The characteristic for offset mho is a circle where two points on the circle are the
setting parameters
ZxFwd
and
ZxRev
. The vector
ZxFwd
in the impedance plane has
the settable angle
ImpedanceAng
and the angle for
ZxRev
is
Imped180°
.
The condition for operation at phase-to-phase fault is that the angle β between the two
compensated voltages Ucomp1 and Ucomp2 is between 90° and 270° (figure
). The
angle will be 90° or 270° for fault location on the boundary of the circle.
The angle β for L1-to-L2 fault can be defined according to equation
.
L1L2
L1L2
U I
ZxFwd
Arg
U ( I
Zx Re v)
æ
ö
-
×
b =
ç
÷
ç
÷
- -
×
è
ø
IECEQUATION2320 V2 EN
(Equation 34)
where
U
EQUATION1800 V1 EN
is the U
L1L2
voltage
ß
U
I
L1L2·
X
I
L1L2·
R
1 2
×
ZxFwd
L L
I
1 2
-
×
ZxRev
L L
I
1 2
1 2
2
(
)
=
+
×
ZxRev
L L
L l
Ucomp
U
I
1 2
1 2
1
(
)
=
-
×
ZxFwd
L L
L L
Ucomp
U
I
IEC09000174_2_en.vsd
IEC09000174 V2 EN
Figure 58:
Simplified offset mho characteristic and voltage vectors for phase L1-
to-L2 fault.
Operation occurs if 90≤β≤270.
Section 7
1MRK 504 135-UEN A
Impedance protection
136
Technical manual
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