Operation P54x/EN OP/La4
MiCOM P543, P544, P545 & P546
(OP) 5-35
OP
Key: Zs = Source impedance behind the relay location
The MiCOM P54x does not allow the polarizing to be selected as entirely self polarized, or
entirely memory polarized. Vpol always contains the directly measured self-polarized
voltage, onto which a percentage of the pre-fault memory voltage can be added. The
percentage memory addition is settable within the range 0.2 (20%) to 5 (500%).
Setting 20% means that the majority of the polarizing will be self-polarizing, with minimal
mho circle expansion, and just enough memory to counteract any CVT transients. Setting
500% means that in the overall polarizing mix the ratio would be 1 part self polarizing to 5
parts memory. Such a high memory content would offer large dynamic expansion, covering
83% of the source impedance (Zs) behind the relay.
Mho expansion = [(Polarizing Setting)/(Setting + 1)] . Zs
This characteristic is used for Zones 1, P (optionally reversed), 2, 4 and Zone 3 if the offset
is disabled.
The characteristic is generated by a phase comparison between V/-Z and the polarizing
signal Vpol
Where:
V is the fault voltage
Vpol is a user selected mix of the fault voltage and prefault memory
is the fault current
Z is the zone reach setting (including residual compensation for
ground fault elements)
Zs is the source impedance (included in Figure 18 to show the position
of the Vpol phasor)
The polarizing signal Vpol is a combination of the fault voltage and the stored vector taken
from 2 cycles before the fault, which is a representation of the volts at the source.
Vpol =
Zs + V
or
Vpol/
= Zs + V/
Operation occurs when the angle between the signals is greater than 90°, which is for faults
inside the circle.
The validity of the voltage memory in MiCOM P54x extends to 16 cycles after loss of the VT
input voltage. If no memory is available, the polarizing signal is substituted by cross
polarizing from the unfaulted phase(s). For example if Vamem is unavailable, the voltages
measured on phases B and C now are used, phase-shifted as necessary.
To produce the reversed zones (Zone 4 and, optionally, Zone P), the impedance Z is
automatically set to a negative value.
1.11.1 Switch on to fault action for zone 1 (Distance option only)
Operation
of the distance elements is generally prevented if the polarizing signal magnitude
is insufficient (less than 1V). The exception is for Zone 1, which following breaker closure is
allowed to operate with a small (10%) reverse offset. This is to ensure operation when
closing on to a close-up three phase fault (Scenario: earthing/ground clamps inadvertently
left in position).
In addition Z4 reverse operation is held if it operates in memory.
Other zones may have their zone time delays bypassed for SOTF/TOR, as detailed in the
application notes.
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