ABB REG670 Applications Manual page 231

1MRK 502 071-UEN -
Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
Application manual
Parallel line out of service and not earthed
A
Z
0m
Z< Z<
IEC09000254_1_en.vsd
IEC09000254 V1 EN
Figure 103: Parallel line is out of service and not earthed
When the parallel line is out of service and not earthed, the zero sequence on that line
can only flow through the line admittance to the earth. The line admittance is high
which limits the zero-sequence current on the parallel line to very low values. In
practice, the equivalent zero-sequence impedance circuit for faults at the remote bus
bar can be simplified to the circuit shown in figure
The line zero sequence mutual impedance does not influence the measurement of the
distance protection in a faulty circuit. This means that the reach of the underreaching
distance protection zone is reduced if, due to operating conditions, the equivalent zero
sequence impedance is set according to the conditions when the parallel system is out
of operation and earthed at both ends.
-
Z Z
0 0m
I
0
A
-
Z Z
0 0m
B
IEC09000255 V1 EN
Figure 104: Equivalent zero-sequence impedance circuit for a double-circuit line
with one circuit disconnected and not earthed
The reduction of the reach is equal to equation 51.
( )
1
× × + +
2 Z Z
1 0 E
3
=
K
U
( )
1
× × + +
2 Z Z
1 0
3
EQUATION1284 V1 EN
This means that the reach is reduced in reactive and resistive directions. If the real and
imaginary components of the constant A are equal to equation
= × × +
Re( ) A R 0 (2 R 1
EQUATION1285 V1 EN
B
Z
0m
I
0
C
IEC09000255_1_en.vsd
R
Z
f
= -
m 0
1
(
× × +
Z 2 Z Z
R
1
0
f
+ × - ×
R 0 3 Rf ) X 0 ( 0 2 X
Section 8
Impedance protection
88
2
)
+
3 R
0
f
52 and equation 53.
+ ×
X 1)
(Equation 97)
(Equation 98)
225