In close-in faults the system voltage on secondary side may fall under several volts or close to nothing.
In such case when measured voltage is absent, the fault direction is not possible to be solved. As
backup, un-directional protection can be used for tripping, but in such cases selectivity of network will
reduce. To prevent this from happening, an angle memory for voltage can be used. Adjustable voltage
level with pre-fault voltage angles can be used as a reference for fault direction/distance, for duration
that can be set manually. Due to con gurable voltage memory, even time delayed backup tripping can
be initiated.
Voltage memory activation and all related settings are found at
Measurements
→
Transformers
→
VT
Module (3/4U) 1
→
Voltage memory
(activated/disabled)
Activation of voltage memory depends of following criteria’s:
1. All used line-to-line or line-to-neutral voltages need to be below set “VMEM activation voltage”.
2. Least one phase current must be higher than set “Measured current condition 3I>”. This setting
limit is optional.
Figure. 5.4.22. - 107. Distance protection characteristics and directional overcurrent.
When above mentioned criteria are met, voltage memory will activate. VMEM will use “VMEM
activation voltage” as voltage amplitude even, when actual measured voltage has decreased lower or
close to nothing. Used angle is captured a moment before the fault has occurred and voltage memory
has been activated. When VMEM is activate, a speci c output “Voltage memory on” is switched on.
This signal can be found under device IO matrix.
Voltage memory blocking signal can be found among other stage related settings in “VT Module (3/4U)
1” -menu. In the blocking element the block signal is checked in the beginning of each program cycle.
Measured input values
Function block uses analog voltage and current measurement fundamental frequency RMS values.
Table. 5.4.22. - 172. Analogic magnitudes used by the Voltage angle memory function.
Signal
Description
Time base
IL1RMS
Fundamental RMS measurement of phase L1/A current
5 ms
IL2RMS
Fundamental RMS measurement of phase L2/B current
5 ms
IL3RMS
Fundamental RMS measurement of phase L3/C current
5 ms
U
1
RMS
Fundamental RMS measurement of voltage U
1
/V
5 ms
U
2
RMS
Fundamental RMS measurement of voltage U
2
/V
5 ms
U
3
RMS
Fundamental RMS measurement of voltage U
3
/V
5 ms
AQ-F255
Instruction manual
Version: 2.00
183
© Arcteq Relays Ltd