ABB Relion 670 series - Page 259

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 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
25.
max
e
pn
V
f
V
=
ANSIEQUATION1268 V1 EN (Equation 152)
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 153 and equation 154.
X X
0 1
3≤ ⋅
EQUATION1269 V4 EN (Equation 153)
R X£
0 1
EQUATION1270 V4 EN (Equation 154)
Where
R
0
is the zero sequence resistance
X
0
is the zero sequence reactance
X
1
is the positive sequence reactance
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
1MRK 506 369-UUS - Section 8
Impedance protection
Line distance protection REL670 2.2 ANSI 253
Application manual