ABB RELION RET670, Руководство по применению

Страница: 298 / 1016

Effectively grounded networks
SEMOD154680-40 v4
A network is defined as effectively grounded if the ground fault factor fe is less than
1.4. The ground fault factor is defined according to equation
157 .
max
e
pn
V
f
V
=
ANSIEQUATION1268 V1 EN-US
(Equation 157)
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
158 and equation 159 .
X X
0 1
3
≤ ⋅
EQUATION1269 V4 EN-US
(Equation 158)
R X
£
0 1
EQUATION1270 V4 EN-US
(Equation 159)
The magnitude of the ground fault current in effectively grounded networks is high
enough for impedance measuring element to detect 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
SEMOD154680-58 v3
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.
This type of network is many times operated in radial, but can also be found operating
meshed.
Typically, for this type of network is that the magnitude of the ground fault current is
very low compared to the short circuit current. The voltage on the healthy phases will
get a magnitude of √3 times the phase voltage during the fault. The zero sequence
Section 8
1MRK 504 163-UUS A
Impedance protection
292 Transformer protection RET670 2.2 ANSI
Application manual