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
e
is less than
1.4. The ground-fault factor is defined according to equation
52.
ANSIEQUATION1268 V1 EN (Equation 243)
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
53 and equation 54.
EQUATION2122 V1 EN (Equation 244)
EQUATION2123 V1 EN (Equation 245)
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
Section 3 1MRK504116-UUS C
IED application
318
Application manual
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