Siemens siprotec 7SA522 User Manual page 205

6.2.4 Distance Protection with MHO Characteristic
6.2.4.1 Method of Operation
Basic Circle
7SA522 Manual
C53000-G1176-C119-2
The Distance Protection 7SA522 may optionally be provided with polygonal tripping
characteristic or with a MHO circle characteristic, or with both depending on which ver-
sion was ordered. If both characteristics are available, they may be selected separate-
ly for phase–phase loops and phase–earth loops. If only the polygonal tripping char-
acteristic is used, this Sub-section 6.2.4 is of no further interest.
One MHO circle is defined for each distance zone, which represents the tripping char-
acteristic of the corresponding zone. In total there are five independent and one addi-
tional controlled zone for each fault impedance loop. The basic shape of a MHO circle
for one zone is shown in Figure 6-30 as an example.
Line
racteristic
Figure 6-30 Basic shape of a MHO circle characteristic
The MHO circle is defined by the line of its diameter which intersects the origin of the
coordinate system and the magnitude of the diameter which corresponds to the im-
pedance Z which determines the reach. The incline of the diameter line corresponds
r
to the line angle ϕ
. A load trapezoid with the setting parameters R
line
be used to cut the load area out of the circle. The reach Z
each zone; the inclination angle ϕ
and ϕ
are common to all zones. As the circle intersects the origin of the coordinate
load
system, a separate directional characteristic is not required.
jX
Line
racteristic
Z
r
Diameter
ϕ
line
R
load
cha-
as well as the load impedance parameters R
line
cha-
ϕ
load
and ϕ
load
may be separately set for
r
Functions
R
may
load
,
load
6-57