ABB RET670 - Page 197

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
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
52.
max
e
pn
V
f
V
=
ANSIEQUATION1268 V1 EN (Equation 99)
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
100 and equation 101.
X X
×
0 1
= 3
EQUATION1269 V3 EN (Equation 100)
R R£
0 1
EQUATION1270 V3 EN (Equation 101)
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
way as for solid grounded networks, 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 this case.
Fault infeed from remote end
All transmission and most all sub transmission networks are operated meshed. Typical
for this type of network is that we will have fault infeed from remote end when fault
1MRK504116-UUS C Section 3
IED application
191
Application manual