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6 Technical data
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 measurement, as it is much smaller than other uncertainties):
2
2
2
𝛿𝑃,𝑄 ≅ √𝛿𝑈ℎ
+ 𝛿𝐼ℎ
+ 𝛿𝑝ℎ
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. 8 shows the phase error between voltage and current harmonics for analyzers
(without clamps and transducers).
Tab. 8. Phase error of PQM-702/703/710/711 analyzers, depending on the frequency.
Frequency range
Error
Phase difference error
40..70 Hz
70..200 Hz
≤0.5
≤1
200..500 Hz
≤2.5
500 Hz..1 kHz
≤4
1..2 kHz
≤7
2..3 kHz
≤10
Phase error introduced by transducers and clamps may be usually found in their technical
documentation. 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
voltage transducers, clamps, and the analyzer.
The uncertainty of the harmonics active power measurements may be calculated according to
the following formula:
𝛿𝑝ℎ = 100 (1 −
cos(𝜑+Δ𝜑)
𝑐𝑜𝑠𝜑
) [%], cos 𝜑 ≠ 0
On the other hand, the uncertainty of the harmonics reactive power measurement may be calculated according to the following formula:
𝛿𝑝ℎ = 100 (1 −
sin(𝜑−Δ𝜑)
𝑠𝑖𝑛𝜑
) [%], sin 𝜑 ≠ 0
In both formulas,  means the actual phase shift angle between the current and voltage components, 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. 57.
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