ABB RET670 - Page 291

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 199)
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, see equation
200 and equation 201.
X X×
0 1
= 3
EQUATION1269 V3 EN (Equation 200)
R R£
0 1
EQUATION1270 V3 EN (Equation 201)
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
1MRK504116-UUS C Section 3
IED application
285
Application manual