ABB ret650, Technical Manual

Page: 142 / 670

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
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 .
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