Siemens siprotec 7SA6 User Manual page 229

Measured Value
Correction for
Parallel Lines
7SA6 Manual
C53000-G1176-C133-1
Table 6-8 Evaluation of measured loops for a multiple pick-up in non-earthed systems
Fault detection
Loops
L1–E, L2–E, (L1–L2)
L2–E, L3–E, (L2–L3)
L1–E, L3–E, (L3–L1)
L1–E, L2–E, (L1–L2)
L2–E, L3–E, (L2–L3)
L1–E, L3–E, (L3–L1)
L1–E, L2–E, (L1–L2)
L2–E, L3–E, (L2–L3)
L1–E, L3–E, (L3–L1)
L1–E, L2–E, (L1–L2)
L2–E, L3–E, (L2–L3)
L1–E, L3–E, (L3–L1)
During earth faults on parallel lines, the impedance values calculated by means of the
loop equations are influenced by the coupling of the earth impedance of the two
conductor systems (Figure 6-27). Unless special measures are employed, this results
in measuring errors in the result of the impedance computation. A parallel line
compensation may therefore be activated. In this manner the earth current of the
parallel line is taken into consideration by the line equation and thereby allows for
compensation of the coupling influence. The earth current of the parallel line must be
connected to the device for this purpose. The loop equation is then modified as shown
below, refer also to Figure 6-24
⋅ ⋅
Z Z
I – I – I
L3 L E E EP
Z
⋅ ⋅ ⋅
Z Z ------ - I
I – I
L3 L E L
Z
where I is the earth current of the parallel line and the ratio Z
EP
parameter, resulting from the geometry of the double circuit line and the nature of the
ground below the line. These line parameters are input to the device — along with all
the other line data — during the parameterisation of the device. The line impedance
is calculated with the equation below similar to the calculation shown earlier.
U
Z --------------------------------------------------------------------------------
=
⁄ ⋅
L
Z Z
I –
L3 E L
Figure 6-27 Earth fault on a double circuit line
Evaluated
Loop(s)
L2–E
L2–E
L3–E
L1–E
L2–E
L3–E
L2–E
L3–E
L1–E
L1–E, L2–E
L2–E, L3–E
L3–E, L1–E
⋅
Z U
=
M
L3–E
Z
E M
⋅ ⋅
Z ------- U
– =
EP L
L3–E
Z
L L
L3–E
⁄ ⋅
Z Z
I – I
E M L EP
A
I
Z I
L L3
U
L3-E
Z I
E E
Z
I
M EP
II
Setting
Parameter 1221
3+$6( 35()SKH =
/ / DF\FOLF
3+$6( 35()SKH =
/ / F\FOLF
3+$6( 35()SKH =
/ / F\FOLF
3+$6( 35()SKH =
$OO ORRSV
/Z is a constant line
M L
B
e.g. L3–E
Functions
6-47