ABB RELION 670 Series Applications Manual page 331

1MRK 505 343-UEN B
Solidly earthed networks
In solidly earthed systems, the transformer neutrals are connected directly to earth without
any impedance between the transformer neutral and earth.
IEC05000215 V2 EN-US
Figure 200: 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 357:
=
3I
0
Z
EQUATION1267 V3 EN-US
Where:
U
L1
Z
1
Z
2
Z
0
Z
f
Z
N
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
A network is defined as effectively earthed if the earth-fault factor f
fault factor is defined according to equation 358:
U
f
max
=
e
U
EQUATION1268 V4 EN-US
Application manual
×
3 U U
=
L1
+ + + +
Z Z 3Z Z
1 2 0 f 1
is the phase-to-earth voltage (kV) in the faulty phase before
fault.
is the positive sequence impedance (Ω/phase).
is the negative sequence impedance (Ω/phase).
is the zero sequence impedance (Ω/phase).
is the fault impedance (Ω), often resistive.
is the earth-return impedance defined as (Z
pn
L1
+
Z Z
N f
-Z )/3.
0 1
Section 7
Impedance protection
GUID-6870F6A8-EB28-47CF-AF26-7CE758BF934E v1
(Equation 357)
GUID-613296E6-A612-4B8B-9F23-311D07CFDB0E v2
is less than 1.4. The earth-
e
(Equation 358)
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