Amprobe DMIII-Multitest Power-Quality-Recorder - Manual (Page 100)

AMPROBE
DMIII MULTITEST
EN-98
16.9. MEASURING METHOD: OUTLINES
The instrument is able to measure: voltages, currents, active powers, inductive and
capacitive reactive powers, apparent powers, inductive and capacitive power factors,
analogic or impulse parameters. All these parameters are analyzed in a digital way: for
each phase (voltage and current), 6 x 128 samples are acquired on a module of 16 x
20ms, repeated for the three phases.
16.9.1. Integration periods
The storage of all the data would require a huge memory capacity.
Therefore we’ve tried to find out a storage method that compresses the information to be
memorized, while providing significant data.
The chosen method is
that of integration: after a certain period called “integration period”,
which can be set from 5 seconds to 60 minutes (3600sec), the instrument extracts from
the sampled values the following data:

Minimum value of the parameter during the integration period (harmonics excluded)

Medium value of the parameter (intended as arithmetic average of all the values
registered during the integration period)

Maximum value of the parameter during the integration period (harmonics excluded)
Only this information (repeated for each parameter to be memorised) is saved in the
memory along with starting time and date of the integration period.
Once these data are memorized, the instrument restarts to take measurements for a new
period.
16.9.2. Power factor calculations
According
to the standards in force, the medium power factor can’t be calculated as
average of the instantaneous power factors. It must be obtained from the medium values
of active and reactive power.
Each single medium power factor (of phase or total) is therefore calculated, at the end of
each integration period, on the medium value of the corresponding powers independently
on the fact that they must be registered or not.
Besides, for a better analysis of the type of load present on the line and in order to have
terms of comparison when studying the invoicing of the low cos

, the values of inductive
and capacitive cos

are treated as independent parameters.
16.9.3. Voltage Anomalies and Periods Analysis comparison
If You want to compare Voltage Anomaly and Periods Analysis results You should note that::
1. Voltage anomalies calculate RMS values over 10ms time while Periods Analysis
calculate MAX, MIN, RMS value over a module of 16 waveforms (16 x 16.66ms =
0.266ms). This generally implicates different values between Voltage Anomalies and
Periods Analysis results because the "Integration" time is different.
Example:
Integration Time = 60sec
If Your signal consists of 3600 waveforms (7200 half waveforms) and only the first half
waveform reach 246,6V while 7199 half waveforms stay at 240.6V You will get:
Voltage anomaly = 246.6V
Max of Periods analysis = 240.79V
AVG of Periods analysis = 240.6008V
MIN of Periods analysis = 240.6V