IEC05000215 V2 EN-US
Figure 122: 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 122)
Where:
U
L1
is the phase-to-earth voltage (kV) in the faulty phase before
fault
Z
1
is the positive sequence impedance (Ω/phase)
Z
2
is the negative sequence impedance (Ω/phase), is considered
to be equal to Z
1
Z
0
is the zero sequence impedance (Ω/phase)
Z
f
is the fault impedance (Ω), often resistive
Z
N
is the earth-return impedance defined as (Z
0
-Z
1
)/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 solidly earthed networks makes it possible to use
impedance measuring techniques to detect earth faults. However, 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 those
cases.
Effectively earthed networks
M17048-38 v6
A network is defined as effectively earthed if the earth-fault factor f
e
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 123)
Section 7
1MRK 505 343-UEN B
Impedance protection
198
Application manual
Содержание Relion 670 series
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