Application – 2
2–49
87 PICKUP
________ Amps
87 SLOPE
________ %
87 DELAY
________ Cycles
87 Phase Differential
The phase differential function (87) is a percentage
differential with an adjustable slope of 1–100%.
Although this protection is used to protect the
machine from all internal winding faults, single-
phase to ground faults in machines with high
impedance grounding may have currents less than
the sensitivity of the differential relay (typically
between 3 and 30 primary amps). Ranges and
increments are presented in Table 2-28.
Turn-to-turn faults are not detected by differential
relays because the current into the generator equals
the current out. Even though the percentage
differential relay is more tolerant of CT errors, all
CTs should have the same characteristics and
accuracies.
To provide restraint for CT saturation at high offset
currents, the slope is automatically adjusted (at a
restraining current equal to two times nominal current)
to four times the slope setting, see Figure 2-20
below.
For very high currents in large generators, the
proximity of CTs and leads in different phases can
cause unbalanced currents to flow in the secondaries.
These currents must be less than the minimum
sensitivity of the relay.
N
O
I
T
C
N
U
F
E
G
N
A
R
T
N
I
O
P
T
E
S
T
N
E
M
E
R
C
N
I
)
7
8
(
t
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f
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p
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k
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m
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M
A
0
0
.
3
o
t
A
0
2
.
0
)
A
0
6
.
0
o
t
4
0
.
0
(
A
1
0
.
0
e
p
o
l
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t
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%
0
0
1
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t
1
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1
y
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C
0
6
1
8
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1
2
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1
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1
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Table 2-28
Differential (87) Setpoint Ranges
@ I
RES
=2xI
NOM
Restraint Current
(IA+Ia)/2, (IB+Ib)/2, (IC+Ic)/2
(IA-Ia),
(IB-Ib),
(IC-Ic)
MIN PU
TRIP
SLOPE
(set)
SLOPE
(4xset)
BLOCK
Where I
A
and I
a
are generator high side and neutral side currents respectively.
Figure 2-20
Differential Relay (87) Operating Characteristics
A typical setting is 0.2 to 0.3 amps.
A typical setting is 10%.
A typical setting is 3 to 5 cycles. Typical settings given above assume
matched current transformer performance, and that transformer inrush
of the unit transformer does not cause dc saturation of the generator
CTs. If there is a significant difference in current transformer ratings
(C800 vs C200, for example), or if saturation of the generator CTs is
expected during energizing of the step up transformer, more appropriate
settings might be 0.5 A pick up, 20% slope, and a delay of 5 to 8 cycles.
Summary of Contents for M-3425
Page 1: ...Instruction Book M 3425 Generator Protection ...
Page 14: ... 13 M 3425 Generator Protection Relay Figure 1 External Connections ...
Page 33: ...x M 3425 Instruction Book This Page Left Intentionally Blank ...
Page 89: ...M 3425 Instruction Book 2 52 This Page Left Intentionally Blank ...
Page 125: ...1 3 A B C M 3425 Instruction Book 4 26 This Page Left Intentionally Blank ...
Page 187: ...M 3425 Instruction Book 6 50 This Page Left Intentionally Blank ...
Page 207: ...M 3425 Instruction Book C 4 This Page Left Intentionally Blank ...
Page 209: ...D 2 M 3425 Instruction Book Figure D 1 Volts Hz 24 Inverse Curve Family 1 Inverse Square ...
Page 210: ...Inverse Time Curves Appendix D D 3 Figure D 2 Volts Hz 24 Inverse Family Curve 2 ...
Page 211: ...D 4 M 3425 Instruction Book Figure D 3 Volts Hz 24IT Inverse Curve Family 3 ...
Page 212: ...Inverse Time Curves Appendix D D 5 Figure D 4 Volts Hz 24IT Inverse Curve Family 4 ...
Page 215: ...D 8 M 3425 Instruction Book Figure D 5 Definite Time Overcurrent Curve ...
Page 216: ...Inverse Time Curves Appendix D D 9 Figure D 6 Inverse Time Overcurrent Curve ...
Page 217: ...D 10 M 3425 Instruction Book Figure D 7 Very Inverse Time Overcurrent Curve ...
Page 218: ...Inverse Time Curves Appendix D D 11 Figure D 8 Extremely Inverse Time Overcurrent Curve ...
Page 223: ...D 16 M 3425 Instruction Book This Page Intentionally Left Blank ...
