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Date Code 20010625
Example Calculations for 87L Settings
J-1
SEL-311L Instruction Manual
APPENDIX J: EXAMPLE CALCULATIONS FOR 87L
SETTINGS
87LANG S
ETTING
C
ONSIDERATIONS
This section describes calculations useful in determining the optimal Alpha plane angle setting:
87LANG. The Alpha plane characteristic angle is adjustable from 90° to 270°. Remember that
while a larger angle setting does permit greater security for out-of-section faults, making
87LANG too large affects the dependability for internal faults.
Source S
Source R
Relay 1
Relay 2
Line
BKR 1
BKR 2
M311L142
Figure J.1: Example System Single Line
Source S
Z
1S
E
S
∠δ
°
E
R
∠
0°
Source R
Relay 1
m • Z
1L
(1 - m) • Z
1L
Relay 2
Z
1R
I
1S
= I
1L
I
1R
M311L143
Z1S = Source S Positive-Sequence Impedance
Z1R = Source R Positive-Sequence Impedance
Z1L = Line Positive-Sequence Impedance
m
= Per-unit distance from Breaker 1 (BKR 1)
I1S = Positive-Sequence current measured by Relay 1 (local relay current, I
1L
, for our
example)
I1R = Positive-Sequence current measured by Relay 2 (remote relay current, I
1R
, for our
example)
Figure J.2: Sequence Connection Diagram for an Internal Three-Phase Fault
Figure J.2 shows the positive-sequence connection diagram for an internal three-phase fault
shown in Figure J.1. If I
1S
and I
1R
are equal in magnitude and phase, then I
AR
/ I
AL
= 1
∠
0°. For
this to occur given a fault placed at m = 0.5, the system twist angle (
δ
) must be zero, and Sources
S and R must have equal strength (where m = per-unit distance from Bus S). How do source
impedances,
δ
, and fault location m effect I
AR
/ I
AL
?
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