Functions
2.5 Distance Protection
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
156
The arc resistance would be:
and the tower footing resistances R
TF
= 3 Ω
As a result, the resistance must be set to
primary:
or secondary
:
In practice, the ratio between resistance and reactance setting is situated in the ranges shown below
(see also /5/):
Note
The following must be kept in mind for short lines with a high R/X ratio for the zone setting: The angle errors of
the current and voltage transformers cause a rotation of the measured impedance in the direction of the R axis.
If due to the polygon, R
E
/R
L
and X
E
/X
L
settings the loop reach in R direction is large in relation to the X direction,
there is an increased risk of external faults being shifted into zone Z1. A grading factor of 85 % should only be
used up to R/X ≤ 1 (loop reach). For larger R/X settings, a reduced grading factor for zone 1 can be calculated
with the following formula (see also /5/)
The reduced grading factor is calculated from:
GF = Grading factor = reach of zone Z1 in relation to the line length
R = Loop reach in R direction for zone Z1 = R1 · (1+R
E
/R
L
)
X = Loop reach in X direction for zone Z1 = X1 · (1+X
E
/X
L
)
δ
U
= Voltage transformer angle error (typical: 1°)
δ
I
= Current transformer angle error (typical: 1°)
In addition or as an alternative, it is also possible to use the setting 1607 Zone Reduction, to modify the
inclination of the zone Z1 polygon and thus prevent overreach (see Figure 2-52).
Type of Line R/X Ratio of the Zone Setting
Short underground cable lines (approx. 0.5 km to 3 km / 0.3 to 1.88 miles) 3 to 5
Longer underground cable lines (> 3 km / 1.88 miles) 2 to 3
Short overhead lines < 10 km (6.25 miles) 2 to 5
Overhead lines < 100 km (62.5 miles) 1 to 2
Long overhead lines between 100 km and 200 km (62.5 miles and 125
miles)
0.5 to 1
Long EHV lines > 200 km (125 miles) ≤ 0,5
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