ABB RET670 Applications Manual page 268

Section 3
IED application
262
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 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 52.
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 179 and equation 180.
×
X = 3 X
0 1
EQUATION1269 V3 EN
£
R R
0 1
EQUATION1270 V3 EN
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 should therefore always be complemented with other
protection function(s) that can carry out the fault clearance in this case.
High impedance grounded networks
In high impedance networks the neutral of the system transformers are connected to the
ground through high impedance, mostly a reactance in parallel with a high resistor.
This type of network is many times operated in radial, but can also be found operating
meshed networks.
1MRK504116-UUS C
is less than
e
(Equation 178)
(Equation 179)
(Equation 180)
Application manual