ABB RELION 650 Series Applications Manual page 108

Section 7
Impedance protection
The earth-fault current at single phase-to-earth in phase L1 can be calculated as equation 31:
=
3I
0
Z
EQUATION1267 V3 EN-US
Where:
U
L1
Z
1
Z
2
Z
0
Z
f
Z
N
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 32.
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
< ×
X 3 X
0
EQUATION2122 V1 EN-US
£
R
R
0 1
EQUATION2123 V1 EN-US
Where
R
0
X
0
102
×
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
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.
1
is the resistive zero sequence of the source
is the reactive zero sequence of the source
L1
+
Z Z
N f
-Z )/3
0 1
1MRK 505 363-UEN A
(Equation 31)
GUID-39CAF169-315E-4E3E-9EE6-28CBF624B90E v5
is less than 1.4. The earth-
e
(Equation 32)
33 and 34.
(Equation 33)
(Equation 34)
Application manual