Schweitzer Engineering Laboratories SEL-311L, Instruction Manual, Page: 725 / 734

Date Code 20010625 Example Calculations for 87L Settings J-3
SEL-311L Instruction Manual
Calculating I
2R
/I
2L
for Ground Faults
Equation J.4 and Equation J.5 show the local terminal negative-sequence currents as a function
of total negative-sequence current (I
2T
). Equation J.6 shows the ratio of remote to local negative-
sequence currents shown in Figure J.3. Note that Equation J.6 differs from Equation J.3 only by
the ratio of local and remote source voltages. This means that the negative-sequence ratio is not
affected by load flow magnitude. Because the phase, negative- and zero-sequence 87L elements
all use 87LANG to establish the restrain characteristic angle, the limiting case is that set by the
phase differential elements. Any increase in Alpha plane coverage caused by (E
R
∠
0°/ E
S
∠δ
°)
serves to increase the security of the 87L2 and 87LG elements without sacrificing sensitivity.
( )
[ ]
[ ]
R 2 L 2 S 2
R 2 L 2
T 2 S 2
Z Z Z
Z Z • m 1
• I I
+ +
+ −
=
Equation J.4
[ ]
[ ]
R 2 L 2 S 2
S 2 L 2
T 2 R 2
Z Z Z
Z Z • m
• I I
+ +
+
=
Equation J.5
[ ]
( )
[ ]
L 1 R 1
L 1 S 1
S 2
R 2
Z • m 1 Z
Z • m Z
I
I
− +
+
=
Equation J.6