3.26
SEL-351A Relay
Instruction Manual
Date Code 20080213
Overcurrent, Voltage, Synchronism Check, and Frequency Elements
Time-Overcurrent Elements
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
To understand the operation of
, follow the explanation given for
in
Phase Time-Overcurrent Elements on page 3.16
, substituting
negative-sequence current:
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)
for maximum phase current I
P
and like settings and Relay Word bits.
q
From Figure 4.23. Directional Control logic input is constantly asserted to
logical 1 in SEL-351A-1
Figure 3.21
Negative-Sequence Time-Overcurrent Element 51QT (With
Directional Control Option in SEL-351A)
IMPORTANT:
Appendix F: Setting Negative-Sequence
for information on setting negative-sequence
overcurrent elements.
where:
a =
∠
120°
a
2
= 1
∠
–120°
51QP
31
2
Setting
51QTC
Torque Control Switch
Directional Control
(asserted to logical 1
continuously if E32 = N)
q
(Level 1)
SEL
OGIC
Torque Control
51QT Negative Sequence
Time-Overcurrent Element
Curve Timing and Reset Timing
Settings
51QP
Pickup
51QC
Curve
Type
51QTD
Time
Dial
51QRS
Electromechanical
Reset? (Y/N)
Pickup
Curve
Timeout
Reset
51Q
51QR
51QT
Torque
Control
TCQ State
Switch Position
Logical 1
Closed
Logical 0
Open
Logic Point TCQ Controls the Torque Control Switch
Setting
51QRS =
Reset Timing
Y
Electromechanical
N
1
Cycle
TCQ
Relay
Word
Bits
SEL
OGIC
Setting
Summary of Contents for SEL-351A
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