ABB RELION 670 SERIES Applications Manual page 363

1MRK 506 369-UUS -
Line distance protection REL670 2.2 ANSI
Application manual
The voltage on the healthy phases during line to ground fault is generally lower than 140%
of the nominal phase-to-ground voltage. This corresponds to about 80% of the nominal
phase-to-phase voltage.
The high zero-sequence current in solidly grounded networks makes it possible to use
impedance measuring techniques to detect ground faults. However, distance protection
has limited possibilities to detect high resistance faults and should therefore always be
complemented with other protection function(s) that can carry out the fault clearance in
those cases.
Effectively grounded networks
A network is defined as effectively grounded if the ground-fault factor f
The ground-fault factor is defined according to equation 25.
V
=
f max
e
V
pn
ANSIEQUATION1268 V1 EN
Where:
V is the highest fundamental frequency voltage on one of the healthy phases at single phase-
max
to-ground fault.
V is the phase-to-ground fundamental frequency voltage before fault.
pn
Another definition for effectively grounded network is when the following relationships
between the symmetrical components of the network impedances are valid, see equation
106 and 107.
< ×
X 3 X
0 1
EQUATION2122 V1 EN
£
R
R
0 1
EQUATION2123 V1 EN
Where
R is the resistive zero sequence of the source
0
X is the reactive zero sequence of the source
0
R is the resistive positive sequence of the source
1
X is the reactive positive sequence of the source
1
Section 8
Impedance protection
is less than 1.4.
e
(Equation 290)
(Equation 291)
(Equation 292)
357