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If the impedance settings are in use, it is important that the settings closely match the
impedances of used conductor types. The impedance settings are given in primary ohms
[ohm/pu] and the line section lengths in per unit [pu]. Thus, impedances can be either
given in ohm/km and section length in km, or ohm/mile and section length in miles. The
resulting Flt distance matches the units entered for the line section lengths.
Positive-sequence impedance values
Fault location requires accurate setting values for line impedances. Positive-sequence
impedances are required both for location of short circuits and ground 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
GUID-B7F3697A-7C8E-4BF6-A63C-7BFD307DD128 V2 EN
(Equation 151)
ω
n
2 × π × f
n
, where f
n
= fundamental frequency [Hz]
a
en
a
a
a
12
23
31
3
⋅
⋅
(
)
GUID-DA850ABF-239A-4AB5-B63B-F0B54557CF2E V2 EN
the geometric average of phase distances [m]
a
xy
distance [m] between phases x and y
r
radius [m] for single conductor
A
B
C
A
B
C
A
B
C
GUID-F884BB08-6948-4327-843A-176A412EFF0B V1 EN
Figure 416:
Typical distribution line tower configurations
Example values of positive-sequence impedances for typical medium voltage overhead-
lines are given in the following tables.
1MAC059074-MB A
Section 5
Protection related functions
615 series ANSI
815
Technical Manual
Содержание Relion 615 series
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