IEC05000215 V2 EN-US
Figure 146: 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 ZZ
×
= =
+ + + + +
EQUATION1267 V3 EN-US (Equation 228)
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)
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 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 should therefore always
be complemented with other protection function(s) that can carry out the fault
clearance in those cases.
Effectively earthed networks
GUID-5F4CCC18-2BAC-4140-B56C-B9002CD36318 v1
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
229.
1MRK 506 369-UEN B Section 8
Impedance protection
Line distance protection REL670 2.2 IEC 291
Application manual
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