Siemens siprotec 7SA522 User Manual page 206

Functions
Polarized
MHO Circle
Characteristics of
the MHO Circle
6-58
As is the case with all characteristics that pass through the origin of the coordinate sys-
tem, the MHO circle boundary around the origin itself is also not defined as the meas-
ured voltage is zero or too small to be evaluated in this case. For this reason, the MHO
circle is polarized. The polarization determines the lower zenith of the circle, i.e. the
lower intersection of the diameter line with the circle. The upper zenith which is deter-
mined by the reach setting Z
the short-circuit voltage is disturbed by transients; the voltage memorized prior to fault
inception is therefore used for polarization. This causes a displacement of the lower
zenith by an impedance corresponding to the memorized voltage (refer to Figure 6-
31). When the memorized short-circuit voltage is too small, a quadrature voltage (from
an unfaulted loop) is used. In theory this voltage is perpendicular to the voltage of the
faulted loop for both phase–earth loops as well as phase–phase loops. This is taken
into account by the calculation by means of a 90° rotation. The unfaulted loop voltages
also cause a displacement of the lower zenith.
jX
k · Z
S
Figure 6-31 Polarized MHO circle
As the quadrature or memorized voltage (without load transfer) equals the corre-
sponding generator voltage E and does not change after fault inception (refer also to
Figure 6-32), the lower zenith is shifted in the impedance plane by the polarizing quan-
tity k·Z = k · E /I . The upper zenith is still defined by the setting value Z
S1 1 1
fault location F (Figure 6-32a) the short-circuit location is in the forward direction and
1
the source impedance is in the reverse direction. All fault locations, right up to the de-
vice location (current transformers) are clearly inside the MHO circle (Figure 6-32b). If
the current is reversed, the zenith of the circle diameter changes abruptly (Figure 6-
32c). A reversed current I
which is determined by the source impedance Z
changed; it now is the lower boundary of the circle diameter. In conjunction with load
transport via the line, the zenith vector may additionally be rotated by the load angle.
remains unchanged. Immediately after fault inception,
r
Line-
characteristic
Z
r
Diameter
R
load
now flows via the measuring location (current transformer)
2
S2
Z = Source impedance
S
k = Weighting factor 0.15 for
polarizing voltage
+ Z . The zenith Z remains un-
L r
7SA522 Manual
C53000-G1176-C119-2
R
. For the
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