IEC05000215 V2 EN-US
Figure 70: Solidly earthed network
The earth fault current is as high or even higher than the short-circuit current. The series
impedances determine the magnitude of the fault current. The shunt admittance has very
limited influence on the earth fault current. The shunt admittance may, however, have some
marginal influence on the earth fault current in networks with long transmission lines.
The earth fault current at single phase -to-earth in phase L1 can be calculated as equation
L1
L1
0
1
2
0
f
1
N
f
3 U
U
3I
Z
Z
Z
3Z
Z
Z
Z
×
=
=
+
+
+
+
+
EQUATION1267 V3 EN-US
(Equation 42)
Where:
UL1
is the phase-to-earth voltage (kV) in the faulty phase before
fault
Z1
is the positive sequence impedance (Ω/phase)
Z2
is the negative sequence impedance (Ω/phase)
Z0
is the zero sequence impedance (Ω/phase)
Zf
is the fault impedance (Ω), often resistive
ZN
is the earth return impedance defined as (Z0-Z1)/3
The voltage on the healthy phases is generally lower than 140% of the nominal phase-to-earth
voltage. This corresponds to about 80% of the nominal phase-to-phase voltage.
The high zero sequence current in solid earthed networks makes it possible to use impedance
measuring technique to detect earth-fault. However, distance protection has limited
possibilities to detect high resistance faults and must, therefore, always be complemented
with other protection function(s) that can carry out the fault clearance in those cases.
Effectively earthed networks
SEMOD168232-40 v2
A network is defined as effectively earthed if the earth-fault factor fe is less than 1.4. The
earth-fault factor is defined according to equation
f
U
U
e
pn
=
max
EQUATION1268 V4 EN-US
(Equation 43)
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.
Section 7
1MRK 505 343-UEN B
Impedance protection
146
Application manual
Summary of Contents for Relion 670 series
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