Functions
2.5 Distance Protection
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
165
Figure 2-64 Polarized MHO characteristic
Properties of the MHO Characteristic
As the quadrature or memorized voltage (without load transfer) equals the corresponding generator voltage E
and does not change after fault inception (refer also to Figure 2-65), the lower zenith is shifted in the impedance
diagram by the polarization quantity k·Z
S1
=k·E
1
/I
1
. The upper zenith is still defined by the setting value Z
r
. For
the fault location F
1
(Figure 2-65a), the short-circuit is in the forward direction and the source impedance is in
the reverse direction. All fault locations right up to the device mounting location (current transformers) are
clearly inside the MHO characteristic (Figure 2-65b). If the current is reversed, the zenith of the circle diameter
changes abruptly (Figure 2-65c). A reversed current I
2
which is determined by the source impedance Z
S2
+ Z
L
now flows via the measuring location (current transformer) . The zenith Z
r
remains unchanged; it now is the
lower boundary of the circle diameter. In conjunction with load transport via the line, the zenith vector may ad-
ditionally be rotated by the load angle.
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