ABB RELION 670 Series Applications Manual page 230

Section 7
Impedance protection
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 equation163:
=
3I
0
EQUATION1267 V3 EN-US
Where:
UL1
Z1
Z2
Z0
Zf
ZN
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
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 164.
U
f
max
=
e
U
EQUATION1268 V4 EN-US
Where:
U
max
U
pn
Another definition for effectively earthed network is when the following relationships between
the symmetrical components of the network impedances are valid, see equation
equation 166.
≤ ⋅
X
0
EQUATION1269 V4 EN-US
224
×
3 U
=
L1
+ + +
Z Z Z 3Z
1 2 0 f
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 (Z0-Z1)/3
pn
is the highest fundamental frequency voltage on one of the healthy phases at single
phase-to-earth fault.
is the phase-to-earth fundamental frequency voltage before fault.
3
X
1
U
L1
+ +
Z Z Z
1 N f
1MRK 504 152-UEN B
(Equation 163)
SEMOD154680-40 v4
(Equation 164)
165 and
(Equation 165)
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