ABB RELION RET670 Applications Manual page 265

1MRK 504 163-UUS A
Transformer protection RET670 2.2 ANSI
Application manual
Effectively grounded networks
A network is defined as effectively grounded if the ground-fault factor f
1.4. The ground-fault factor is defined according to equation 110.
V
=
f max
e
V
pn
ANSIEQUATION1268 V1 EN-US
Where:
V is the highest fundamental frequency voltage on one of the healthy phases at single
max
phase-to-ground fault.
V is the phase-to-ground fundamental frequency voltage before fault.
pn
Another definition for effectively grounded network is when the following
relationships between the symmetrical components of the network impedances are
valid, see equation 111 and equation 112.
< ×
X 3 X
0 1
EQUATION2122 V1 EN-US
£
R
R
0 1
EQUATION2123 V1 EN-US
Where
R is the zero sequence source resistance
0
X is the zero sequence source reactance
0
R is the positive sequence source resistance
1
X is the positive sequence source reactance
1
The magnitude of the ground-fault current in effectively grounded networks is high
enough for impedance measuring elements to detect ground faults. However, in the
same way as for solidlygrounded 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.
Section 8
Impedance protection
is less than
e
(Equation 110)
(Equation 111)
(Equation 112)
M17048-38 v6
M17048-57 v6
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