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 solidly grounded networks makes it possible to use
impedance measuring techniques to detect ground faults. 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
e
is less than 1.4.
The ground-fault factor is defined according to equation
36.
ANSIEQUATION1268 V1 EN (Equation 36)
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 37 and equation 38.
EQUATION2122 V1 EN (Equation 37)
EQUATION2123 V1 EN (Equation 38)
Where
R
0
is the resistive zero sequence reach
X
0
is the reactive zero sequence reach
R
1
is the resistive positive sequence reach
X
1
is the reactive positive sequence reach
1MRK 506 334-UUS A Section 6
Impedance protection
115
Application manual
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