Functions
2.5 Distance Protection
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
164
Figure 2-63
Basic shape of an MHO characteristic
Polarised MHO characteristic
As is the case with all characteristics that pass through the origin of the coordinate system, the MHO charac-
teristic boundary around the origin itself is also not defined as the measured voltage is zero or too small to be
evaluated in this case. For this reason, the MHO characteristic is polarized. The polarization determines the
lower zenith of the circle, i.e. the lower intersection of the diameter line with the circumference. The upper zenith
which is determined by the reach setting Z
r
remains unchanged. Immediately after fault inception, the short-
circuit voltage is disturbed by transients; the voltage memorized prior to fault inception is therefore used for po-
larization. This causes a displacement of the lower zenith by an impedance corresponding to the memorized
voltage (refer to Figure 2-64). When the memorized short-circuit voltage is too small, an unfaulted voltage is
used. In theory, this voltage is perpendicular to the voltage of the faulted loop for both phase-to-earth loops as
well as phase-to-phase loops. This is taken into account by the calculation by means of a 90° rotation. The un-
faulted loop voltage also causes a displacement of the lower zenith of the MHO characteristic.
