f
Upol
IL1 (Ref)
en06000472_2.vsd
IEC06000472 V2 EN
Figure 70: Simplified offset mho characteristic and vector diagram for phase L1-
to-earth fault
Operation occurs if 90≤β≤270.
Offset mho
The characteristic for offset mho at earth fault is a circle containing the two vectors
from the origin ZPE and ZRevPE where ZPE and ZrevPE are the setting reach for the
positive sequence impedance in forward respective reverse direction. The vector ZPE
in the impedance plane has the settable angle AngZPE and the angle for ZRevPP is
AngZPE+180°.
The condition for operation at phase-to-earth fault is that the angle β between the two
compensated voltages Ucomp1 and Ucomp2 is greater or equal to 90° see figure
71.
The angle will be 90° for fault location on the boundary of the circle.
The angle β for L1-to-earth fault can be defined as
L1 L1L
L1 L1
U -I
arg
U -(-I Re )
ZPE
Z vPE
×
b =
×
æ ö
ç ÷
è ø
EQUATION1802 V1 EN
(Equation 41)
EQUATION1
805 V1 EN
is the phase L1 voltage
Section 7 1MRK502052-UEN B
Impedance protection
190
Technical manual
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