faults. As data sheet impedance per unit values are generally valid only for
a certain tower configuration, the values should be adjusted according to the
actual installation configuration. This minimizes the fault location errors caused
by inaccurate settings.
The positive-sequence reactance per unit and per phase can be calculated with a
following approximation equation which applies to symmetrically transposed three-
phase aluminium overhead lines without ground wires.
X
a
r
km
n
en
1
4
10
2
0 5
≈
⋅
⋅
+
−
ω
ln
.
[
/
]
Ω
(Equation 166)
ω
n
2 × π × f
n
, where f
n
= fundamental frequency [Hz]
a
en
a
a
a
12
23
31
3
⋅
⋅
(
)
the geometric average of phase distances [m]
a
xy
distance [m] between phases x and y
r
radius [m] for single conductor
Figure 449: Typical distribution line tower configurations
Example values of positive-sequence impedances for typical medium voltage
overhead-lines are given in the following tables.
Table 812: Positive-sequence impedance values for typical 11 kV conductors,
“Flat” tower configuration assumed
Name
R1 [Ω/km]
X1 [Ω/km]
ACSR 50 SQ.mm
0.532
0.373
ACSR 500 SQ.mm
0.0725
0.270
Protection related functions
1MRS757644 H
846
620 series
Technical Manual
