4.88
SEL-751 Relay Instruction Manual Date Code 20170927
Protection and Logic Functions
Group Settings (SET Command)
51P1P := 6.00 A
51AP := 10.00 A
51BP := 10.00 A
51CP := 10.00 A
HBL2DO := 0.03 sec
51P1TC := 1
51ATC := 1
51BTC := 1
51CTC := 1
TR := …+ 51P1T * !HBL2T + 51AT + 51BT + 51CT +…
ER := …+ /51P1P + /51P1T +…
In this example, 51P1T is allowed to assert regardless of the state of the
second-harmonic blocking element. However, 51P1T cannot cause a trip if
HBL2T is asserted. Dropout timer HBL2DO ensures that the blocking
condition is maintained until 51P1T deasserts. If electromechanical reset is
disabled (51P1RS := N), 51P1T remains asserted for 1 cycle after the phase
current falls below pickup setting 51P1P. HBL2DO may be increased to
provide additional security should the second-harmonic current fall below
the pickup threshold before the fundamental frequency current falls below
the overcurrent element pickup. Because the relay may not trip when 51P1T
asserts, the ER Event Report Trigger SEL
OGIC control equation is modified to
trigger an event report. This event report can be used to evaluate the
effectiveness of the harmonic blocking and determine if setting
adjustments are necessary.
EXAMPLE 4.15 Changing the Pickup of a Time-Overcurrent
Element
Use the second-harmonic blocking elements to increase the pickup current
of a time-overcurrent element during inrush conditions without changing
the time delay characteristics. For example,
51P1P := 6.00 A
51AP := 15.00 A
51BP := 15.00 A
51CP := 15.00 A
50P3P := 12.00 A
51P1TC := !HBL2T + 50P3
51ATC := 1
51BTC := 1
51CTC := 1
TR := …+ 51PT + 51AT + 51BT + 51CT +…
In this example, the maximum-phase time-overcurrent element operates if
the harmonic blocking element is deasserted or the phase current exceeds
the Level 3 phase instantaneous overcurrent setting. If harmonic blocking is
asserted and the phase current is below the Level 3 phase instantaneous
overcurrent setting, the time-overcurrent element 51P1P does not operate.
Thus the pickup of the maximum-phase time-overcurrent element 51P1 is
increased from 6 amperes secondary to 12 amperes secondary during
inrush. Once the maximum phase current exceeds 50P3P, the timing of the
51P1 element does not change, so coordination is maintained for large faults.
As shown in Figure 4.6, if the torque control equation 51PTC deasserts, the
Level 1 phase time-overcurrent element may fully or partially reset. When
second-harmonic blocking elements are included in torque control
equations for time-overcurrent elements, the element will need to time
from reset after the blocking element deasserts. Consider this when
evaluating time-overcurrent coordination and when reviewing event reports
in which harmonic blocking has operated.
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