Theory Of Operation - ABB Relion Technical Manual

1MRK 502 048-UUS A
7.2.6.4
Technical manual

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 ImpedanceAng+180°.
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 69). The
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 38.
æ
- ×
V I ZxFwd
b =
Arg ç AB
ç
- - ×
V ( I Zx Re v)
è
AB
ANSIEQUATION2320 V2 EN
where
is the V
AB
V
EQUATION1801 V1 EN
ö
÷
÷
ø
voltage
Section 7
Impedance protection
(Equation 38)
165