ABB RELION RET670, Applications Manual

Page: 197 / 1016

Z0 is the zero sequence impedance (Ω/phase)
Zf is the fault impedance (Ω), often resistive
ZN is the ground return impedance defined as (Z0-Z1)/3
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 solid grounded networks makes it possible to use
impedance measuring technique to detect ground-fault. However, distance protection
has limited possibilities to detect high resistance faults and must, therefore, always be
complemented with other protection function(s) that can carry out the fault clearance in
those cases.
Effectively grounded networks
SEMOD168232-40 v2
A network is defined as effectively grounded if the ground-fault factor fe is less than
1.4. The ground-fault factor is defined according to equation
30 .
max
e
pn
V
f
V
=
ANSIEQUATION1268 V1 EN-US
(Equation 30)
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, as shown in equation
31 and equation 32 .
X X
0 1
3
≤ ⋅
EQUATION1269 V4 EN-US
(Equation 31)
R X
£
0 1
EQUATION1270 V4 EN-US
(Equation 32)
The magnitude of the ground fault current in effectively grounded networks is high
enough for impedance measuring element to detect ground-fault. However, in the same
1MRK 504 163-UUS A
Section 8
Impedance protection
Transformer protection RET670 2.2 ANSI 191
Application manual