ABB Relion 670 Series Technical Reference Manual page 252

Section 6
Impedance protection
246
I X
·
AB
I ZPP
×
AB
ANSI07000109 V1 EN
Figure 118: Simplified mho characteristic and vector diagram for phase A-to-B fault
Offset Mho
The characteristic for offset mho is a circle where two points on the circle are the
setting parameters ZPP and ZRevPP. The vector ZPP in the impedance plane has the
settable angle AngZPP and the angle for ZRevPP is AngZPP+180°.
The condition for operation at phase-to-phase fault is that the angle β between the two
compensated voltages Vcomp1 and Vcomp2 is greater than or equal to 90° (figure
119). The angle will be 90° for fault location on the boundary of the circle.
The angle β for A-to-B fault can be defined according to equation 35.
1MRK505222-UUS C
-
=V I ZPP
V
×
comp AB AB
ß
V
pol
V
AB
I ·R
AB
en07000109_ansi.vsd
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