ABB Relion REG670, Applications Manual

Page: 221 / 780

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 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
e
is less than 1.4.
The earth-fault factor is defined according to equation 34 :
f
U
U
e
pn
=
max
EQUATION1268 V4 EN
(Equation 82)
Where:
U
max
is the highest fundamental frequency voltage on one of the healthy phases at single phase-
to-earth fault.
U
pn
is the phase-to-earth fundamental frequency voltage before fault.
Another definition for effectively earthed network is when the following relationships
between the symmetrical components of the network impedances are valid, see
equations
35 and 36 :
0 1
X 3 X
< ×
EQUATION2122 V1 EN
(Equation 83)
0 1
R
R
£
EQUATION2123 V1 EN
(Equation 84)
Where
R
0
is the resistive zero sequence of the source
1MRK 502 071-UEN -
Section 8
Impedance protection
Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062 215
Application manual