58 Function Description
WAGO-I/O-SYSTEM 750 XTR
750-495/040-00x 3-Phase Power Measurement Module 690 V
Manual
Version 1.2.0
4.4.1
2-Wattmeter Method (Aron Circuit/Blondel’s Theorem)
The measurement principle behind the 2-wattmeter method is based on the fact
that the sum of the line-to-line currents is zero (
𝐼𝐼
1
+
𝐼𝐼
2
+
𝐼𝐼
3
= 0
). The
measurement errors increase if this is not the case, e.g., due to flowing leakage
currents that can be caused by the type of installation.
In the “3-Wire Wye/Delta” topology, the line-to-line voltage U
L3L1
calculated
independently by the I/O module can have measurement errors greater than
indicated in the section “Measurement Accuracy.”
4.4.2
3-Wattmeter Method (Artificial Star Point – ASP)
The measurement principle behind the 3-wattmeter method is based on the sum
of the line-to-line currents being zero (
𝐼𝐼
1
+
𝐼𝐼
2
+
𝐼𝐼
3
= 0
). The measurement errors
increase if this is not the case.
Furthermore, measurement errors are dependent on the symmetry of the mains
voltages. Due to deviations caused by the structure and the asymmetry of real
mains voltages, phase voltages, individual powers and individual energies are
measured with sometimes significantly increased measurement errors. In
practical terms, only currents, external line voltages and overall outputs can be
measured with the values specified in the “Measurement Accuracy” section.
4.4.3
Harmonic Analysis
The upper harmonic components are determined on the basis of a number of
individual values measured within the current curve shapes for voltage and
current. From these, the included components of the current fundamental
frequency (45 Hz to 65 Hz) and its integer harmonics are calculated.
Upper harmonic components in the current curve shapes for measured voltages
and currents with non-integer multiples of the fundamental frequency can affect
the accuracy of the calculation results for “neighboring harmonics” (the so-called
“leakage effect”).
Abrupt changes to the line frequency cause time delays in the calculation of the
harmonic analysis. Therefore, line frequency jumps greater than 2 Hz should be
avoided.
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