Operation P54x/EN OP/La4
MiCOM P543, P544, P545 & P546
(OP) 5-81
OP
AR Timer Block
(3rd & 4th stages only)
1
Phase Overcurrent Timer Block
(Independent for each stage)
Fast VTS
Block
Directional
Check
&
1
1
Directional
Check
&
Directional
Check
&
IDMT/DT
&
C Phase Start
C Phase Trip
IDMT/DT
&
IDMT/DT
&
B Phase Start
B Phase Trip
A Phase Start
A Phase Trip
VAB Polarizing
Memory
VAB Polarizing
Voltage
C Phase
Overcurrent
VCA Polarizing
Memory
VCA Polarizing
Voltage
B Phase
Overcurrent
VBC Polarizing
Memory
VBC Polarizing
Voltage
A Phase
Overcurrent
P1628ENb
Figure 54 Directional overcurrent logic
Any of the four overcurrent stages may be configured to be directional noting that IDMT
characteristics are only selectable on the first two stages. When the element is selected as
directional, a VTS Block option is available. When the relevant bit is set to 1, operation of
the Voltage Transformer Supervision (VTS), will block the stage if directionalized. When set
to 0, the stage will revert to non-directional upon operation of the VTS.
1.26 Synchronous polarization
For a
close up three-phase fault, all three voltages will collapse to zero and no healthy phase
voltages will be present. For this reason, the MiCOM P54x relays include a synchronous
polarization feature that stores the pre-fault voltage information and continues to apply it to
the directional overcurrent elements for a time period of 3.2 seconds. This ensures that
either instantaneous or time delayed directional overcurrent elements will be allowed to
operate, even with a three-phase voltage collapse.
1.27 Thermal overload protection
The relay
incorporates a current based thermal replica, using rms load current to model
heating and cooling of the protected plant. The element can be set with both alarm and trip
stages.
The heat generated within an item of plant, such as a cable or a transformer, is the resistive
loss (2R x t). Therefore, heating is directly proportional to current squared. The thermal
time characteristic used in the relay is therefore based on current squared, integrated over
time. The relay automatically uses the largest phase current for input to the thermal model.
Equipment is designed to operate continuously at a temperature corresponding to its full load
rating, where heat generated is balanced with heat dissipated by radiation etc.
Over-temperature conditions therefore occur when currents in excess of rating are allowed to
flow for a period of time. It can be shown that temperatures during heating follow
exponential time constants and a similar exponential decrease of temperature occurs during
cooling.
The relay provides two characteristics that may be selected according to the application.
Thermal overload protection may be disabled by DDB 478 Inhibit Thermal > .
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