Effectively grounded networks
A network is defined as effectively grounded if the ground-fault factor f
e
is less than 1.4.
The ground-fault factor is defined according to equation 25.
ANSIEQUATION1268 V1 EN (Equation 232)
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 source impedances are valid, see
equation
106 and equation 107.
EQUATION2122 V1 EN (Equation 233)
EQUATION2123 V1 EN (Equation 234)
Where
R
0
is the resistive zero sequence source impedance
X
0
is the reactive zero sequence source impedance
R
1
is the resistive positive sequence source impedance
X
1
is the reactive positive sequence source impedance
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.
1MRK 506 369-UUS - Section 8
Impedance protection
Line distance protection REL670 2.2 ANSI 311
Application manual
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