ABB REL670 2.2 IEC, Applications Manual

Page: 298 / 908

f
U
U
e
pn
=
max
EQUATION1268 V4 EN-US
(Equation 229)
Where:
U
max
is the highest fundamental frequency voltage on one of the healthy phases at single
phase-to-earth fault.
U
pn
is the phase-to-earth fundamental frequency voltage before fault.
Another definition for effectively earthed network is when the following
relationships between the symmetrical components of the network source
impedances are valid, see equation
230 and equation 231 .
0 1
X 3 X
< ×
EQUATION2122 V1 EN-US
(Equation 230)
0 1
R
R
£
EQUATION2123 V1 EN-US
(Equation 231)
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 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
GUID-099F53F9-7CD8-4D96-811D-7B55EF249FFF v1
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.
Section 8
1MRK 506 369-UEN B
Impedance protection
292 Line distance protection REL670 2.2 IEC
Application manual