ABB RELION RET670 Applications Manual page 337

1MRK 504 163-UUS A
Transformer protection RET670 2.2 ANSI
Application manual
A
21
ANSI05000221 V1 EN-US
Figure 154: Class 1, parallel line in service.
The equivalent zero sequence circuit of the lines can be simplified, see figure 155.
-
Z
Z
0 0m
A
-
Z
Z
0
0m
B
IEC09000253 V1 EN-US
Figure 155: Equivalent zero sequence impedance circuit of the double-circuit,
parallel, operating line with a single phase-to-ground fault at the
remote busbar.
When mutual coupling is introduced, the voltage at the relay point A will be changed
according to equation 209.
æ
= × +
V Z 1 I 3 I
ç
ph 0
ph L
è
EQUATION1276 V4 EN-US
By dividing equation 209
the impedance present to the relay at A side as:
æ
×
3 0 I
= +
Z Z 1 1
ç
L
+
I ph 3 0
è
EQUATION1277 V3 EN-US
Where:
KNm = Z0m/(3 · Z1L)
FAULT
Z0m
Z
0m
IEC09000253_1_en.vsd
-
Z
Z 0 Z 1
× +
3 I
L L
0 p
×
3 1 3
Z
L
by equation 208 and after some simplification we can write
ö
KNm
÷
×
I KN
ø
Impedance protection
B
21
en05000221_ansi.vsd
C
ö
0
m
÷
×
1
Z
ø
L
Section 8
(Equation 209)
(Equation 210)
331