power System and Protective Relays Commissioning

228 Practical Power System and Protective Relays Commissioning

18.6.7 POWER SWING BLOCKING RELAY
This feature along with distance protection to block the distance function in
the relay during power swing conditions. It is utilizes an outer characteristic
for relay Zone 3 and the rate of change of impedance between the two char-
acteristics means z power swing and Zone 3 will be monitored if the change
of impedance is slow and the impedance lies between the two characteristics
for a time longer than the power swing time then the power swing relay will
block the distance function of the relay, but if the rate of change of imped-
ance is very fast and the impedance passes between the two characteristics
very quickly then the power swing relay recognizes this condition as a fault
and does not block the distance function and allows the relay to trip (refer to
Fig. 18.6.28).
The basic criterion for power swing condition is dZ , if it is high then it is
dt
a fault condition but if it is low then it is a power swing condition.

18.6.8 VOLTAGE TRANSFORMER SUPERVISION (FUSE
FAILURE)
This feature is incorporated with the distance relay to monitor the secondary
voltage of the VT and to give an alarm in case of losing voltage or to block
the operation of the distance relay.
This function can work in a zero-sequence principle or negative-sequence
principle to monitor the secondary voltage of the VT and in case of loss of
voltage to block operation of different relay functions which depend on
voltage as distance protection, synchro check function, and under voltage
function, etc., the blocking function is issued after a time delay of the order
of 3 5 seconds.
The criteria for fuse failure detection are as follows:
1. Detection based on the rise of the value of zero-sequence voltage 3V 0 or
negative-sequence voltage V 2 above the setting value.
2. The value of the zero-sequence currents 3I 0 or negative-sequence current
is less than the setting value.
However, in case of detecting high unsymmetrical zero-sequence currents
3I 0 during the blocking operation of distance function due to fuse failure
detection then this block will be released.
Also, some manufacturers of relays are switching the distance function
during a fuse failure blocking condition to an emergency overcurrent
relay.