Siemens siprotec 7SD5 User Manual

2.5 Distance Protection
153
7SD5 Manual
C53000-G1176-C169-1
Figure 2-50 Polarized MHO characteristic with quadrature or memorized voltages
Selecting
Polarization
Incorrect directional decisions may be reached with short lines resulting in tripping or
blocking in spite of a reverse fault. This occurs because their zone reach is set very
small. Therefore their loop voltages are also very small, resulting in the phase angle
comparison between difference voltage and loop voltage being insufficiently accurate.
If phase angle comparison is performed using a polarization voltage consisting of a
loop voltage component recorded before the fault and a component of the current loop
voltage, these problems may be avoided. The following equation shows the polariza-
tion voltage U
P
for a Ph-E loop:
U
P
=(1–k
Pre
)·U
L-E
+k
Pre
·U
Ph-EMemorized
The evaluation (factor k
Pre
) of the prefault voltage may be set separately for Ph-E and
Ph-Ph loops. In general the factor is set to 15 %. The memory polarization is only per-
formed if the RMS value of the corresponding memorized voltage for Ph-E loops is
greater than a 40 % of the nominal voltage U
N
(address 204) and greater than a 70 %
of U
N
for Ph-Ph loops.
If there is no prefault voltage due to a sequential fault or energization onto a fault, the
memorized voltage can only be used for a limited time. For single-pole faults and two-
pole faults without earth path component a voltage which is not involved in the fault
may be used for polarization. This voltage is rotated by 90° in comparison with the
fault-accurate voltage (cross polarization). The polarization voltage U
P
is a mixed
voltage which consists of the valid voltage and the corresponding unfaulted voltages.
The following equation shows the polarization voltage U
P
for a Ph-E loop:
U
P
=(1–k
Cross
)·U
L-E
+k
Cross
·U
L-EUnfaulted
The cross polarization is used if there is no memorized voltage available. The evalu-
ation (factor k
Cross
) of the voltage may be set separately for Ph-E and Ph-Ph loops. In
general the factor is set to 15 %.
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Siemens siprotec 7SD5 Specifications

General

Brand	Siemens
Model	siprotec 7SD5
Category	Relays
Language	English