ABB RELION RET670 Applications Manual page 204

Section 8
Impedance protection
198
Z0
A
Z0
B
IEC99000038 V1 EN-US
Figure 73: 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 IED point A is changed,
according to equation 36.
æ
= × +
V Z 1 I 3 I
ç
ph 0
ph L
è
EQUATION1276 V4 EN-US
By dividing equation 36
impedance present to the IED 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)
The second part in the parentheses is the error introduced to the measurement of the
line impedance.
If the current on the parallel line has negative sign compared to the current on the
protected line that is, the current on the parallel line has an opposite direction
compared to the current on the protected line, the distance function overreaches. If the
currents have the same direction, the distance protection underreaches.
Maximum overreach occurs if the fault infeed from remote end is weak. If we consider
a single phase-to-ground fault at "p" unit of the line length from A to B on the parallel
-
Z0
m
Z0 m
-
Z0
m
-
Z 0 Z 1 Z
× +
3 I
L L
0 p
×
3 Z 1 3
L
by equation 35 and after some simplification we can write the
ö
KNm
÷
×
I KN
ø
1MRK 504 163-UUS A
C
99000038.vsd
ö
0
m
÷
×
Z 1
ø
L
Transformer protection RET670 2.2 ANSI
Application manual
(Equation 36)
(Equation 37)