Functions
2.5 Distance Protection
SIPROTEC, 7SD5, Manual
C53000-G1176-C169-5, Release date 02.2011
166
Figure 2-65
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 com-
ponent recorded before the fault and a component of the current loop voltage, these problems may be avoided.
The following equation shows the polarization 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 performed 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 reasons of accuracy. 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 polarisation. This voltage is rotated
by 90° in comparison with the fault-accurate voltage (cross polarization). The polarisation 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 polarisation is used if no memorized voltage is available. The evaluation (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 %.
Note
When switching onto a three-pole fault with the MHO characteristic, there is no memory voltage or unfaulted
loop voltage available. To ensure fault clearance when switching onto three-pole close-up faults, please make
sure that in conjunction with the configured MHO characteristic the instantaneous tripping function is always
enabled.