ABB RET650 Technical Manual page 142

Section 7
Impedance protection
136
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 ImpedanceAng+180°.
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 58). 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 34.
æ
- ×
U I ZxFwd
b =
Arg L1L2
ç
ç
- - ×
U ( I Zx Re v)
è
L1L2
IECEQUATION2320 V2 EN
where
is the U
L1L2
U
EQUATION1800 V1 EN
I X
L1L2·
× ZxFwd
I
L L 1 2
- × ZxRev
I
L L 1 2
IEC09000174 V2 EN
Figure 58: Simplified offset mho characteristic and voltage vectors for phase L1-
to-L2 fault.
Operation occurs if 90≤β≤270.
ö
÷
÷
ø
voltage
=
Ucomp U
1
L L 1 2
ß
U
Ucomp
1MRK 504 135-UEN A
- × ZxFwd
I
( )
L L 1 2
= + × ZxRev
2 U ( I )
L L 1 2 L l 1 2
I R
L1L2·
IEC09000174_2_en.vsd
Technical manual
(Equation 34)