
5-10
Applications
Date Code 20010515
SEL-2BFR, -2/BFR Instruction Manual
secondary
A
9.09
600
primary
A
5456
I
ph
3
=
=
The 50FT element accuracy specification is ±0.025 A secondary ±5% of setting. Thus the worst-
case error for a setting of 9.09 A secondary is:
50FT error
= 0.025 A + (0.05) • (9.09 A)
= 0.479 A secondary
To ensure that 50FT asserts for all Zone 1, three-phase faults, we set the 50FT element:
50FT
= 9.09 A – 0.479 A = 8.61 A secondary
Note:
If Scheme 6 is selected, the 50FT overcurrent element has a 12 percent transient
overreach. This is due to the digital filter turning on suddenly after the 62TT or 62FC
pickup timers time out and then turning off suddenly one cycle later. As an example, if
setting 50FT = 8.00 A secondary and the current is 7 A secondary, when the 50FT
overcurrent element is enabled (digital filter turned on), the 50FT overcurrent element
can still pick up:
8 A secondary – [8 A secondary • (12% / 100%)] = 7 A secondary
In this case, even though setting 50FT = 8.00 A secondary, the 50FT overcurrent element
can still pick up for 7 A secondary.
Select Scheme Timer Settings
Select the scheme timer settings based upon the following factors:
Maximum permissible fault duration
Protective relay operating time
Protected breaker operating time
50FT element reset time, if required by the selected protection scheme
Operating times of electrically adjacent breakers
Desired breaker failure operating safety margin
If an auxiliary interposing relay is used between the line protective relay trip output and the trip
input to the SEL-2BFR Relay, you must account for the operating time of the auxiliary relay as
well. Be aware that different models of auxiliary relays have a wide variety of operating times.
Most scheme operating considerations are listed in Table 5.2. Additional timing considerations
are listed in Table 5.3 by scheme number.
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