Schweitzer Engineering Laboratories SEL-351 - Accuracy; Negative-Sequence Time-Overcurrent Element

Date Code 20001006 Overcurrent, Voltage, Synchronism Check, Frequency, and Power Elements 3-27
SEL-351 Instruction Manual
See Section 9: Setting the Relay for additional time-overcurrent element setting information.
Accuracy
Pickup: ±0.05 A secondary and ±3% of setting (5 A nominal phase current inputs, IA, IB, IC)
±0.01 A secondary and ±3% of setting (1 A nominal phase current inputs, IA, IB, IC)
Curve Timing: ±1.50 cycles and ±4% of curve time for currents between (and including) 2
and 30 multiples of pickup
Negative-Sequence Time-Overcurrent Element
Figure 3.20: Negative-Sequence Time-Overcurrent Element 51QT (With Directional Control
Option)
IMPORTANT: See Appendix F for information on setting negative-sequence overcurrent
elements.
To understand the operation of Figure 3.20, follow the explanation given for Figure 3.14 in the
preceding Phase Time-Overcurrent Elements subsection, substituting negative-sequence current
3I
2
[3I
2
= I
A
+ a
2
· I
B
+ a · I
C
(ABC rotation), 3I
2
= I
A
+ a
2
· I
C
+ a · I
B
(ACB rotation), where
a = 1 ∠ 120° and a
2
= 1 ∠ -120°] for maximum phase current I
P
and like settings and Relay Word
bits.
Courtesy of NationalSwitchgear.com