ABB RELION RET670 Applications Manual page 404

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