ABB REL670 2.2 IEC, Applications Manual

Page: 367 / 908

f
U
U
e
pn
=
max
EQUATION1268 V4 EN-US
(Equation 335)
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 impedances are
valid, see equations
336 and 337 :
0 1
X 3 X
< ×
EQUATION2122 V1 EN-US
(Equation 336)
0 1
R
R
£
EQUATION2123 V1 EN-US
(Equation 337)
Where
R
0
is the resistive zero sequence of the source
X
0
is the reactive zero sequence of the source
R
1
is the resistive positive sequence of the source
X
1
is the reactive positive sequence of the source
The magnitude of the earth-fault current in effectively earthed networks is high
enough for impedance measuring elements to detect earth faults. However, in the
same way as for solidly 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-123D2D9D-8055-4A2F-9CC2-6EAE0C9F905B v2
In this type of network, it is mostly not possible to use distance protection for
detection and clearance of earth faults. The low magnitude of the earth-fault
current might not give start of the zero-sequence measurement elements or the
sensitivity will be too low for acceptance. For this reason a separate high sensitive
earth-fault protection is necessary to carry out the fault clearance for single phase-
to-earth fault.
1MRK 506 369-UEN B
Section 8
Impedance protection
Line distance protection REL670 2.2 IEC 361
Application manual