Schweitzer Engineering Laboratories SEL-352-1 - Close Resistor Energy; Overpower Pickup (Oppu)

Date Code 20010731 Breaker Logic 3-31
SEL-352-1, -2 Instruction Manual
The minimum breaker resistance as calculated for 37OP with resistors in circuit is found when
the trip and close resistors are inserted in parallel.
For the example breaker:
R
TRIP
W=W¼= 2201102
R
CLOSE
W=W¼= 70352
R
MIN
primary
70220
70220
W
+
¼
=
SULPDU\ W=
Line-charging current is 350 A primary; thus the minimum resistor in circuit difference voltage
across the breaker is:
V
MIN
VHFRQGDU\9

$
=
W¼
=
To make sure the 87TH picks up for this minimum difference voltage condition, set 87TH = 4.3
or less. If differential measurements immediately after a successful close operation pick up the
87TH element longer than the OPpu time, the SEL-352 Relay will incorrectly indicate heating of
the resistors. The 87TH setting should be raised if this condition exists.
Overpower Pickup (OPpu)
For the example protected breaker, we select an OPpu setting as follows:
62OP = 3 cycles
If you are unsure of the amount of time necessary to ride through the transients, use the SEL-352
Relay event report to see how long the 87TH and 37OP bits are asserted after a switching
operation. Refer to Section 10: Event Reports and SER for a description of the event reports.
Close Resistor Energy
Next, calculate the energy capacity of the close resistors. In the example breaker, the close
resistors consist of 28 ceramic disks that have a volume per disk of 225 cm
3
and a maximum
thermal capacity of 700 J/cm
3
. Maximum thermal stress per head is calculated as follows:
TS
MAX
= 28 disks ¼ 225 cm
3
¼ 700 J/cm
3
TS
MAX
= 4.41 MJ primary per breaker pole
TS
SEC
=
37
5
&7
5
SULPDU\0-
¼
= 1.71 J secondary
Thermal failure settings should prevent a normal close operation from adding enough energy to
damage resistors after thermal failure elements assert.