5-18
Applications
Date Code 20010515
SEL-2BFR, -2/BFR Instruction Manual
To ensure that 59FO asserts when a resistor is stuck in service
59FO = 4.33 - 0.31 = 4.02 V secondary
Set flashover failure timer setting, 62FF, based on the disruptive effects of breaker flashover on
load and generation equipment and system stability. Set the flashover pending failure timer
setting, 62FP, shorter than 62FF.
For the example protected breaker:
62FP = 25 cycles
62FF = 30 cycles
Select Phase Discordance/Failure to Close Settings
87UB Unbalance Current Ratio
The 87UB setting defines the divisor the relay uses to determine whether an unbalance exists
between the three-phase current magnitudes. Lower unbalance ratio settings make the relay more
sensitive to current unbalance. An example follows.
Assume 87UB equals eight and one per-unit current flows in each breaker pole upon closure. If
A-phase and B-phase breaker poles close while the C-phase pole remains open, the relay makes
the following comparisons:
8
current
unit
-
per
2
pu
0
Ic
8
current
unit
-
per
2
pu
1
Ib
8
current
unit
-
per
2
pu
1
Ia
8
I
I
I
pu
1
Ia
c
b
a
<
=
>
=
>
=
+
+
>
=
87UC asserts because C-phase current is less than the sum of phase current magnitudes divided
by 87UB. C-phase current must rise above 0.25 per unit before 87UC drops out. If 87UB equals
64, 87UC drops out when C-phase current reaches 0.031 per unit. Thus, higher 87UB settings
permit greater phase current unbalance.
For the example protected breaker, 87UB = 16.
62UC Time Delay Pickup Timer Setting
Set the 62UC timer slightly higher than the nominal closing time of the protected breaker. For
the example breaker, assuming a closing time of 10 cycles, we select a setting:
62UC = 12 cycles
The additional two cycles allow for possible pole scatter upon closure.
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