U
4
RMS
Fundamental RMS measurement of voltage U
4
/V
5 ms
Voltage measurement mode 3LN and 3LL use three voltage inputs channels U
A
, U
B
and U
C
. When
voltage mode is set to 2LL only two channels U
A
and U
B
are being used. In this mode voltage memory
is based on line-to-line voltages U12 and U23. In case 2LL+U0 mode is used, voltage memory is based
on calculated phase-to-neutral voltages.
Pick-up characteristics
VMEM activation voltage and measured current condition 3I>
When voltage memory is enabled, it activated when all line voltages drop below “VMEM activation
voltage” threshold limit. This limit is settable between 2...50VAC. In case “Measured current condition
3I>” is used, activation based on mere voltage is not enough. Then, also at least one of three phase
currents need to raise above the set current pick-up setting. Voltage memory can be active for period
of set “VMAX active time”
VMEM max active time
VMEM function supports de nite time (DT) delay type. How long voltage memory should be used
depends of the application. Normally during massive bolted faults, the fault should be cleared, and
breaker opened as soon as it is possible. Therefore, normally a short operating time for voltage
memory is applied. Typical delay for voltage memory is 0.5 to 1.0 seconds. When operating time
passes and voltage memory is no longer used, directional overcurrent/distance protection goes to
unidirectional mode to secure safe tripping. Longer operating times for VMEM are used in case backup
protection is applied, for example in distance protection a zone(s) further away. VMEM max active time
is settable between 0.02...50.00 seconds.
Figure. 5.4.22. - 108. Voltage angle drift is approximately 1 degree in a second while voltage memory is in use.
While voltage memory is active the voltages are absent and therefore angle measurement is not
possible. Healthy state angles (before fault) are being used during fault. Due to this, a drift between
assumed voltage angle and actual measured phase current angle is taking place. While voltage
memory is used the angle of phase currents drifts for approximately 1 degree per each passing
second.
Forced CT f tracking on VMEM
While xed frequency tracking is used, all protection stage-based sampling (Except frequency
protection) is based on set xed frequency like 50/60Hz. In case frequency drops massively during fault
and angle memory is used, it is also possible that frequency of the system starts to fluctuate. In such
cases, if current sampling of used protection stages is based on 50/60Hz, there could be an error in
current magnitude and angle measurement. To minimize errors, it is preferable that while voltages are
gone, it is better to measure frequency, and to also perform protection-based sampling from the
current.
AQ-F255
Instruction manual
Version: 2.00
© Arcteq Relays Ltd
184