ABB RELION RET670 - Page 404

The voltage on the healthy phases 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
GUID-613296E6-A612-4B8B-9F23-311D07CFDB0E v2
A network is defined as effectively grounded if the ground-fault factor f
e
is less than
1.4. The ground-fault factor is defined according to equation
305:
max
e
pn
V
f
V
=
ANSIEQUATION1268 V1 EN-US (Equation 305)
Where:
V
max
is the highest fundamental frequency voltage on one of the healthy phases at single
phase-to-
ground
fault.
V
pn
is the phase-to-ground fundamental frequency voltage before fault.
Another definition for effectively grounded network is when the following
relationships between the symmetrical components of the network impedances are
valid, see equations
306 and 307:
0 1
X 3 X< ×
EQUATION2122 V1 EN-US (Equation 306)
0 1
R R£
EQUATION2123 V1 EN-US (Equation 307)
Where
R
0
is the resistive zero sequence of the source
X
0
is the reactive zero sequence of the source
R
1
is the resistive positive sequence of the source
X
1
is the reactive positive sequence of the source
Section 8 1MRK 504 163-UUS A
Impedance protection
398 Transformer protection RET670 2.2 ANSI
Application manual