ABB RELION RET670 Applications Manual page 298

Section 8
Impedance protection
292
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 157.
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 equation 158 and equation 159.
X 3 X
≤ ⋅
0 1
EQUATION1269 V4 EN-US
£
R X
0 1
EQUATION1270 V4 EN-US
The magnitude of the ground fault current in effectively grounded networks is high
enough for impedance measuring element to detect 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.
Typically, for this type of network is that the magnitude of the ground fault current is
very low compared to the short circuit current. The voltage on the healthy phases will
get a magnitude of √3 times the phase voltage during the fault. The zero sequence
1MRK 504 163-UUS A
(Equation 157)
(Equation 158)
(Equation 159)
Transformer protection RET670 2.2 ANSI
Application manual
SEMOD154680-40 v4
SEMOD154680-58 v3