ABB RELION RET670 - Page 329

Effectively grounded networks
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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
201.
max
e
pn
V
f
V
=
ANSIEQUATION1268 V1 EN-US (Equation 201)
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 202 and equation 203.
0 1
X 3 X< ×
EQUATION2122 V1 EN-US (Equation 202)
0 1
R R£
EQUATION2123 V1 EN-US (Equation 203)
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
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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 504 163-UUS A Section 8
Impedance protection
Transformer protection RET670 2.2 ANSI 323
Application manual