2.2 Distance protection
95
7SA522 Manual
C53000-G1176-C155-3
Figure 2-27 Polarized MHO circle with quadrature or memorized voltages
Selecting Polariza-
tion
False directional decisions may be made (tripping or blocking in spite of a reverse
fault) in short lines the zone reach of which must be very small and in small loop volt-
ages the phase angle comparison of which becomes inaccurate between difference
voltage and loop voltage. If phase angle comparison is performed using a polarization
voltage consisting of a loop voltage component recorded before the fault and a com-
ponent 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
Ph-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 ) 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
Ph-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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