ABB RELION 670 SERIES Applications Manual page 172

Section 8
Impedance protection
166
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 line
for the case when the fault infeed from remote end is zero, we can draw the voltage V in
the faulty phase at A side as in equation 33.
(
V = Z 1 I + K
×
p
A L ph N
EQUATION1278 V4 EN
Notice that the following relationship exists between the zero sequence currents:
3 0 3 0
I Z I Z
⋅ = ⋅
0 L p
EQUATION1279 V3 EN
Simplification of equation 34, solving it for 3I0p and substitution of the result into
equation 33 gives that the voltage can be drawn as:
æ
V Z 1 ç I
= × +
p
A L ph
è
EQUATION1280 V2 EN
If we finally divide equation
the IED as
)
I K I
× + ×
3 3
0 Nm 0 p
0 2 ( )
p
−
L
I
3
K I K 0
× + ×
3
N 0 Nm
-
2
35 with equation 30
1MRK 506 369-UUS -
ö
×
p
÷
ø
p
we can draw the impedance present to
Line distance protection REL670 2.2 ANSI
Application manual
(Equation 33)
(Equation 34)
(Equation 35)