User manual
PQM-702, PQM-703 Operating Manual
96
6.4.10 Estimating measurement uncertainty values for power and energy
The total measurement uncertainty for power, active and reactive energy and harmonics is
based on the following relation (for energy we ignore the additional uncertainty due to time meas-
urement, as it is much smaller than other uncertainties):
where:
P,Q
– measurement uncertainty for active or reactive power,
Uh
– total measurement uncertainty of voltage harmonic amplitude (analyzer, transducers),
Ih
– total measurement uncertainty of current harmonic amplitude (analyzer, transducers),
ph
– additional uncertainty of the measurement of the phase between voltage and current
harmonics.
ph
uncertainty may be calculated when the phase angle is known for the considered frequency
band. Tab. 7 shows the phase error between voltage and current harmonics for PQM-702 and PQM-
703 analyzers (without clamps and transducers).
Tab. 7. Phase error of PQM-702 and PQM-703, depending on the frequency.
Phase difference error
Frequency range
40..70 Hz
70..200 Hz
200..500 Hz
500 Hz..1 kHz
1..2 kHz
2..3 kHz
Error
≤0.5
≤1
≤2.5
≤4
≤7
≤10
Phase error introduced by transducers and clamps may be usually found in their technical doc-
umentation. In this case, we need to estimate the resultant phase error between the voltage and
the current for a given frequency caused by all elements of the measuring circuit: current and volt-
age transducers, clamps, and the analyzer.
The uncertainty of the harmonics active power measurements may be calculated according to
the following formula:
,
On the other hand, the uncertainty of the harmonics reactive power measurement may be cal-
culated according to the following formula:
,
In both formulas, means the actual phase shift angle between the current and voltage com-
ponents, and means the total phase error for a given frequency. The conclusion which can be
drawn from these relationships is that power measurement uncertainty for the same phase error
very clearly depends on the displacement power factor between current and voltage. It is shown in
Fig. 43.