ABB RELION 670 Series Applications Manual page 255

1MRK 504 152-UEN B
Effectively earthed networks
A network is defined as effectively earthed if the earth-fault factor f
fault factor is defined according to equation 208.
U
f max
=
e
U
EQUATION1268 V4 EN-US
Where:
U
max
U
pn
Another definition for effectively earthed network is when the following relationships between
the symmetrical components of the network source impedances are valid, see equation
and equation 210.
< ×
X 3 X
0
EQUATION2122 V1 EN-US
£
R
R
0 1
EQUATION2123 V1 EN-US
Where
R
0
X
0
R
1
X
1
The magnitude of the earth-fault current in effectively earthed networks is high enough for
impedance measuring element to detect earth-fault. However, in the same way as for solid
earthed 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 earthed networks
In high impedance networks, the neutral of the system transformers are connected to the
earth 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
networks.
What is typical for this type of network is that the magnitude of the earth 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 voltage (3U
have the same magnitude in different places in the network due to low voltage drop
distribution.
The magnitude of the total fault current can be calculated according to equation 211.
Application manual
pn
is the highest fundamental frequency voltage on one of the healthy phases at single
phase-to-earth fault.
is the phase-to-earth fundamental frequency voltage before fault.
1
is the resistive zero sequence source impedance
is the reactive zero sequence source impedance
is the resistive positive sequence source impedance
is the reactive positive sequence source impedance
Section 7
Impedance protection
GUID-5F4CCC18-2BAC-4140-B56C-B9002CD36318 v1
is less than 1.4. The earth-
e
(Equation 208)
209
(Equation 209)
(Equation 210)
GUID-099F53F9-7CD8-4D96-811D-7B55EF249FFF v1
) will
0
249