Schweitzer Engineering Laboratories SEL-2BFR, Instruction Manual

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Date Code 20010515 Applications 5-15
SEL-2BFR, -2/BFR Instruction Manual
secondary W 2.52 V 4.33 •
600
A 350
P
MIN
= =
Consider 37OP element accuracy when selecting a setting. Element accuracy is specified: 37OP
±2.25 mW ±10.25% of measured power, ±2.63% of measured voltage, ±9.45% of measured
current. At 37OP pickup, the maximum error is calculated:
( ) [ ]
( )
[ ]
mW 429 ∆ P
mW 54.9 mW 114 mW 258 mW 2.25 ∆ P
W 58 . 0 • 9.45% 33 . 4 • 2.63% W 2.52 • 10.25% mW 2.25 ∆ P
W I • 9.45% V • 2.63% P • 10.25% mW 2.25 ∆ P
MIN
MIN
MIN
c MIN MIN MIN
=
+ + + =
+ + + =
+ + + =
To ensure that the 37OP element asserts under minimum resistor in circuit power and maximum
error conditions, we select a 37OP element setting:
37OP = 2.52 W – 0.429 W = 2.09 W secondary.
26TF and 26TP Thermal Element Settings
Next, calculate the energy capacity of the trip and close resistors. In the example breaker, the trip
resistors consist of 88 ceramic disks, having a volume per disk of 225 cm
3
and a maximum
thermal capacity of 700 Joules/cm
3
. The maximum thermal stress per head is found:
secondary J 5.37
PTR • CTR
primary MJ 86 . 13
TS
pole breaker per primary MJ 86 . 13 TS
cm / J 700 • cm 225 • disks 88 TS
SEC
MAX
3 3
MAX
= =
=
=
Thermal failure settings should prevent a normal trip and close operation from adding enough
energy to damage resistors after thermal failure elements assert. Setting the relay conservatively,
we select the trip resistor fail energy, 26TF:
26TF = 85% • TS
SEC
= 4.56 J secondary.
For the example, we select the trip resistor thermal pending failure element, 26TP, to assert when
the thermal model reaches 75 percent of the 26TF value, or
26TP = 75% • 26TF = 3.42 J secondary.
Calculate close resistor energy capacity the same way. In the example breaker, the close resistors
consist of 28 ceramic disks. Maximum thermal stress per head is calculated as follows: