Siemens SIMEAS R-PMU, Manual, Page: 44 / 296

6
6.1 Measured Value Acquisition and Processing
44
Digital Fault Recorder, SIMEAS R-PMU, Manual
E50417-H1076-C360-A5, Release 10.2012
Reference frequency mode
The measured frequency is used as reference frequency for all cycle-based calculations. The
reference frequency is distributed to the modules in the device by the internal frequency master.
The following three setting options can be selected for the device using the OSCOP P parame-
terization software:
‰
Auto mode:
Depending on the signal quality and the voltage/current level, a reference
voltage/current channel is determined automatically from all DAUs that are located in the
system. The reference frequency channel with the best signal quality (lowest total harmonic
distortion (THD)) is determined automatically from this voltage/current group. All other
voltage or current channels in the system use the reference frequency from the reference
frequency channel.
‰
Stand Alone
mode: Every DAU automatically determines its own reference voltage/current
channel depending on the signal quality and signal level. Every further voltage or current
channel of the corresponding DAU uses the reference frequency from the reference
frequency channel.
‰
Slot, channel group
mode: The user defines a reference voltage group for a VCDAU or
VDAU in the device. The reference frequency channel with the best signal quality (lowest
THD value) is determined automatically from this voltage group. All other voltage or current
groups in the system use the reference frequency from the reference frequency channel. The
reference frequency cannot be defined for a mere CDAU system.
If no channel with the required voltage/current level (
<
10 % of the nominal value) is provided
in the selected reference voltage group, e. g., because a malfunction has occurred, the
system automatically switches to the
Auto
mode (see above). The return to the
Slot,
channel group
mode is also done automatically, as soon as the required voltage or current
level (
<
10 % of the nominal value) is provided again.
Frequency calculation
For the correct calculation of measured values and trigger values frequency is measured pre-
cisely. The measured frequency is determined per voltage or current group within the range of
f
n
/2 < f < f
n
+ 10. For this purpose, the THD is used to determine the channel of the group with
the best signal quality periodically.
The group frequency is marked as invalid, if:
−
all voltages or currents of the group are smaller than 10 % V
nom
or I
nom
,
−
all voltages or currents of the group change in form of a jump,
−
the given frequency range is left.
When leaving the given frequency range the frequency measurement displays the last valid
value.
The reference frequency will be marked as invalid if no valid group frequency is available
(depending on the reference frequency mode).
Note
For the reference frequency modes
Auto
and
Slot, channel group
all DAUs have to be oper-
ated with one signal frequency (e. g. connected to one bus bar).
For the reference frequency mode
StandAlone
, the DAUs may be operated with different signal
frequencies (e. g. DAU1 connected to an unsynchronized generator, DAU2 connected to the
net). The power calculation of coupled DAUs does not make sense for different signal frequen-
cies.