4.9
Date Code 20100129
Instruction Manual
SEL-751A Relay
Protection and Logic Functions
Basic Protection
When a core-balance CT is not available, use the 50G residual overcurrent
elements.
.
The relay offers four negative-sequence overcurrent elements to detect phase-
to-phase faults, phase reversal, single phasing, and unbalance load.
Time-Overcurrent
Elements
One level of inverse time element is available for phase A, B, C, and negative-
sequence overcurrent. Also, two levels of inverse time elements are available
for maximum phase, neutral, and residual overcurrent. See
through
for available settings.
You can select from five U.S. and five IEC inverse characteristics.
and
show equations for the curves and
through
show the curves. The curves and equations shown do not account
for constant time adder and minimum response time (settings 51_CT and
51_MR respectively, each assumed equal to zero). Use the 51_CT if you want
to raise the curves by a constant time. Also, you can use the 51_MR if you
want to ensure the curve times no faster than a minimum response time.
RES OC TRIP DLY
0.00–5.00 sec
50G4D := 0.50
RES OC TRQ CON
SEL
OGIC
50G4TC := 1
a
For I
NOM
= 5 A.
b
For I
NOM
= 1 A.
Table 4.7
Negative-Sequence Overcurrent Settings
Setting Prompt
Setting Range
Setting Name :=
Factory Default
NSEQ OC TRIP LVL
OFF, 0.50–100.00 A
a
,
0.10–20.00 A
b
a
For I
NOM
= 5 A.
b
For I
NOM
= 1 A.
50Q1P := OFF
NSEQ OC TRIP DLY
0.1–120.0 sec
50Q1D := 0.2
NSEQ OC TRQ CON
SEL
OGIC
50Q1TC := 1
NSEQ OC TRIP LVL
OFF, 0.50–100.00 A
,
0.10–20.00 A
50Q2P := OFF
NSEQ OC TRIP DLY
0.1–120.0 sec
50Q2D := 0.2
NSEQ OC TRQ CON
SEL
OGIC
50Q2TC := 1
NSEQ OC TRIP LVL
OFF, 0.50–100.00 A
,
0.10–20.00 A
50Q3P := OFF
NSEQ OC TRIP DLY
0.1–120.0 sec
50Q3D := 0.2
NSEQ OC TRQ CON
SEL
OGIC
50Q3TC := 1
NSEQ OC TRIP LVL
OFF, 0.50–100.00 A
,
0.10–20.00 A
50Q4P := OFF
NSEQ OC TRIP DLY
0.1–120.0 sec
50Q4D := 0.2
NSEQ OC TRQ CON
SEL
OGIC
50Q4TC := 1
Table 4.6
Residual Overcurrent Settings
(Sheet 2 of 2)
Setting Prompt
Setting Range
Setting Name :=
Factory Default
NOTE:
Phase CT ratios are typically
higher than core-balance (CB) CT
ratios. For this reason, the relay
sensitivity to ground faults is less
when the residual overcurrent
element is used instead of the CB
element. A separate ground fault
detection method should be used if a
CB CT is not available in applications
where resistance grounding reduces
the available ground fault current.
Summary of Contents for 751A
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