7.2.6.4
Theory of operation
The mho algorithm is based on the phase comparison of a operating phasor and a
polarizing phasor. When the operating phasor leads the polarizing phasor by more than 90
degrees, the function operates and gives a trip output.
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 Vcomp1 and Vcomp2 is between 90° and 270° (figure
angle will be 90° or 270° for fault location on the boundary of the circle.
The angle β for A-to-B fault can be defined according to equation
.
AB
AB
V I
ZxFwd
Arg
V ( I
Zx Re v)
æ
ö
-
×
b =
ç
÷
ç
÷
- -
×
è
ø
ANSIEQUATION2320 V2 EN
(Equation 38)
where
V
EQUATION1801 V1 EN
is the V
AB
voltage
1MRK 502 048-UUS A
Section 7
Impedance protection
165
Technical manual