ABB 615 Series - Page 821

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
AB
C
AB
C
AB
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