Siemens siprotec 7SD5 User Manual

2.5 Distance Protection
151
7SD5 Manual
C53000-G1176-C169-1
Figure 2-48 Basic shape of an MHO characteristic
Polarized MHO
Characteristic
As is the case with all characteristics that pass through the origin of the coordinate
system, the MHO characteristic 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 circle. The
upper zenith which is determined by the reach setting Z
r
remains unchanged. Imme-
diately after fault inception, 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 2-49). 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-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 of the MHO characteristic.
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Siemens siprotec 7SD5 Specifications

General

Brand	Siemens
Model	siprotec 7SD5
Category	Relays
Language	English