3.11
Date Code 20110408 Instruction Manual SEL-551 Relay
Relay Elements and Logic
Instantaneous Overcurrent Elements
Residual Ground
Instantaneous
Overcurrent Elements
Two residual ground instantaneous overcurrent elements (50G1 and 50G2) are
available (see Figure 3.7). Their pickup settings (50G1 and 50G2,
respectively) are compared to the magnitude of the residual ground current
(I
G
= 3I
0
, derived from I
A
, I
B
, and I
C
).
Figure 3.7 Residual Ground Instantaneous Overcurrent Elements 50G1 and
50G2
Example 50G1 element operation:
I
G
> pickup setting 50G1P, then Relay Word bit 50G1 = logical 1
I
G
≤ pickup setting 50G1P, then Relay Word bit 50G1 = logical 0
If pickup setting 50G1P is set to 50G1P = OFF, then element 50G1 is
disabled. Relay Word bit 50G1 equals logical 0 at all times.
The second residual ground instantaneous overcurrent element (50G2)
operates similarly.
Negative-Sequence
Instantaneous
Overcurrent Elements
Two negative-sequence instantaneous overcurrent elements (50Q1 and 50Q2)
are available (see Figure 3.8). Their pickup settings (50Q1 and 50Q2,
respectively) are compared to the magnitude of the negative-sequence current
(3I
2
, derived from I
A
, I
B
, and I
C
).
Figure 3.8 Negative-Sequence Instantaneous Overcurrent Elements 50Q1
and 50Q2
Example 50Q1 element operation:
3I
2
> pickup setting 50Q1P, then Relay Word bit 50Q1 = logical 1
3I
2
≤ pickup setting 50Q1P, then Relay Word bit 50Q1 = logical 0
If pickup setting 50Q1P is set to 50Q1P = OFF, then element 50Q1 is
disabled. Relay Word bit 50Q1 equals logical 0 at all times.
The second negative-sequence instantaneous overcurrent element (50Q2)
operates similarly.
Relay
Word
Bits
Settings/
Currents
50G1P
I
G
50G2P
I
G
50G1
50G2
(residual)
(residual)
IMPORTANT: See Appendix F for
information on setting negative-
sequence overcurrent elements.
Relay
Word
Bits
Settings/
Currents
50Q1P
31
2
50Q2P
31
2
50Q1
50Q2
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