Signal
Description
Time base
U
3
RMS
RMS measurement of voltage U
3
/V
5ms
U
4
RMS
RMS measurement of voltage U
4
/V
5ms
Voltage measurement modes 3LN and 3LL use three voltage inputs: channels U
A
, U
B
and U
C
. When
the voltage mode is set to 2LL, only two channels (U
A
and U
B
) are in use, and the memory is based on
the line-to-line voltages U
12
and U
32
. When the mode 2LL+U0 is used, the memory is based on
calculated phase-to-neutral voltages.
Pick-up
VMEM activ
VMEM activaation v
tion volta
oltage
ge and Mea
Measur
sured curr
ed current condition 3I>
ent condition 3I>
When the voltage memory function is enabled, it activates when all line voltages drop below the "VMEM
activation voltage" threshold limit. This limit can be set to be anything between 2...50 V AC. When
"Measured current condition 3I>" is used, activation cannot be based on just the voltage. Therefore, at
least one of the three-phase currents must also rise above the set current pick-up setting.
VMEM ma
VMEM max activ
x active time
e time
Voltage memory can be active for a specific period of time, set in "VMAX active time". It can be
anything between 0.02...50.00 seconds. The function supports the definite time (DT) delay type. It
depends on the application for how long the memory should be used. During massive bolted faults, the
fault should be cleared and the breaker opened as soon as possible; therefore, a short operating time
for voltage memory is usually applied. A typical delay for voltage memory is between 0.5...1.0 s. When
the operating time passes and voltage memory is no longer used, directional overcurrent and/or
distance protection goes to the unidirectional mode to secure a safe tripping. The memory uses longer
operating times when a backup protection is applied (e.g. in distance-protection zones are
farther away).
FFor
orced C
ced CT f track
T f tracking on VMEM
ing on VMEM
While fixed frequency tracking is used, all protection stage-based sampling (apart from frequency
protection) is based on a set fixed frequency such as 50 Hz or 60 Hz. When the frequency drops
massively during a fault while angle memory is in use, it is also possible that the frequency of the
system starts to fluctuate. In such cases, if current sampling of used protection stages is based on 50/
60 Hz, there could be an error in current magnitude and in angle measurement. To minimize these
errors, it is recommended that the frequency is measured and protection-based sampling from the
current is performed while voltages are gone.
When the "Forced CT f tracking" parameter is activated and voltages are gone, the frequency from the
selected current-based reference channel 3 (the current from IL3) is used for current sampling. This
eliminates any possible measurement errors in the fixed frequency mode.
Figure. 5.3.20 - 156. Frequency reference channels.
For example, let us say a 500 A current is measured on the primary side while the fixed frequency is set
to 50 Hz. This results in the frequency dropping to 46 Hz, while the actual current measurement would
be 460 A. Therefore, the system would have an error of 40 A.
A
AQ
Q-T215
-T215
Instruction manual
Version: 2.04
240
